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Circulation of blood

              Many substances like water, oxygen, soluble food material, and waste products are exchanged between the parts of the body. To facilitate this exchange these substances have to be transported. The function of transportation is carried out by the circulatory system.

                Blood is the fluid connective tissue in human beings. It carries out the function of transport of various materials in the body. A Pumping organ is needed to push the body around the body along with a network of tubes to reach all the tissues and a system to repair the network from time to time if it is damaged. The volume of the blood in an average adult as 4.7 liters.

              The soluble substances produced in the digestive system are absorbed by the blood and get dissolved in its water content. These dissolved food materials are then supplied to all cells in the body through the blood.
                 Once oxygen from the air enters the body it, too, is absorbed by the blood and supplied to all cells through blood. Chemical like hormones, which are essential for life processes also enter the blood. blood transport them to all cells.waste products from all cell are transported to the excretory system by blood, to be expelled from the body.

                  In higher animals and man, there is a separate system meant for the circulation of blood. This is closed system and consist of the heart, blood vessels and capillaries.
                In mammals, the heart is divided into four parts. The heart is a muscular organ which throbs continuously. This cause the blood to circulate continuously through the blood vessels. The blood vessels are a kind of tubes through which blood flows all the time. The vessels which carry blood from the heart to the different parts of the body are called 'arteries' while those that carry it back to the heart from all the parts of the body are called 'veins'. arteries provide pure blood to the body. Veins bring the impure blood from all parts of the body back to the heart. As arteries spread to the different parts of the body, they branch out and their diameter become smaller and smaller till they become as fine as hairs. These fine blood vessels are called capillaries. capillaries reach every cell in the body. capillaries have thin walls. This facilitates the exchange of substances between them and the cells. After that capillaries begin to join each other and their diameter becomes bigger and bigger. These bigger blood vessels are what we call veins. During the exchange of substances, cells get oxygen, food materials, hormones and vitamins from the blood and the waste product in the cells enter into the blood. Blood, depleted of oxygen, is called impure blood.

HEART - The involuntary pump:-
                    The human heart is a muscular organ which pumps blood. The heart is covered by the pericardial membrane. It is of the size of a human fist and weight about 360 gm. As oxygen and carbon dioxide both have to be transported by the blood, the heart has has different chambers, the left and right, to prevent oxygen rich blood from mixing with the blood containing carbon dioxide. The left half carries oxygenated blood. Such separation allows a highly efficient supply of oxygen to the body.This is very essential in animals that have high energy needs, such as birds and mammals, which constantly use energy to maintaining their body temperature. Each half is further divided into two chambers. The upper one is called atrium and the lower one is termed as the ventricle. therefore the human heart has four chambers.

  • The body temperature of the cold blooded animals varies according to environmental temperature. Such animals can tolerate some mixing of oxygenated and deoxygenated  blood. such animals have three chambered heart e.g. amphibians and many reptiles.

  • fish have only a two chambered heart so blood through the heart only once.

  • There are about 97000 kilometer of blood vessels in the human body.

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