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Human Circulatory System

Updated on April 3, 2010


Circulating Fluids

Blood is the life stream of the human body. It has four main parts: white blood cells, red blood cells, platelets and plasma. Blood consists of blood cells and a liquid portion called plasma.

Plasma is mostly water. It also contains proteins, inorganic minerals and some organic substances. Major proteins in the plasma are albumin which helps keep the plasma inside the blood vessels, fibrinogen which Plays an important role in clotting; and globulin which helps fight infections.

Red Blood Cells (erythrocytes) contain a pigment called hemoglobin which gives a red color to the blood cells. Hemoglobin contains iron and ithas strong attraction to oxygen. Because of hemoglobin content, red blood cell is best adapted for carrying oxygen to the body cells. Red blood cells are made in the bone marrow.34

White Blood Cells (leukocytes) are generally larger than the red blood cells but are fewer. They are made in the bone marrow and some are made in the lymph nodes. The main function of white blood cells is defending the body against diseases carrying organisms and other harmful substances that enter the body.

Blood Platelets (thrombocytes) are disk-like structures that develop from cells produced in the marrow. They are not cell for they don't have nuclei. They help prevent the loss of blood from damaged vessels and play an important role in the clotting of blood.

Blood Types

Blood types play an important role in blood transfusions. Human blood have been classified into various types or groups based on the presence or absence of certain antigens on the membrane of red blood cells. Blood type A have antigen A and anti-body B. Blood type B have antigen B and anti-body A. Blood type AB have antigens A and B and no antibody. While blood type O have no antigen but its antibodies are A and B. A patient cannot have the antibody against his won antigen because it will result in the clumping of red blood cells and results in serious illness or even death. Blood type O is called the universal donor because it has no antigen, so it can be given to any type but it cannot accept any type except O. While blood type AB is called the universal recipient because it has no antibody so it can accept any type of blood.

Blood Pressure

Blood pressure is the pressure that the blood exerts against the walls of the arteries. The amount of pressure depends upon the strength and rate of the heart's contraction, volume of the blood in the circulatory system and the elasticity of the arteries. The instrument used in taking blood pres­sure is called sphygmomameter. Systolic pressure is the blood pressure when the heart is relaxing.

Blood Vessels

Blood circulates in the body through a system of tubes called blood vessels. There are three kinds of blood vessels: vein, arteries and capillaries.

Veins are blood vessels that carries the blood towards the heart. Most veins return the blood to the heart after it has given out nourishment to the tissues and taken up wastes and poisons. Blood in the veins are called venous blood. Blood carried by the veins has lost much of its oxygen and is dull, brownish-red in color.

Arteries are blood vessels through which blood is pumped away from the heart to the various parts of the body. Blood carried by the arteries is bright red in color because it has picked up oxygen while passing through the lungs. Blood in the arteries is called arterial blood.

Capillaries are the smallest blood vessels in the body, seen only un­der the microscope. They make a network of blood vessels through all parts of the body. They have thin walls into the tissues. Wastes from the tissues also pass into the blood through the capillary walls.

Lymphatic System

Lymphatic system is a network of small vessels that resembles blood vessels. It returns fluid from the body tissues to the blood stream. This process is necessary because fluid pressure in the body continuously causes water, protein and other materials to seep out of the capillaries.

Parts of the Lymphatic System

1.   Lymph vessels. Like the blood vessels, they are found throughout the body. Lymph flows from the tiny vessels with many branches into larger vessels. Lymph from all but the upper right quarter of the body reaches the thoracic duct, the largest lymph vessel. Lymph from the upper right quarter of the body flows into the right lymph duct.

2.    Lymph is chemically similar to the plasma, but lymph contains half as much protein as a plasma because large protein molecules do not seep through the blood vessel walls so easily.

3.    Lymph nodes may be found at many places along the lymph vessels. They look like bumps and they resemble knots in a string of lymph vessels.35 Nodes are bunched together in the neck and armpits, above the groin and near various organs and large blood vessels. Lymph nodes contain large cells called macrophages that absorb harmful matter and dead tissues.

4.    Lymphocytes are white blood cells produced in the lymph nodes. They defend the body against infection.

5.    Lymphoid tissue resembles the tissue of the lymph node. Adenoid, tonsils, spleen, and thymus gland consists of lymphoid tissue that pro­duces and contains lymphocyctes and aids in the body's defense against infection.

6.    Heart is a large hollow muscle, a busy machine that pumps blood to all parts of the body.

Human Heart

Our heart has four chambers: two atria and two ventricles. Atria re­ceive blood from the various parts of the body, thus they are called the receiving chambers of the heart. Ventricles pump the blood to the differ­ent parts of the body, thus they are called the pumping chambers of the heart. The heart is enclosed by a thin sac called pericardium which pro­tects the heart from rubbing against the lungs and the walls of the chest. A muscular wall called septum divides the heart lengthwise. Valves control the flow of blood through the heart. Tricuspid valve controls the flow of blood from the right atrium to the right ventricle. Mitral valve controls the flow of blood between the left ventricle. Semilunar valve controls the flow of blood from the ventricles to the arteries.


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