THE LIVING WORLD
Unit Six. Animal Life
About 5% of your body mass is composed of the blood circulating through the arteries, veins, and capillaries of your body. This blood is composed of a fluid called plasma, together with several different kinds of cells that circulate within that fluid.
Blood Plasma: The Blood's Fluid
Blood plasma is a complex solution of water with three very different sorts of substances dissolved within it:
1. Metabolites and wastes. If the circulatory system is the highway of the vertebrate body, the blood contains the traffic traveling on that highway. Dissolved within its plasma are glucose, vitamins, hormones, and wastes that circulate among the cells of the body.
2. Salts and ions. Like the seas in which life arose, plasma is a dilute salt solution. The chief plasma ions are sodium, chloride, and bicarbonate. In addition, trace amounts of other salts, such as calcium and magnesium, as well as metallic ions, including copper, potassium, and zinc, are present in plasma. The composition of the plasma is not unlike that of seawater.
3. Proteins. Blood plasma is 90% water. Passing by all the cells of the body, blood would soon lose most of its water to them by osmosis if it did not contain as high a concentration of proteins as the cells it passes. Some of the proteins blood plasma contains are antibodies that are active in the immune system. More than half the amount of protein that is necessary to balance the protein content of the cells of the body consists of a single protein, serum albumin, which circulates in the blood as an osmotic counterforce. Human blood contains 46 grams of serum albumin per liter—that’s over half a pound of it in your body. Starvation and protein deficiency result in reduced levels of protein in the blood. This lack of plasma proteins produces swelling of the body because the body’s cells, which now have a higher level of solutes than the blood, take up water from the albumin-deficient blood. A symptom of protein deficiency diseases such as kwashiorkor is edema, a swelling of tissues, although other factors can also result in edema.
The liver produces most of the plasma proteins: albumin; the alpha and beta globulins, which serve as carriers of lipids and steroid hormones; and fibrinogen, which is required for blood clotting. When blood in a test tube clots, the fibrinogen is converted into insoluble threads offibrin that become part of the clot. You can see a blood clot forming in figure 23.9. Red blood cells, the disk-shaped cells, are becoming trapped in and among the threads of fibrin. The fluid that’s left, which lacks fibrinogen and so cannot clot, is called serum.
Figure 23.9. Threads of fibrin.
This scanning electron micrograph (x1,430) shows fibrin threads among red blood cells. Fibrin is formed from a soluble protein, fibrinogen, in the plasma to produce a blood clot when a blood vessel is damaged.
Blood Cells: Cells That Circulate Through the Body
Although blood is liquid, nearly half of its volume is actually occupied by cells. The three principal cellular components of blood are erythrocytes (red blood cells), leukocytes (white blood cells), and cell fragments called platelets. The fraction of the total volume of the blood that is occupied by red blood cells is referred to as the blood’s hematocrit. In humans, the hematocrit is usually about 45%.
Erythrocytes Carry Hemoglobin. Each microliter (1 pl) of blood contains about 5 million erythrocytes. Each human erythrocyte, pictured at the top of figure 23.10, is a flat disk with a central depression on both sides, something like a doughnut with a hole that doesn’t go all the way through. Erythrocytes carry oxygen to the cells of the body. Almost the entire interior of an erythrocyte is packed with hemoglobin, a protein that binds oxygen in the lungs and delivers it to the cells of the body.
Figure 23.10. Types of blood cells.
Erythrocytes, leukocytes (neutrophils, eosinophils, basophils, monocytes, and lymphocytes), and platelets are the three principal cellular components of blood in vertebrates.
Mature mammalian erythrocytes function like boxcars rather than trucks. Like a vehicle without an engine, erythrocytes contain neither a nucleus nor the machinery to make proteins. Because they lack a nucleus, these cells are unable to repair themselves and therefore have a rather short life; any one human erythrocyte lives only about four months. New erythrocytes are constantly being synthesized and released into the blood by cells within the soft interior marrow of bones.
Leukocytes Defend the Body. Less than 1% of the cells in mammalian blood are leukocytes, also called white blood cells. Leukocytes (the somewhat transparent cells shown in figure 23.10 with large or odd-shaped nuclei) are larger than red blood cells. They contain no hemoglobin and are essentially colorless. There are several kinds of leukocytes, each with a different function. Neutrophils are the most numerous of the leukocytes, followed in order by lymphocytes, monocytes, eosinophils, and basophils. Neutrophils attack like kamikazes, responding to foreign cells by releasing chemicals that kill all the cells in the neighborhood— including themselves. Monocytes give rise to macrophages, which attack and kill foreign cells by ingesting them (the name means “large eater”). Lymphocytes include B cells, which produce antibodies, and Tcells, which kill infected body cells.
All of these white blood cell types, and others, help defend the body against invading microorganisms and other foreign substances, as you will see in chapter 27. Unlike other blood cells, leukocytes are not confined to the bloodstream; they are mobile soldiers that also migrate out into the fluid surrounding cells.
Platelets Help Blood to Clot. Certain large cells within the bone marrow, called megakaryocytes, regularly pinch off bits of their cytoplasm. These cell fragments, called platelets (shown at the bottom of figure 23.10), contain no nuclei. Entering the bloodstream, they play a key role in blood clotting. In a clot, a gluey mesh of fibrin protein fibers (shown in figure 23.9) sticks platelets together to form a mass that plugs the rupture in the blood vessel. The clot provides a tight, strong seal, much as the inner lining of a tubeless tire seals punctures. The fibrin that forms the clot is made in a series of reactions that start when circulating platelets first encounter the site of an injury. Responding to chemicals released by the damaged blood vessel, platelets release a protein factor into the blood that starts the clotting process.
Key Learning Outcome 23.4. Blood is a collection of cells that circulate within a protein- rich, salty fluid called plasma. Some of the cells circulating in the blood carry out gas transport; others are engaged in defending the body from infection.