The human body

What does blood do?

The blood is a vitally important fluid for the body. It is thicker than water, and feels a bit sticky. The temperature of blood in the body is 38° C, which is about one degree higher than body temperature. How much blood you have depends mostly on your size and weight. A man who weighs about 70 kg (about 154 pounds) has about 5 to 6 liters of blood in his body. Blood has three important functions:

Transportation

The blood transports oxygen from the lungs to the cells of the body, where it is needed for metabolism. The carbon dioxide produced during metabolism is carried back to the lungs by the blood, where it is then exhaled. Blood also provides the cells with nutrients, transports hormones and removes waste products, which the liver, the kidneys or the intestine, for example, then get rid of.

Regulation

The blood helps to keep certain values of the body in balance. For instance, it makes sure that the right body temperature is maintained. This is done both through blood plasma, which can absorb or give off heat, as well as through the speed at which the blood is flowing. When the blood vessels expand, the blood flows more slowly and this causes heat to be lost. When the environmental temperature is low the blood vessels can contract, so that as little heat as possible is lost. Even the so-called pH value of the blood is kept at a level ideal for the body. The pH value tells us how acidic or alkaline a liquid is. A constant pH value is very important for bodily functions.

Protection

If a blood vessel is damaged, certain parts of the blood clot together very quickly and make sure that a scrape, for instance, stops bleeding. This is how the body is protected against losing blood. White blood cells and other messenger substances also play an important role in the immune system.

The individual parts of blood

Blood is 55% blood plasma and about 45% different types of blood cells. The blood plasma is a light yellow liquid. Over 90% of blood plasma is water, while less than 10% is dissolved substances, mostly proteins. Blood plasma also contains electrolytes, vitamins and nutrients such as glucose and amino acids. Over 99% of the solid particles present in blood are cells that are called red blood cells (erythrocytes) due to their red color. The rest are pale or colorless white blood cells (leukocytes) and platelets (thrombocytes).

Illustration: Blood cells and blood vessel

Red blood cells look like discs with indentations on top and on the bottom. They can bend easily to “squeeze through” narrow blood vessels. Red blood cells have no nucleus, in contrast to many other cells. Each red blood cell contains hemoglobin, which can transport oxygen. In tiny blood vessels in the lung the red blood cells pick up oxygen from inhaled air and carry it through the bloodstream to all parts of the body. When they reach their goal, they release it again. The cells need oxygen for metabolism, which also creates carbon dioxide as a waste product. The red blood cells then pick up the carbon dioxide and transport it back to the lung. There we exhale it when we breathe out.

Red blood cells can also pick up or release hydrogen and nitrogen. When picking up or releasing hydrogen they help to keep the pH level of the blood steady; by releasing nitrogen the blood vessels expand, and blood pressure falls. Red blood cells have a life cycle of about 120 days. When they are too old or damaged they are broken down in the bone marrow, spleen or liver.

White blood cells (leukocytes) have a cell nucleus and do not contain hemoglobin. There are different types of white blood cells. They are classified according to how their nucleus is shaped and what the inside of the cell looks like under a microscope. Granulocytes have small granules in their cytoplasm. Monocytes and lymphocytes also contain granules, but these granules are extremely small and cannot be seen under a microscope. There are many more red than white blood cells in the blood.

White blood cells play an important role in the immune system. Here the different blood cells have different functions: some fight intruders such as bacteria, viruses, parasites or fungi themselves and render them harmless. Others produce antibodies, which specifically target foreign objects or germs like viruses. Leukocytes also have a part in allergic reactions: they make sure, for instance, that someone with a house dust allergy gets a runny nose when he or she comes into contact with dust. Certain lymphocytes can also kill cancerous cells that have been produced elsewhere in the body. Most of the white blood cells have a lifespan of only a few hours to several days. Some lymphocytes can remain in the body for many years, though.

Blood platelets (thrombocytes) also look like little discs, as do the red blood cells, and they also have no cell nucleus. However, they are much smaller than the red blood cells. They play an important role in blood clotting: if a blood vessel is damaged – for instance when you are cut by a knife – the healing process begins with blood platelets binding closely together on the inside of the damaged wall of the blood vessel. This causes a plug to form quickly that closes the wound temporarily: thrombocytes usually live only 5 to 9 days. Old thrombocytes are mainly disposed of in the spleen.

Creation of blood cells

All solid parts of the blood originate from common parent cells, the so-called stem cells. In adults blood cells are produced mainly in the bone marrow. The various blood cells develop in several stages from stem cells to blood cells or blood platelets. White blood cells such as lymphocytes do not mature only in the bone marrow, but also in the lymph nodes. When the cells are completed, they are released into the bloodstream. In addition to these mature cells, the blood still contains a small number of precursor cells.

Certain messenger substances regulate the production of blood cells. The hormone erythropoietin, which is produced in the kidneys, promotes the production of red blood cells, while so-called cytokines stimulate the production of white blood cells.

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