Chapter 2: Cardiovascular System

Section 2a: Circulation

The heart is a muscular pump with four chambers, which drive two circuits of blood round the body. If the circulation fails the body sacrifices less important cells and tissues in order to maintain the brain. It receives used blood from all the major organs e.g. brain, internal organs and limbs, pumps this blood through a moderately high pressure system round the lungs where the blood obtains oxygen and carbon dioxide is expelled into the lungs. This blood is received back into the left side of the heart where it is pumped round the main organs of the body to provide them with oxygen and other essential items. The vessels, which deliver the blood to these organs, are called arteries. They have thick muscular walls and deliver blood in pulses under high pressure. A pulse can be felt by an examining finger at various sites in the body e.g. neck (carotid), wrist (radials) and groins (femorals). The significance of all these sites is that when a subject is talking to you they must have a carotid pulse as the heart clearly is delivering blood to the brain. If they do not have a radial pulse there is a significant reduction in the blood pressure (shock). If they do not have a femoral pulse then the reduction in blood pressure is even more serious. Pressures required to produce pulses in the various areas are given below.

Normal blood pressure is 120/80mmHg. (systolic/diastolic BP mmHg)

The reason for the normal blood pressure being expressed in this way is that it is in the form of a pressure wave. The highest figure is the high point of the wave and the lower figure is the minimum level to which the arterial pressure drops. If someone has lost a significant amount of blood the first change one would note in the blood pressure would be a change from 120/80 to 120/90. This is because the muscular arteries squeeze the remaining blood to increase the pressure to normal and for this reason the lower figure is raised slightly. If even more blood is lost both figures drop alarmingly. Once the pressure drops to levels of about 80-60 then the circulation to the kidneys is at risk and replacement of blood loss or even just giving plasma or fluids to increase the blood pressure could rescue the kidneys at this point. Blood is delivered to the main organs by the arterial system it is then delivered to the cells of these organs by a tree like branching of increasingly smaller blood vessels, the smallest of which are known as capillaries. The thinness of these vessels allows the blood to be spread over a very large area and also allows oxygen and nutrients to easily pass from the blood to the tissues.

Once the blood has been delivered to the organs, it then returns to the head by the low-pressure collection system known as the venous system, which returns to the right side of the heart where once again it is pumped back to the lungs. The heart is situated slightly to the left of the midline within the chest. It is well protected by the ribs. Its main vulnerability in a diving situation is that if a diver sustains a pneumothorax which goes under tension, this can impair the hearts function to the point where the pulses are progressively lost i.e. radial, then femoral then ultimately carotid resulting in death. Its inability to pump can also be impaired if gas is propelled into the main vessels as this will seriously impair the hearts ability to propel blood it will simply act to compress the gas within it's chambers rather than pump blood.

The blood volume is body weight in Kg x 80mls i.e. for a 70Kg man, a blood volume of 5.6L would be present.

The blood itself consists of about 50% red cells which are the component of the blood designed to carry oxygen round the body and is responsible for it's colour. The other portion of the blood is mainly fluid which contains factors which control clotting and also contain the various nutrients, and waste products propelled around the body.