The Circulatory System:
The circulatory system consists of the heart, the arteries, the veins and the capillaries. It is about circulating blood across the body and then returning it back to the heart.
The heart pumps out blood through the aorta and into the arteries. As heart beats every second so the blood is pushed forward every second as well. The arteries have thick walls and have thick elastic muscles near the heart. This allows movement of blood without the arteries bursting. The blood vessels are made of endothelial cells and have thick, muscular and elastic walls. The arteries carry the blood to all parts of the body providing oxygen and food molecules to the cells and also proving hormones and excretory products such as urea.
The arteries branch out and form arterioles. These arterioles then branch out into smaller capillaries. The small size of the capillaries help in diffusion of the substances out of the vessels. The blood then travels through the venules and flow to the veins. The veins then return the blood back to the heart. The veins are different than arteries. The veins are a bit flat and have valves inside it which is shaped like half moon which is why it is called the semi lunar valves.
The veins carry de-oxygenated blood without any pressure which is why it runs smoothly. It does not have any strong muscular or elastic walls. The valves are present to prevent back flow of the blood. The movement of blood is supported by the skeletal muscles on the veins.
Double circulation system In Mammals:
In mammals there is a double circulation system of blood. The first being the pulmonary circulation. The pulmonary arteries extend from the heart to the lungs which then carries oxygen back to the heart through the pulmonary veins.
The heart then pumps oxygenated blood through the main circulatory system, the systemic circulation.
There are another set of tubes called the lymph capillaries which collect the tissue fluid. This fluid is then called lymph. The lymph is then transported to the veins where it diffuses into the blood.
This forms the lymphatic system. This helps in keeping the blood composition stable. It also helps in keeping the amount of tissue fluid in the body constant and takes off excess and supplies back to the blood at the veins near the heart.