Cardiovascular System and Blood Help (page 2)
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.
Platelets, or thrombocytes, are small cellular fragments that originate in the bone marrow from a giant cell, a megakaryocyte. Megakaryocytes form platelets by pinching off bits of cytoplasm and extruding them into the blood. Platelets contain several clotting factors, calcium ions, ADP, serotonin, and various enzymes; they play an important role in hemostasis (the arrest of bleeding).
The major events in hemostasis are:
- constriction of the blood vessels;
- plugging the wound by aggregated platelets; and
- clotting of the blood into a mass of fibrin that augments the plug in sealing the wound and providing a framework for repair.
In the event of a vessel defect or injury, platelets aggregate to form a plug. Adenosin diphosphate (ADP) and thromboxane A2 released from the platelets further enhance platelet aggregation. The platelet plug aids in reducing blood loss at the site by:
- physically sealing the vessel defect,
- releasing chemicals that cause vasoconstriction, and
- releasing other chemicals that stimulate blood clotting. There are multiple clotting factors produced in the liver involved in clotting.
There are five types of leukocytes (white blood cells).
Blood plasma is composed of the following:
- Proteins (albumins, globulins, and fibrinogens)
- Electrolytes (Na+, K+, Ca2+, Mg2+, Cl–, HCO3 –, HPO42–, SO42–)
- Nutrients (glucose, amino acids, lipids, cholesterol, vitamins, trace elements)
- Dissolved gasses (CO2, O2, N)
- Waste products (urea, uric acid, creatinine, bilirubin)
Albumins, the smallest and most abundant proteins in the blood, maintain the osmotic pressure of the blood, buffer the blood, and contribute to the viscosity of blood. The globulin proteins in the blood function in transport, enzymatic action, clotting, and immunity. The electrolytes are necessary for membrane transport, blood osmolarity, and neurological function.
Practice problems for these concepts can be found at:
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