Arteries, veins and capillaries
An adult human has 5 to 6 liters of blood in his body. This is pumped around the heart in a blood vessel system. At rest, the blood circulates through our body in about 1 minute, with heavy exertion this takes 20 seconds. What does that blood vessel system actually look like?
Blood must constantly flow to carry oxygen and nutrients to organs and tissues. The blood also ensures that waste products are returned. In total, the body has 100,000 kilometers of blood vessels for this. From the heart, these vessels branch off in a number of main streams, and the further they enter the body, the more intricate the network becomes.
circulatory system
The circulatory system consists of the small and the large circulatory system. In the small, deoxygenated blood enters the right atrium of the heart. Then the valve between the right atrium and ventricle opens and blood flows to the right ventricle. The heart pumps blood through the right ventricle and the pulmonary artery to the lungs. In the lungs, the blood gives off carbon dioxide and takes in oxygen. This oxygenated blood flows through the pulmonary veins back to the heart.
The major circulatory system goes from the heart to all parts of the body. The left ventricle pumps the oxygenated blood through the aorta, the large body artery, into the body. The aorta then branches into smaller and smaller vessels.
blood vessels
There are three types of blood vessels: the arteries (arteries), veins (veins), and capillaries (capillaries). They all have a different function and are therefore also built differently. Blood flows from the heart to the arteries, then to the capillaries and back to the heart through the veins.
arteries
The arteries carry oxygen and nutrients from the heart to the rest of the body. The pulmonary arteries are the only exception to this – they carry deoxygenated blood to the lungs.
With each heartbeat, the heart pushes blood through the arteries under great pressure. They have a thick elastic wall to absorb this pressure. Basically, the walls of the artery are smooth so that the blood can flow easily. The narrower and more irregular the walls, the less well the blood flows. Arteries also have muscle cells in their walls that allow them to contract to control blood flow.
Aorta
The aorta is, as already mentioned, the major body artery and also the largest blood vessel in the body. It has the diameter of a garden hose. At rest, the blood flows through the aorta at a speed of 50 centimeters per second: this speed can increase further with exertion. Branches of the aorta, arteries (arteries), smaller arteries (arterioles) and capillaries (capillaries) distribute the blood among the organs.
coronary arteries
The heart muscle itself withdraws little oxygen from the blood flowing through the ventricles and atria. It gets oxygen and nutrients from the coronary arteries, a network of blood vessels that runs around the heart. These arteries are also known as coronary arteries. Corona is the Latin word for wreath. Two coronary arteries arise from the aorta: the left mainly supplies the left half of the heart and the right naturally supplies the right side. The left coronary artery soon bifurcates in two.
capillaries
They are the smallest blood vessels: the capillaries. They have a thin wall and a microscopic cross section, about ten times as thin as a hair.
The capillaries ensure the blood flow to the tissues and they therefore run through the muscles and organs as a finely branched network. In the capillaries the blood flows quite slowly so that substances are easily exchanged through the thin vessel wall. Oxygen and nutrients are released, waste products are absorbed and carried along.
Excess fluid is also squeezed out of the blood in the capillaries. Thus, the fluidity of the blood can be regulated. The excess fluid is drained through the lymphatic system.
veins
After the metabolism in the capillaries, the blood flows back to the heart via the veins. This blood mainly contains carbon dioxide and waste products and little oxygen. The pulmonary veins are an exception to this. They carry oxygenated blood to the heart.
Veins are under less pressure than arteries and therefore have a thinner wall. They can hold more blood than the arteries because the wall can expand. This is often clearly visible on your hand: sometimes you can see the veins on the back of the hand. The bluish color shows that the blood is low in oxygen. The system of veins is an important storage place for blood.
The pumping action of the heart is no longer noticeable in the veins and the blood hardly flows in the veins. Thanks to vein valves, the blood still goes in the right direction; those valves prevent blood from flowing back down with gravity. Muscle movements and the pumping action of the arteries push the blood further up. The artery wall does not contain muscle cells.
The veins drain near the heart into the superior and inferior vena cava, the largest veins in the body. There are also coronary arteries around the heart, which carry the oxygen-poor blood from the heart muscle. Both vena cavae (venae cavae) drain into the right atrium. Then the circulation starts again.
In picture
This video of the Heart Foundation shows the blood circulation and the function of the different blood vessels very clearly:
