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53 4 Homeostasis in the mammalian cardiovascular system depends importantly on the interactions among blood pressure, blood flow, resistance to blood flow, and other hemodynamic variables. Moreover, there are several important reflexes such as the baroreceptor reflex and the Bainbridge reflex that help maintain an equilibrium in the above hemodynamics and that try to restore homeostasis when it is disturbed. The mammalian cardiovascular system is best understood by analyzing its component parts. In the simplest terms, these are the heart, the blood vessels, and the blood. The hearts of all mammals have four chambers: a left and a right atrium and left and right ventricles. The atria are separated by a thin wall of muscle called the interatrial septum and the ventricles are separated by the thicker interventricular septum. There are effectively two cardiovascular systems in mammals: the pulmonary and systemic (peripheral) circulatory systems. Each is composed of a pump (right or left ventricle) and a set of blood vessels. The ventricles function as the pump mechanism for generating pressure and volume work, and need to create high enough internal pressures to overcome the arterial resistance to blood flow. The ventricles must also eject a sufficient volume of blood to initiate and sustain circulation. The main differences between the two cardiovascular systems are that () the pulmonary circulatory system supplies blood flow only to the lungs whereas the systemic circulatory system provides blood flow to all remaining organs and tissues, and () the pulmonary is a low-pressure system while the systemic is a high-pressure system. The pulmonary cardiovascular system begins at the right ventricle and ends at the left atrium. The systemic or peripheral cardiovascular system begins at the left ventricle and ends at the right atrium. All other organs and tissues of the body are interposed between these two chambers. All vessel components between the two end points in either system are an integral part of that particular system. The Cardiovascular System and the Blood This means that under normal conditions, maximum ventricular/arterial pressures in the systemic circulatory system are about mmHg whereas corresponding pressures in the pulmonary circulatory system are about mmHg. This is important because the systemic circulation and left ventricle work against a high resistance. The pulmonary circulation and right ventricle do not. The Blood Blood is a tissue. A tissue is a group of like-behaving cells that serve a common purpose. In the case of blood, the purpose is delivery of nutrients and gases and removal of by-products of metabolism. Blood is composed of a variety of cell types found in a fluid matrix. The cells are red blood cells (RBCs or erythrocytes ), white blood cells (WBCs or leukocytes), and blood platelets (fragments of larger, more primitive blood-forming cells). In adult humans, neither the platelets nor the RBCs have nuclei. All WBCs have nuclei, but the nuclei have a wide range of morphologies and staining characteristics. Some do not stain and are called neutrophils. Others bind basic stains (blue) and are called basophils. Still others bind acidic stains (red) and are called eosinophils. The fluid matrix in which the cells are suspended is called the plasma. By both weight and volume, plasma is mostly water, about  percent. Because plasma is mostly water and because water is the universal chemical solvent, plasma is considered a solvent. It contains a wide range of solutes including but not limited to macromolecules such as the immunoglobulins (antibodies) and other proteins, electrolytes, vitamins and minerals, carbohydrates, lipids, and amino acids. One of the important constituents is fibrinogen, a protein that is essential to the blood-clotting mechanism. Under conditions where increased clotting is necessary, fibrinogen is broken down to fibrin monomers that polymerize to help form fibrin clots. These serve as vascular plugs to minimize loss of blood from severed blood vessels. When fibrinogen and its breakdown products are removed from the plasma, it (plasma) will no longer clot and is then known as serum. The two terms plasma and serum are frequently misused. Clinically speaking, if withdrawn plasma is stored and intended for reinfusion, it is either anticoagulated or has the fibrinogen removed before storage. In this latter case, the plasma becomes serum. The health of the blood is a good indicator of the overall health of the body, and it can be assessed in a number of ways. One of the most important quantifications of the blood is what we call hematocrit. Hematocrit is the percentage of whole blood (cells plus plasma) that is...

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