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The Heart - Blood Flow
The heart is a hollow muscular organ that is roughly the size of its owners fist, weighing around 225g in women and 310g in men. It is positioned in the thoracic cavity between the lungs, a little more to the left than the right, see heart and lung diagram. It is a muscular sac that is not under voluntary control that pumps blood around the body, it maintains blood pressure in the lungs and in general circulation.
Its structure consists of three layers of tissue; -
Pericardium - which forms the outer tissues, a fibrous layer to maintain the heart in position and a fluid filled membrane that helps smooth out the heartbeats
Myocardium - which is specialised cardiac muscle found only in the heart that is not consciously controlled.
Endocardium - which forms the lining of the myocardium and the heart valves. It is a thin, smooth membrane that permits smooth flow of blood inside the heart.
The interior of the heart is divided into a left and right side by the Septum and an Atrioventricular Valve divides each side into two chambers, the upper chambers called the Atrium and the lower chamber called the Ventricle, see blood flow diagram. (The heart can be described as two pumps within one unit). The hearts valves are basically formed by double folds of endocardium strengthened by fibrous tissue, some have two flaps (known as cusps) some have three. These valves allow blood to flow only one way (non-return valves), they are prevented from opening the wrong way and becoming damaged, by tendinous cords called chordae tendineae which attach the cusps to papillary muscles.
The hearts valves operate by differences in pressure: -
As the atrium contracts (atrial systole), pressure in the atrium becomes higher than the pressure in the ventricle, this causes (a) the pulmonary and aortic valves (the outlet valves) to close thus preventing blood from flowing back, (b) the left and right atrioventricular valves (inlet valves) to open thus allowing blood to flow from the atrium to the ventricle.
As the ventricle then contracts (ventricular systole) pressure in the ventricle becomes higher that the pressure in the atrium, this causes (a) the left and right atrioventricular valves (inlet valves) to close, preventing back flow, (b) the pulmonary and aortic valves (the outlet valves) to open allowing flow through the pulmonary arteries and the aorta.
Whilst at rest, this cardiac cycle ranges between 60 and 80 times per minute, so taking an average of 74; each beat would take just 0.8 seconds: -
Atrial Systole – contraction of the Atria at 0.1 seconds.
Ventricular Systole – contraction of the Ventricles at 0.3 seconds.
Complete Cardiac Diastole – relaxation of the Atria and Ventricles at 0.4 seconds.
The above uses 74 beats per minute as an average but there are several factors that can affect this rate: -
Position. The pulse rate is normally higher when a person is standing as opposed to lying down.
Age. The pulse rate in children is more rapid than in adults.
Sex. The pulse rate tends to be more rapid in females than in males.
Exercise. Any exercise will increase the pulse rate, though normal rate should be restored soon after exercise has stopped.
Emotion. Strong emotional states can increase ones pulse rate, e.g., excitement, fear, anger or grief.
As earlier mentioned, the heart theoretically consists of two pumps that continually pump blood around two separate circulation systems, pulmonary circulation and systemic circulation, as in Circulation System diagram (there is a third system, portal circulation which is not dealt with on this particular assignment).
Pulmonary Circulation. Here, the right side of the heart pumps blood to the lungs where gas exchanges occur; waste CO2 (carbon dioxide) leaves the blood and enters the lung ready to be exhausted, fresh O2 (oxygen) leaves the lung and enters the blood ready to be circulated.
Systemic Circulation. Here, the left side of the heart pumps freshly oxygenated blood throughout all our body tissues, where again, exchanges take place. Tissue waste enters the blood and oxygen and nutrients are extracted from the blood.
Our hearts tirelessly pump throughout our lives, varying by demand, supplying a continuous flow of blood, hence oxygen and nutrients, to all our body cells. Without this supply our cells would become inadequate, damage could occur, death could eventually follow.
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