This re- oxygenated red blood then returns to the left side of the heart through the pulmonary veins, which brings the red blood into the left atrium. It then goes through the valve to the left ventricle and is pumped with high pressure through the aorta artery. This pressure then drives the blood back out to the head, the neck the arms and legs and all the organs and the cycle is complete. This shows how the heart is responsible for your blood pressure.
Complete circulation of blood
Blood travels from the right atrium > to the right ventricle > through the pulmonary artery
The pulmonary artery divides into two above the heart, half goes to the left lung and half goes to the right lung. Within the lungs, the pulmonary artery branch out like a tree and continue branching until the smallest branches, which are called capillaries. Capillaries are so slim that they can only fit one blood cell through them at a time. It is within the capillaries that the processes of blood exchanging waste produce for fresh oxygen takes place. The capillaries then connect up to make veins, which join more veins until they get bigger and become the pulmonary veins, which carry red oxygenated blood back into the left atrium of the heart. The red oxygenated blood flows from the left atrium to the left ventricle chamber, which then pumps the red blood back out of the heart through the aorta. The aorta branches out to smaller arteries, which carry the red blood out to all the body parts again. The oxygen from the red blood is distributed to all the bodies tissues, in exchange the red blood collects all the waste products, as in the lungs this process takes place in the capillaries, which connect all the small arteries to the small veins. As the oxygen is gradually used up by tissue cells, the colour of the blood changes to blue again. The blue blood collects in veins and returns to the right atrium of the heart. His completes one full cycle of blood circulating around the body controlled by the heart.
The valves of the heart
In order to ensure blood only flows in one direction, there are four one-way valves within the heart. Making the blood in the entire body continually follow one direction. The most important are the two that control the blood flow in and out of the left ventricle, called the mitral and aortic valves. The corresponding two within the right ventricle are called the tricuspid and pulmonary valves. They work simply like doors that only open one way, ensuring that blood cannot go the wrong way. The left ventricle draws blood into it by expanding, and opens the mitral valve lying between it and the left atrium. At the same time, the aortic valve closes. When the left ventricle contracts, it squeezes blood out through the aortic valve, and the mitral valve connecting the ventricle and the atrium is pushed shut.
Coronary Arteries
As well as the rest of the body, the heart also needs a continues supply of oxygen, this oxygen is supplied by the two coronary arteries which branch of from the aorta immediately after it leaves the heart. It is from these left and right coronary arteries that the heart muscle is provided with the oxygen it needs to function. This is why disease of the coronary arteries is the commonest cause of death or disability in the western world because it stops the heart – the body’s engine, from doing its job.
Nerves and hormones
The heart muscle differs from other muscles in the body because it is myogenic. This means it beats on its own. It does not rely on a nerve impulse to make it contract. A heart beat starts with an electrical signal from an area of muscle in the wall of the right atrium called the sinoatrol node (SAN) or pacemaker. This electrical signal sets the rate at which the heart will beat. Every time the muscle cells in the SAN beat, they send out a wave of electrical activity, which spreads over the surface of the atria, causing the muscles in the atrial wall to contract. This wave of excitation starts at the top of the atria and spreads towards the ventricles, making the atrial wall contract in a way that will force blood into the ventricles.
The ventricle muscle cannot start contracting until the atrial muscles have finished contracting and squeezed all the blood into the ventricles. The delay in the ventricle contraction is brought about by delaying the passage of the excitation wave through to the ventricle. A ring of fibrous tissue between the atria and the ventricles prevents its spread and it can only pass through in one region, the antrioventricular node (AVN). After a short delay here, the wave passes down specialised conducting fibres in the wall (septum) between the left and right ventricles. These fibres form the “bundle of His”, and they conduct the excitation wave very rapidly to the base of the ventricles. Then it spreads upwards through the muscle in the wall of the ventricle. This ensures that the ventricles contract from the base upwards, squeezing blood into the arteries.
Cardiac output can be adjusted to meet the needs of the body. The heart responds to hormones, to electrical impulses passing down nerves from the brain and to changes for blood returning to the heart through the veins.
The cardiac output is the amount of blood that is pumped out of the left side of the heart. When someone exercises there heart rate increases and so does there cardiac output. The heart responds to hormones, in particularly adrenalin, which is secreted by the adrenal glands, which are found at the top of your kidneys.Adrenilin travels in the blood to all areas of the body. One of its many effects is to increase the heart rate by increasing the rate at which the SAN sends out its waves of electrical activity.
The electrical impulses passing through nerves from the brain that go to the heart belong to a system called the autonomic nervous system. The autonomic nervous system consists of two groups of nerves called sympathetic and parasympathetic nerves. These are the nerves that control functions of the body that we are not usually aware of, for example pupil dilation and contraction, contractions of the bladder, and the increase and decrease of heart rate.
The three ways in which cardiac output can be adjusted. These are caused by the heart responding to hormones, nerve impulses from the brain or changes to the volume of blood returning to the heart.
