Heart structure and function
Created | Updated Jan 28, 2002
Heart structure and function.
The heart is a sort of upside down cone shape with bluntd edges. On the outside there is a layer of fat, and across it's surface are a network of veins and arteries, known as the coronary vessels, which keep the muscle suppiled with blood.
The heart's job is to pump blood around the body to where it is needed. Consequently, the heart is made up of cardiac muscle, which can contract often without tiring.
The heart is made up of four chambers. The top two are atria (singular: atrium) which take in blood from blood vessels. (The Vena Cave feed into the right atrium, the pulmonary vein into the left). The atria contract (atrial systole) and force the blood down into the two lower chambers, the ventricles. When these contract (ventricular systole) they push the blood into the arteries. (The right ventricle pushes blood into the pulmonary artery, the left supplies the Aorta). The blood entering the Aorta must travel all the way around the body, supplying all the organs except the lungs with blood, while the blood going into the pulmonary artery only goes to the lungs and back. Consequently, the muscle under the left ventricle is much thicker than that under the right, allowing it to contract with greater force.
The left and right sides of the heart are seperated by the septum, a wall of muscle and cardiac tissue.
Thje valves which stop the blood flowing back from the the ventricles to the atria are known as the atrio-ventricular (AV) valves. On the left are the Bicuspid and on the right the Tricuspid. There are also valves which stop the blood from the arteries flowing back into the ventricles. These are called semi-lunar valves.
PICTURE of heart with labels.
The cardiac cycle.
Systole refers to the contraction of the muscle.
Diastole refers to the relaxation of the muscle.
1. Atrial systole (which lasts around 0.1 seconds).
The atria are full of blood.
The AV valves are closed.
The atria contract, raising the pressure in the atria higher than the pressure in the ventricles.
The AV valves open and blood is forced into the ventricles.
2. Ventricular systole (which lasts around 0.3 seconds).
The ventricles contract almost immediately after atrial systole.
Ventricular pressure rises.
As the pressure rises above that in the atria, the AV valves are pushed closed to prevent blood flowing back into the atria. (The LUB heart sound is the blood hitting the AV valves).
The pressure in the ventricles is now higher than that in the arteries and the atria.
Blood is forced into the arteries.
3. Diastole (which lasts around 0.4 seconds).
The atrial and ventricular walls relax.
Ventricular pressure falls as walls relax.
Semilunar valves close as arterial pressure is higher than that in the ventricles. they shut to prevent blood flowing back into the the ventricles. (The DUB heart sound is the blood hitting the valves).
As the walls of the atria relax blood from the veins enters the atria.
PICTURE of a heart beating.
The cycle then begins again. The whole process lasts around 0.8 seconds.
Co-ordination of the heartbeat
The heart is myogenic (muscle starting) and will beat on it's own without instruction from the brain. The sinoatrial node (SAN) sends out impulses causing the heart to contract.
The brain can send messages down nerves to make the heart beat faster (via tha accelerater nerve) or slower (via the Vagus nerve).
The impulse fro the SAN spreads first across the atria, causing them to contract, the a sructure in the septum called the atrioventricular node (AVN) pauses the impulse for a fraction of a second in order to make sure the ventricles contract together. The impulse then continues down the bundles of His, into the Purkinje (sometimes called Purkyne) tissue. (N.B. The sides of the heart do not carry the impulse down into the ventricles).
