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Understand the structure and function of the circulatory system

The heart is a muscular pump, roughly the size of a man’s clenched fist, which pushes blood into the tissues. Located behind and to the left of the sternum, it has 4 chambers: 2 atria and 2 ventricles and is predominantly made of cardiac muscle (myocardium).

Heart Circulation

The right-hand side of the heart receives blood from the upper and lower body via veins. Blood enters the right atrium through either the inferior or superior vena cava saturated with carbon dioxide (CO2) (deoxygenated blood) and into the right ventricle through the tricuspid (atrioventricular) valve.

It is ejected by the right ventricle through the semilunar/ pulmonary valve to the lungs via the pulmonary artery. In the pulmonary capillaries, carbon dioxide diffuses into the lungs to be expired. Oxygen(O2) enters the blood (oxygenated) and travels via the pulmonary vein to the left atrium of the heart, through the bicuspid/mitral (atrioventricular) valve the left ventricle.

The left ventricle then ejects the blood and O2 via the aortic valve and aorta, to the tissues of the body (system circulation).

Please note: arteries carry blood away from the heart and veins always carry blood to the heart (see below for a diagram)

Structure of the Heart

The Valves of the Heart

Unwanted backflow into the chambers is prevented by a number of valves which open and close in response to changes in pressure as the heart contracts and relaxes. They are fundamental to effective circulation as any backflow will compromise the efficiency of each heartbeat.

The main valves are:

  • Atrioventricular (AV) valves located between the atria and ventricles and prevent backflow from the ventricles to the atria. As the ventricles contract, pressure rises and forces the valves to snap shut and directing blood through the arteries
  • Semilunar (SL) valves located at the base of the arteries leaving the heart. After each contraction there is a relative drop in pressure in the ventricles as they relax. As blood moves back towards the ventricles the SL valves snap shut so blood cannot re-enter

As the valves snap shut they are anchored in place by tendon-like chords known as chordae tendinea which stop the valve flaps from being pushed too far into the atria and going ‘inside out’.

Control of the Heart - Conductive System

The heart is stimulated to contract by a complex series of integrated systems. The natural pacemaker – the sinoatrial (SA) node, initiates the contraction and can be found in the wall of the right atrium. The myocardium is stimulated to contract about 72 times per minute by thE SA node. The stimulation is part of the autonomic nervous system

Coronary Circulation

Coronary circulation is the term used to describe circulation of blood to the heart. The heart has its own network of blood vessels to supply it with constant oxygen and nutrients via 2 coronary arteries (left and right coronary arteries). The arteries branch out from the base of the aorta and divide into a crown-like network of blood vessels across the heart wall.

Blood is circulated through the superficial and deep tissues of the heart before being drained away by the coronary veins. The right coronary artery supplies blood to the myocardium of the right ventricle, whilst the left coronary artery divides into 2 branches to supply blood to the left ventricle and the posterior of the heart. Coronary arteries are susceptible to heart disease due to blood clots, fatty plaques or spasms in the smooth muscle in the vessel walls. These complications reduce blood flow and oxygen and nutrients to the heart muscle causing the heart to fail.

Reduced blood flow is called ischaemia, therefore Myocardial ischaemia is a reduction in blood flow to the heart. It can result in myocardial infarction (heart attack), infarction meaning death of an area of tissue as a result of interrupted blood supply

The effect of Disease Processes on the Blood Vessels

Blood flows freely to reach its target tissue or organ in a healthy blood vessel, however vascular disease narrows the blood vessels, which has a negative impact on their performance. Vascular disease is one of the main causes of death in the developed world and is caused by inflammation in the blood vessels and accumulation of mineral, protein and fat deposits creating a build-up of plaque on vessel walls. If an artery becomes inflamed or damaged, plaque will form to attempt to repair the artery. As the plaque builds up the artery becomes thicker, harder and less elastic – therefore narrower and less able to stretch in response to blood flow. As a result the blood flow (and oxygen) is reduced and can causes target tissue death unless those tissues are supplied by alternative arteries. This build up restricts or completely prevents blood flow to tissue and organs, starving the structures of vital nutrients and oxygen.

Symptoms include reduced ability to exercise, episodes of chest pain and possibly heart attacks. It is also linked to strokes and kidney disease. Narrowed arteries in the brain can also become blocked by clots resulting in a stroke.

Blood Pressure and Health Risks

Blood pressure (BP) is a measure of force applied by blood to the walls of the arteries as it flows through them. It is an expression of the arterial blood flow and peripheral resistance the blood encounters as it flows around the body.

Blood pressure = cardiac output x total peripheral resistance

It is measured in mmHg using a sphygmomanometer and is expressed as systolic and diastolic.  Optimal BP is below 120/80 mmHg and Hypertension is classed as 140/90 mmHg or higher.

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