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Human Heart and The Circulatory System


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Ciculatory System


The human circulatory system consists of a heart, blood, and blood vessels. The circulatory system is also called the cardiovascular system. The circulatory system carries oxygen, nutrients, and hormones to the tissues of the body, and removes waste products, like carbon dioxide and other toxic elements.


The heart is a muscular organ, located in between the lungs, slightly left to the middle of the chest. Its size is of the fist. Its main function is to pump blood through the network of arteries and veins, supply it to different organs of the body. In a normal human heart beats about 60 to 80 times per minute to pump blood throughout the body. It consists of four chambers:

  • Right atrium: receives impure blood from the veins and pumps it to the right ventricle.
  • Right ventricle: pumps blood to the lungs for purification through the pulmonary artery.
  • Left atrium: receives oxygenated blood from the lungs and pumps it to the left ventricle.
  • Left ventricle: pumps oxygenated blood to all parts of the body.

The chambers of the heart are divided by a partition called the septum. Atriums are divided by an inner auricular septum and ventricles are divided by an inner ventricular septum.

How does a valve function in the heart?

There are mainly four valves in the human heart. Valves enable the blood to flow in the right direction through the heart. They act as either entry or exit points for the unidirectional smooth flow of the blood. The purpose of these valves is to prevent the backward flow of blood. Valves are thin flaps called leaflets or cusps, made of strong tissues, allowing the blood to either enter the ventricle that is a lower right chamber or leaving the ventricle that is a lower left chamber. These four valves are:

  • Tricuspid valve: The right atrium and right ventricle are separated by a valve called the tricuspid valve having three leaflets or cusps. Blood flows from the right atrium into the right ventricle when the tricuspid valve gets open. When the right ventricle is full, the tricuspid valve gets closed that keeps the blood from flowing backward into the right atrium again.
  • Bicuspid valve: The left atrium and the left ventricle are separated by a valve called the bicuspid valve (mitral valve). It has only two leaflets. When the left ventricle is full, the bicuspid valve closes which keeps blood from flowing backward into the left atrium when the ventricle contracts.
  • Aortic valve: This is the valve that separates the left ventricle from a large blood vessel called aorta that carries blood to different organs of the body. It has three leaflets. The aortic valve gets open when the left ventricle contracts. Blood is pumped out of it and enters into the aorta which branches into many arteries thereby providing blood to different parts of the body.
  • Pulmonic valve: This is the valve that separates the right ventricle from a large blood vessel i.e pulmonary artery that carries impure blood to the lungs for oxygenation. It also possesses three cusps. When the right ventricle contracts the pulmonic valve is forced open as the tricuspid valve is closed. So the blood is pumped out of the right ventricle through the pulmonic valve and enters the pulmonary artery that carries blood to the lungs for oxygenation.

Pericardium: Heart is protected by a membrane surrounding it called the pericardium.

Blood Circulation in the Heart

Blood Circulation in the Heart



Blood is the main fluid content and the main circulating medium of the body. It has four main components:

  • Plasma: Plasma is a light yellow color fluid consists mainly of water with proteins, ions, nutrients, and waste products.
  • White blood cells: They respond to the immune system, their main job is to fight infection.
  • Red blood cells: They are responsible for carrying oxygen and carbon dioxide.
  • Platelets: These are cell fragments responsible for blood clotting.

Blood flows through a tubular structure called blood vessels.

Blood Vessels

Blood vessels consist of arteries, veins, and capillaries. The system of blood vessels can be compared to a tree trunk. Here, the trunk is the largest artery that branches into arteries and further into arterioles that further branches into capillaries thus forming a capillary network that combines to form venules, venules to veins, veins to pulmonary veins. Their functions are as under:

  • Arteries: usually carry oxygenated blood from the heart to the body, the pulmonary artery being an exception carries impure blood from the heart to the lungs for oxygenation. Another exception is the Umbilical artery in females that carries deoxygenated blood from the fetus to the placenta. Arteries have thick walls and no valves since the heart pumps blood with such a force that blood is able to flow in one direction, it never flows backward. Aorta is the largest artery in a human's body. The coronary artery, the very first branch of Aorta, supplies blood to the heart muscle itself.
  • Veins: carry oxygen-depleted blood from the body back to the heart, pulmonary veins being an exception that carries oxygenated blood to the heart from the lungs. They have a thin layer of elastic fibers and muscles. The two largest veins in the body are venae cavae - the superior vena cava, delivers blood from the upper part i.e. head, neck, arms and chest to the right atrium, and the inferior vena cava—the largest vein in the human body—delivers blood from the lower body parts i.e legs, back, abdomen, pelvis back to the right chamber of the heart (atrium).
  • Capillaries: exchange blood, they are microscopic blood vessels - only one cell thick, connect arteries and veins through arterioles and venules, their tiny respective branches.
Blood Vessels

Blood Vessels

How does the circulatory system work?

The circulatory system is divided into two types of circulation—systemic circulation and pulmonic circulation.

Blood circulation starts during the relaxation phase of the heart i.e between two successive heartbeats: The blood flows from both the upper two chambers of the heart into the lower two chambers i.e. from atriums to their respective ventricles, which then expand. In the next phase, both the ventricles pump the blood into the large arteries.

In the systemic circulation, blood reaches all the tissues, cells, and organs of the body through arteries, that branches into arterioles and arterioles into capillaries. In the systemic circulation, the left ventricle pumps oxygen-rich blood into the main artery called aorta via the aortic valve. The blood travels from the aorta to larger and smaller arteries, then into the capillary network. There the blood releases oxygen, nutrients and other important substances, takes up carbon dioxide and other waste products. Now the oxygen content in the blood is low. Deoxygenated blood is collected in veins through capillaries, travels to the right atrium entering the right ventricle.

Now the pulmonary circulation begins: the right ventricle pumps deoxygenated blood into the pulmonary artery, which branches off into smaller arteries, arteries into arterioles and arterioles into capillaries. These capillaries form a fine network around the pulmonary (lungs) vesicles, little air sacs at the end of the airways. In the sacs, carbon dioxide is released from the blood into the air and fresh oxygen enters the bloodstream. These pulmonary vesicles are the sites of blood exchange. During exhale, carbon dioxide is released from the body. Oxygen-rich blood travels through the pulmonary veins, enters the left atrium, then into the left ventricle. The next heartbeat starts a new cycle of systemic circulation. In this way, the circulatory system works.


For each question, choose the best answer. The answer key is below.

  1. Q.1 Which blood vessel carries deoxygenated blood?
    • Pulmonary vein
    • Pulmonary artery
    • 1 and 2 both
    • none
  2. Q.2 The main and largest artery in human body is ....
    • Pulmonary artery
    • Aorta
    • Umbilcal artery in females
    • None
  3. Q.3 A membrane that encloses the heart and the roots of the major heart vessels
    • Epicardium
    • Pericardium
    • Endocardium
    • None
  4. Q.4 The largest vein in human body is ......
    • Pulmonary vein
    • Superior vena cava
    • Inferior vena cava
    • None of the above
  5. Q.5 Which of the following ststement is true?
    • heart have valves
    • arteries contain valves
    • Veins contain valves
    • 1 and 3 both
  6. Q. 6 Human heart have .... chambers.
    • 5
    • 3
    • 4
    • 2
  7. Q.7 Human heart has ...... valves.
    • three
    • two
    • five
    • four
  8. Q.8 Heart valves are .....
    • uni-directional
    • bi-directional
    • 1 and 2 both
    • None
  9. Q.9 Which of the following artery supplies blood to the heart muscles itself?
    • Pulmonary artery
    • Aorta
    • Coronary artery
    • None

Answer Key

  1. Pulmonary artery
  2. Aorta
  3. Pericardium
  4. Inferior vena cava
  5. 1 and 3 both
  6. 4
  7. four
  8. uni-directional
  9. Coronary artery


  1. InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care (IQWiG); 2006-. How does the heart work? 2011 Dec 6 [Updated 2019 Jan 31]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK279249/

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2019 sonal