CIRCULATION OF THE BLOOD.
The blood maintains a suitable environment for the individual cells of the body by transporting the necessary substance to and from that environment. The mechanism that ensure an adequate flow of blood to the tissues are therefore vital parts of the homeostatic process. since the need of any cell fro blood -borne substances varies with its metabolic activity and active muscle uses more oxygen than a resting muscle there must be a way to increase its blood supply when necessary. the amount of blood available, however, is limited and any increase in the blood flow and distribution of blood requires changes in those factors that are responsible for moving the blood. the flow of blood and the factors that control it constitute what we know as HEMODYNAMIC.
There are three basic elements in the movement of blood: flow, pressure, and resistance. they are so closely interrelated that one cannot consider one without the others. flow blood is caused by pressure generated by the ventricle and is directly proportion to the pressure. flow is impeded by any resistance along the way , and is inversely proportional to that resistance.
The blood flow is the volume of blood passing a point in a given period of time, usually a minute, and is expressed in milliliters or liters per minute. it may be the blood flow to a particular organ,or the blood flow in the entire circulatory system. the total blood flow is the volume of blood leaving the left ventricle each minute, and is the same cardiac output. and five liters per minute, this distributed among the organs of the body according to their needs. the organs in the adbominal cavity receive nearly half the total cardiac output.
PRESSURE AND RESISTANCE
The force imparted to the blood by the heart is opposed by various types of resistance to it flow, and if the pressure developed by the resistance, there is little or no blood flow and the circulation virtually ceases.
It is not the pressure itself that causes blood to flow, but the pressure gradient. this can be illustrated with a mechanical system in which a reservoir provides the head of pressure for a fluid filled system. when the outlet is closed, the pressure is the same throughout, as indicated by the identical fluid heights in the vertical tubes; there is pressure , but no pressure gradient and no flow.
Not all of the energy expended by the heart to develop the pressure is used to cause blood to flow. because the blood in arteries is under pressure, it pushes outward against the artery wall. the artery resists this force by pushing in to balance the outward force of the blood.
ESSENTIAL HUMAN ANATOMY AND PHYSIOLOGY. BY BARBARA R LANDAU.
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