Circulatory System

It is a vast network of organs and vessels that is responsible for the flow of blood, nutrients, hormones, oxygen and other gases to and from cells. Powered by the heart, it is the body’s distribution system to organs with oxygen, hormones and essential nutrients that helps it function properly. The circulatory system helps to fight off disease, helps the body maintain a normal body temperature, and provides the right chemical balance to provide the body’s homeostasis, or state of balance among all its systems.

Organs in Circulatory system

  1. Blood
  2. Veins
  3. Arteries
  4. Heart


Blood is the medium of transport in the body. The fluid portion of the blood, the plasma, is a straw-colored liquid composed primarily of water. All the important nutrients, the hormones, and the clotting proteins, as well as the waste products, are transported in the plasma. Red blood cells and white blood cells are also suspended in the plasma.

  1. Red blood cells: Red blood cells are also called erythrocytes. These are disk-shaped cells produced in the bone marrow. Red blood cells have no nucleus, and their cytoplasm is filled with haemoglobin
  2. White blood cells: White blood cells are referred to as leukocytes. They are larger than red blood cells and have clearly defined nuclei. They are also produced in the bone marrow and have various functions in the body. Certain white blood cells called lymphocytes are essential components of the immune system.

Blood Group

A has only the A antigen on red cells (and B antibody in the plasma)

B has only the B antigen on red cells (and A antibody in the plasma)

AB has both A and B antigens on red cells (but neither A nor B antibody in the plasma)

O has neither A nor B antigens on red cells (but both A and B antibody are in the plasma)

In addition to the A and B antigens, there is a third antigen called the Rh factor, which can be either present (+) or absent ( – ). In general, Rh negative blood is given to Rh-negative patients, and Rh positive blood or Rh negative blood may be given to Rh positive patients.

  1. The universal red cell donor has Type O negative blood type.
  2. The universal plasma donor has Type AB blood type.


Veins are the blood vessels that carry deoxygenated blood from parts of our body back to the heart. Veins serve a critical function within our bodies. When blood has been pumped by the heart to various parts of the body, it must return back to the heart. In a metaphorical sense, veins are the return portion of a round-trip plane ticket. The veins serve the purpose of delivering the blood, now bluish in color, back to the right atrium (chamber) of our heart. In the heart, blood will collect more oxygen and prepare to be pumped back out through arteries.  The largest vein in the body is called the vena cava.


The blood from the heart is carried through the body by a complex network of blood vessels. Arteries take blood away from the heart. The main artery is the aorta that branches into other major arteries, which take blood to different limbs and organs. These major arteries include the carotid artery, which takes blood to the brain; the brachial arteries, which take blood to the arms; and the thoracic artery, which takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidneys, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then into smaller vessels called arterioles, to reach more deeply into the muscles and organs of the body.


The heart is a muscular organ about the size of a closed fist that functions as the body’s circulatory pump. Heart weighs about 10 - 12 ounces (or 280 - 340 grams) in man, and 8 -10 ounces (or 230 - 280 grams) in woman. In an adult, the heart beats at an average of 60-80 times per minute. It takes in deoxygenated blood through the veins and delivers it to the lungs for oxygenation before pumping it into the various arteries (which provide oxygen and nutrients to body tissues by transporting the blood throughout the body). The heart is located in the thoracic cavity medial to the lungs and posterior to the sternum.

Parts of heart and their function

Pericardium: This is a fibrous covering that wraps around the heart and holds it in place. This special membrane also contains a fluid which lubricates the heart in the pericardial space or cavity to prevent friction. The pericardium has two layers, consisting of a visceral layer directly coving the heart and a parietal layer, which forms a sac containing the fluid in the pericardial cavity.

Heart Wall:

  1. Epicardium: It is the outermost layer of the heart, is a thin layer of membrane that lubricates and protects the outside portion of the heart.
  2. Myocardium: It is the muscular layer of the heart wall, consists of the muscle tissue. It consists of the majority of the thickness of the heart and is responsible for the pumping action of the heart.
  3.  Endocardium: It is the simple squamous endothelium layer that lines the inside of the heart. The endocardium is very smooth and is responsible for keeping blood from sticking to the inside of the heart and forming potentially deadly blood clots.

Chambers of the Heart

The heart has four chambers:

  1. ŸRight atrium
  2. ŸLeft atrium
  3. ŸRight ventricle
  4. ŸLeft ventricle

Atria are smaller than ventricles and have thin, less muscular walls. They are the receiving chambers of the blood, which is delivered to the heart by the large veins. Ventricles are the larger, more muscular pumping chambers that push blood out to the circulation. They are connected to large arteries that carry blood to the circulation.

The right atrium and the right ventricle are smaller than the corresponding chambers on the left. They have less muscle in their walls compared to the left side of the heart. The difference in size is related to their functions. Blood from the right side of the heart goes to the pulmonary circulation while blood from the left chambers is pumped to the rest of the body.


These are fibrous tissue flaps found between the cardiac chambers and within the veins. They serve as gates that ensure one-way flow and prevent the backflow of blood.

  1.  Atrioventricular valves are found between each atrium and ventricle. The valve between the right atrium and ventricle is the tricuspid valve, while that found between the left atrium and ventricle is called the mitral valve.
  2. Semilunar valves are found between the ventricles and the large arteries. There is an aortic valve between the left ventricle and the aorta and a pulmonary valve between the right ventricle and the pulmonary artery.

Blood Flow through the Heart

Deoxygenated blood returning from the body first enters the heart from the superior and inferior vena ceva. The blood enters the right atrium and is pumped through the tricuspid valve into the right ventricle. From the right ventricle, the blood is pumped through the Pulmonary Semilunar valve into the pulmonary trunk

The pulmonary trunk carries blood to the lungs where it releases carbon dioxide and absorbs oxygen. The blood in the lungs returns to the heart through the Pulmonary veins. From the pulmonary veins, blood enters the heart again in the left atrium.

The left atrium contracts to pump blood through the bicuspid (mitral) valve into the left ventricle. The left ventricle pumps blood through the aortic semilunar valve into the aorta. From the aorta, blood enters into systemic circulation throughout the body tissues until it returns to the heart via the vena cava and the cycle repeats.

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