The Structure of the heart
The heart consists of two muscular pumps. The left side of the heart deals with oxygenated blood from the lungs and pumps this oxygenated blood to the whole body. The right side of the heart deals with deoxygenated blood from the rest of the body and pumps this blood to the lungs, where it will be oxygenated again
Each pump has 2 chambers:
- The atrium - thin walled, elastic & stretches as it collects blood. It only has to pump a short distance to the ventricle, so only has a thin muscular wall
- The ventricle - much thicker muscular wall, has to pump blood quite a distance, either to lungs or the rest of the body. The left ventricle has a thicker muscular wall than the right because it has to contract powerfully to pump blood to the rest of the body. The right ventricle only has to pump blood to the lungs (shorter distance).
There are two types of valve:
- Atrioventricular, left =bicuspid. right =tricuspid - link the atria to the ventricles, stop blood flowing back into the atria when the ventricles contract
- semi-lunar valves- link the ventricles to the pulmonary artery and aorta, stop blood flowing black into the ventricles after they contract
The Cardiac Cycle
Ongoing sequence of contraction and relaxation of the atria and ventricles that keeps blood continously circulating around the body.
- Atrial systol (ventricles relax, atria contract) - the ventricles are relaxed. The atria contract, decreasing the volume and increasing the pressure in the chamber. This forces blood into the ventricles. There's an increase in ventricular pressure & volume as the ventricles receive the blood.
- Ventricular systol (ventricles contract, atria relax) - the atria relax. The ventricles contract decreasing volume & increasing pressure. The pressure becomes higher in the ventricles than the atria, which forces the atrioventricular valves shut ('lub' sound), to prevent back flow. The pressure in the ventricles is also higher than in the pulmonary artery and aorta, which forces open the semi-lunar valves and blood is forced out.
- Diastole (relaxation of the the heart)- atria & ventricles relax. The higher pressure in the pulmonary artery and aorta closes the semi-lunar valves to prevent backflow. ('dub' sound) Blood returns to the heart and the atria fill again due to the high pressure in the vena cava & pulmonary vein. In turn this starts to increase the pressure of the atria. As the ventricles continue to relax, their pressure falls below the pressure of the atria and so the atrioventricular valves open. This allows blood to flow passively into the ventricles.
How the Cardiac Cycle is Controlled
Cardiac (heart) muscle is 'myogenic'- this means that it can contract and relax without nervous impulses. This pattern of contractions controls the regular heartbeat
- The process starts in the sino-atrial node (SAN), which is in the wall of the right atrium
- The SAN is like a pacemaker - it sets the 'rhythm' of the heartbeat, by sending out waves of electrical activity across both atria
- This causes the right and left atria to contract at the same time
- A layer of non-conductive collagen tissue (the atrioventricular septum) prevents these waves of electrical activity from crossing to the ventricles
- The wave of electrical activity is allowed to pass through the atrioventricular node (AVN), which lies between the atria
- The AVN, after a short delay, conveys a wave of electrical activity between the ventricles along a series of specialised muslce fibres called the bundle of His (Purkyne fibres). There is a short delay to ensure the ventricles contract after the atria have emptied
- The bundle of His conducts the wave through the atrioventricular septum to the base of the ventricles, where the bundle branches into smaller fibres
- The wave of electrical activity is released from these fibres, causing the ventricles to contract quickly at the same time, from the apex(bottom) upwards, forcing blood up & out.
Cardiac output in the volume of blood pumped by the heart per minute (measured in cm3 per minute). it is calculated using this formular:
- Cardiac Output = Stroke Volume x Heart Rate
- Heart rate= the number of heartbeats per minute.
- Stroke volume = the volume of blood pumped during each heartbeat, measured in cm3
You need to be able to interpret data on the cardiac cycle; if you know how pressure & volume effect the cardiac cycle then this should be easy
An Atheroma is a fatty deposit that forms within the inner lining of an artery
- The endothelium (inner lining) that is usually smooth and unbroken is damaged e.g. by high blood pressure
- White blood cells (mostly macrophages) that have taken up low-density lipoproteins (LDL's) clump together under the lining to form fatty streaks
- These streaks enlarge, (more WBCs, lipids & connective tissue- dead muscles) to form an irregular patch, or 'atheromatous plaque'.
- This plaque bulges out into the lumen, narrowing the artery, which reduces the amount of blood flow, which causes blood pressure to increase. (as the volume of the artery is decreased)
Atheromas are the main cause of thrombosis and aneurysm,
An Aneurysm is a balloon-like swelling of the artery
- Atheroma plaques damage and weaken arteries. They also narrow arteries, increasing blood pressure.
- When blood travels through a weakened artery at high pressure, it may push the inner layers of the artery through the outer elastic layer to form a balloon-like blood filled swelling, which is an aneurysm
- This aneurysm may burst, causing a haemorrhage (bleeding)
- a haemorrhage in the brain is also known as a stroke
The formation of a blood clot
- An atheroma plaque can rupture (burst through) the endothelium (inner lining) of an artery.
- This damages the artery wall and leaves a rough surface that interrupts the smooth flow of blood
- Platelets and fibrin (a protein) accumulate at the site of damage and form a blood clot (a thrombus)
- This blood clot can cause a complete blockage of the artery, or it can become dislodged and block a blood vessel elsewhere in the body
- Debris from the rupture can cause another blood clot to form further down the artery
more commonly known as a 'heart attack'
- This refers to a reduced supply of oxygen to the muscle (myocardium) of the heart.
- if a coronary artery becomes completely blocked (e.g. by a blood clot) an area of the heart muscle will be totally cut off from it's blood supply, receiving no oxygen
- This causes a myocardial infarction
- a heart attack can cause damage and death of the heart muscle
- If this occurs close to the junction of the coronary artery and the aorta, the heart will stop beating because its blood supply will be completely cut off
- if the blockage is further along the coronary atery the symptoms will be milder because a smaller area of muscle will suffer oxygen deprivation
Risk- High Blood Cholesterol & Poor Diet
- If blood cholesterol level is high (above 240mg per 100cm3) then the risk of coronary heart disease is increased
- This is because cholesterol is one of the main constitutents of the fatty deposits that cause atheromas
- atheromas can lead to increased blood pressure and blood clots
- this could block the flow of blood to coronary arteries, which could cause a myocardial infarction
- a diet high in saturated fat is associated with high blood cholesterol levels
- a diet high in salt also increases the risk of cardiovascular disease because it increases the risk of high blood pressure
Risk- Cigarette Smoking
- Both carbon monoxide and nicotine, found in cigarette smoke, increase the risk of coronary heart disease
- Carbon monoxide combines with haemoglobin and reduces the amount of oxygen transported in the blod, and so reduces the amount of oxygen available to tissues
- If heart muscle doesn't receive enough oxygen it can lead to a myocardial infarction
- Smoking also decreases the amount of antioxidants in the blood- these are important in protecting cells from damage. Fewer antioxidants means cell damage in the coronary artery walls is more likely, and this can lead to atheroma formation
- Nicotine in cigarettes stimulates th production of the hormone adrenaline, which increases heart rate and raises blood pressure. Nicotine also amkes the red cells in the blood more 'sticky' and this leads to a higher risk of thrombosis and hence of strokes or myocardial infarction
Risk- High Blood Pressure
- High Blood Pressure increases the risk of damage to the artery walls
- Damaged walls have an increased risk of atheroma formation, causing a further increase in blood pressure
- Atheromas can also cause blood clots to form
- A blood clot could block flow of blood to the heart muscle possibly resulting in a myocardial infarction
- higher blood pressure within the arteries means that they are more likely to develop an aneurysm and burst, causing haemorrhage
- To resist the higher pressure within them, the walls of the arteries tend to become thickened and may harden, restricting the flow of blood and making the arteries more brittle; atheromas more likely to form
- So anything that increases blood pressure also increases the risk of CHD e.g. being overweight, not exercising and excessive alcohol consumption