What is double circulation and its importance in human circulatory system


Double circulation is an important characteristic feature of the human circulatory system. This article gives an account of the role of the circulatory system in animals, the pattern of flow of blood in double circulation. It also gives an account of why the double circulation of blood in humans is advantageous over single circulation. This resource also provides some frequently asked questions related to this topic.

Introduction

In the circulatory system especially of all vertebrates the heart, blood vessels like arteries, veins, and thin blood capillaries are involved circulation of blood. In all vertebrates, we find a closed circulatory system where the blood always flows through blood vessels only. In invertebrates, Annelids (Earthworm) have a closed circulatory system where the blood flows in blood vessels only. But in certain Invertebrates like Arthropods (Cockroach) and Molluscs (Snails) have an open circulatory system where the blood flows not in blood vessels but in open spaces within the body. Similarly, certain lower groups of animals show single circulation and the higher animals show double circulation of blood.

Importance of circulatory system in humans

  1. It carries digested nutrients like sugars amino acids to all parts of the body through the circulatory medium blood.

  2. It carries oxygen from the lungs to all tissues of the body and transport CO2 from all tissues of the body to the lungs through blood for expiration.

  3. All metabolic wastes from cells and tissues of the body are transported by the circulatory system to the kidneys for their elimination.

  4. It transport water, inorganic ions, and chemicals to all the cells and tissues of the body according to their needs.

  5. It transports hormones secreted by endocrine glands into the blood to the target organs to trigger them into action.

  6. It regulates the blood pressure by changing the volume of blood.

  7. By transporting blood from deeper parts to the surface of the body, it regulates body temperature.



Double circulation of the blood in humans

Circulation of blood two times through the heart before it gets distributed throughout the body is known as Double Circulation. Double circulation is a characteristic feature of blood circulation in human beings as well as higher vertebrates.
A flow chart showing double circulation in human
Flow chart designed by the Author

The two main vein blood vessels-superior vena cava (pre caval) and inferior vena cava (post caval) collect blood from the upper part of the body and lower part of the body respectively. The de-oxygenated blood carried out by these two major blood vessels and dump the blood into the right auricle. Due to the pressure exerted by the blood-filled in the right auricle, the blood gets pushed into the right ventricle by the opening of the tricuspid valve or right auriculo-ventricular aperture. As soon as the right ventricle is filled with blood from the right auricle, the tricuspid valve closes and it produces a heartbeat sound. After the closure of the tricuspid valve, due to pressure, the blood in the right ventricle gets pushed into the pulmonary artery by the opening of the pocket-shaped semilunar valves in the pulmonary artery. Through this, the de-oxygenated blood gets flown into the lungs for oxygenation. Then the oxygenated blood from the lungs will be carried to the left auricle by four pulmonary veins. As the left auricle gets filled with blood it exerts pressure to open the bicuspid valve or left auriculo-ventricular valve. Through the opening of the valve, the oxygenated blood moves into the left ventricle. As the right ventricle gets filled with oxygenated blood, the bicuspid valve closes. The pressure exerted by the left ventricle now pushes the oxygenated blood to move through the aorta by the opening of semilunar valves present at its base. The oxygenated blood moves out of the heart through the aorta and gets distributed to all parts of the body through its branches.

The major blood vessel aorta that originates from the left ventricle gives off so many branches that supply oxygenated blood to different parts of the body. Carotid arteries of the aorta supply oxygenated blood to the head, and subclavian arteries to the upper limbs. The aorta then runs down as dorsal aorta and then its branches supply oxygenated blood to the organs of thoracic and abdominal regions. Renal arteries of the aorta supply oxygenated blood to kidneys, iliac arteries to the abdominal region, and the femoral arteries to hind limbs.

The flow of blood into the auricles and the ventricles occur simultaneously. When the right auricle expands to receive the blood from the vena cavae, the left auricle expands to receive blood from the lungs by pulmonary veins. The expansion of auricles to receive blood and its branches supply oxygenated blood is known as auricular diastole while contraction of auricles to pump blood into their respective ventricles is called auricular systole.

The pumping of blood into the pulmonary artery and aorta occur simultaneously by the contraction of the right and left ventricles. This is known as ventricular systole. After this for a period of time, the ventricles expand to get filled up by the blood. This is known as ventricular diastole. One systole and one diastole form a cardiac cycle and that takes about 0.85 seconds.

Circulation of the blood in humans occurs through distinct paths called Pulmonary Circulation and Systemic Circulation. Pulmonary circulation and Systemic circulation we together call as double circulation.

Pulmonary circulation

It is the path of circulation in which the deoxygenated blood from the right ventricles is carried to the lungs by the pulmonary artery for oxygenation and then the oxygenated blood from the lungs is transported to the left auricle by four pulmonary veins. This is the shortest route of circulation of blood in the human circulatory system.

Systemic Circulation

It is the circulation of oxygenated blood from the left ventricle through the aorta to all parts of the body by its branches and the return of deoxygenated blood from all parts of the body back to the right auricle through superior and inferior vena cavae. This is the longest route of circulation of blood in the human circulatory system.

Significance of double circulation in humans

  • It prevents mixing of oxygenated and deoxygenated blood unlike in animals with a single circulation of blood.

  • Separation of oxygenated and deoxygenated blood without mixing will help in the efficient supply of oxygen to cells and tissues of the body.

  • The blood can be pumped at high pressure due to the double circulatory system.

  • Body temperature is also maintained perfectly due to the efficient flow of blood to all parts of the body through double circulation.

  • It helps in generating a large amount of energy by the body through the efficient supply of oxygen to cells and that allows the body to cope up with all life activities.


Most frequently asked questions


  1. Name the largest vein and largest artery involved in double circulation of blood.
    Ans. The largest vein is the Vena cava and the largest artery is the aorta

  2. Name the blood vessels that carry oxygenated blood from the lungs to the left atrium.
    Ans. pulmonary veins.

  3. State any two advantages of double circulation of blood in humans.
    Ans. Separation of oxygenated and deoxygenated blood without mixing and efficient distribution of oxygen to all tissues of the body.

  4. Name the blood vessel that circulates deoxygenated blood to the lungs for oxygenation.
    Ans. Pulmonary vein.

  5. What is a cardiac cycle?
    Ans. Systole and diastole together constitute a cardiac cycle.

  6. What are the two circulations in double circulation?
    Ans. Pulmonary circulation and Systemic circulation.


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