When we get a prescription from a doctor, the drug usually comes with instructions for how to use it. These include when we take the drug (before or after a meal), how many times a day and for how long we take it, and what we have to avoid while using the drug. Doctors also ask if we use any other medication or if we have any chronic disease before prescribing any new medication. Most medicines look similar, but drugs themselves can be very different. Have you thought about what happens to a drug when it gets into our body?

All body functions and disease processes—as well as most drug reactions—occur at the cellular level. Drugs are chemicals that alter basic processes in body cells. They cannot add functions and activities but can only affect normal cellular functions and activities. Drugs given for systemic effects must reach adequate concentrations in the blood and other tissue fluids surrounding our cells to have an effect. Drugs must circulate through their body, reach their intended cells, and, after affecting the cells, be eliminated from the body. But how do all these processes occur? How do systemic drugs reach, interact with, and leave the body’s cells?

The answers to these questions are answered by one of the main domains of pharmacology—pharmacokinetics. Pharmacokinetics is derived from the Greek words pharmakon (drug) and kinetikos (movement) and means “what the body does to a drug.” It refers to the movement of a drug into, through, and out of the body.

Knowledge of pharmacokinetic principles helps healthcare providers to adjust dosages more accurately and rapidly. Application of pharmacokinetic principles to individualize pharmacotherapy is termed therapeutic drug monitoring and aims to give the exact amount of drug to each person.

The four main parts of the pharmacokinetic processes are

• Absorption
• Distribution
• Metabolism
• Elimination

These processes are abbreviated as ADME.

Figure. Entry and movement of drug molecules through the body to sites of action, as well as metabolism and excretion 

Absorption

Absorption is the process that occurs when a drug enters the body until it enters the bloodstream to be circulated. The duration of a drug’s action is largely determined by the rate of absorption, and intensity is determined by the extent of absorption.

Numerous factors affect the rate and extent of drug absorption, including dosage, form, route of administration, blood flow to the site of administration, gastrointestinal function, and the presence of food or other drugs.

Most oral drugs must be swallowed, dissolved in gastric fluid, and delivered to the small intestine (which has a large surface area for absorption of nutrients and drugs) before they are absorbed. Liquid medications are absorbed faster than tablets or capsules because they need not be dissolved. Rapid movement through the stomach and small intestine may increase drug absorption by promoting contact with absorptive mucous membrane; it also may decrease absorption because some drugs may move through the small intestine too rapidly to be absorbed. For many drugs, the presence of food in the stomach slows the rate of absorption and may decrease the amount of drug absorbed. That is why some drugs are taken before meals and some after.

Drugs cannot be absorbed well when a patient has diarrhea and the intestines are working excessively; the effect of the drug will be reduced. Absorption may be increased if the patient has constipation and the intestines are working very slowly; this could have unforeseen adverse effects.

Drugs injected into subcutaneous (under the skin) or intramuscular (in the muscles) tissues are usually absorbed more rapidly than oral drugs because they move directly from the injection site to the bloodstream. Absorption is rapid from muscle sites because muscle tissue has abundant blood supply. Drugs injected intravenously (into the veins) do not need to be absorbed because they are placed directly into the bloodstream.

Other absorptive sites include the skin, mucous membranes, and lungs. Most drugs applied to the skin are given for local effects (e.g., muscle relaxant creams, itch relief ointments, and burn ointments). Systemic absorption is minimal from intact skin but may be considerable when the skin is inflamed or damaged. Also, a number of drugs have been formulated in adhesive skin patches for absorption through the skin. Some drugs applied to mucous membranes also are given for local effects. However, systemic absorption occurs from the mucosa of the oral cavity, nose, eyes, etc. Drugs absorbed through mucous membranes pass directly into the bloodstream. For example, nitroglycerin sublingual tablets are used to treat episodes of angina (chest pain) in people who have coronary artery disease. It is also used just before activities that may cause episodes of angina to prevent symptoms. It works by relaxing the blood vessels so the heart does not need to work as hard and therefore does not need as much oxygen. The lungs can be a good absorption site, as they have a large surface area for absorption of anesthetic gases and a few other drugs.

Distribution

Distribution involves the transport of drug molecules within the body. Once a drug is injected or absorbed into the bloodstream, it is carried by the blood and tissue fluids to its sites of pharmacologic action, metabolism, and excretion. Most drug molecules enter and leave the bloodstream at the capillary level, through gaps between the cells that form capillary walls.

Distribution depends largely on the adequacy of blood circulation. Drugs are distributed rapidly to organs receiving a large blood supply, such as the heart, liver, and kidneys. Distribution to other internal organs, muscle, fat, and skin is usually slower.

An important factor in drug distribution is protein binding. Most drugs form a complex with plasma proteins such as albumin. These proteins act as carriers for the drugs. Drug molecules bound to plasma proteins are pharmacologically inactive because the large size of the complex prevents their leaving the bloodstream through the small openings in capillary walls and reaching their sites of action. Only the free or unbound portion of a drug acts on the body’s cells. As the free drug acts on cells, the decrease in plasma protein levels causes some of the bound drug to be released.

Protein binding allows part of a drug dose to be stored and released as needed. Some drugs also are stored in muscle, fat, or other body tissues and released gradually when plasma drug levels fall. These storage mechanisms reduce the risk of toxicity. Drugs that are highly bound to plasma proteins or stored extensively in other tissues have a long duration of action. This affects the frequency of intake of a drug and is why some drugs are taken more frequently.

Drug distribution into the central nervous system (CNS) is limited because of the blood–brain barrier, which is composed of capillaries with tight walls and limits the movement of drug molecules into brain tissue. This barrier has been created to act as a selectively permeable membrane to protect the CNS. However, it also can make drug therapy for CNS disorders more difficult because drugs must pass through the cells of the capillary wall rather than between cells. As a result, only drugs that are lipid soluble or have a transport system can cross the blood–brain barrier and reach therapeutic concentrations in brain tissue.

Drug distribution during pregnancy and lactation is also unique. During pregnancy, most drugs cross the placenta and may affect the fetus. During lactation, many drugs enter breast milk and may affect the nursing infant. This barrier is also important and protects the fetus from the adverse effects of drugs.

If we have valuable things at home or at work, we put them in extra safe places and use extra security systems to protect them. So it is with our body: our brain and a fetus are extra protected by these barriers.

Metabolism

Metabolism is the chemical reaction by which drugs are inactivated or bio-transformed by the body. Most often, an active drug is changed into one or more inactive metabolites, which are then excreted. Some active drugs yield metabolites that are also active and that continue to exert their effects on the body’s cells until they are metabolized further or excreted. Other drugs (called prodrugs) are initially inactive and exert no pharmacologic effects until they are metabolized.

Most drugs are lipid soluble, a characteristic that aids their movement across cell membranes. However, the kidneys, which are the primary excretory organs, can excrete only water-soluble substances. Therefore, one function of metabolism is to convert fat-soluble drugs into water-soluble metabolites.

Hepatic drug metabolism—or clearance—is a major mechanism for terminating drug action and eliminating drug molecules from the body. Most drugs are metabolized by enzymes in the liver, red blood cells, plasma, kidneys, lungs, and gastrointestinal mucosa.

With long term administration, some drugs stimulate liver cells to produce larger amounts of drug metabolizing enzymes (a process called enzyme induction). Enzyme induction accelerates drug metabolism because larger amounts of the enzymes allow larger amounts of a drug to be metabolized during a given time. As a result, larger doses of the rapidly metabolized drug may be required to produce or maintain therapeutic effects. Metabolism also can be decreased or delayed in a process called enzyme inhibition, which most often occurs with concurrent administration of two or more drugs that compete for the same metabolizing enzymes. In this case, smaller doses of the slowly metabolized drug may be needed to avoid adverse reactions and toxicity from drug accumulation. Thus, doctors always ask about any medications already being used before prescribing a new medication. Also, some foods (eg. grapefruit), drinks (alcohol), and smoking affect the activity of the enzymes which are metabolizing the drugs in the liver. Thus, we must be careful and follow a doctor’s instructions while using any medication.

The rate of drug metabolism is also reduced in infants (their hepatic enzyme system is immature), in people with impaired blood flow to the liver or severe hepatic or cardiovascular disease, and in people who are malnourished or on low-protein diets. Metabolism in  elderly people and pregnant women also changes a lot; thus, the dose given to each individual should be carefully calculated—especially in children, the elderly, and pregnant women.

Excretion

Excretion refers to the elimination of a drug from the body. Effective excretion requires adequate functioning of the circulatory system and of the organs of excretion (kidneys, bowel, lungs, and skin). Most drugs are excreted by the kidneys and eliminated unchanged or as metabolites in urine. Some drugs or metabolites are excreted in bile, then eliminated in feces; others are excreted in bile, reabsorbed from the small intestine, returned to the liver (called enterohepatic recirculation), metabolized, and eventually excreted in urine. Some oral drugs are not absorbed and are excreted in the feces. The lungs mainly remove volatile substances, such as anesthetic gases. The skin has minimal excretory function. Factors impairing excretion, especially severe renal disease, lead to accumulation of numerous drugs and may cause severe adverse effects if dosage is not reduced. Doctors always ask about any chronic diseases before prescribing any medication.

The process of developing a drug is long and difficult. The instructions for usage are decided after many experimental and clinical trials. None of the instructions given with medications are decided randomly; thus, we must completely follow the instructions to maximize the positive effects of a drug and to avoid any possible adverse effects.

References

• Abrams, Anne Collins., Carol Barnett Lammon, and Sandra Smith Pennington. Clinical Drug Therapy: Rationales for Nursing Practice. 7th ed. Philadelphia: Lippincott Williams & Wilkins, 2007.
• Basic&Clinical Pharmacology, 12^th Edition, McGrawHill Lange.
• Modern Pharmacology with Clinical Applications, Sixth Edition, Charles R. Craig and Robert E. Stitzel, Lippincott Williams &Wilkins.

There are many ongoing studies into fasting, a practice prescribed across many religions. Increasingly, there is evidence to support that intermittent fasting is beneficial to human health.

The opposite of fasting – overeating – has been revealed to be a major culprit in many illnesses, including cancer, obesity, and heart disease. For many years, the medical consensus was that fasting, i.e. prolonged hunger, too, could have deleterious effects on the human body. From kidney failure to loss of muscle, fasting was believed to be harmful – thus calling into question the benefits of this widespread religious commandment.

People of faith – including Muslims – have long believed that God wouldn’t recommend a practice that was harmful to the body. The hardships and troubles that accompany a religious practice are not too extreme for people aware of their servitude to God: they regard their trials as a testament of their faith. Part of that faith is the certainty that God wouldn’t recommend a harmful or unbearable practice.

In this article, we will share with you two unknown benefits of fasting that have been recently discovered.

Regenerating stem cells

There is a substantial body of evidence showing that staying hungry during certain periods of the day heals the body’s metabolism, hastens loss of fat, decreases oxidative stress,* and improves the functions of the tissues that make up various organs including the liver, the intestines, and the brain. The first of the two new discoveries about the underlying processes, however, provides a missing piece of the puzzle by helping us understand the incredible changes hunger triggers in stem cells.

Researchers at MIT, Duke University School of Medicine, and Whitehead Institute for Biomedical Research in Cambridge published an article recently on stem cells, indicating that part of stem cell’s mystery could lie in the oxidation (burning) of fat in the mitochondria [1]. Omer H. Yilmaz and his fellow researchers found that a 24-hour fast hastens fat breakdown in intestinal stem and special progenitor cells of rats.

To run the study, Yilmaz and colleagues let the mice go hungry for 24 hours to study the state of their stem cells. They found that the functions of intestinal stem cells increased, and fat metabolism quickened, in both young and aged mice, even during early periods of hunger. They saw that the body’s tapping into fat for its energy needs maintained the health and strength of the intestinal stem cells. Moreover, they noticed that if the aged mice did not fast, they started to lose their ability to break down and use fats for energy expenditure.

The researchers obtained more interesting results as the studies progressed. It was found that a single period of hunger for 24 hours boosted renewal of intestinal cells – and the stem cell functions increased even more significantly in aged mice. Another interesting finding was that mice with damaged intestines that were fasted recovered faster than those that were fed.

“My lab is really interested in understanding how diet, in general, can be used to improve tissue function,” Yilmaz said. “One of the tissue types I study is the intestine. In my lab we study the intestine because it’s one of the largest organs in the body. It’s also a tissue that experiences rapid cellular turnover.” [2]

The intestine is lined by a single layer of cells, Yilmaz explains, that turns over every 5 to 7 days. The workhorses of the intestinal lining and this cellular turnover are intestinal stem cells. These cells must retain a high level of function or cellular health in order to replenish the intestinal epithelium on a regular basis. Intestinal stem cells are particularly important in terms of repairing intestinal damage caused by gut infections and chemotherapy, for example.

The single layer of epithelial cells needs to be renewed every 5-7 days: the aids and enzymes secreted in the intestine for digestive and absorptive activities damage cells despite the protective mucus layer, and some other cells already burst and die as they empty their secretions. Moreover, some medications, particularly chemotherapy, cause the destruction and breakdown of the epithelial cell layer. However, fast-multiplying stem cells replenish the epithelial cells. Stem cells are very active and young and have the ability to divide and multiply continuously.

Dr. Yilmaz also says:

“As we age, stem cells in the intestine as well as in many other tissues of the body, including in the blood and nervous systems, become less functional. We believe that reduced adult stem cell function contributes to some of the decline of function associated with old age. My lab is very interested in studying low-calorie interventions to delay this decline. As a field, we’ve known for over 100 years that low-calorie states such as fasting or caloric restriction can have positive effects on tissue health and aging. We’ve seen evidence that fasting during times of intestinal infections that lead to diarrhea may promote healing of the intestinal lining, for example.” [2]

This quote emphasizes the importance of the issue. Yilmaz adds that despite all this knowledge, the cellular mechanisms of this renewal have not been discovered, and he and his team are working to find out how fasting and hunger enable this recovery.

Stem cells become happy in fat!

The researchers discovered through the experiments that the stem cell function could be brought about in hungry mice by the burning (oxidation) of fatty acids in intestinal cells. When they stopped the fat metabolism through genetic engineering, they noticed that the benefits of fasting on intestinal stem cells were negligent.

In the present dietary conditions, we obtain nearly 60-70% of our energy from carbohydrates or sugar, 20% from fats and 10% from amino acids. Yet an interesting finding revealed by Dr. Yılmaz and his research team is that we essentially obtain much greater energy from using fats once we fast. According to their experiments on mice, during fasting, the intestinal stem cells in both young and aged mice switch from carbohydrates to fats as the primary source of energy, and this shift enables improvement in stem cell functions.

It is not yet known what underlies the fat metabolism that boosts stem cell functions in response to fasting, but it is observed that stem cells work better when they burn fat. The ability to metabolize fats efficiently decreases with age.

It is likely that this hastened metabolism – encouraged by certain diets, like the keto diet, where the amount of fat ingested is raised to 70% and carbohydrate intake is limited to 5% – helps with epileptic seizures and similar neurological disorders: the ketone bodies generated during metabolism of fats are used as energy sources by the brain.

According to Dr. Yilmaz, if fasting can improve the functions of intestinal stem cells through metabolism of fats, the key is the mitochondria, the powerhouses of the cell responsible for this function. Fat metabolism, or the immediate breaking down or “burning” of the fat entering the cell, is carried out in the mitochondria. Disrupted energy generation associated with aging and decreasing mitochondria can be a reason for the brain’s susceptibility to age-related illnesses. Positive developments in brain functions can therefore be viewed in connection with the correlation between fasting and the oxidation of fatty acids.

Cleaning by fasting

Just like the spring cleaning in our homes, our cells need a thorough cleaning to function properly. Wrongly folded protein particles, remains of damaged organelles, broken molecular pieces, and aged cells that can no longer divide should be disposed out of our cellular structure. Fasting perfectly performs the task of cleaning these wastes and clearing the area in the cell.

“Everything has zakat (a means of cleaning), and the zakat of the body is fasting.” (Ibn-i Majah, Siyam: 44)
“Fasting is a protective shield.” (Bukhari, Sawm: 2)

What is autophagy?

Autophagy is the compound of two ancient Greek words: auto (self) and phagos (eating). What is meant by “self-eating,” is the breaking down and recycling of protein waste and old, impaired molecules by intracellular digestive organelles called lysosomes. In this way, proteins or cellular organelles are digested and taken out of circulation. We can liken autophagy to garbage collection.

Japanese researcher Yoshinori Ohsumi’s study, which brought him the 2016 Nobel Prize for Medicine, found that the autophagy that occurs inside the cell due to fasting or starvation plays an important role in preventing ageing, infections, and tumors. If autophagy breaks down, many illnesses may be triggered, including cancer. Conversely, if autophagy activity is regular, tumors may be suppressed – depending on the stage of development and type of tumor. Cancer research has long focused on channeling these autophagic activities. Restriction of food intake through fasting shows promise: it may protect normal cells while triggering autophagy and thus increasing the effect of cancer treatments. Autophagy might offer solutions or treatment options for other illnesses, too, including inflammatory diseases [3], neurodegeneration [4], metabolic and cardiovascular diseases [5], obesity [6], and metabolic disorders.

Preclinical studies have shown that dietary restrictions by fasting contribute to the increase of a person’s lifespan and slow the development of age-related diseases such as cancer and neurodegenerative and cardiovascular diseases [7].

* Oxidative stress: The damage caused as a result of excessive proliferation of free oxygen radicals released from foods that spike blood sugar (with high glycemic index) as metabolic waste. A good example of oxidative stress is the browning of certain foodstuff such as apples, bananas, etc. sometime after they are peeled.

References

1. Yilmaz, Omer H. et al. 2018. “Fasting Activates Fatty Acid Oxidation to Enhance Intestinal Stem Cell Function during Homeostasis and Aging.” Cell Stem Cell, Vol. 22, Issue 5, May 3, pp. 769–778.
2. Paige Brown Jarreau. 2018. “Eating (Or rather, Fasting) Our Way to Rejuvenated Stem Cells?” in Life and Tech @ LifeOmic. June 7. A Medium Corporation.
3. Cadwell K. 2016. “Crosstalk between autophagy and inflammatory signaling pathways: balancing defence and homeostasis.” Nat Rev Immunol.16 (11): 661–75.
4. Menzies FM, Fleming A, Caricasole A, Bento CF, Andrews SP, Ashkenazi A et al. 2017. “Autophagy and Neurodegeneration: Pathogenic Mechanisms and Therapeutic Opportunities.” Neuron. 93 (5):1015–34.
5. Bravo-San Pedro JM, Kroemer G, Galluzzi L. 2017. “Autophagy and Mitophagy in Cardiovascular Disease.” Circ Res. 120((11)):1812–24.
6. Lavallard VJ, Meijer AJ, Codogno P, Gual P. 2012: “Autophagy, signaling and obesity.” Pharmacol Res. 66 (6):513–25.
7. O’Flanagan CH, Smith LA, McDonell SB, Hursting SD. 2017. “When less may be more: calorie restriction and response to cancer therapy.” BMC Med. 15(1):106.

It has been well known for a long time that the metabolisms of smaller animals are faster when compared to the metabolisms of larger animals relative to their body size. In 1883, German physiologist Max Rubner tried to define a scaling principle based on the laws of thermodynamics and geometry. The metabolism of an organism works like a perfect machine that continuously converts one form of energy into others. It also releases energy while doing so. Metabolic speed can be described as the speed at which cells convert nutrients into energy. This energy is utilized for the execution of cellular functions and the construction of new cells.

Calculations have shown that the speed of metabolism is directly proportionate to body mass. For instance, the biomass of a hamster is eight times bigger than a mouse. According to this ratio, one would expect the metabolism of a hamster to be eight times faster than that of a mouse. Similarly, the body mass of a hippopotamus is 125,000 times larger than a mouse; therefore its metabolism would be estimated to be 125,000 times faster.

The problem is that a hamster generates eight times more heat than a mouse. Furthermore, the total body surface area, which is how the heat energy leaves the hamster?s body, is four times bigger than the surface area a mouse has. Consequently, as the body of an organism grows, its surface area develops more slowly compared to its mass.

This situation is shown in Figure 1. Here a mouse, a hamster, and a hippopotamus are represented in spheres. As the spheres get larger, their volume and surface area also gets bigger. In geometry we know that the volume of a sphere is given as? ?(r is the radius of the sphere). The surface area of a sphere is also expressed as . In this case we can say that while the volume of a sphere is scaled with the cubed radius, the surface area of a sphere is scaled with the squared radius. In other terms, the volume of a sphere is directly proportionate to the cubed radius, as is the surface area of the sphere to the squared radius.

Figure 1. The scaled features of a mouse, hamster, and hippopotamus. Taken from Complexity: A Guided Tour by Melanie Mitchell, Oxford University Press.

Figure 1 displays the first model used by scientists when the relation between metabolic speed, body mass, and surface area was being investigated. According to this early model, the association between body mass, surface area, and volume was studied with spheres that encased organisms. As the volume of the sphere approximately provided the volume of the organism, the sphere?s surface area also represented the surface area of the organism (of their skin). According to this, the radius of the sphere containing the hamster is nearly twice as large as the radius of the sphere with the mouse. Therefore, it can be claimed that the volume of a hamster is approximately eight times the volume of a mouse, and its surface area is four times larger. When it comes to the hippopotamus; the radius of the sphere is 50 times larger than the one with the mouse; this shows it has a 2500 times wider surface area and a 125,000 times larger volume compared to the mouse.

As it can be seen with these examples, while the radius of the sphere increases, its surface area does, too, but at a slower ratio compared to the volume ? in other terms, it gets scaled.

Based on this model, since the surface area is scaled (enlarged) with the square of the radius, and the volume with the cube of the radius, it can be estimated that the surface area is scaled with 2/3 the power of the volume.

The interesting point that all these rough calculations take us to is that the surface-area width of the living things is adjusted in a way to permit the release of energy they generate into their surroundings in a healthy pattern. If a hamster generated more heat than the amount corresponding to four times its surface area, this would lead to the hamster overheating.

In a similar way, if you elevated the heat in a hippopotamus, which generates 125,000 times more heat than a mouse does, by eight, the hippopotamus would suffer from heat exhaustion. This is because the surface area of a hippopotamus is only 2500 times bigger than a mouse. This is called the ?surface hypothesis,? and until it was discovered that it does not correlate well with the experimental data, scientists carried out investigations based on this model for nearly 50 years.

Around 1930, Swiss animal scientist Max Kleiber performed a series of studies involving the metabolic speed of various animals. The data he collected showed that the metabolic speed is scaled to ? the power of body mass, or Metabolic speed = body mass3/4.? This is called the power law in science. Instead of the 2/3 power in the first model, the correlation of 3/4 power with the experiments demonstrated that animals, especially large ones, have higher metabolic speeds than the first model predicted.

In summary, the establishment and maintenance of this equilibrium requires knowledge beyond geometric calculations, and makes it impossible to explain via random occurrences.?

In Figure 2, the scaling of different animals in regard to their body mass is charted. The horizontal axis shows the body mass in kilograms, whereas the vertical axis marks the speed of their average basal metabolism in watts. The symbols displayed as dots are real values measured from different animals and the fact that these dots align almost on a line show the correlation of the metabolic speeds of organisms with 3/4 the power of their body masses. This points to an extraordinary order in the universe, one that has been created with perfect harmony built into it. ?

One feature of the ?power law? is that when the two axes are drawn logarithmically, the relation between the two physical magnitudes appears as a line. Here, a similar situation is present and this power law is called the ?Kleiber Law.? This law successfully provides the metabolic speeds of mammals, birds, fish, plants, and even single celled organisms.

There are also other scaling relations pertaining to these that have confounded biologists for a long time. For example, the bigger a mammal is, the longer its life span is. There are examples, like humans, who do not fit into this general principle. However this is applicable for many mammals. The life of a mouse lasts two years, typically, whereas the life of a pig is around 10 years ? and elephants live for 50 years. If you chart the average life span of many different species across their body masses, the value of the power law is seen as ?. In other terms, for mammals, the average life span is scaled to ? of the body mass (directly proportioned).

As another example of proportion and scale, if you draw a graphic displaying how the average heart beat speed changes against the body mass of different species, you will once again find the scale of the power law to be ?. This means that the larger the body mass of a mammal, the lower its heart beat is.

Biologists are trying to solve the mysteries among the relations of many power laws like this one. There is no doubt among scientists regarding the fact that these power laws manifest certain common features in living things as an important sign pointing to the presence of very significant common features for all organisms.

Figure 2. Metabolic speeds of different animals according to their body masses. Taken from Complexity: A Guided Tour by Melanie Mitchell, Oxford University Press.

The creation of living things according to a power law, which is a relation between their metabolic speeds and body sizes, is a wisdom that requires thanksgiving. To safely release their generated energy, the metabolic speeds of living things must be scaled to ? the power of their body mass. A lower value would cause a higher body heat, and a higher value would lead to very low body heat. In both situations it would not be possible for organisms to sustain their lives in the physical world. Our bodies are perfectly scaled for us to survive.

Note

Experimental values were obtained from the book of – K. Schmidt-Nielsen, Scaling: Why is animal size so important?, Cambridge University Press, 1984.

Reference

• Complexity – A Guided Tour, Melanie Mitchell, Oxford University Press, 2009.

By examining the heart of a crocodile, researchers have discovered how it is that an air-breathing land animal can manage to glide through murky waters for several hours without the need to surface.

There is that one scene in documentaries that we often come across on television: Crocodiles gliding gracefully inside the water, waiting for hours for the perfect time to pounce and snatch their prey from their necks and into the water. But how do these animals manage to stay underwater for almost two hours without surfacing for air even though they, just like human beings and other land animals, live on pulmonary respiration and are in need of the free oxygen in the air?

A member of the reptiles class, crocodiles do not have gill nor can they have skin respiration since their skin is covered with a thick and airtight keratin armor. Just as every living organism are provided with a suitable anatomic and physiological character for their survival, crocodiles are also granted with a system that facilitates their long stay in the water.

Crocodiles are bestowed with a special heart anatomy different to other reptiles like lizards, tortoises, and snakes. The hearts of other reptiles are designed to contain three sections including two atriums and one ventricle. The right atrium, which collects the returned oxygen-deprived (deoxygenated) blood and the left atrium which collects the oxygen-rich (oxygenated) blood retrieved from pulmonary arteries of the lung, transports the blood to one common ventricle. Because there is only one ventricle to receive and combine oxygenated and deoxygenated blood, a mixture of less oxygenated blood is pumped to their body. Depending on outside temperature, the body temperature of a reptile increases or decreases. Their metabolism slows down, almost to a halt, while their body temperature decreases when outside temperature drops near or beyond freezing conditions. Hibernation begins as a result. Frogs and reptiles stop hibernating as soon as their body temperature increases depending on the outside temperature when the weather gets warm. These organisms are called cold blooded animals (with variable body temperatures) because of this feature.

The heart of a crocodile is different to other reptiles in that it has four chambers just like birds and mammals. Blood is sent to the lungs for gas exchange from the right, and from the left ventricle it is pumped to the body. Thus the two types of blood do not mix in the heart. However, what is interesting is that blood is mixed as soon as it leaves the heart via a valve (foramen of panizza) placed in between the right and left aorta.

What could be the purpose of blood, which does not normally mix in the heart, mixing through the medium of a hole? Does this opening in between two aortas indicate a flaw? It is understood after some research that this hole in fact is not a flaw or an anomaly; on the contrary, it is a necessity for a metabolism suited perfectly to the lifestyle of the crocodile.

Warm blooded vertebrates like birds and mammals with a four chamber heart have faster metabolic speeds and higher blood pressures. For these organisms can only supply the energy they consume during their daily activities via such a fast metabolism and a high level of oxygen provided with oxygenated blood.

If the metabolism of a crocodile was fast like mammals all throughout the year, it would have to continuously be nourished and use oxygen. Furthermore, because crocodiles do not have much predators, they could have also lead to the extinctions of some species by overpopulating if they featured a faster metabolism. The low ratio of heart-body mass in crocodiles (0.15%) compared to mammals and birds (0.40%-0.50%) cause the movements of crocodiles to be relatively slower. The Almighty, who creates everything with his wisdom, lowers the blood oxygen ratio and the metabolic speed of crocodiles by creating a valve that combines the two aortas. Thus eliminating the possibility of crocodile overpopulation.

Crocodiles have two aortic arches whereas mammals only have a left, and birds have one right aortic arch. The left aortic arch, despite some contact with the returned blood via foramen of panizza, delivers the oxygenated blood towards intestines, stomach, spleen and the liver after receiving it from the left ventricle of the heart. This is because the digestive system of a crocodile requires oxygen-rich blood. Deoxygenated blood while exiting the right ventricle goes towards the pulmonary arteries of the lung for exchange and mixes with oxygenated blood coming from the right aorta, feeding other organs that are instrumental for its slow metabolism.

Under the water, separated oxygen-rich and oxygen-poor blood mixes when exiting the heart and switches route, thus making oxygenated blood vessels start to carry oxygen poor blood. So what is the reason behind this switch in the direction of the bloodstream under the water? See at this point, the extraordinary features of the crocodile blood circulation system kick in. The two anatomical features belonging only to only crocodile hearts is what enables them to stay under water without breathing. Because of little or no lung use under the water, a big portion of the blood stream is diverted away from lungs; therefore oxygen poor blood is pumped back to the body. As one feature of the two, foramen of panizza restricts (does not close) upon signals coming from nostril sensors under the water but expands and remains open on land. The two aortic arches connects with each other via foramen of panizza as soon as they leave the heart but merge completely in the lower parts of the body away from the heart (anastomosis).

The second feature stems from a serrated valve. Refilling of pumped blood is stopped via a passive, thin leaf-shaped valve which is located at the tip of the pulmonary artery exiting the right ventricle. Thus, one-way direction of blood flow in the heart is maintained. These valves, which carry nodules made of connective tissue, constrict during the dive and blood flow to the lungs is reduced greatly. Therefore blood rejoins the systemic circulation from the right aortic arch.

The blood circulation of crocodiles is similar to birds, mammals, and humans while they are active on land. Oxygen-deprived blood is sent to lungs for gas exchange. The only difference is the turning of the right aorta to the left and the left aorta to the right. Oxygen rich blood not only flows through the left aorta but also through the right aorta via foramen of the panizza as well causing distribution via two channels into the body. However, the foramen of the panizza being open is not sufficient for these two channels to be used. At the same time, the pressure of the blood within the left ventricle needs to be higher as well. This way, high pressure oxygenated blood flows into the right aorta through the opening of the panizza, applying pressure to the valve at the tip of the right aorta to close it in order to prevent the mixing of the oxygen-poor blood into this route. As a result, oxygenated blood gets distributed quickly by each aortic arch without mixing with the used blood. Thus, oxygen-poor and oxygen-rich blood follows the following route on land

* Deoxygenated blood: Body – superior and inferior pulmonary veins – right atrium – right ventricle – lung pulmonary artery – lungs.

* Oxygenated blood: Lung pulmonary vein – left atrium – left ventricle – right aorta and left aorta via panizza valve (both aortas are active) and body.

The opening of the panizza narrows with the help of signals coming from the nostrils when crocodiles submerge. At the same time serrated valves at the tip of pulmonary artery that transports the blood to the lungs also constrict. While this serrated valve is at work, a majority of the blood returning from the body is not sent to the lungs because they are not functioning at the time. This serrated valve also increases the pressure of the right ventricle. This pressure, along with elevated resistance in pulmonary circulation and lowered pressure of systemic circulation, leads to the opening of normal valves at the tip of the left aorta. In the end, the left aorta which normally carries oxygenated blood on land starts carrying oxygen deprived blood, and there is a route switch.

The most beneficial part of this switch is to re-route the deprived blood back to the body via a different route, that is, the left aorta. This by-passes the lungs and prevents time loss. Despite the fact that blood of the left aorta mixes with the oxygenated blood of the right aorta to some degree via the panizza valve, the main function of this opening while submerged is to supply blood flow to the arteries feeding the heart and brain through the transfer of some poor blood from the left aorta into the right aorta; this way vital organs are not left without blood.

Blood returning from the body is not sent to the lungs for gas exchange when crocodiles are under the water. However, existing oxygen in the blood can be delivered to the tissues quickly by a route switch. The amount of bicarbonate ions that is important in the transport of CO2 in the blood increases when oxygen pressure in the tissues drop. Anaerobic respiration of tissues increases. This leads to an increase in lactic acid levels and reduces pH. Eventually, it facilitates the release of oxygen carried by hemoglobin. In the end, oxygen that is bonded with hemoglobin is used more efficiently. In the meantime, the body temperature of a crocodile submerged under water decreases and slows down its metabolism, reducing the need for oxygen. Oxygen stored in the blood can be sufficient up to two hours under the water. However when these reserves are consumed, crocodiles have to resurface to breathe even though it may cost a prey to escape.

Crocodiles can live on land and in the water and adapt to their environments with ease and efficiency thanks to the ponderous working of their mechanisms under water, the change in blood circulation, slow blood flow, reduced body temperature and metabolic speed bestowed upon them. Just like humans in sleep, crocodiles can remain submerged for long periods (4-6 minutes in usual dives; up to 2 hours when pressed) with this perfect system granted to them. Crocodiles use these mechanisms not only when under water, but also while resting or for periods after heavy feeding.

Hierarchy of hematopoietic stem cells

The blood system, also known as hematopoietic system, has enormous regenerative capacity to maintain functional mature blood cells that arise from highly proliferative but short-lived progenitor cells. Those progenitors in turn are generated from very rare blood stem cells called hematopoietic stem cells (HSCs). HSCs are one of the most studied stem cells in our body which has greatly shaped our thinking on the features of adult stem cells. These stem cells are kept at the bone marrow in close proximity to bone cells and other supporting cells forming the specialized home known as niche. In several aspects, a niche resembles a cellular stronghold that a queen lives in a safe and protected environment.

The interaction of stem cells with the niche is crucial as this prevents exhaustion of stem cells from uncontrolled cellular divisions and proliferation. While active progenitors account for the generation of mature blood cells, hematopoietic stem cells function as a reserved cell population. Interestingly, we observe the importance of the balance between those two cell populations in the aging process. Although the number of HSCs increases in aged animals, there is a decline in self-renewal of HSCs.

Other protective mechanisms include the low proliferation rates of HSCs in a relatively quiescent state, residing in a low oxygen environment [3], a relatively low degree of metabolism and preferential use of glycolysis as energy source, and additional protection mechanism against oxidative stress.

Low in oxygen but a good place to be!

Stem cells as the cell bank of the body are protected against internal and external insults by a number of mechanisms. Stem cell niche not only provides an environment that they can survive but also poses the lesser degree of internal and external insults. Those possible stresses on cells include, but not limited to, UV exposure, radiation, toxic chemicals, and free oxygen species that cause various damages in the cell including mutations in DNA (Fig. 3). Cells respond to those external and internal issues by various ways such as senescence (loss of stem cell activity), cell death or DNA repair. For example, blood stem cells mainly house in the bone marrow next to osteoblastic lining (blood-forming cells) and endothelial cells where they form the hypoxic (low oxygen tension) endosteal region. This hypoxic niche of HSCs provides lower levels of oxygen so that there are lower levels of free oxygen radicals that mainly arise from electrons leaking from mitochondria during oxidative phosphorylation. In addition, it has been shown that HSC express higher levels of hypoxia inducible factor-1α, a master regulator at low oxygen tension with hundreds of downstream targets regulating various aspects of metabolism including defense against oxidative stress and survival at low oxygen environment. It has also been shown that hypoxia increases self-renewal abilities of HSCs, thus keeps them healthy and functional for longer periods.

Protection from detrimental effects of reactive oxygen species (ROS)

Excess amounts of reactive oxygen species are detrimental to cells. ROS are found to cause hematopoietic stem cell defects as shown in mouse lacking FoxO and Atm genes. In those mutant mice, the hematopoietic defects could be rescued by the use of an antioxidant N-acetyl-cysteine. Anti-oxidants are one of the scavengers that diminish unwanted effects of reactive oxygen species. A number of fruits and vegetables such as beans, blueberry, strawberry, and apple are known with their high content of anti-oxidants. It is amazing to observe anti-oxidants being placed into our sustenance just as much as in some special genes (such as SOD2 and Hypoxia Inducible factor-2α) that provide additional protection for cells. Amazingly, stem cells show high levels of ROS scavenger genes.

Low metabolism provide protection for stem cells

Recent studies demonstrate that hematopoietic stem cells have lower rates metabolism as measured by lower oxygen consumption, lower ATP content and higher lactate production (an end product of cytoplasmic glycolysis) [4]. This means that stem cells produce and consume lesser energy (ATP) compared to more differentiated cells and the by-products of the energy production are kept lower. As higher energy demand brings higher rates of internal insults like production of ROS which is associated with aging and cellular damages, HSCs are granted with another protective mechanism by preferential use of glycolysis (anaerobic) instead of oxidative phosphorylation (aerobic).

Hematopoietic stem cells are quiescent

Another defense mechanism is the quiescence of stem cells which is associated with slow cell-cycle progression. Quiescence of stem cells means that they are kept at a resting, inactive state thus sustaining a self-renewing HSC compartment for life. Because when cell divides, they have to undergo thousands of chemical reactions including making a copy of the three billion letter long DNA, which puts cells at risk to get mutations. Thus, they don’t undergo division unless there is a stimulus. In addition, it has been found that HSCs divide only once every 145 days on average.

There are a number of studies indicating that there are signals in the niche that keeps HSCs in a quiescent state. Tie2/Ang-1 signaling, for instance, has been demonstrated to contribute to the maintenance of HSCs by inducing quiescence. While Ang-1 is expressed in the mesenchymal/stromal cells of niche, its receptor Tie2 is expressed at HSCs. In addition, it has been shown that Ang-1 can inhibit HSC division in culture and promote quiescence of HSCs in the bone marrow [5].

It is also reported that the cell adhesion molecules that allow physical interaction between stem cells and their niche components may participate in regulation of stem cell quiescence through a process called contact dependent inhibition of proliferation. For instance, it has been found that cell adhesion molecules such as N-cadherin, β1-integrin, and osteopontin might be involved in the regulation of cell cycle status of HSCs [6].

One advantage of quiescence of HSC comes from the lower susceptibility of slowly proliferating cells to radiation than other cells due to the expression of cell cycle inhibitors like p21 and anti-apoptotic (controlled cell death) machinery like ATM in HSCs. In addition, studies in p21 (a cell cycle inhibitor gene) knockout mice suggest that maintaining cell cycle quiescence is directly linked to self-renewal of HSCs [7].

Toxics are exported from hematopoietic stem cells

There are other issues concerning external insults against toxics and unwanted chemicals. An HSC population described as side population has been equipped with a number of transporters such as ATP Binding Cassette (ABC) transporters, P-glycoprotein (P-gp/ABCB1) and Breast Cancer Resistance Protein (BCRP/ABCG2) on their membrane providing high efflux ability [8]. They play an important role in the excretion of drugs and endogenous compounds. Those transporters work actively when there is an entrance or excess of such chemicals thus keeping damage minimal.

HSCs are placed in such an environment that even minimum damages by internal and external insults are prevented by different defense mechanisms including residing HSCs in the hypoxic niche, expression of ROS scavenger genes, preferential use of glycolytic metabolism, quiescence nature of HSCs, and removal of toxins by ABC transporters. It is very wise to home such an important cell in a place where it can prosper with a carefully balanced rate of cell division and metabolism. Hypoxic niche seems key to the protection of hematopoietic stem cells by supporting self-renewal and preservation of hematopoietic functions both at the same time. The presence of these protective systems that are graciously placed in our cells with perfect measurements provides an elusive mechanism to ensure healthy life-long reservoir of HSCs.

Ali Fethi Toprak is a PhD candidate at Southwestern Medical Center, Texas University.

Selected References

1. Kobayashi, C.I. and T. Suda, Regulation of reactive oxygen species in stem cells and cancer stem cells. J Cell Physiol, 2012. 227(2): p. 421-30.

2. Li, L. and H. Clevers, Coexistence of quiescent and active adult stem cells in mammals. Science, 2010. 327(5965): p. 542-5.

3. Eliasson, P. and J.I. Jonsson, The hematopoietic stem cell niche: low in oxygen but a nice place to be. J Cell Physiol. 222(1): p. 17-22.

4. Simsek, T., et al., The Distinct Metabolic Profile of Hematopoietic Stem Cells Reflects Their Location in a Hypoxic Niche. Cell Stem Cell, 2010. 7(3): p. 380-390.

5. Arai, F., et al., Tie2/angiopoietin-1 signaling regulates hematopoietic stem cell quiescence in the bone marrow niche. Cell, 2004. 118(2): p. 149-61.

6. Yamashita, Y.M., D.L. Jones, and M.T. Fuller, Orientation of asymmetric stem cell division by the APC tumor suppressor and centrosome. Science, 2003. 301(5639): p. 1547-50.

7. Cheng, T., et al., Hematopoietic stem cell quiescence maintained by p21cip1/waf1. Science, 2000. 287(5459): p. 1804-8.

8. Huls, M., F.G. Russel, and R. Masereeuw, The role of ATP binding cassette transporters in tissue defense and organ regeneration. J Pharmacol Exp Ther, 2009. 328(1): p. 3-9.

9. Antioxidant Riches Found in Unexpected Foods. Retrieved from http://www.webmd.com/food-recipes/news/20040617/antioxidants-found-unexpected-foods, January 31, 2012.