Great “roads” and “highways” connect all kinds of big "cities", and food and other necessary supplies are delivered to them.
Such large pathways in the body are the arteries - vessels that carry blood from the heart and veins through which the blood returns to the heart.However, in the “small towns” and “villages” there is also need for food and supplies. Small roads and streets (the capillaries) reach to the villages and houses in the human body, even to the smallest stations.
What are Arteries?The arteries lie deep in the tissues, except in the area of the wrist, on the upper side of the foot, at the temples and on the sides of the neck.
From any of these places, we can feel the pulse, which is very important for the doctors.
Major arteries have valves at the places from where they come out from the heart.
These vessels are built from many elastic fibers and muscles that can expand and contract.
Arterial blood is bright red and moves through the arteries in waves.
What are Veins?Veins lie near the surface of the skin and blood in them is darker and moves smoothly: they have valves at certain distances along their whole length.
Far from the heart, into all parts of the body, large blood vessels become tiny, thin as hairs, and there are called capillaries.
What are Capillaries?Capillary is 50 times thinner than the thinnest of human hairs!
It is so thin that blood cells pass through the capillaries just one after the other, which means that the blood passes through the capillaries very slowly.
What happens when blood flows through a capillary?
Blood leaves the capillary, but the capillary wall is thin and consists of a single layer of cells. Through the wall of the blood gives up its oxygen to the surrounding tissues, and on return it receives carbon dioxide which is released from the tissues around the capillaries. At the same time, the blood exchanges other substances that provide food to the tissue, and “unnecessary” products enter into the capillary.
Interesting facts about the human heart
- Adult heart is the size of a fist, and weighs about 300 grams, yet it for 24 hours can produce energy sufficient to lift about 70 tons (or one locomotive) to 33 centimeters above the ground!
- During the day, the heart tightens and relaxes approximately 100,000 times.
- The heart of adult men in 24 hours, can pump 16.5 liters of blood.