D. Hildbrand
The blood vessels and their function
The introductory part of the film illustrates one important principle of life. Just as the water in a river flows so the blood constantly circulates around our body. This constant movement of the blood can be observed especially in the capillaries, the smallest blood vessels in our body. Altogether the blood vessels are about 90000 km long, more than twice the circumference of the earth. Every day more than 7200 litres of blood are pumped through the blood vessels. The fluid is kept on the move by a muscular pump called the heart.
The blood is pumped away from the left ventricle in the aorta, which divides up into smaller vessels called arterioles near the different organs. Then the blood enters the capillary network of the different organs. Capillaries are extremely tiny vessels for carrying the blood through the tissues. Every tissue, therefore, contains a large capillary network so that every cell is less than 0.1 mm from a capillary. As the blood passes through these capillaries, some of the plasma passes out and enters the spaces between the tissue cells. Here the exchange of gases and other substances takes place: The cells remove food and oxygen from this tissue fluid and excrete carbon dioxide and other wastes into it.
Then small venules which join up to form the veins (vena cava superior and inferior) carry the blood back to the right part of the heart.
Arteries are thickish-walled vessels lined by a smooth endothelium because they have to withstand the very high pressure caused by the heart pumping blood into them. The aorta for example is about 30 mm in diameter on the outside and 10 mm in the inside. The walls are made up of elastic tissue and involuntary smooth muscle.
The capillaries reunite to form slightly larger vessels called venules. These join up to form the veins which take the blood back to the heart. Veins also have walls of smooth muscle, but there is much less muscle than in the arteries because the blood pressure is much lower there. They are elastic in nature and can easily swell to take the large volume of blood that has to be returned to the heart. In fact it is said that 60% of our blood is within our veins. Most veins are sandwiched between muscles and when these contract, the veins are squashed, forcing the blood in them to move .To make sure it moves in the correct direction, the veins contain one way valves (pocket valves).
In the legs where the blood must move up against the force of gravity the pocket valves make sure that the blood moves in the correct direction. If these valves are damaged or destroyed by inflammation the blood may pushed permanently against the walls of some veins and they stretch. Then we get so-called varicose veins in which blood accumulates in pouches. There the blood flow is slowed down and a blood clot called a thrombus may form. If it breaks free and reaches the lungs it may block the tiny capillaries there and cause great difficulties breathing.
The following part of the film illustrates the connection between the structure and the function of the arteries more closely. The muscles in the arteries are able to contract, thus squeezing the blood into a smaller space within the arteries. (vasoconstriction). When the left ventricle contracts a great quantity of blood is rhythmically squeezed into the aorta. Then the aorta first dilates and then slowly contracts transforming the rhythmic blood flow into a continuous blood flow. The following experiment with the elastic tube demonstrates this transformation of the rhythmic into a continuous blood flow. Vasoconstriction can also be useful to reduce bleeding when our tissues are wounded.
Words given
circumference - Umfang
wastes - Abfallstoffe
to swell - anschwellen
to be sandwiched - eingeklemmt sein, eingezwängt sein
to squash - zusammendrücken
inflammation - Entzündung
varicose veins - Krampfadern
pouches - here: Aussackungen
[zum Seitenanfang] |