The Heart

The Basics

• Consists of two muscular pumps
• Right side pumps deoxygenated blood to the lungs
• The Right side is where the: pulmonary artery, superior vena cava, inferior vena cava, right atrium, semi-lunar valve, right atrioventircular valve and the right ventricle are found.

• Left side pumps oxygenated blood to the whole body
• The Left side is where the: aorta, pulmonary vein, left atrium, semi-lunar valve, left atrioventricular valve and the left ventricle are found.

• The Left ventricle of the heart has thicker, more muscular walls than the right  ventricle, because it needs to contract powerfully to pump blood all the way round the body. The right side only needs to get blood to the lungs, which are nearby.

• The ventricles have thicker walls than the atria, because they have to push blood out of the heart whereas the atria just need to push blood a short distance into the ventricles 

• Atrioventricular (AV) valves link the atria to the ventricles and stop blood blood flowing back into the atria when the ventricles contract.

• The semi-lunar (SL) valves link the ventricles to the pulmonary artery and aorta, and stop blood flowing back into the heart after the ventricles contract.
• The cords attach the AV valves to the ventricles to stop them being forced up into the atria when the ventricles contract.

• Cardiac muscle is 'myogenic' this means that it can contract and relax without receiving signals from nerves.

Heartbeat: 

• Starts with the sino-atrial node (SAN), which is in the wall of the right atrium
• SAN could be called a pacemaker, i.e. it sets the rhythm.
• A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles
• Instead the waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN)
• The AVN is then responsible for transporting the waves to a bundle of His. There is a slight delay before the AVN reacts to make sure the ventricles contract after the atria has emptied.
• "Bundle of His" is a group of muscle fibres responsible for conducting the waves of electrical activity to the Purkyne fibres
• These fibres carry the waves into the muscular walls of the right and left ventricles, causing them to contract simultaneously from the bottom up                                                                             

Cardiac output=stroke volume x heart rate

• Heart rate- the number of hearbeats per minute.

• Stroke Volume- the volume of blood pumped during each heartbeat, measured in cm3

NB: Quite obviously Cardiac output increases with exercise.

1) Ventricles relax, atria contract

• The ventricles are relaxed. The atria contract, decreasing the volume of the chamber and increasing the pressure inside the chamber. This pushes the blood into the ventricles. There's a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria.

2) Ventricles contract, atria relax

• The atria relax. The ventricles contract (decreasing their volume), increasing their pressure. The pressure becomes higher in the ventricles than the atria, which forces the AV valves shut to prevent back-flow. The pressure in the artery, which forces open the SL valves and blood is forced out into these arteries.

3) Ventricles relax, atria relax

• The ventricles and the atria both relax. The higher pressure in the pulmonary artery and aorta close the SL valves. Blood returns to heart and the atria fill again due to higher pressure in the vena cava and pulmonary vein. Pressure of ventricles falls below that of the atria so the AV valves open and blood flows passively into the ventricles from the atria

Atheroma

• The wall of an artery is made up of several layers
• The endothelium (inner lining) is usually smooth and unbroken.
• If damage occurs to the endothelium (e.g. by high blood pressure), white blood cells (mostly marcophages) and lipids (fat) from the blood, clump together under the lining to form fatty streaks
• Over time, more white blood cells, lipids and connective tissue build up and harden to form a fibrous plaque called an atheroma.
• This plaque partially blocks the lumen of the artery and restricts blood flow, which causes blood pressure to increase.

Aneurysm

• 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 an aneurysm
• Aneurysms may burst, causing a haemorrhage