21.8: Blood is a mixture of cells and fluid.

Figure 21.14: What makes up blood?

If circulatory systems are like highways throughout our bodies, routes for the transport of goods and garbage, then blood is the traffic. Blood is a salty, protein-rich mixture of cells and fluid, with a consistency like that of motor oil. The average human body has 4–5 quarts (3.7–4.7 liters) of blood, which makes up just under 10% of our total body weight. Blood’s functions revolve primarily around its transport and delivery capabilities, including the transport of (1) respiratory gases such as oxygen and carbon dioxide, (2) vitamins and minerals, (3) nutrients, (4) hormones, (5) immune system cells, and (6) metabolic wastes. Blood also helps to maintain body temperature and homeostasis (see Chapter 20).

Blood has several distinct components, which we can identify by putting a sample of blood in a test tube and spinning it rapidly in a centrifuge (FIGURE 21-14). The lightest-weight part of the blood, the creamy yellow layer in the test tube, is the plasma, the liquid part of the blood. Plasma is 90% salty water. Dissolved within this water is a huge variety of molecules: metabolites and wastes, salts and ions, and hundreds of plasma proteins that serve to transport lipids, vitamins, and a host of other chemicals that need molecular escorts to get to the tissues where they are required. Most of the carbon dioxide produced in tissues as a by-product of cellular respiration is carried to the lungs dissolved in the plasma.

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A thin middle layer in the centrifuged tube, called a buffy coat, contains white blood cells mixed with platelets. And the heaviest components of blood are at the bottom of the tube. This layer consists of tightly packed blood cells. The proportion of the blood that consists of cells is called the hematocrit. In humans, a hematocrit of about 45% is normal. Individuals living at high elevations for a few weeks or longer, however, have hematocrits of around 48% or 49%. Why? The increase is a response to the reduced oxygen concentrations in the air at high elevations. To continue to deliver enough oxygen to the body’s tissues, there must be more blood cells to carry it.

Blood cells are made in the bone marrow (the material that fills the interior of our bones) by specialized cells, called stem cells, that are able to develop into a diverse range of cell types. Stem cells throughout the bones in our body produce blood cells at a rate of about two million cells per second. There are two types of blood cells suspended in the plasma, red blood cells and white blood cells, as well as platelets, which are cellular fragments (FIGURE 21-15).

Figure 21.15: Erythrocytes, leukocytes, and platelets.

1. Red blood cells, also called erythrocytes, are the workhorses of the circulatory system and the most common blood cells. In a human being, about 95% of the blood cells circulating at any given time are red blood cells. They are oxygen-transporting specialists, and their structure maximizes their effectiveness. Externally, they are shaped like flexible disks, so they can squeeze through capillaries in single file. Internally, they have hardly any organelles—no nucleus, mitochondria, or protein-making machinery. What are they filled with? Each red blood cell contains about 250 million molecules of hemoglobin, an oxygen-carrying protein molecule (which we discuss in detail in Section 21-13). Because it lacks almost all internal cellular machinery for repair and upkeep, a red blood cell doesn’t last long, remaining in circulation for about four months. (For comparison, many brain cells and heart muscle cells are never replaced and last for your entire life. Fat cells often last for a decade or more.) During its short life, a red blood cell will travel about 900 miles, repeatedly picking up oxygen in the lungs and releasing it to body cells that need it. We cover more on how blood cells transport oxygen later in this chapter, in the sections on respiration.

When an individual has too few red blood cells, anemia develops. Iron deficiency is the most common cause of this condition. Because red blood cells deliver oxygen to the body’s cells, one consequence of anemia is a reduction in the oxygen available to cells, essentially suffocating them. This causes people with anemia to feel tired and run-down. A reduction in the number of red blood cells is also associated with an increased susceptibility to infection, apparently by weakening the immune system’s ability to mount a response to pathogens. Iron is a critical element that enables oxygen to be carried by red blood cells. If iron is in short supply, red blood cells can’t deliver sufficient oxygen to the tissues where it’s needed. Both men and women can be anemic, but anemia affects women much more commonly, because of the blood loss during menstruation. An old folk remedy for anemia—which actually worked—was sipping liquid daily from a jug containing rusty iron nails and water. Even food cooked in an iron skillet has a significantly increased iron content (FIGURE 21-16).

Q

Question 21.5

What is anemia? Why are women more susceptible than men?

Figure 21.16: Cooking in a cast-iron skillet can significantly increase the iron content of foods!
Figure 21.17: Fighting blood loss. Platelets patch damaged blood vessels. Shown here: Elena Reid during a world championship boxing bout.

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2. White blood cells, also called leukocytes, are the defenders of the body and the primary components of the body’s immune response system. Five different types of white blood cells can be found circulating in the blood: neutrophils, lymphocytes, monocytes, eosinophils, and basophils (see Chapter 26 for more on these white blood cells and the immune system). Like red blood cells, white blood cells arise from stem cells in bone marrow. Once in the bloodstream, they patrol for pathogens, disease-causing foreign organisms circulating in the blood. White blood cells also spend much of their time outside the circulatory system, diffusing out of the capillaries and moving about in the interstitial fluid, where many pathogens such as viruses and bacteria may be and where the white blood cells may encounter and destroy cancerous body cells or cells infected by pathogens. The number of leukocytes circulating in an individual can vary greatly, depending on his or her health. Under normal conditions, there is approximately one leukocyte for every thousand red blood cells, but the number of leukocytes can increase up to threefold during an infection.

3. With more than 50,000 miles of blood vessels in our bodies, it is inevitable that there will be occasional cuts or punctures (FIGURE 21-17). Fortunately, the platelets are ready to swing into action when this happens. Platelets are cellular fragments rather than full-fledged cells. In the bone marrow, large cells called megakaryocytes repeatedly pinch off little bits of cytoplasm that have no nuclei or other organelles. These cell fragments, the platelets, are filled with enzymes and chemicals that are essential for patching damaged blood vessels. Hundreds of thousands of platelets circulate at any given time, with each platelet generally lasting about a week. You can envision platelets as jars full of Super Glue. When they bump into the edge of a cut in a blood vessel, they aggregate and release their contents, initiating constriction of the blood vessel and production of fibrin threads that form a blood clot to reduce blood loss.

Some individuals lack platelets with properly functioning clotting factors and experience uncontrolled bleeding from even minor cuts or scrapes. These problems can be due to an inherited malfunctioning gene (as in hemophilia), or they can be environmental. If the liver, where many of the clotting enzymes are produced, is damaged by disease (such as cirrhosis, which can result from alcoholism), uncontrolled bleeding can occur. Conversely, problems of blood clotting too readily can also lead to health problems. Thrombosis, for example, is the formation of clots of coagulated blood within a blood vessel. When such clots block circulation in vessels that supply blood to the muscle tissue of the heart itself, a heart attack occurs.

TAKE-HOME MESSAGE 21.8

Blood is a salty, protein-rich mixture of cells and fluid that transports (1) respiratory gases, (2) vitamins and minerals, (3) nutrients, (4) hormones, (5) components of the immune system, and (6) metabolic wastes. Blood also helps maintain a constant internal environment, including body temperature. Blood cells are produced throughout life by stem cells in bone marrow. The plasma contains two types of cells, red blood cells (oxygen transport) and white blood cells (defense from infections), as well as cellular fragments called platelets (repair).

What types of substances are transported in blood, and which portion of the blood is responsible for this transport?

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