STRUCTURE AND FUNCTION OF BLOOD VESSELS

INTRODUCTION

The main transport systems are the circulatory systems, in which substances are dissolved or suspended in liquid and carried from one part of the body to another in a system of tubes called vessels.

There are two main circulatory systems:

The blood circulatory system (sometimes called the cardiovascular system) and the LYMPHATIC SYSTEM.

The blood circulatory system is the main method of transporting oxygen, carbon dioxide, nutrients and metabolic breakdown products, cells of the immune and other defence systems, chemical messengers (hormones), other important substances (e.g. clotting factors).

The lymphatic system drains extra-cellular fluid from the tissues returning it to the blood circulatory system after passage through lymph nodes. This system is also involved in absorption of nutrients from the gut.

THE BLOOD CIRCULATORY SYSTEM

There are three types of blood circulatory system, two of which (systemic circulation and pulmonary circulation) depend on a pump, the heart, to push the blood around. The third type of circulation is known as a portal system. These are specialised channels that connect one capillary bed site to another but do not depend directly on a central pump. The largest of these in the human is the hepatic portal system which connects the intestines to the liver.

The systemic circulation transfers oxygenated blood from a central pump (the heart) to all of the body tissues (systemic arterial system) and returns deoxygenated blood with a high carbon dioxide content from the tissues to the central pump (systemic venous system).

As briefly mentioned above the systemic circulation supplies all the body tissues, and is where exchange of nutrients and products of metabolism occurs. All the blood for the systemic circulation leaves the left side of the heart via the aorta.

This large artery then divides into smaller arteries and blood is delivered to all tissues and organs. These arteries divide into smaller and smaller vessels each with its own characteristic structure and function. The smallest branches are called arterioles.

The arterioles themselves branch into a number of very small thin vessels, the capillaries, and it is here that the exchange of gases, nutrients and waste products occurs.

Exchange occurs by diffusion of substances down concentration and pressure gradients.

The capillaries then unite to form larger vessels, venules, which in turn unite to form fewer and larger vessels, known as veins.

The veins from different organs and tissues unite to form two large veins. The inferior vena cava (from the lower portion of the body) and the superior vena cava (from the head and arms), which return blood to the right side of the heart. Thus there are a number of parallel circuits within the systemic circulation.

The pulmonary circulation is where oxygen and carbon dioxide exchange between the blood and alveolar air occurs. The blood leaves the right side of the heart through a single artery, the pulmonary artery, which divides into two - one branch supplying each LUNG. Within the lung, the arteries divide, ultimately forming arterioles and capillaries; venules and veins return blood to the left side of the heart.

Portal circulation. Normally there is only one capillary bed for each branch of a circuit; however, there are a few instances where there are two capillary beds, one after each other, in series. These are known as portal systems or portal circulations. One example of this is in the liver. Part of the blood supply to the liver is venous blood coming directly from the QASTROINTENTINAL tract and spleen via the hepatic portal vein. This arrangement enables the digested and absorbed substances from the gut to be transported directly to the liver, where many of the body's metabolic requirements are synthesised. Thus there are two micro-circulations in series, one in the gut and the other in the liver.

The force required to move the blood through the blood vessels in the two circulations is provided by the heart, which functions as two pumps, the left side of the heart supplying the systemic circulation and the right side the pulmonary circulation.

The systemic circulation is much larger than the pulmonary circulation and thus the force generated by the left side of the heart is much greater than that of the right side of the heart. However, as the circulatory system is a closed system, the volume of blood pumped through the pulmonary circulation in a given period of time must equal the volume pumped through the systemic circulation - that is, the right and left sides of the heart must pump the same amount of blood. In a normal resting adult, the average volume of blood pumped simultaneously is approximately 5 litres per min. As there are approximately 5 litres of blood in an adult, this means that the blood circulates around the body approximately once every minute. During heavy work or EXERCISE, the volume of blood pumped by the heart can increase up to 25 litres per min (or even 35 litres per min in top class athletes).