Blood is a fluid that nurtures life, contains many enzymes and hormones, and transports O2 and CO2. In physiological conditions it may be considered as incompressible. This complex mixture has a number of functions in the body. It may be considered as a connective tissue consisting of formed elements suspended in plasma. Plasma is mostly water but it also contains proteins, salts, nutrients, hormones, gases, vitamins, and metabolic wastes. Plasma behaves as a Newtonian fluid whereas whole blood has non-Newtonian characteristics. The specific gravity of plasma is 1.03.
The three types of formed elements are the RBC’s (erythrocytes), WBC’s (leukocytes), and platelet. A cubic millimeter of blood normally contains about:
5 million RBC’s, 7000 WBCs, and 250,000 platelet
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| From Hickman figure 8.8 - Demonstrates the development of white blood cells from a pluripotent stem cell. |
Red Blood Cells. Mature red blood cells are biconcave discs packed with hemoglobin. Its shape and content are adapted to transport oxygen. Their thanes enables them to bent easily hence passing through narrow passageways and its biconcave shape increases its surface area for gas exchange. The average volume of a RBC is 83µm3.Specific gravity of red blood cells is about 1.10.
As red blood cells develop they lose their nuclei, in addition to their mitochondria, and ribosomes they have a 120 day life span. As they move through the cardiovascular system at times they are pounding against artery walls or are squeezing through tiny capillaries. They are normally destroyed in the liver or spleen. Most of its components are recycled for future use.
The red cells contain a very high concentration of hemoglobin and really nothing else. The hemoglobin molecule is composed of four polypeptide chains. Hemoglobin exists in two functional forms of oxyhemoglobin and deoxyhemoglobin. Since carbon monoxide (CO) binds more strongly to heme groups than oxygen, poisoning occurs.
When red blood cells are placed in distilled water or a hypotonic solution, water enters and the cell swells. Swelling occurs until a critical point is reached where the membrane abruptly becomes permeable to hemoglobin, which leaves it almost completely and enters the external medium (hemolysis). If the cell is given subsequent exposure to hypertonic saline will shrink the cell back towards the original volume.
Erythropoietin is the hormone that is responsible for stimulating red blood cell production in the bone marrow.
White Blood Cells. White blood cells are larger but less numerous than the red blood cells. These nucleated cells protect against injury, infection, and cancer by either engulfing foreign agents or releasing biochemical that destroy them. The five varieties of WBCs are:
Neutrophils, eosinophils, basophils, lymphocytes, monocytes.
WBC originate in the bone marrow and liver for about one year, spending only 3 or 4 days in the blood. Elevated or diminished numbers of WBC can provide clues regarding the type of infection or disease present.
Platelets. Platelets are small, colorless cell fragments responsible for blood clotting. They originate as part of a huge bone marrow cell called a megakaryocytes. They last for about one week. In healthy circulatory system they travel freely within the vessels and last for about one week.
Blood pH. The normal pH of arterial blood is 7.4, while the pH of venous blood is 7.35 due to the extra quantities of carbon dioxide. The lower limit for survival is 6.8 and the upper limit is 8.0 (Note: pH = -log[H+]).
Design Considerations. The biconcave shape of the erythrocyte makes it difficult to measure accurately the dimensions other than the maximum diameter. There is no doubt that the surface area is much greater than if it would be for a sphere of the same volume as the red blood cell. Since the respiratory gases may have to reach the hemoglobin by diffusion through the contents of the red cell, a shape which allows diffusion to the innermost parts in the shortest times is advantageous. It is probable that the actual mixing of the contents of the red blood cells occurs during passage of the red cells, one at a time, through the smallest capillaries. Therefore, the process of exchange is not dependant on diffusion alone.
There is a wide variation in shapes of the red blood cells in the blood of a single animal. In abnormal physiological conditions and a disease, different forms are recognized by pathologists. A well known variation in shape, is that of sickle cell anemia. Here, a large percentage of the cells look like crescent moons in profile and are called meniscocytes. This represents a state in which the oxygen tension of the blood is sufficiently increased, and the cells become indistinguishable in shape from the normal cells. The change in molecular force field enables the closely packed hemoglobin molecules in the red cell to become organized in a semicrystalline state, which imposes the sickle cell shape on the entire cell.
