Most of the Oxygen in Blood Is Carried in Chemical Combination with the Protein Hemoglobin Within Red Blood Cells
Of the 20 mL of O2 normally carried in each 100 mL of oxygenated blood, only 1.5% (0.3 mL) is carried in dissolved form. The remaining 98.5% is carried in chemical combination with hemoglobin (in RBCs).
Oxygenated hemoglobin (oxyhemoglobin, HbO2) is bright red. VXTien O2 is released, HbO2 becomes reduced hemoglobin (Hb), which is dark bluish red. 'Γhe adequacy of oxygenation of an animal’s blood can be judged somewhat by looking at the color of its nonpigmented epithelial membranes (e.g., gums, nostrils, or inside surfaces of eyelids). Well- oxygenated tissues appear pink. Poorly oxygenated tissues appear bluish (cyanotic) because of the prevalence of reduced I lb.The ability of blood to carry oxygen is determined by the amount of hemoglobin in the blood and by the chemical characteristics of that Hb. For example, each deciliter (dL) of normal dog blood contains about 15 g of Hb. Each gram of Hb can combine with 1.34 mL of O2, when fully saturated. Thus, each deciliter of fully oxygenated, normal blood carries 20 mL of O2. Several disease states result in the synthesis of chemically abnormal Hb, with a diminished capacity to bind O2. In addition to these hemoglobinopathies, several common toxins, including carbon monoxide (CO) and nitrates, cause life-threatening alterations in the ability of Hb to bind O2.
Because hemoglobin is localized inside RBCs, it is possible to derive several clinically useful relationships among the blood Hb content, RBC count, Hb content of each RBC, and hematocrit. For example, if a dog has 15 g of Hb/dL of blood and RBC count of 6 million cells per microliter (μl.) blood, it follows that each RBC (on average) contains 25 picograms (pg) of Fib:
The value calculated in this way is called the mean corpuscular hemoglobin (MHC).
An easier calculation, which serves the same purpose, is to determine how much hemoglobin is contained in each deciliter of packed RBCs. For example, if a dog has 15 g of Hb in each deciliter of blood and has a hematocrit of 50%, the Hb concentration in the RBC portion of the blood must be 30 g of Hb/dL of packed RBCs:
The value calculated in this way is called the mean corpuscular hemoglobin concentration (MCHC). For simplicity, the calculation is often summarized as follows:
MCHC = (HemogIobinlZHematocrit
The brackets denote concentration.
An abnormally low value of MCH or MCHC is clinically important because it points to a deficit in hemoglobin synthesis (i.e., not enough Hb being made to load up each RBC). In contrast, an abnormally low value for Hb by itself is less helpful; hemoglobin concentration in the blood could fall below normal for several reasons, including a deficit in Hb synthesis, a deficit in RBC synthesis, or a “watering down” of the blood either by addition of excess plasma fluid or by loss of RBCs.
Deviations from a normal hematocrit (Hct) can have important consequences in terms of the ability of blood to carry oxygen. Hematocrit also affects the viscosity of blood, as shown in Figure 18-6. Viscosity is a measure of resistance to flow. For example, honey is more viscous (more resistant to flow) than water. Plasma, by itself, is about 1.5 times more viscous than water because of the presence of plasma protein molecules (albumin, globulin, fibrinogen). The presence of cells in blood has an even greater effect on viscosity. Blood with a Hct of 40% has twice the viscosity of plasma. For Hct exceeding 50%, viscosity increases rapidly. An abnormally high hematocrit is called polycythemia, which literally means “many cells in the blood.” The blood of a patient with polycythemia can carry more than the normal 20 mL of O2∕dL of blood (provided that the MCHC is normal), and this may be viewed as beneficial.
However, the increased viscosity makes it difficult for the heart to pump the blood. Therefore, polycythemia creates a heavy workload for the heart and can lead to heart failure, particularly if the cardiac muscle is not healthy.The opposite problem, in which the hematocrit is too low, is called anemia. Anemia literally means “no blood,” but the word is used to refer to any condition in which there are abnormally few RBCs in blood. Each deciliter of blood of an
FIGURE 18-6 Plasma is more viscous than water because of the presence of plasma proteins. Blood is more viscous than plasma because of the presence of blood cells. Blood viscosity increases sharply when the fraction of cells (hematocrit) increases above 50%.
FIGURE 18-7 Relative size and shape of the major constituents of blood.The figure emphasizes that the plasma protein molecules are huge compared with the other plasma solutes, such as glucose, Na*, and Cl. Furthermore, the blood cells (red and white) are huge compared with plasma protein molecules. Numbers under constituents are their molecular weights (in daltons).The scale (upper left) indicates a length of 10 nm. In comparison, the diameter of the red blood cell is 7.5 μm (7500 nm).
anemic patient carries less than the normal 20 mL of O2. Therefore, cardiac output must be increased above normal to deliver the normal amount of O2 to the tissues each minute. This need to increase cardiac output also imposes an increased workload on the heart and can lead to the failure of a diseased heart. Thus, Hct in the range of 40% to 50% provides the blood with enough Hb to carry an adequate amount of O2 without putting an undue workload on the heart. For additional information about the transport of O2 and CO2 in blood, see Chapter 48.
Figure 18-7 provides an idea of the relative sizes and shapes of the major constituents of blood. The plasma proteins are much, much larger than the ions and nutrient molecules that are dissolved in plasma. RBCs and WBCs are many, many times larger than the plasma proteins. In fact, as mentioned earlier, blood cells are so large that they can barely squeeze through a typical capillary.