The Functioning of the Circulatory System.

Another feature of Hb is that when it binds to oxygen; it can actually bind to four oxygen molecules. As soon as the first oxygen molecule successfully bint to Hb the four subunits rearrange themselves to make it easier for other three to bind to it. The higher the partial pressure of oxygen the higher is the affinity of Hb for oxygen and this is demonstrated in the 'oxygen dissociation curve'. At high partial pressures of oxygen the curve becomes level because it reaches almost 100% alternatively at lower partial pressure of oxygen the curve is very steep because a very small drop in the concentration of oxygen causes huge change in the affinity of Hb for oxygen. Foetal haemoglobin will be to the right of this standard curve because it has much higher oxygen affinity.

Our heart is myogenic organ which means that it can actually beat in it was removed from our bodies and placed in an appropriate nutrients medium. The cardiac muscle of the heart responds to the waves of excitation which are produced by the SAN. The waves make the muscles of the atriums contract and so forcing the blood from the atriums which come in from the Vena cava as well as the Pulmonary Vein. The blood is forced into the Ventricles. The wave of excitation is slightly delayed from reaching the ventricles by the AVN because it contains a layer of non-conducting cells. The impulse travels down the nerve in the bundle of His and starts to contract the ventricles from the bottom upwards. This is an advantage because all of the blood is forced out this way. The blood will be forced in to Pulmonary vein and the Aorta. Blood will go to the lungs and then around the body again. It returns back to the heart before going around the body because it needs a pressure boost so the Pressure filtration process in the kidneys is carried out successfully.

Even though the heart can beat independently out of our bodies the heart rate needs to be controlled to suit the occasion. For example if a person starts doing exercises his heart rate will increase because the demand by the muscles for oxygen and glucose will increase and this is brought about by the increasing rate at which the SAN produces the waves of excitation this will cause the heart to beat faster. The way SAN knows when to speed up the pace is by the nervous impulses from the Medulla in the brain. The two stimuli for this to happen is a high level of carbon dioxide in our blood which is sensed by the chemoreceptor in the aorta and also by the stretch receptors in the heart. The heart muscle also starts to receive 180% more blood then it does before the body began to exercise which enables the heart to work much more efficiently.

There are three parts to the hearts cycle. The atrial systole forced blood into the ventricles that is when the atrial ventricular valves open because the pressure in atriums exceeds that in the ventricles so blood flows into the ventricles.

The next stage is the Ventricular systole when the ventricles of the heart contract forcing the blood through the semi lunar valves into the blood vessels. The reason the valves open is because when the heart beats the pressure is raised beyond the pressure in the aorta and pulmonary artery.

The next step is the diastole and that is when the heart rests and it is quite long about 0.4 sec. during this the atriums begin to fill with blood again and the entire cycle repeats.

The breathing rate also has to be maintained in order to keep homeostasis. The way this is done the respiratory centre in the medulla receives information about the level of carbon dioxide in the body and the degree of inflation of the lungs; medulla responds by sending an electrical impulse to the intercostal muscles and also the diaphragm so that they both increase the rate and also the depth of breathing. As soon as this happens the blood will be richer in oxygen and will be able to satisfy the oxygen demand of the respiring muscles.

The medulla itself acts as a receptor because the cells of the medulla respond to increasing carbon dioxide levels.

The nerves that act on the rate of the heart are antagonistic nerves because they have a reverse effect on the rate of the heart beating. They are both going from the cardiovascular centre in the medulla to the sino-atrial node in the top region of the heart.

Cardiac output = stroke volume + number of beats per minute

(Stroke volume is the volume pumped in one beat)

Which means if a person is an athlete for example then their number of beats per minute will be less but the cardiac output will be the same therefore the athletes heart increases the stroke volume which means that at one beat of the heart the ventricles push out more blood to maintain oxygen levels in the blood.