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The Human Circulatory System

Human Circulatory System Basics


Each and every cell in the body requires energy to perform various activities. For performing these activities oxygen and nutrients should be transported to these cells. Waste materialsgenerated by the result of metabolic activities of the cells should be transported to the excretory organs.

Parts of the Circulatory System
The important parts of the circulatory system are:

  • Heart
  • Blood vessels
  • Blood

1.Heart is of the size of the fist and is built with the muscles which is situated in the rib cage between the lungs.

  • Chambers of the Heart:  It consists of four chambers of which upper two chambers are called as auricles and lower two chambers as ventricles.
  • Auricles are divided as right and left auricles by a thin membrane know as intra auricular membrane. Similarly ventricles are divided into right and left ventricles by a thick membrane called as intraventricular membrane. Auricles and ventricles are separated by a auriculoventricular membrane.
  • Pacemakers namely sinoauricular node (SA node) and auriculoventricular node (AV node) are present which stimulate the functioning of the heart. Right auricle receives the deoxygentaed blood from various parts of the body. Oxygentaed blood from lungs is brought to left auricle by the pulmonary vein. Deoxygentaed blood from right auricle is pumped into right ventricle and this blood is pumped to lungs by the pulmonary artery. Oxygentaed blood from left auricle is pumped to left ventricle and this blood is supplied to all body parts.

2. Blood Vessels: Blood vessels are thin pipe like structures which supply the blood to the heart and receives impure blood from body parts. Blood vessels are of two types namely arteries and veins.

  • Arteries: Supply oxygentaed blood from the left ventricle to all parts of the body. These arteries divide into arterioles and end as capillaries in the cells. Pulmonary artery supplies deoxygenated blood from right ventricle to lungs.
  • Veins: Collect deoxygenated blood from all parts of the body. They start as capillaries, then to veinlets and to larger veins finally. Pulmonary vein brings oxygenated blood from lungs to left auricle.

3. Blood: Blood is a fluid connective tissue which is red in colour due to the presence of haemoglobin. Our body contains 5-6 litres of blood. The main components of the blood are plasma and blood cells.

  • Plasma: Plasma constitutes 55% of the blood and is light yellow coloured liquid containing water, salts and plasma proteins.
  • Blood cells: Blood cells constitute 45% of the blood. They float in plasma and are of three types. They are red blood cells, white blood cells and blood platelets.
  • Red Blood Corpuscles: They are also known as erythrocytes. RBC are round in shape and disc shaped with constriction in the middle. They contain a pigment called as haemoglobin. They do not contain nucleus. The life span of RBC is 120 days.
  • White Blood Corpuscles: They are also known as leukocytes. They are of differenet shapes. WBC contains nucleus. WBC are classified as granulocytes and agranulocytes based on the presence of granules in their cells. They do not contain haemoglobin and hence are white in colour. They attack the microorganisms which enter our body and protects us from diseases. Their lifespan is 12 days.
  • Blood platelets are called as thrombocytes and are small and oval in shape. They do not contain the pigment and are white in colour. The life span is 3 -10 days. Blood platelets are useful for formation of the blood clot during bleeding when wound occurs in the body.
  • The aorta is the largest single blood vessel in the body. It is approximately the diameter of your thumb. This vessel carries oxygen-rich blood from the left ventricle to the various parts of the body.
  • The superior vena cava is one of the two main veins bringing deoxygenated blood from the body to the heart. Veins from the head and upper body feed into the superior vena cava, which empties into the right atrium of the heart.
  • The inferior vena cava is one of the two main veins bringing de-oxygenated blood from the body to the heart. Veins from the legs and lower torso feed into the inferior vena cava, which empties into the right atrium of the heart.
  • The right atrium receives de-oxygenated blood from the body through the superior vena cava (head and upper body) and inferior vena cava (legs and lower torso). The sinoatrial node sends an impulse that causes the cardiac muscle tissue of the atrium to contract in a coordinated, wave-like manner. The tricuspid valve, which separates the right atrium from the right ventricle, opens to allow the de-oxygenated blood collected in the right atrium to flow into the right ventricle.
  • The right ventricle receives de-oxygenated blood as the right atrium contracts. The pulmonary valve leading into the pulmonary artery is closed, allowing the ventricle to fill with blood. Once the ventricles are full, they contract. As the right ventricle contracts, the tricuspid valve closes and the pulmonary valve opens. The closure of the tricuspid valve prevents blood from backing into the right atrium and the opening of the pulmonary valve allows the blood to flow into the pulmonary artery toward the lungs.
  • The left atrium receives oxygenated blood from the lungs through the pulmonary vein. As the contraction triggered by the senatorial node progresses through the atria, the blood passes through the mitral valve into the left ventricle.
  • The left ventricle receives oxygenated blood as the left atrium contracts. The blood passes through the mitral valve into the left ventricle. The aortic valve leading into the aorta is closed, allowing the ventricle to fill with blood. Once the ventricles are full, they contract. As the left ventricle contracts, the mitral valve closes and the aortic valve opens. The closure of the mitral valve prevents blood from backing into the left atrium and the opening of the aortic valve allows the blood to flow into the aorta and flow throughout the body.
  • The pulmonary arteries are the vessels transporting de-oxygenated blood from the right ventricle to the lungs. A common misconception is that all arteries carry oxygen-rich blood. It is more appropriate to classify arteries as vessels carrying blood away from the heart.
  • The pulmonary veins are the vessels transporting oxygen-rich blood from the lungs to the left atrium. A common misconception is that all veins carry de-oxygenated blood. It is more appropriate to classify veins as vessels carrying blood to the heart.
  • The chordae tendineae are string-like in appearance and are sometimes referred to as “heart strings.” They connect the heart muscles to the valves.
  • The tricuspid valve separates the right atrium from the right ventricle. It opens to allow the de-oxygenated blood collected in the right atrium to flow into the right ventricle. It closes as the right ventricle contracts, preventing blood from returning to the right atrium; thereby, forcing it to exit through the pulmonary valve into the pulmonary artery.
  • The mitral valve (bicuspid valve) separates the left atrium from the left ventricle. It opens to allow the oxygenated blood collected in the left atrium to flow into the left ventricle. It closes as the left ventricle contracts, preventing blood from returning to the left atrium; thereby, forcing it to exit through the aortic valve into the aorta.
  • The pulmonary valve separates the right ventricle from the pulmonary artery. As the ventricles contract, it opens to allow the de-oxygenated blood collected in the right ventricle to flow to the lungs. It closes as the ventricles relax, preventing blood from returning to the heart.
  • The aortic valve separates the left ventricle from the aorta. As the ventricles contract, it opens to allow the oxygenated blood collected in the left ventricle to flow throughout the body. It closes as the ventricles relax, preventing blood from returning to the heart.
Tags : science
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