Dear Peter, in order for you to understand me better, keep in mind a basic principle about the functioning of living organisms-they all depend on energy use. If no energy enters a living system, then no metabolic activity, no life function can be carried out. Think about a car without fuel. No matter how great the car is, it simply won’t work without any fuel in the tank. The human body is no different. Plants and animal products which people consume as food provide the body with fuel. However, you cannot make use of the energy in nutrients in the form in which you take them in. They need to undergo a process so that they become usable fuel for us, like crude oil being refined into gasoline to make a car work. This is roughly what my duty is. Without my functioning, you would be devoid of the energy to move a finger, and eventually die. Do you understand now how important a set of pipes I am? You just think that I look like a soft and hollow canal and misjudge me as simple. Well, I know that I don’t have such complex parts as the heart, lungs, and kidneys, but I’m created as a perfect work of art in plain design.

Although the hoses you use for watering your garden wear out and break in a relatively short time, my walls made of four layers keep functioning through a lifetime without any holes unless I contract a disease like cancer. My outer layer consists of a durable connective tissue, the next one consists of two sets-one horizontally and one vertically laid-of straight muscles, the next layer under that consists of glands spread in a soft connective tissue, and the innermost layer is the epithelial mucosa where the actual absorption takes place.

Now, let’s come to how I achieve digestion, one of your body’s vital activities. Actually, there is no place for me to take any pride in it; I’m just doing as I am ordered. Anyway, the complex processes occurring within my simple-looking walls are just fascinating! Every one of my cells producing the particular enzymes to break up each nutrient is like a separate factory. Some of these enzymes break proteins into different levels of peptides, some break the peptides into amino acids, some break fats into fat acids and glycerin, whereas some break carbohydrates down into glucose. All of these particular enzymes have their sub-branches within themselves. For example the enzymes breaking down fructose (fruit sugar), lactose (milk sugar), and starch are all different. In order for the enzymes to be effective, my inside needs to have the right pH level; the enzymes work in very sensitive conditions. To give you an idea, the enzymes in the stomach-which happens to be the second station the nutrients are destined for before they come to me-work in an acidic environment (pH: 2.5–3). In my case however, basic fluids are secreted and this strong acidity is neutralized for my enzymes to work.

My overall length is around 8.5 meters from the first entrance at the stomach to the last exit. The small intestine is nearly 7 meters long and the remaining 1.5-meter section is the large intestine. Although the small intestine is the longest section of the digestive tract, it is still called small since it is smaller in diameter than the large intestine.

The small intestine is also divided into three sections. The very short (25–30cm) and relatively thicker part right after the stomach is the duodenum. Bile-which works like detergent and facilitates breaking up fats-produced by the liver and digestive enzymes from the pancreas enter the duodenum. Thus, the nutrients are digested one step further and pass on to the second section (jejunum) and then to the third (ileum). You cannot easily tell apart these final two sections. As blood circulation is more intense in the second section, this section is more reddish and the contractions here are faster and stronger. The third section is narrower and has thinner walls. The blood circulation here is relatively lower and the movements are more limited. The thin membrane of connective tissue (mesentery) around me which attaches me to the abdomen wall and prevents me from knotting up is relatively fatty in this third section.

My most vital parts are the villi-tiny nipples covering the curly surface of my inner wall like a carpet. Shaped like the fingers of a glove, villi yield an enormously large inner surface. They contain a net of capillaries and lymph canals. In addition to the glands secreting the enzymes to break down nutrients, the secretion of certain glands protects me against the destructive effect of the stomach acid. Some cells secrete mucus for lubrication and protection of the passing nutrients. As some cells of the villi secrete digestive enzymes, some of my cells absorb the nutrients broken down until the final phase and pass them to the bloodstream.

Peter, how can some guys mistake such a splendid mechanism as a work of unconscious nature? What I’m telling you about is a manifestation of such great knowledge and might that it leaves you spellbound. I know the characteristics of foods, I know about the other organs’ needs, I adjust various enzymes and an absorption system, I fit them in a limited space… In addition, I do all these in the most ideal way, without any waste or flaw! C’mon Peter, can all these happen by themselves? Now, if I were to start telling everyone about the absorption mechanism in detail, they would probably see those cells as divine beings! The One who assigned special carrier molecules and a system for every nutrient molecule, has placed two transfer systems as blood and lymph pathways in every single one of those millions of villi! The blood pathway passes amino acids, water and salts into the blood directly, whereas the lymphatic pathway absorbs fats to pass them to the blood indirectly. After absorption, the nutrients become a property of the body and they are carried in the bloodstream to all the cells waiting for them in need.

Well, what about the waste then? Since everything you eat is not beneficial and usable, and some things are even toxic, they should be disposed of as soon as possible. The unabsorbed remnants are still too watery to be disposed of; sending them away as they are will be a waste of water and minerals. But don’t worry, everything is perfectly planned! Now the large intestine comes on duty. In this 1.5-meter section, the water of the waste and certain minerals are absorbed, and the waste solidifies. The large intestine is also divided into three sub-sections. The pouch connected to the junction of the small and large intestines is named the cecum and there’s the appendix at its end. This end sometimes festers and you have to have it removed in an appendectomy. Now, there’s this made-up story that the appendix was once longer since your ancestors only ate plants, that it has evolved into a shorter form for I now eat more meat, so on and so forth… Bah! Nothing is created in vain, Peter. If it didn’t have a duty, it simply wouldn’t be created. Only after some time did it dawn on them, after researchers proved that it is so necessary, that as a lymphoid organ, rich in blood vessels, it produces antibodies to fight the germs which somehow make their way into me.

The rest of the large intestine is the colon and the rectum. The mucosa covering my inner surface is rather smooth. It secretes mucus to facilitate the removal of waste. In addition, useful bacteria are made to work in abundance in the large intestine for your needs. These bacteria synthesize the group B vitamins like B12, thiamin, and riboflavin, along with vitamin K. You see how all the processes are carried out so splendidly? If it weren’t for vitamin K, your blood would fail to coagulate, and the slightest injury to your blood vessels would kill you. Could you ever have imagined that what looks to you like a sewage canal could produce vitamins of vital significance? Your Creator has infinite wisdom.

Peter, now you may wonder how the acts of this organ which resembles a long hose are regulated, how the nutrients inside are propelled, and then thrown out. To put it briefly, the “willful” part of your brain does not even know about it. Indeed, if it knew, it would be constantly busy with me and unable to do anything else.

Under the control of the autonomous nervous system, the straight muscles of my walls gradually contract in waves-this is squeezing act is called peristalsis. The nerve fibers connected to me fall into two basic categories-sympathetic and parasympathetic. As the sympathetic fibers pressure me to slow down, parasympathetic fibers stimulate me to act. Thus, I try to keep a balanced functioning between these two opposite effects. When the waste material I propel this way assumes a state to be disposed of, it reaches the rectum, and when the walls here strain, I make a natural call to you that I need to get rid of garbage. This is the step where your will has a partial interference.

Colon cancer, which troubles many people today, appears in this final section. The major reason is consuming too much meat and fatty foods, lack of movement, and leading a stressful life. When these are combined, I fail to function properly. If you want to help me at that, you should consume fiber-rich foods such as fruit and vegetables, and also lead a peaceful life. My web of nerves is amazingly rich and complex. Therefore, I am sensitive to nervous changes. If you feel down or sad, and if you suffer too much stress, I begin to go into spasms. Then I fail to dispose of waste, the toxic material inside me begins to damage my inner walls and eventually increases your cancer risk. Therefore, you’d better take up the habit of a glass of warm water when you get up in the morning, and try to have regular meals at the same times of the day. Most importantly, always have fresh green vegetables on your table, reduce meat intake… and it would be great if you could afford to consume olive oil rather than any other.

Hey, wait! I was about to forget the most important point. If you don’t have any peace of mind, all of these will be useless. This doesn’t mean that you will never worry; after all, this world is a testing ground and you are a human being like anyone else. However, if you give in to troubles and get overcome by feelings like panic, fatigue, and hopelessness, then my functioning will be upset. So, troubles faced with active patience and effort without giving up hope do not harm me much.

Peter, I do not wish you to wait until you see colon cancer patients disposing of waste through a hole in their belly into a plastic bag before you feel grateful for the blessings you enjoy. Actually, maybe I have told you at most a tenth of what I know about myself. Anyway, I think even this much will give you an idea of what a work of art I am. Thanks for listening to me, Peter!

Irfan Yilmaz is a professor of biology at Dokuz Eylül University, Izmir, Turkey.

The muscle cells are responsible for the strength and tone of the vessels. Today, we know that the role of the endothelial layer goes beyond a simple physical barrier. In addition, there are twenty-five different substances secreted by the endothelial cells that play a role in blood clotting, cell proliferation, the regulation of vessel permeability, and the functioning of the immune system. The endothelial cells are 10-15 &μm wide and 20-25 μm long. They are located in the inner vessel wall in a single-cell layer. The total endothelial area in the body of an adult is around 5000-6000 m², and it weighs around 2.5 kg. Endothelial cells during inflammation Capillaries consist of an endothelial structure; they can only be seen under a microscope, but their total length is nearly 96,000 km. The blood brought by the arteries is conveyed to the vein through capillaries. At this stage, the gas, liquids, and nutrients are brought out through the vessels and the cells and tissues around are supplied with oxygen and nutrients. In return, the liquids that they discharge and other waste matter are conveyed to the vein through capillaries. This matter-exchange, which occurs both inside and outside of the capillaries, is regulated thanks to the permeability of the endothelial cell layer and the pressure balance of the capillary system. During cardiac failure and inflammation, liquid release is increased due to a pressure imbalance and the liquid retrieval is not sufficient to make up for the amount lost. This results in swelling in the area in question. Here, we need to underline that inflammation, which appears with symptoms such as edema, redness, fever, and pain, is not a harmful process. On the contrary, it is a miraculous defensive mechanism granted to our body; inflammation protects the body against serious damage. For instance, the inflammation that forms around a bee sting prevents the venom from spreading throughout the body. The endothelial cells are given an important role in the inflammation as well. The chemical molecules secreted by the endothelial cells in the inflamed spot cause the vessels to react by enlarging and thus perfusion is increased. Later, the endothelial layer becomes ready for leukocytes to settle; these are used in neutralizing the substance that caused the inflammation in the first place.

Balancing blood pressure

The layer of smooth muscle cells is stimulated with chemicals secreted by the endothelial cells and the tone of the vessels are controlled through the constriction and relaxation of the vessels. Therefore, an important duty in the regulation of blood pressure is given to these cells. During aninfection, bacteria circulate in the blood stream and the blood pressure falls extremely low. Tissue nutrition is upset (septic shock) and an excess of muscle-relaxing substance is released by the endothelial structure. Veins and arteries become too relaxed and there is a considerable drop in blood pressure (hypotension). On the other hand, with problems like atherosclerosis, the endothelial cells cannot fulfill their duty and due to a deficiency in nitrogen oxide, they become immune to the stimulus to relax the muscles. The resulting problem in this situation is hypertension.

Endothelial cells prevent hemorrhage

In order for a hemorrhage to stop the vessels that are bleeding need to narrow down. This is very important in the first stages of blood loss, particularly when there is a problem with blood clotting. When a hemorrhage begins, the endothelial cells are ordered to excrete a substance called endothelin. This starts the narrowing down of the bleeding vessels. Endothelin is not excreted in normal vessels. When the umbilical cord of a newborn is cut, it prevents the baby from losing blood.

Endothelial cells in blood clotting

The duty of endothelial cells can prevent or facilitate blood clotting, depending on the situation. First of all, they prevent the blood cells from adhering to the vessel walls and prevent clotting inside the vessels. Imagine water flowing through a pipe. The speed of the flow is greater in the center and lower at the periphery. Therefore, in the long run, some residue forms inside the pipe. In the veins and arteries, the flow of blood near the walls is also slower. To prevent the formation of any residue, both the endothelial cells and the blood cells are created with negative loaded surfaces and the blood cells are pushed towards the center. In addition, a substance called prostocyclin (PGI2) is excreted and the thrombocytes change their structure. As a result, residue formation and clotting is prevented along the vessel walls. In a case of any long term damage to the endothelium (e.g. due to smoking, diabetes, or hypertension), the relevant protection mechanism fails, and clotting inside the vessels results in thrombosis. Some serious cases can even necessitate the amputation of a limb. The endothelial cells can also facilitate clotting when necessary. In case of bleeding due to a wound, they function contrarily and help the blood to clot to prevent blood loss.

Endothelial cells in the bone marrow, the liver, and spleen

As a divine blessing, the endothelial cells form a looser layer in these organs and vessel permeability is increased. Thanks to this increase, matter Exchange with blood is easily realized; blood reaches these organs, which are responsible for the constant control of the contents of the blood, easily.

Endothelial cells in the brain and eyes

The endothelial cells in organs like the brain and eyes are very closely integrated forming a barrier between the blood and the organs. This is to such an extent that the major nutrients of the brain, like glucose and oxygen, pass without any obstacles, but several chemicals, including medication, are blocked by the selective-permeability of this protective mechanism. Research has proven that various substances injected into the bloodstream reach almost all the tissues except for the brain. Thanks to the efficient protective mechanism that has been given to these minute cells, the brain is saved from a great deal of negative effects. Even a single cell is not left to chance and nothing happens randomly. As can be seen throughoutthe universe, opposites are made to work hand in hand in the human body as well in a splendid harmony for the continuation of life.

References

• Vinay Kumar, Abul K. Abbas, Nelson Fausto, Richard Mitchell, Robbins Basic Pathology, W.B. Saunders; 8th edition, 2007. Hall, John E., Arthur C. Guyton, Textbook of Medical Physiology,

Viking I was launched from earth orbit in August 1975. It traced seven hundred million kilometres in an interplanetary spiral to reach Martian orbit the following June. In July 1976 a car size tripod lander, dropped from the space craft, alighted on the rocky surface of Mars some 1200 miles from the Martian Mariner canyon system. This was the location of man’s first on-site search for life on another planet.

The successful Viking missions to Mars supplied us with most valuable scientific information. Although Mars is a rock-strewn desert, its rocks are enriched with minerals that may support life forms with water, air, and raw materials. This was the verdict of the Viking 1 and 2 missions to Mars. Mars may once have had rivers and lakes, but now the temperatures and the atmospheric pressures are so low that water can only exist in the form of vapour or ice. Scientists also suspect that the planet’s giant volcano, Olympus Mons, overlies a hot spot. Although this volcano may still erupt every 10,000 years, the red planet has essentially been frozen to death. However, there is still hope, because the experiments conducted with Martian soil produced significant results. During the experiments, Martian soil was fed with nutrients and water. The results were most surprising. Unlike the lunar dust, Martian soil consumed the nutrients. Not only did apparent consumption of the nutrients give ‘rise to a steady rate of carbon dioxide production but the introduction of water vapour resulted in a most unexpected surge in oxygen levels.

Eighteen years have passed since these experiments and mankind has not taken a step on Mars yet. Will it ever be possible for man to achieve a settlement on Mars in the future? Firstly, such settlements on the planet need to be as self-sufficient as possible. The first Mars invaders will need to begin a search for crucial supplies such as water and oxygen. Since Mars has little nitrogen in its soil to sustain plants, scientists would need to inject the soil with earthly micro-organisms to free up the crucial element. A base independent from earth is impossible without these necessities of life, particularly water and power sources and raw materials for building an ecosystem. Although the atmosphere is only 0.03 % water, the air is saturated with water most of the time, due to the low temperature. However, rain is impossible because the atmosphere is too thin and it is generally cloudless.

A Martian day is 24 hours and 37 minutes, which is very close to earth’s. The fact that gravity is one third of earth’s, is also an advantage. However, what is important is to produce resources like ammonia (a plant nutrient), hydrazine (for rocket fuel), formic acid (for storing electricity), nitric acid (for oxygen storage) and methane (natural gas). We do not know what will actually be found on Mars when it is completely explored, but the important fact about Mars is that, unlike the moon, it contains all the raw materials that are crucial for a Martian base.

At present the planet is an Arctic wasteland, but scientists say this has not always been the case. Much can be learned from a small core sample of the planet’s polar ice. Scientists need to know what happened to its climate, if it resembled the earth’s in the distant past. The topographical features and the river channels suggest that a few million years ago Mars was a warm planet and probably had a thick atmosphere. The Viking findings and the analysis of some of the data suggest that Mars contains more water than once thought. We must also keep in mind that the Viking probes did not completely rule out the possibility of life on the planet. It might still be possible that Mars supports some kind of microbes. It may be that we have not yet looked in the right place. There are many questions about Mars that remain to be answered. For example: we do not know anything about the interior of the planet, whether its core is liquid or frozen or maybe an earth-like core or if the largest volcanoes in the solar system are still alive. A manned mission to Mars and building bases on its surface is quite possible but we are not sure when it will actually take place.

When we think about the planets in our solar system and their conditions, it is impossible to overlook the fact that the earth was designed purposely for life forms to exist. One has to be out of one’s mind to assume that all this order and harmony is completely coincidental. One important thing we have to understand is that we (living organisms) need the earth and its conditions to survive. It is the only planet in the solar system that could support us. For all we know, it might even be the only one in the galaxy. It is obvious that the earth was specifically designed for human beings, because all other life forms on earth serve mankind. By this token we can even claim that the whole universe was created for the benefit of man. The only reason for this claim is that everything would be completely meaningless without an intelligent thinking being.

Our neighbour, the red planet, might be a hope for the future. Whether we will indeed be able to build bases, grow plants, produce valuable gases and materials, begin settlements and colonies or maybe even achieve the birth of the first human being on Mars, remains to be seen, but until then we have to take care of this blue planet of ours and its inhabitants, since we are its trustees.