Blood | Research & Encyclopedia Articles

The life fluid of the body is blood. All animals, including humans, require that nutrients and oxygen be available for metabolism and that wastes be removed. In animals that measure 1 millimeter or less in diameter, these substances are transported within the body by diffusion between the cells and nearby body parts. In larger, more complex animals, circulatory systems have evolved with arteries, veins, and capillaries to transport respiratory gases, nutrients, waste products, hormones, antibodies, and salts to parts of the body.

Blood, the medium for transporting nutrients and waste products, is both a tissue and a fluid containing many specialized types of cells. It is a tissue because it is a collection of similar cells that serve a particular function. These cells are suspended in a liquid matrix called plasma, which allows the blood to act as a fluid.

Blood plays an important role in nearly all body functions. Oxygen is one of the crucial substances that enters the blood. Oxygen passes through the walls of the lungs, gills, or skin of the animal. The blood picks up and carries oxygen to all parts of the body. As the oxygen-laden blood moves through the circulatory system, it passes through cell walls and provides fuel for the working parts of the body.

Blood also carries digested food from the intestines to the muscle cells. When the muscles work, they produce waste products that must be disposed of. These waste products pass through the walls of the circulatory system into the blood. The blood then carries wastes to the kidneys, where they are eliminated from the body. The work of the muscles creates heat, which is transferred by blood throughout the body. In warm-blooded birds and mammals, blood maintains the temperature of the body.

Blood plays a critical part in the fight against diseases in animals. Blood contains many kinds of disease-fighting substances such as antibodies and white blood cells. Blood tests can reveal a great deal about how well the body is working.

The blood of mammals—including humans—is complex. About half of the volume of blood is made up of blood cells, which originate in the bone marrow. Blood cells begin as stem cells, then develop into many other kinds of cells—red cells, white cells, and platelets. Blood is composed of 55 percent plasma and 45 percent other elements.

Plasma is the watery part of the blood. Plasma is 90 percent water and carries most of the chemicals in the blood. These chemicals include minerals such as sodium, potassium, vitamins, hormones, enzymes, and glucose. Some of these substances are manufactured in the body; others enter through the lungs or with food. Plasma also carries dissolved gasses, especially oxygen, carbon dioxide, and nitrogen.

Most stem cells become red blood cells, or erythrocytes. Human blood contains 4.8 to 5.4 billion red blood cells per milliliter of blood. Red blood cells' primary function is to carry oxygen from the lungs to every cell throughout the body. The outer layer, or membrane, of the red blood cell is flexible and can bend in many different directions without breaking.

Red cells have an iron-containing substance or pigment known as hemoglobin. As hemoglobin passes through the lungs, it picks up oxygen, forming a red-colored compound known as oxyhemoglobin, which gives the blood a distinctive red color. As the blood passes through body tissues, hemoglobin releases oxygen to cells throughout the body. During this passage, the hemoglobin gives up some of its oxygen. In response, the tissues send a waste gas, carbon dioxide, into the blood.

White blood cells, or leukocytes, form a wandering system of protection for the body. Composed of granulocytes, monocytes, and lymphocytes, these cells originate in the bone marrow, where there is a ratio of one white cell to 700 red cells. Two-thirds of white cells are granulocytes, which travel to places in the body where bacteria or other foreign substances are located and swallow up these invaders. Monocytes, another type of white cell, also swallow up foreign substances and assist the body in overcoming and resisting infections. Lymphocytes produce antibodies, which are released into the blood to target and attach to foreign substances.

The smallest of the blood cells are called platelets. These cells assist in blood clotting by sticking together and plugging small holes in the walls of the blood vessels. As these tiny platelets flow out of a cut on the wall of the blood vessel, they release a chemical known as thromboplastin. This self-sealing characteristic of blood is critical to an animal's survival.

One-celled organisms have no need for blood. They are able to absorb nutrients, expel wastes, and exchange gases with their environment through aprocess called diffusion. In some invertebrates, such as flatworms and cnidarians, oxygen is dissolved in the plasma. Simple multicelled marine animals such as sponges, jellyfish, and anemones use seawater to bathe cells and perform the function of blood. The immune system of invertebrates is less developed than that of vertebrates, lacking the white blood cells and antibody system found in mammals.

Differing oxygen requirements play a significant role in the composition of blood and the design of animals' circulatory systems. Crustaceans and other arthropods have an open type of circulatory system, while more complex vertebrates—including humans—have a closed circulatory system. Larger and more complex animals have greater oxygen needs and have developed respiratory pigments to help transport oxygen in the blood. These specialized compounds, hemoglobin or hemocyanin, are able to carry greater amounts of oxygen because of the metal atoms in the pigments reacting with and transporting additional atoms of oxygen.

The red pigment hemoglobin contains iron, transports oxygen, and is found in all vertebrates as well as some invertebrates with a closed circula-tory system, such as earthworms. The blue pigment hemocyanin, which contains copper, is found in some animals with an open circulatory system, including some crustaceans such as crabs, and in some mollusks. This pigment transports oxygen to body tissues and gives the blood a bluish color. The blood of insects is clear or yellow. The red fluid from some squashed insects actually comes from blood they have eaten, not from their own blood, as they have no pigments.

Although the blood of complex animals tends to be similar to human blood, there are differences at the cellular level. For example, reptiles, fish, and amphibians have red blood cells with a nucleus, unlike humans and other mammals. Some arctic fish are able to produce a specialized protein that acts as a type of antifreeze, allowing them to survive where the blood of other animals would freeze.

Plasma is unquestionably essential for the survival of human beings. Along with carrying important minerals and dissolved salts like calcium, sodium, and potassium among others, disease-fighting antibodies are contained in plasma.

Circulatory System.

Hickman, Cleveland, Larry Roberts, and Frances Hickman. Integrated Principles of Zoology, 8th ed. St. Louis, MO: Times Mirror/Mosby College Publishing, 1990.

Mill, P. J. Respiration in the Invertebrates. London: Macmillan Press, 1972.

Randall, David, Warren Burggren, and Kathleen French. Eckert Animal Physiology: Mechanisms and Adaptations, 4th ed. New York: W. H. Freeman and Company, 1997.

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