Blood

Physiology and Circulation

Britannica Educational Publishing, Kara Rogers

• Publisher: Britannica Educational Publishing
• Year: 2010
• eBook ISBN: 9781615302505
• Topic: Biological Sciences
• Subtopic: Physiology
• Index: Biological Sciences

Although its ubiquity in the human body may make it seem unremarkable, simply put, blood makes life possible. It nourishes cells throughout the body and transports carbon dioxide to the lungs. Without it, the body would be unable to fight disease and infection or function at all. Readers are invited to follow the course of this extraordinary fluid as it circulates through the body and learn about its component parts. Detailed diagrams supplement the text and allow readers a glimpse into the anatomy and life sustaining properties of human blood.

CHAPTER 1

THE FLUID OF LIFE

Blood is a fundamental component of human life. Within the adult body, approximately 4 to 5 litres (1 to 1.3 gallons) of blood circulates continuously through an intricate network of vessels, driven by the powerful contractions of the beating heart. As blood moves away from the lungs and heart, passing through large arteries and winding into increasingly narrower and more complex networks of small vessels, it comes into contact with the individual cells of tissues. At this level, its primary function is to feed these cells, delivering to them a multitude of nutrients, including oxygen—the most basic element necessary for human survival—which it has been carrying since its departure from the lungs. In exchange for these beneficial nutrients, blood picks up and carries away cellular wastes, such as carbon dioxide, that will ultimately be removed from the body as the blood travels back to the lungs.

The basic components of blood consist of specialized cells and fluids. Each of these components performs one or more well-defined functions physiologically, and each can be isolated and tested in a laboratory, thereby providing vital information about a person’s health. Indeed, blood is one of the most easily accessed and readily examined tissues of the human body. Blood analysis has played a key role in the diagnosis of disease as well as the success of multiple lifesaving procedures, including blood transfusions and bone marrow transplantations. Although much is known about blood, it remains a subject of intense scientific investigation, fueled especially by the desire for an improved understanding of the role of white blood cells in pathological processes and in defense against infection.

The heart’s forceful contractions propel blood through the body. Veins carry blood to the heart and lungs, where oxygen is replenished before arteries pump it back through the body. Shutterstock.com

BLOOD

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Technically, blood is a transport liquid pumped by the heart to all parts of the body, after which it is returned to the heart to repeat the process. Blood is both a tissue and a fluid. It is a tissue because it is a collection of similar specialized cells that serve particular functions. These cells are suspended in a liquid matrix (plasma), which makes the blood a fluid. If blood flow ceases, death will occur within minutes because of the effects of an unfavourable environment on highly susceptible cells.

The constancy of blood’s composition is made possible by the circulation, which conveys blood through the organs that regulate the concentrations of its components. In the lungs, blood acquires oxygen and releases carbon dioxide transported from the tissues. The kidneys remove excess water and dissolved waste products. Nutrient substances derived from food reach the bloodstream after absorption by the gastrointestinal tract. Glands of the endocrine system release their secretions into the blood, which transports these hormones to the tissues in which they exert their effects. Many substances are recycled through the blood. For example, iron released during the destruction of old red cells is conveyed by the plasma to sites of new red cell production where it is reused. Each of blood’s numerous components is kept within appropriate concentration limits by an effective regulatory mechanism. In many instances, feedback control systems are operative. Thus, a declining level of blood sugar (glucose) leads to accelerated release of glucose into the blood so that a potentially hazardous depletion of glucose does not occur.

Unicellular organisms, primitive multicellular animals, and the early embryos of higher forms of life lack a circulatory system. Because of their small size, these organisms can absorb oxygen and nutrients, as well as discharge wastes, directly into their surrounding medium by simple diffusion. Sponges and coelenterates (e.g., jelly-fish and hydras) also lack a blood system, so the means to transport foodstuffs and oxygen to all the cells of these larger multicellular animals is provided by water, sea or fresh, pumped through spaces inside the organisms. In larger and more complex animals, transport of adequate amounts of oxygen and other substances requires some type of blood circulation. In most such animals, the blood passes through a respiratory exchange membrane, which lies in the gills, lungs, or even the skin. There the blood picks up oxygen and disposes of carbon dioxide.

The cellular composition of blood varies from group to group in the animal kingdom. Most invertebrates have various large blood cells capable of amoeboid movement, some of which aid in transporting substances while other are capable of surrounding and digesting foreign particles or debris (phagocytosis). Compared with vertebrate blood, however, that of the invertebrates has relatively few cells. Among the vertebrates, there are several classes of amoeboid cells (white blood cells, or leukocytes) and cells that help stop bleeding (platelets, or thrombocytes).

Oxygen requirements have played a major role in determining both the composition of blood and the architecture of the circulatory system. In some simple animals, including small worms and mollusks, transported oxygen is merely dissolved in the plasma. Larger and more complex animals, which have greater oxygen needs, have pigments capable of transporting relatively large amounts of oxygen. The red pigment hemoglobin, which contains iron, is found in all vertebrates and in some invertebrates. In almost all vertebrates, including humans, hemoglobin is contained exclusively within the red blood cells (erythrocytes). The red cells of the lower vertebrates (e.g., birds) have a nucleus, whereas mammalian red cells lack a nucleus. Red cells vary markedly in size among mammals. For example, those of the goat are much smaller than those of humans, but the goat compensates by having many more red cells per unit volume of blood. The concentration of hemoglobin inside the red cell varies little between species. Hemocyanin, a copper-containing protein chemically unlike hemoglobin, is found in some crustaceans. Hemocyanin is blue in colour when oxygenated and colourless when oxygen is removed. Some annelids have the iron-containing green pigment chlorocruorin, whereas others have the iron-containing red pigment hemerythrin. In many invertebrates the respiratory pigments are carried in solution in the plasma, but in higher animals, including all vertebrates, the pigments are enclosed in cells. If the pigments were freely in solution, the pigment concentrations required would cause the blood to be so viscous as to impede circulation.

BLOOD COMPONENTS

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In humans, blood is an opaque red fluid, freely flowing but denser and more viscous than water. The characteristic colour is imparted by hemoglobin, a unique iron-containing protein. Hemoglobin brightens in colour when saturated with oxygen (oxyhemoglobin) and darkens when oxygen is removed (deoxyhemoglobin). For this reason, the partially deoxygenated blood from a vein is darker than oxygenated blood from an artery. The red blood cells constitute about 45 percent of the volume of the blood, and the remaining cells (white blood cells and platelets) less than 1 percent. The fluid portion, plasma, is a clear, slightly sticky, yellowish liquid. After a fatty meal, plasma transiently appears turbid. Within the body the blood is permanently fluid, and turbulent flow assures that cells and plasma are fairly homogeneously mixed.

Blood is made up of multiple components, including red blood cells, white blood cells, platelets, and plasma. Encyclopædia Britannica, Inc.

The total amount of blood in humans varies with age, sex, weight, body type, and other factors, but a rough average figure for adults is about 60 to 65 ml per kg (about 1 ounce per 1.2 pounds) of body weight. This figure can be broken down into plasma volume and red cell volume. Thus, a young male may have a plasma volume of about 35 ml per kg (0.6 ounce per 1.2 pounds) and a red cell volume of about 30 ml per kg (0.5 ounce per 1.2 pounds) of body weight. There is little variation in the blood volume of a healthy person over long periods, although each component of the blood is in a continuous state of flux. In particular, water rapidly moves in and out of the bloodstream, achieving a balance with the extravascular fluids (those outside the blood vessels) within minutes.

The normal volume of blood provides such an adequate reserve that appreciable blood loss is well tolerated. Withdrawing 500 ml (about 1 pint) of blood from normal blood donors is a harmless procedure, because blood volume is rapidly replaced after blood loss. Within hours, plasma volume is restored by movement of extravascular fluid into the circulation. Red cells are completely replaced within several weeks. The vast area of capillary membrane, through which water passes freely, would permit instantaneous loss of the plasma from the circulation were it not for the plasma proteins—in particular, serum albumin. Capillary membranes are impermeable to serum albumin, the smallest in weight and highest in concentration of the plasma proteins. The osmotic effect of serum albumin retains fluid within the circulation, opposing the hydrostatic forces that tend to drive the fluid outward into the tissues.

PLASMA

The liquid portion of the blood, the plasma, is a complex solution containing more than 90 percent water. Plasma serves as a transport medium for delivering nutrients to the cells of the various organs of the body and for transporting waste products derived from cellular metabolism to the kidneys, liver, and lungs for excretion. It is also a transport system for blood cells, and it plays a critical role in maintaining normal blood pressure. Plasma helps to distribute heat throughout the body and to maintain homeostasis, or biological stability, including acid-base balance in the blood and body. The water of the plasma is freely exchangeable with that of body cells and other extracellular fluids and is available to maintain the normal state of hydration of all tissues. Water, the single largest constituent of the body, is essential to the existence of every living cell.

Of all the constituents of plasma, the proteins are the most abundant, making up about 7 percent of the plasma by weight. In fact, the principal difference between the plasma and the extracellular fluid of the tissues is the high protein content of the plasma. Plasma protein exerts an osmotic effect by which water tends to move from other extracellular fluid to the plasma. When dietary protein is digested in the gastrointestinal tract, individual amino acids are released from the polypeptide chains and absorbed. The amino acids are transported through the plasma to all parts of the body, where they are taken up by cells and assembled in specific ways to form proteins of many types. These plasma proteins are released into the blood from the cells in which they were synthesized. Much of the protein of plasma is produced in the liver.

The major plasma protein is serum albumin, a relatively small molecule synthesized in the liver, the principal function of which is to retain water in the bloodstream by its osmotic effect. Serum albumin constitutes approximately 60 percent of all of the plasma proteins. The amount of serum albumin in the blood is a determinant of the total volume of plasma. Depletion of serum albumin permits fluid to leave the circulation and to accumulate and cause swelling of soft tissues (edema). Serum albumin binds certain other substances that are transported in plasma and thus serves as a nonspecific carrier protein. Bilirubin, for example, is bound to serum albumin during its passage through the blood. Serum albumin has physical properties that permit its separation from other plasma proteins, which as a group are called globulins. In fact, the globulins are a heterogeneous array of proteins of widely varying structure and function, only a few of which will be mentioned here. The immunoglobulins, or antibodies, are an important class of proteins that are secreted by cells of the immune system known as B cells (or B lymphocytes). The immunoglobulins provide most of the body’s supply of protective antibodies and are produced in response to a specific foreign substance, or antigen. For example, administration of polio vaccine, which is made from killed or attenuated (weakened) poliovirus, is followed by the appearance in the plasma of antibodies that react with poliovirus and effectively prevent the onset of disease. Antibodies may be induced by many foreign substances in addition to microorganisms; immunoglobulins are involved in some hypersensitivity and allergic reactions.

Another group of molecules found in the plasma consists of small, short-lived proteins called cytokines, which are synthesized by cells of various organs and by cells found in the immune system and bone marrow. They serve as intercellular chemical messengers that regulate blood cell formation (hematopoiesis), though they are perhaps best known for the roles they play in the immune system’s defense against disease-causing organisms. One cytokine called erythropoietin, synthesized by specialized kidney cells, stimulates bone marrow blood progenitor cells to produce red blood cells. Other cytokines stimulate the production of white blood cells and platelets. Another protein system in the plasma, called complement, is important in mediating appropriate immune and inflammatory responses to a variety of infectious agents.

Many proteins are involved in highly specific ways with the transport function of the blood. Blood lipids are incorporated into protein molecules as lipoproteins, substances important in lipid transport. Iron and copper are transported in plasma by unique metal-binding proteins (transferrin and ceruloplasmin, respectively). Proteins called alpha and beta globulins transport lipids such as cholesterol as well as steroids and sugar. Vitamin B₁₂, an essential nutrient, is bound to a specific carrier protein. Although hemoglobin is not normally released into the plasma, a hemoglobin-binding protein (haptoglobin) is available to transport hemoglobin to the reticuloendothelial system should hemolysis (breakdown) of red cells occur. The serum haptoglobin level is raised during inflammation and certain other conditions; it is lowered in hemolytic disease and some types of liver disease.

Another critical group of plasma proteins is the coagulation proteins and their inhibitors, synthesized primarily in the liver. When blood clotting is activated, fibrinogen circulating in the blood is converted to fibrin, which in turn helps to form a stable blood clot at the site of vascular disruption. Coagulation inhibitor proteins help to prevent abnormal coagulation (hypercoagulability) and to resolve clots after they are formed.

Lipids are present in plasma in suspension and in solution. The concentration of lipids in plasma varies, particularly in relation to meals, but ordinarily does not exceed 1 gram per 100 ml. The largest fraction consists of phospholipids, complex molecules containing phosphoric acid and a nitrogen base in addition to fatty acids and glycerol. Triglycerides, or simple fats, are molecules composed only of fatty acids and glycerol. Free fatty acids, lower in concentration than triglycerides, are responsible for a much larger transport of fat. Other lipids include cholesterol, a major fraction of the total plasma lipids. These substances exist in plasma combined with proteins of several types as lipoproteins. The largest lipid particles in the blood are known as chylomicrons and consist largely of triglycerides; after absorption from the intestine, they pass through lymphatic channels and enter the bloodstream through the thoracic lymph duct. The other plasma lipids are derived from food or enter the plasma from tissue sites.

Some plasma constituents occur in plasma in low concentration but have a high turnover rate and great physiological importance. Among these is glucose, or blood sugar. Glucose is absorbed from the gastrointestinal tract or may be released into the circulation from the liver. It provides a source of energy for tissue cells and is the only source of energy for some cells, including the red cells. Glucose is conserved and used and is not normally excreted. Amino acids also are so rapidly transported that the plasma level remains low, but they are required for all protein synthesis throughout the body. Urea, an end product of protein metabolism, is rapidly excreted by the kidneys. Other nitrogenous waste products, such as uric acid and creatinine, are similarly removed.

Several inorganic materials are essential constituents of plasma, and each has special functional attributes. The predominant cation (positively charged ion) of the plasma is sodium, an ion that occurs within cells at a much lower concentration. Because of the effect of sodium on osmotic pressure and fluid movements, the amount of sodium in the body is an influential determinant of the total volume of extracellular fluid. The amount of sodium in plasma is controlled by the kidneys under the influence of the hormone aldosterone, which is secreted by the adrenal gland. If dietary sodium exceeds requirements, the excess is excreted by the kidneys. Potassium, the principal intracellular cation, occurs in plasma at a much lower concentration than sodium. The renal excretion of potassium is influenced by aldosterone, which causes retention of sodium and loss of potassium. Calcium in plasma is in part bound to protein and in part ionized. Its concentration is under the control of two hormones: parathyroid hormone (or parathormone), which causes the level to rise, and calcitonin, which causes it to fall. Magnesium, similar to potassium, is a predominantly intracellular cation and occurs in plasma in low concentration. Variations in the concentrations of these cations may have profound effects on the nervous system, the muscles, and the heart, effects normally prevented by precise regulatory mechanisms. Although iron, copper, and zinc are required in trace amounts for synthesis of essential enzymes, much more iron is needed in addition for production of hemoglobin and myoglobin, the oxygen-binding pigment of muscles. These metals occur in plasma in low concentrations. The principal anion (negatively charged ion) of plasma is chloride, and sodium chloride is its major salt. Bicarbonate participates in the transport of carbon dioxide and in the regulation of pH. Phosphate also has a buffering effect on the pH of the blood and is vital for chemical reactions of cells and for the metabolism of calcium. Transported through plasma in trace amounts, iodide is avidly taken up by the thyroid gland, which incorporates it into thyroid hormone.

The hormones of all the endocrine glands are secreted into the plasma and transported to their target organs, the organs on which they exert their effects. The plasma levels of these agents often reflect the functional activity of the glands that secrete them. In some instances, measurements are possible when concentrations are extremely low. Among the many other constituents of plasma are numerous enzymes, some of which appear simply to have escaped from tissue cells and have no functional significance in the blood.

BLOOD CELLS

There are four major types of blood cells: red blood cells, platelets (thrombocytes), lymphocytes, and phagocytic cells. Collectively, the lymphocytes and phagocytic cells constitute the white blood cells. Eac...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Introduction
• Chapter 1: The Fluid of Life
• Chapter 2: The Dynamics of Blood
• Chapter 3: Blood Group Systems
• Chapter 4: Blood Analysis and Therapeutic Applications
• Chapter 5: Diseases of Red Blood Cells and Hemoglobin
• Chapter 6: Diseases of White Blood Cells and Platelets and Disorders of Coagulation
• Conclusion
• Appendix: Tables
• Glossary
• Bibliography
• Index