Gases and Nutrients

The most important blood gases are oxygen and carbon dioxide. Plasma also contains a considerable amount of dissolved nitrogen, which ordinarily has no physiological function. Chapter 19 (pp. 807-812) discusses blood gases and their transport.

discussed later in this chapter under the section titled "Blood Coagulation." Table 14.7 summarizes the characteristics of the plasma proteins.

O List three types of plasma proteins.

^9 How do albumins help maintain water balance between the blood and the tissues?

As a rule, blood gases are evaluated using a fresh sample of whole blood obtained from an artery. This blood is cooled to decrease the rates of metabolic reactions, and an anticoagulant is added to prevent clotting. In the laboratory, the levels of oxygen and carbon dioxide of the blood are determined, the blood pH is measured, and the plasma bicarbonate concentration is calculated. Such information is used to diagnose and treat disorders of circulation, respiration, and electrolyte balance. Appendix C (p. 1030) lists average values for these laboratory tests.

The plasma nutrients include amino acids, simple sugars, nucleotides, and lipids absorbed from the digestive tract. For example, plasma transports glucose from the small intestine to the liver, where it may be stored as glycogen or converted to fat. If blood glucose concentration drops below the normal range, glycogen may be broken down into glucose, as described in chapter 13 (p. 530).

Recently absorbed amino acids are also carried to the liver. Here they may be used to manufacture proteins, or deaminated and used as an energy source (see chapter 18, p. 743).

Plasma lipids include fats (triglycerides), phospho-lipids, and cholesterol. Because lipids are not water soluble and plasma is almost 92% water, these lipids are carried in the plasma by joining with proteins, forming lipoprotein complexes. These lipoproteins are relatively large and consist of a surface layer of phospholipid, cholesterol, and protein surrounding a triglyceride core. The protein constituents of lipoproteins in the outer layer, called apoproteins or apolipoproteins, can combine with receptors on the membranes of specific target cells. Lipoprotein molecules vary in the proportions of the lipids they contain.

Because lipids are less dense than proteins, as the proportion of lipids in a lipoprotein increases, the density of the particle decreases. Conversely, as the proportion of lipids decreases, the density increases. Lipoproteins are classified on the basis of their densities, which reflect their composition. Chylomicrons mainly consist of triglycerides absorbed from the small intestine. Very low-density lipoproteins (VLDL) have a relatively high concentration of triglycerides. Low-density lipoproteins (LDL) have a relatively high concentration of cholesterol and are the major cholesterol-carrying lipopro-teins. High-density lipoproteins (HDL) have a relatively high concentration of protein and a lower concentration of lipids.

Many residents of Tangier Island in the Chesapeake Bay have inherited a deficiency in a particular blood lipoprotein from the original settlers, who arrived in 1686. A low blood level of alpha lipoprotein, which normally transports cholesterol in the blood, leads to low blood cholesterol. However, excess cholesterol accumulates elsewhere, such as in the thymus gland and in scavenging macrophages.

Chylomicrons, discussed further in chapter 17 (p. 720), transport dietary fats to muscle and adipose cells. Similarly, very low-density lipoproteins, produced in the liver, transport triglycerides synthesized from excess dietary carbohydrates. After VLDL molecules deliver their loads of triglycerides to adipose cells, an enzyme, lipoprotein lipase, converts their remnants to low-density lipoproteins. Because most of the triglycerides have been removed, LDL molecules have a higher cholesterol content than do the original VLDL molecules. Various cells, including liver cells, have surface receptors

Lipoprotein

Characteristics

Functions

Chylomicron

High concentration

Transports dietary

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