Cardiovascular System and Blood Help
Functions of Blood
Blood is a fluid connective tissue that is pumped by the heart through the vessels (arteries, arterioles, capillaries, venules, and veins) of the cardiovascular system.
- Transports oxygen, nutrients, and hormones to body tissues, and carbon dioxide and waste materials from tissues to be excreted.
- Acid-base regulation. Controls respiratory acidosis (low pH) or alkalosis (high pH) through the bicarbonate buffer system. H+ combines with bicarbonate to form carbonic acid, which dissociates to form CO2 and H2O. CO2 is exhaled, and blood becomes less acidic.
- Thermoregulation. During hyperthermia, carries excess heat to the body surface.
- Immunity. Leukocytes (white blood cells) are transported to sites of injury or invasion by disease-causing agents.
- Hemostasis. Thrombocytes (platelets) and clotting proteins minimize blood loss when a blood vessel is damaged.
Composition of Blood
Blood is composed of a liquid matrix (blood plasma) and several types of formed elements (red blood cells, white blood cells, and platelets) (see Figure 14-1). The plasma contains a variety of proteins and many other small molecules and ions. Blood minus the formed elements and the clotting proteins is called serum.
An erythrocyte, or red blood cell (RBC), is a flexible, biconcave, anucleated cell. During embryonic development, erthropoiesis (the manufacture of red blood cells) occurs first in the yolk sac. Production then moves to the liver, spleen, and red bone marrow. The main constituent of RBCs is hemoglobin and the essential function is to bring oxygen, carried by hemoglobin, to all parts of the body. The hematocrit is the percentage of total blood volume occupied my erythrocytes. Erythropoiesis is the manufacture of RBC. The sequence of cellular differentiation is as follows:
- hemocytoblast → proerythroblast → erythroblast → normoblast → reticulocyte → erythrocyte
The substances required for erythrocyte production include: proteins, lipids, amino acids, iron, Vitamin B12, folic acid, copper, and cobalt.
RBCs have an average lifespan of 120 days. New RBCs are formed in the bone marrow and old RBCs are destroyed in the liver and spleen. Any condition that decreases oxygen in the body tissues will, by negative feedback, increase erythropoiesis. In response to low oxygen concentration, the kidneys secrete the hormone erythropoietin, which stimulates erythropoiesis in the bone marrow.
The hemoglobin (Hb) molecule contains four iron containing molecules (heme) and four polypeptide chains (globin). Each heme portion can bind four molecules of oxygen. Each RBC has approximately 280 million hemoglobin molecules, and can thus carry over a billion molecules of oxygen. Hemoglobin can also bind CO2 and CO (carbon monoxide) molecules. CO2 and O2 have distinct carry sites on the Hb molecule. CO binds to a heme at the same site as O2 and has a greater affinity for the heme, thus excluding O2 binding at that site. This exclusion of O2 makes CO such a dangerous gas. The breakdown of erythrocytes in the liver and spleen results in molecules used to form bile, a digestive secretion formed in the liver. These by-products are excreted in feces or urine, giving them their distinctive brown or yellow color.
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