Erythrocytes

The major functions of erythrocytes are to carry oxygen taken in by the lungs and carbon dioxide produced by cells. Erythrocytes contain large amounts of the protein hemoglobin with which oxygen and, to a lesser extent, carbon dioxide reversibly combine. Oxygen binds to iron atoms (Fe) in the hemoglobin molecules. The average concentration of hemoglobin is 14 g/100 ml blood in women and 16 g/100 ml in men. Further description of hemoglobin structure and functions is given in Chapter 15, where the transport of oxygen and carbon dioxide is presented.

Erythrocytes have the shape of a biconcave disk— that is, a disk thicker at the edges than in the middle, like a doughnut with a center depression on each side

Vander et al.: Human I III. Coordinated Body I 14. Circulation I I © The McGraw-Hill

Physiology: The Functions Companies, 2001 Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

TABLE 14-1 Reference Table of Plasma Constituents

Constituent

Amount/Concentration

Major Functions

Water

93% of plasma weight

Medium for carrying all other constituents

Electrolytes (inorganic)

Total < 1% of plasma weight

Keep H2O in extracellular compartment; act as

Na+

145 mM

buffers; function in membrane excitability and

K+

4 mM

blood clotting

Ca2+

2.5 mM

Mg2+

1.5 mM

H+

0.0004 mM

cr

103 mM

Hcor

24 mM

Phosphate (mostly HPO42~)

1 mM

SO42"

0.5 mM

Proteins

Total = 7% of plasma weight,

Provide nonpenetrating solutes of plasma; act as

7.3 g/100 ml (2.5 mM)

buffers; bind and transport other plasma constituents

Albumins

4.2 g/100 ml

(lipids, hormones, vitamins, metals, etc.);

Globulins

2.8 g/100 ml

clotting factors; enzymes, enzyme precursors;

antibodies (immune globulins); hormones

Fibrinogen

0.3 g/100 ml

Blood clotting

Gases

CO2

2 ml/100 ml (1 mM)

A waste product

O2

0.2 ml/100 ml (0.1 mM)

Oxidative metabolism

N2

0.9 ml/100 ml (0.5 mM)

No function

Nutrients

(See Chapters 2, 4, and 18)

Glucose and other

100 mg/100 ml (5.6 mM)

carbohydrates

Total amino acids

40 mg/100 ml (2 mM)

Total lipids

500 mg/100 ml (7.5 mM)

Cholesterol

150-250 mg/100 ml

(4-7 mM)

Individual vitamins

0.0001-2.5 mg/100 ml

(0.00005-0.1 mM)

Individual trace elements

0.001-0.3 mg/100 ml

(0.0001-0.01 mM)

Waste products

Urea (from protein)

34 mg/100 ml (5.7 mM)

Creatinine (from creatine)

1 mg/100 ml (0.09 mM)

Uric acid (from nucleic acids)

5 mg/100 ml (0.3 mM)

Bilirubin (from heme)

0.2-1.2 mg/100 ml (0.003-

0.018 mM)

Individual hormones

0.000001-0.05 mg/100 ml

Messengers in control systems

(10~9-10-6 mM)

instead of a hole (Figure 14-2). This shape and their small size (7 ^m in diameter) impart to the erythrocytes a high surface-to-volume ratio, so that oxygen and carbon dioxide can diffuse rapidly to and from the interior of the cell. The plasma membrane of erythro-cytes contains specific polysaccharides and proteins that differ from person to person, and these confer upon the blood its so-called type, or group. Blood groups are described in Chapter 20, in the context of the immune responses that occur in transfusion reactions.

The site of erythrocyte production is the soft interior of bones called bone marrow, specifically the "red" bone marrow. With differentiation, the erythrocyte precursors produce hemoglobin but then they ultimately lose their nuclei and organelles—their machinery for protein synthesis. Young erythrocytes in the bone marrow still contain a few ribosomes, which produce a web-like (reticular) appearance when treated with special stains, an appearance that gives these young erythrocytes the name reticulocyte. Normally, only mature erythrocytes, which have lost these ribosomes,

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

Circulation CHAPTER FOURTEEN

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