Cardiovascular system

The heart is the seat of the emotions. At least, it was long regarded as being so. Although we now know that it is an organ for pumping blood through the body, it still retains a special significance; perhaps this is partly because it is so indispensable. The heart and the brain are the only organs that must not stop working for a minute.

Blood is responsible for the transport of oxygen, defence against germs and the supply of necessary substances to all parts of the body. Although the quantity of blood can vary quite a lot without directly troubling us much (we can give away half a litre without any problem), the composition of the blood is very accurately regulated. This composition is needed to maintain the body’s internal environment. The metabolic processes can take place normally only if all the body substances have a constant composition. Because of the blood’s many functions, its composition reveals a great deal about the state of the body. Measuring the quantities of certain substances in the blood forms one of the most important parts of medical examinations. The blood vessels that carry the blood to the tissues are found everywhere except in places where they would be in the way. There are no blood vessels, for example, in the cornea of the eye, because it must be completely transparent.

An important function of the blood is defence. This is built up in collaboration with the lymphatic system, whose components include thin vessels running through the whole of the body and a large number of lymphatic glands. The body can build up a resistance against millions of substances. Defence is also one of the few functions that medical science can really influence. Vaccination can stimulate the body to produce antibodies against a bacterium or a virus, with the result that we do not become ill if we encounter the organism in question.

Circulation

All parts of the human body require a constant source of oxygen and energy-supplying substances, as well as minerals, hormones and vitamins, to maintain and govern the functioning of various organs and tissues. These organs and tissues also require a drainage system to remove waste products, and to transport these and other substances to various parts of the body for excretion, storage or utilization. These vital functions are provided for by the body’s circulatory system – the network of blood vessels and its driving force, the pumping action of the heart. This system is divided into two parts. The first is the systemic circulation, which transports blood from the left side of the heart round the body (except for the lungs) and back to the right side of the heart. The second is the pulmonary circulation, which takes blood from the right side of the heart to the lungs and then back to the left side of the heart.

Systemic circulation

All vessels that transport oxygenated blood away from the heart to supply tissues are known as arteries. The main artery leading from the left ventricle of the heart is the aorta. It arches round from the top of the heart, over and down through the chest. From its arch, major arteries lead up into the head and neck (the common carotid arteries) and the arms (the brachiocephalic and subclavian arteries). The aorta continues down into the abdomen, giving off, as it goes, branches to the stomach, liver and spleen (the coeliac artery), kidneys (the renal arteries), and intestine (the mesenteric arteries). It finally divides into the two arteries that supply blood to the legs (the common iliac arteries).

All of these arteries branch again and again forming ever smaller and more diverse routes that penetrate deep into the tissues. The smallest end-branches are called arterioles. These in turn divide and lead into minute tubes with walls only one cell thick, called capillaries. All the necessary oxygen and nutrients from the blood can pass through the walls of the capillaries, and waste products pass into the blood to be carried away through the venous system. This consists of a reverse, joining system of tiny veins or venules leading to larger veins and ultimately to the large collecting veins – the inferior vena cava from the lower parts of the body and superior vena cava from the upper parts, both of which pass venous blood into the right side of the heart.

Pulmonary circulation

If you look at the veins in your forearm and hand you may notice that they have a bluish tinge. This is because they carry ‘used’ blood back to the heart. This blood has already circulated through the various body tissues and given up its supply of oxygen to them.

Oxygen is carried in a substance in the blood called haemoglobin, which is bright red when fully charged with oxygen and dark, bluish-red when deoxygenated. The deoxygenated blood returns from the systemic circulation to the right side of the heart, from where it is ejected into the pulmonary artery. This is the main vessel taking blood away from the heart to the lungs and is therefore an artery – the only artery which carries deoxygenated (blue) blood. It branches into the right and left pulmonary arteries and each of these branches again and again, so that the lungs are very richly supplied with blood via a mass of arteries, arterioles and capillaries.

The tiny capillaries in the lung are in close contact with the air spaces inside the air sacs, or alveoli. It is from the alveoli that oxygen diffuses into the blood at the same time that other gases, mainly carbon dioxide, pass the other way and are excreted. Each of these gases passes readily through the thin alveoli and capillary walls. The waste gases are exhaled and with each new breath the alveoli refill with fresh oxygen. The blood – freshly charged with oxygen and now bright red – drains back through the mass of tiny collecting venules, veins and the pulmonary veins into the left side of the heart. These are the only veins in the body that contain oxygenated blood.

Blood pressure

From the left atrium (upper chamber) of the heart the blood passes into the left ventricle (lower chamber) from which it is pumped under pressure by the strong contraction of the heart muscle. With each contraction of the left ventricle a pressure wave travels through the arteries of the systemic circulation. This can be felt at certain points on the body, where arteries are close to the skin surface, as a pulse. At the wrist the radial pulse is felt; the brachial pulse is at the bend of the elbow; the carotid pulse is under the angle of the jaw in the neck; and the femoral pulse is felt in the groin.

The pulse represents a peak of pressure in the artery coinciding with the pumping contraction of the left ventricle. Between contractions the pressure drops away to a minimum pressure, maintained in order to ensure that the blood continues to flow and has sufficient pressure to reach and enter the capillaries. The pressure of the blood in the arteries may be measured by means of a sphygmomanometer, which consists of an inflatable cuff that is wrapped around the upper arm then inflated. The cuff is connected to a mercury column so that the pressure within it may be measured in terms of millimetres of mercury. The cuff is pumped up to sufficient pressure to block the arterial flow in the vessels near the surface of the arm around which it is wrapped. The cuff is then slowly deflated and, as the pressure is released, a point is reached at which the peak pressure of blood is just enough to overcome the cuff pressure and so blood passes beyond the cuff. Because this flow is still partly blocked by the cuff, it is very turbulent, so the person taking the blood pressure can hear, with the aid of a stethoscope, blood flowing through the artery just beyond the cuff. It is heard as a tapping sound in time with the pulse. This maximum pressure is known as the systolic blood pressure, and is normally around 120mm mercury.

As the cuff is released further, blood flows more easily until the minimal pressure in the system overcomes the cuff pressure, the artery is permanently open and blood once more flows smoothly. At this point the sounds fade out. This is called the diastolic blood pressure, which is usually about 80mm of mercury.

Normal adult blood pressure is a combination of normal systolic and normal diastolic pressure; taking the two figures above, this would be written as 120/80. This measurement is very useful because, in conjunction with the pulse rate, it gives doctors some idea of how well blood is being circulated and how hard the heart is working to do this.

If the blood pressure falls too low, for example in the case of severe blood loss, vital organs are deprived of blood and action has to be taken to restore the blood pressure.

Blood pressure in the arteries is governed by the myocardial contractility (the pumping force of the heart), the heart rate and the tone of the blood vessels, which depends on the diameters of the arteries.

Special cells called baroreceptors monitor blood pressure in certain arteries, in the heart muscle itself and in the brain. By a system of chemical and nervous impulses there is a feedback of information to the various controlling mechanisms so that they can maintain the blood pressure at the needed level.