BLOOD AND CIRCULATION 539 



in the blood in the capillary bed. They exert an osmotic pressure that is 

 responsible for the return of water from the tissue fluids. Hydrostatic 

 pressure, i.e., blood pressure, forces the water out of the capillaries into 

 the tissue fluid. These two forces normally just balance and keep the 

 blood volume constant. 



The plasma proteins, together with the hemoglobin in the red 

 blood cells, are also important buffers. A buffer is a mixture of a weak 

 acid and its salt, or of a weak base and its salt. A buffer tends to prevent 

 a change in the pH of a solution when an acid or base is addded. Com- 

 plex animals such as mammals cannot tolerate wide fluctuations in pH, 

 and the pH of the blood is held remarkably constant, at about 7.4. 

 Buffers combine reversibly with the hydrogen ions (H + ) released by the 

 dissociation of acids into their constituent ions. Acidic substances are 

 constantly produced as by-products of cell metabolism and enter the 

 blood. Carbon dioxide, for example, is produced in cellular respiration 

 and tends to increase the acidity of the blood for it combines with water 

 to form carbonic acid, H0CO3. Basic substances, which release hydroxyl 

 ions (OH^), are much less common by-products of metabolism. Buffers 

 neutralize their effects by releasing hydrogen ions, which combine with 

 the hydroxyl ions to form water (H^O). Eventually the acidic or basic 

 substances are removed from the body, carbon dioxide by the lungs and 

 the others by the kidneys. Inorganic buffers such as carbonic acid- 

 bicarbonate are present in the blood, but the blood proteins, especially 

 hemoglobin, are extremely important and abundant buffers. 



229. Red Blood Cells 



The red blood cells, or erythrocytes, are the most numerous of the 

 formed elements of the blood, there being about 5,000,000 of them in 

 each cubic millimeter of blood in an adult human. Those of mam- 

 mals lose their nuclei as they develop, and mature mammalian red cells 

 are biconcave discs. Such a shape provides more surface area than a 

 sphere of equal volume, and the increased surface area in turn facilitates 

 the passage of materials through the plasma membrane. 



Erythrocytes contain the respiratory pigment hemoglobin, which 

 acts as a buffer and is essential for the transport of oxygen and carbon 

 dioxide. As we explained in section 28, hemoglobin (Hb) combines 

 with oxygen in the capillaries of the lungs, where the oxygen tension is 

 high, to form oxyhemoglobin (HbOo), and oxyhemoglobin releases 

 oxygen in the tissue capillaries, where the oxygen tension is low. It has 

 been estimated that we would need a volume of blood 35 times as great 

 or the blood would have to circulate 35 times as fast as it does if all of 

 the oxygen were carried in physical solution instead of in combination 

 with hemoglobin. 



Carbon dioxide diffuses into the blood from the tissues of the body. 

 Some is carried in physical solution in the plasma, but most of it (about 

 95 per cent) enters the erythrocytes. Some of this combines with certain 

 amino groups on the hemoglobin molecule to form carbaminohemo- 

 globin, but most of it combines with water to form carbonic acid. This 



