The human heart is a hollow organ. Its walls are formed from 3 different layers; these are the endocardium, myocardium and the epicardium. The endocardium is the innermost layer; it’s a smooth layer of epithelial cells. The myocardium is the middle layer of the heart. It’s made from cardiac muscle and is the thickest layer. The epicardium is the outside layer and is only a thin membrane. The internal cavity of the heart is divided into four chambers: right atrium, right ventricle, left atrium and the left ventricle. The two atria are thin-walled chambers that receive blood from the veins. The two ventricles have thick walled chambers that pump blood out of the heart. The differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which mirrors the amount of force each chamber is needed to make. The right atrium collects deoxygenated blood from systemic veins; the left atrium receives oxygenated blood from the pulmonary veins.
Cardiac cycle
The cardiac cycle covers the events taking place in the heart during one heartbeat. The average number of heart beats in one minute is 70. To work out the average time for a cardiac cycle you do a simple sum, for example if your heart beats 55 times in one minute you divide 55 by 60 seconds you get the answer 0.9 seconds, this means 0.9 seconds is the average time for a cardiac cycle. The higher the heart rate is the shorter the cardiac cycle, until a limit is reached when the heart does not have time to fill between cycles.
Cardiac output
This is the volume of blood pumped by the heart per minute (ml blood/min). Cardiac output is a function of heart rate and stroke volume. To measure the heart rate you simply count how many times it beats per minute. The stroke volume is the volume of blood pumped out of the heart with each beat; this is measured in (ml). When either the heart rate or stroke volume increases, cardiac output increases with it.
Control of heart
The sinoatrial node (SA node) of the heart is enervated by both sympathetic and parasympathetic nerve fibres. Under conditions of rest the parasympathetic fibres release acetylcholine, which slows the pacemaker potential of the SA node and therefore reduce heart rate. Under conditions of physical or emotional activity sympathetic nerve fibres release norepinephrine, which speeds up the pacemaker potential of the SA node therefore increasing heart rate. Sympathetic nervous system activity also causes the release of epinephrine from the adrenal medulla. Epinephrine enters the blood stream, and is delivered to the heart where it binds with SA node receptors.
The respiratory system is located in the thorax, and is in charge of gaseous exchange between the circulatory system and the outside world. Air is taken in through the upper airways (the nasal cavity, pharynx and larynx) through the lower airways (trachea, primary bronchi and bronchial tree) and into the small bronchioles and alveoli within the lung tissue. The lungs are separated into lobes; the left lung is composed of the upper lobe, the lower lobe and the lingula, the right lung is made up of the upper, the middle and lower lobes. To breathe in, the external intercostal muscles contract making the ribcage move up and out; the diaphragm moves down at the same time, making negative pressure in the thorax. The lungs are held to the thoracic wall by the pleural membranes, and so expand outwards as well. This creates negative pressure within the lungs, so air rushes in through the upper and lower airways.
Exercise benefits greatly in the cardiovascular system due to the improved functioning of the heart and lowering of blood pressure. This is combined with increased maximum oxygen consumption, or lung capacity. An athlete who has not properly trained their cardiovascular system is likely to incur other injuries more easily by the rapid onset of fatigue and the consequent lowering of motivation and mental awareness. Even the athlete
