The Composition and Functions of Blood
When a blood sample is prevented from clotting and spun in a centrifuge tube, it separates into two layers (Fig. 11.1). The lower layer consists of white blood cells (note the buffy layer), blood platelets, and red blood cells. Collectively, these are the formed elements, which make up about 45% of the total volume of whole blood; the percentage of blood attributed to red blood cells is called the hematocrit. The upper layer is plasma, which contains a variety of inorganic and organic molecules dissolved or suspended in water. Plasma accounts for about 55% of the total volume of whole blood.
Functions of Blood
The functions of blood fall into three categories: transport, defense, and regulation.
Blood moves from the heart to all the various organs, where exchange with tissues takes place across thin capillary walls. Blood picks up oxygen from the lungs and nutrients from the digestive tract and transports these to the tissues. It also picks up and transports cellular wastes, including carbon dioxide, away from the tissues to exchange surfaces, such as the lungs and kidneys. We will see that capillary exchanges keep the composition of tissue fluid within normal limits.
Various organs and tissues secrete hormones into the blood, and blood transports these to other organs and tissues, where they serve as signals that influence cellular metabolism.
Figure 11.1 Composition of blood. When a blood sample is prevented from clotting and spun in a centrifuge tube, it forms two layers. The lucent, yellow top layer is plasma, the liquid portion of blood. The formed elements are in the bottom layer. This table describes these components in detail.
Blood defends the body against invasion by pathogens (microscopic infectious agents, such as bacteria and viruses) in several ways. Certain blood cells are capable of engulfing and destroying pathogens, and others produce and secrete antibodies into the blood. Antibodies incapacitate pathogens, making them subject to destruction, sometimes by white blood cells.
When an injury occurs, blood forms a clot, and this prevents blood loss. Blood clotting involves platelets and the plasma protein fibrinogen. Without blood clotting, we could bleed to death even from a small cut.
Blood helps regulate body temperature by picking up heat, mostly from active muscles, and transporting it about the body. If the blood is too warm, the heat dissipates from dilated blood vessels in the skin.
The salts and plasma proteins in blood act to keep the liquid content of blood high. In this way, blood plays a role in helping to maintain its own water-salt balance. Because blood contains buffers, it also helps regulate body pH and keep it relatively constant.
Plasma is the liquid portion of blood, and about 92% of plasma is water. The remaining 8% of plasma is composed of various salts (ions) and organic molecules (Table 11.1). The salts, which are simply dissolved in plasma, help maintain the pH of the blood. Small organic molecules such as glucose, amino acids, and urea are also dissolved in plasma. Glucose and amino acids are nutrients for cells; urea is a nitrogenous waste product on its way to the kidneys for excretion. The large organic molecules in plasma include hormones and the plasma proteins.
The Plasma Proteins
Three major types of plasma proteins are the albumins, the globulins, and fibrinogen. Most plasma proteins are made in the liver. An exception is the antibodies produced by B lymphocytes, which function in immunity. Certain hormones are plasma proteins made by various glands.
The plasma proteins have many functions that help maintain homeostasis. They are able to take up and release hydrogen ions; therefore, the plasma proteins help buffer the blood and keep its pH around 7.40. Osmotic pressure is a force caused by a difference in solute concentration on either side of a membrane. The plasma proteins, particularly the albumins, contribute to the osmotic pressure, which pulls water into the blood and helps keep it there.
There are three types of globulins, designated alpha, beta, and gamma globulins. The alpha and beta globulins, produced by the liver, bind to metal ions, to fat-soluble vitamins, and to lipids, forming the lipoproteins. Antibodies, which help fight infections by combining with antigens, are gamma globulins.
Both albumins and globulins combine with and transport large organic molecules. For example, albumin transports the molecule bilirubin, a breakdown product of hemoglobin. Lipoproteins, whose protein portion is a globulin, transport cholesterol.
Fibrinogen (and also a protein called prothrombin) are necessary to coagulation (blood clotting).