Double Circulation: We know that each and every cell of our body needs oxygen and nutrients for survival or getting energy. And waste material has to be removed from each cell. There should be a proper system that can transfer substances ‘to and fro’ from each cell of the body. This can be achieved by the flow of blood from each cell. But only to and fro movement of blood will not serve the purpose.

Oxygenated or pure and deoxygenated or impure blood should not be mixed.  But how to separate them? To overcome this problem, there occurs Double Circulation in mammals and birds. It is important for students who want to pursue their higher education in medicine especially, to have a strong base in biology.

When complex concepts are made easier for students to understand, it becomes easier for them to relate with other concepts of the same subject. Learning about the human body is essential for students at both the school level and higher education levels. Continue reading the article to know more about Double Circulation.

What is Double Circulation?

A type of blood circulation that occurs in mammals and birds in which blood flows twice through the heart is called double circulation. It comprises systemic and pulmonary circulation.

Double Circulation Diagram

Fig: Double Circulation

Components Involved in Double Circulation

1. Heart– It is four-chambered in humans, i.e., right and left atrium & right and left ventricle.
2. Blood vessels– Arteries, veins and capillaries come under blood vessels. Arteries carry oxygenated blood; veins carry deoxygenated blood except for the pulmonary artery and pulmonary vein.
Each artery and vein consists of three layers, namely, tunica intima, tunica media and tunica externa.
3. Blood- The fluid connective tissue comprises blood cells and plasma.

Fig: Heart Showing Blood Flow

Double Circulation Process

It includes systemic and pulmonary circulation. Systemic circulation involves the movement of blood between all parts of the body and the heart, while pulmonary circulation involves the movement of blood between the heart and the lungs.

Fig: Double Circulation

(i) Systemic Circulation– Our heart is completely divided into four chambers. This comprises actually two separate pumps working together.
The flow of oxygenated blood from the left ventricle to all parts of the body and then the flow of deoxygenated blood from various parts of the body to the right atrium constitutes systemic circulation.
In this circulation, oxygenated blood entering the left atrium is pumped to the left ventricle through the mitral valve and then to all parts of the body through the aorta. After circulation in all tissues of the body, deoxygenated blood is collected by veins and is transferred to the right atrium through the superior and inferior vena cava.
As in systemic circulation, blood is supplied to most of the tissues of the body; it is called greater circulation or peripheral circulation.
Functions of systemic circulation:
a. It carries oxygen and nutrients to all cells of the body.
b. Removes carbon dioxide and other waste products from them.
(ii) Pulmonary Circulation– Deoxygenated blood from the right atrium is moved to the right ventricle through the tricuspid valve. It is then pumped to the pulmonary artery through a pulmonary valve. The pulmonary artery carries deoxygenated blood to the lungs for its oxygenation. After blood gets oxygenated, the pulmonary vein carries this oxygenated blood to the left atrium.
Functions of pulmonary circulation:
a. It helps in purifying deoxygenated blood to oxygenated blood, i.e., deoxygenated blood gives off carbon dioxide and picks up oxygen.

Difference Between Systemic Circulation and Pulmonary Circulation

Systemic Circulation	Pulmonary Circulation
I. It involves the flow of oxygenated blood from the left ventricle to all parts of the body and deoxygenated blood from all parts of the body to the right atrium.	I. It involves the flow of oxygenated blood from the lungs to the left atrium and the flow of deoxygenated blood from the right ventricle to the lungs.
II. Here, the flow of blood covers a long distance.	II. Here, the flow of blood covers a short distance.
III. It happens between the heart and the body.	III. It happens between the heart and the lungs.
IV. In Systemic Circulation, the oxygenated blood is sent back to the heart through pulmonary veins from the lungs.	IV. In Pulmonary circulation, the deoxygenated blood is collected from the system of venules, veins, and sent back to the heart.

Significance of Double Circulation

1. It checks the mixing of deoxygenated and oxygenated blood.
2. Oxygenated blood carries more oxygen which is supplied to all parts of the body.
3. Deoxygenated blood carries more carbon dioxide for its removal from the body.

NCERT Solutions for Class 11 Biology Chapter 17

NCERT Solutions for Class 11 Biology

NCERT Solutions for Class 11 Biology Chapter 19

Circulation Through Special Regions

Portal System

In the case of the portal system, there is a special vein called the portal vein. There are three types of portal systems. Two of which are found in humans and the third one is found in fishes and amphibians.
a. Hepatic Portal system
b. Hypophyseal Portal system
c. Renal Portal system
Each has two sets of capillaries:-
a. Collecting capillaries
b. Distributing capillaries
In humans, hepatic and hypophyseal portal systems are found.
A portal vein is a vein that collects the material from one organ and breaks into the second set of capillaries and offloads that material into an intermediate organ. So, depending upon which organ the offloading of material occurs, it will give its name. In the case of the hepatic portal vein, the organ is the liver, so the name is the Hepatic portal vein.

a. Hepatic Portal System

The artery brings oxygenated blood to the intestines, and from the intestine, simple forms of food are absorbed. Now, this simple form has to go to the liver so that extra glucose can be converted to glycogen and so on.
So, here the hepatic portal vein sends the deoxygenated blood with all the nutrients that have to be offloaded to the liver. Then the deoxygenated blood, which is devoid of nutrients, is sent to the heart.

b. Hypophyseal Portal System

In this hypophyseal portal system, the hypothalamus and posterior pituitary are involved. Artery brings oxygenated blood to the hypothalamus, and from the hypothalamus, neurohormones are secreted to the blood. This neurohormone has to be sent to the posterior pituitary gland.
So, before releasing the deoxygenated blood to the heart, neurohormones are offloaded to the posterior pituitary gland through a special hypophyseal portal vein. Then the deoxygenated blood, which is devoid of neurohormones, is sent to the heart.

c. Renal Portal System

This portal system is very well developed in fishes, amphibians and reptiles. It is vestigial in birds. However, it is absent in mammals. Because of this system, they are able to maintain the osmotic concentration of the blood, which helps in the absorption of oxygen from the water. In this portal system, the kidney is involved, so named the renal portal system.

Coronary Circulation

It refers to the circulation of blood to the wall of the heart. The heart wall is made of cardiac muscles, and it requires continuous blood supply so that it can work properly. So, the blood is supplied to the wall of the heart by coronary arteries, which arise from the aorta just above the semilunar valve. The coronary veins carry deoxygenated blood to the coronary sinus, which brings deoxygenated blood to the right atrium.

Summary

Double circulation is the type of blood circulation in which blood flows twice through the heart. It comprises systemic and pulmonary circulation. The flow of oxygenated blood from the left ventricle to all parts of the body and then the flow of deoxygenated blood from various parts of the body to the right atrium constitutes systemic circulation. The flow of deoxygenated blood from the right ventricle to the lungs and flow of oxygenated blood from the lungs to the left atrium comprises pulmonary circulation.

Double circulation also includes portal systems. In humans, hepatic and hypophyseal portal systems are found. Double circulation ensures that the body has a dedicated supply of oxygen always and it enhances body efficiency.

Other important Biology articles:

Circulatory System	Lymphoid Organs
Respiration and Circulation	Lymphatic System
Circulatory Disorders	Blood Groups
Blood	Blood Vessels

FAQs on Double Circulation

Q1. Why is double circulation necessary in human beings?
Ans: Double circulation is necessary for human beings because of the following advantages:-
1. It checks the mixing of deoxygenated and oxygenated blood.
2. Oxygenated blood carries more oxygen which is supplied to all parts of the body.
3. Deoxygenated blood carries more carbon dioxide for its removal from the body.

Q2. Define double circulation in humans?
Ans: A type of blood circulation that occurs in mammals and birds in which blood flows twice through the heart is called double circulation.

Q3. What are the disadvantages of double circulation?
Ans: Double blood circulation has no disadvantage as it is the most efficient circulation system.

Q4. What is an example of double circulation?
Ans: Human circulatory system is an example of double circulation.

Q5. Do birds have four-chambered hearts?
Ans: Yes, birds have four-chambered hearts.

Q6. Do all tetrapods have double circulation?
Ans: Tetrapods include amphibians, reptiles, birds and mammals. These all have a double circulation system.

We hope this detailed article on Double Circulatory System helps you in your preparation. If you get stuck do let us know in the comments section below and we will get back to you at the earliest.