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Double Circulation, Hepatic Portal System, Practice problems and FAQs

Double Circulation, Hepatic Portal System, Practice problems and FAQs

We are all aware that if the heart stops beating even for a few seconds, it can lead to our death. But why is the heart such an important part of our body? It is because it pumps blood rich in oxygen to different body parts and oxygen is absolutely essential for the cells in our body to carry out cellular respiration and generate energy. Simultaneously, the blood pumped by the heart also helps in collecting the waste carbon dioxide from the cells and takes it to the lungs to be expelled out of the body. 

But does the heart of all vertebrates function in a similar way? Do all vertebrates have the same structure of heart? Does the flow of blood through the heart follow the same pattern in all vertebrate animals? Well, the answer to all these questions is no. While fishes have a two chambered heart, all amphibians and most reptiles have a three-chambered heart and crocodiles, birds and mammals have a four chambered heart. Needless to say, the way these hearts function is quite different from each other. In this article, we will discuss briefly about the circulation in different types of hearts while focussing mostly on the four-chambered heart as seen in humans. Blood through our heart travels twice while completing a full round of the body and hence it is said to complete double circulation. Let us learn more about double circulation through this article.

Table of contents

  • Single circulation
  • Incomplete double circulation
  • Double circulation of blood 
  • Hepatic Portal system
  • Practice problems
  • FAQs

Single circulation

Fishes have a two chambered heart with one auricle and one ventricle. Their heart is considered to be a venous heart as it receives deoxygenated blood (blood rich in CO2) only. The heart pumps the deoxygenated blood to the gills where it is oxygenated (O2 is added and CO2 is removed). The oxygenated (rich in O2) blood is sent to different parts of the body from the gills. Thus, the blood has to pass through the heart only once while completing the full circuit in the body. Such type of circulation is known as single circulation.

Double circulation of blood 

Apart from fishes, other vertebrates have an arteriovenous heart which receives both oxygenated and deoxygenated blood. In amphibians the heart is three chambered with two auricles and one ventricle, in most reptiles the heart is incompletely four chambered and in crocodiles, birds and mammals the heart is four chambered. In all these cases, the blood travels twice through the heart to complete one round of the body and thus this type of circulation is known as double circulation. 

However, in amphibians and most reptiles, there is some degree of mixing of the oxygenated and deoxygenated blood in the single ventricle and hence the circulation in them is considered to be incomplete double circulation. The mixing of oxygenated and deoxygenated blood is kept to a minimum due to the timing of the contractions between the atria.

In crocodiles, birds and mammals, the heart has two auricles and two ventricles which ensures that the oxygenated and deoxygenated blood does not mix during double circulation. This is known as complete double circulation.

Complete double circulation in humans

The human heart is four chambered with two auricles and two ventricles. The deoxygenated blood from all parts of the body enters through the right side of the heart and leaves to go to the lungs for oxygenation. The oxygenated blood from the lungs comes back to the left side of the heart and is sent from there to the tissues in all parts of the body for supplying oxygen. Thus, to complete a full round of the body, the blood passes twice through the heart and this type of blood circulation also allows the separation of oxygenated and deoxygenated blood for a more efficient supply of oxygen in the body. Thus, this type of blood circulation in humans is said to be complete blood circulation in humans.

In complete double circulation, there are two pathways by which the blood flows. These are:

  1. Pulmonary circulation
  2. Systemic circulation

Fig. : Double circulation of blood

Fig. : Double circulation of blood

Pulmonary circulation

Pulmonary circulation is the circulation of blood between the heart and lungs. The main purpose of pulmonary circulation is to carry the deoxygenated blood to the lungs for oxygenation and return the oxygenated blood from the lungs back to the heart. The deoxygenated blood is brought to the right atrium via the superior vena cava (from upper part of body), inferior vena cava (from lower part of body) and the coronary sinus (from heart walls). 

From the right atrium the blood is pumped to the right ventricle from where a great artery arises, named pulmonary aorta or pulmonary arch and divides into two pulmonary arteries. These arteries carry the deoxygenated blood to the lungs for oxygenation

The oxygenated blood is carried back to the heart’s left atrium with the help of four pulmonary veins, two arising from each lung. This flow of blood from the right ventricle to the left atrium of the heart via the route of lungs is termed as ‘pulmonary circulation’.

Fig: Pulmonary circulation

Fig: Pulmonary circulation

Systemic circulation

The oxygenated blood from the left atrium is pumped into the left ventricle which in turn pumps out the oxygenated blood into the great artery named systemic aorta. This aorta branches out into multiple systemic arteries that carry oxygenated blood to all the body organs except the lungs. In the tissues, the arteries branch out into very thin capillaries which allow gaseous exchange. Most of the O2 provided by the blood is utilised by the tissues for energy production via cellular respiration, and the CO2 released in the process diffuses into the blood, making it deoxygenated. 

This deoxygenated blood from the organs is carried by the veins and poured into the two great veins - superior and inferior vena cava. The deoxygenated blood from the heart walls is carried to the lungs via the coronary sinus, which along with the vena cavas, pours the deoxygenated blood into the right atrium. Thus the blood flow from left ventricle to right atrium of the heart via the route of all the body tissues is called ‘systemic circulation’. The best advantage of having systemic circulation is that it provides nutrients, O2 and other essential substances to the tissues and takes CO2 and other harmful substances away for elimination.

Fig: Systemic circulation

Fig: Systemic circulation

Hepatic Portal system

A portal system is the one which begins and ends in capillaries. Hepatic portal system is present in all the vertebrates including humans. It is a specialised vascular connection between the digestive tract and the liver. A vein called ‘hepatic portal vein’ carries the venous blood from the alimentary canal , pancreas and spleen to the liver before it is delivered to other body tissues. The superior and inferior mesenteric veins, the splenic vein, gastric vein, and cystic vein combine to form the hepatic portal vein. This blood from the liver is then carried by a pair of hepatic veins to the inferior vena cava.

Significance of hepatic portal system

The liver acts as a detoxifier for the blood that is directly coming from the digestive tract. Liver also synthesises many useful substances that it adds to the blood passing through it via hepatic portal system.

Fig: Hepatic portal system

Fig: Hepatic portal system

Practice problems

Q1. Complete double circulation occurs in

A. in all vertebrates
B. in crocodiles, birds and mammals
C. in fishes
D. in amphibians and reptiles

Solution: Double circulation occurs in all vertebrates other than fishes. It refers to the type of blood circulation in which blood flows twice through the heart while completing one full round of the body. But amphibians and most reptiles have a three-chambered heart (two auricles and one ventricle) and while blood flows twice through the heart, there is mixing of oxygenated and deoxygenated blood in the ventricle. Thus, they have incomplete double circulation.

Crocodiles, birds and mammals have a four chambered heart (two auricles and two ventricles) through which blood flows twice and the oxygenated and deoxygenated blood remain completely separated. Thus, they are said to have complete double circulation.

Thus, the correct option is b.

Q2. Systemic circulation is

A. circulation of blood from the right ventricle to the whole body and back to the right auricle
B. circulation of blood from the left ventricle to lungs and back to right auricle
C. circulation of blood from the left ventricle to the whole body and back to the right auricle
D.  Circulation of blood from the body tissues to the heart and then to the lungs

Solution: Oxygenated blood from the left ventricle is pumped into the great artery named systemic aorta. This aorta branches out into multiple systemic arteries that carry oxygenated blood to all the body organs except the lungs. In the tissues, the arteries branch out into very thin capillaries which allow gaseous exchange. Most of the O2 provided by the blood is utilised by the tissues for energy production via cellular respiration, and the CO2 released in the process diffuses into the blood, making it deoxygenated. 

This deoxygenated blood from the organs is carried by the veins and poured into the two great veins - superior and inferior vena cava. The deoxygenated blood from the heart walls is carried to the lungs via the coronary sinus, which along with the vena cavas, pours the deoxygenated blood into the right atrium. Thus the blood flow from left ventricle to right atrium of the heart via the route of all the body tissues is called ‘systemic circulation’. 

Fig: Systemic circulation

Fig: Systemic circulation

Q3. The hepatic portal system carries blood from

A. small intestine to the kidneys
B. pancreas to the kidneys
C. small intestine to pancreas
D. small intestine to liver

Solution: A portal system is the one which begins and ends in capillaries. Hepatic portal system is present in all the vertebrates including humans. It is a specialised vascular connection between the digestive tract and the liver. A vein called ‘hepatic portal vein’ carries the venous blood from the alimentary canal , pancreas and spleen to the liver before it is delivered to other body tissues. The superior and inferior mesenteric veins, the splenic vein, gastric vein, and cystic vein combine to form the hepatic portal vein. This blood from the liver is then carried by a pair of hepatic veins to the inferior vena cava.

Thus, the correct option is d.

Fig: Hepatic portal system

Fig: Hepatic portal system

Q4. Identify the correct statement about pulmonary circulation.

(a) deoxygenated blood travels from left ventricle to lungs and back to the body
(b) oxygenated blood travels from right auricle to the whole body and back to the heart
(c) deoxygenated blood travels from right ventricle to lungs and back to left auricle
(d) oxygenated blood travels from left auricle to lungs and then to the whole body

Solution: In pulmonary circulation, deoxygenated blood from the  the right ventricle goes into the great artery named pulmonary aorta or pulmonary arch which further divides into two pulmonary arteries. These arteries carry the deoxygenated blood to the lungs for oxygenation

The oxygenated blood is carried back to the heart’s left atrium with the help of four pulmonary veins, two arising from each lung. This flow of blood from the right ventricle to the left atrium of the heart via the route of lungs is termed as ‘pulmonary circulation’.

Fig: Pulmonary circulation

Fig: Pulmonary circulation

FAQs

Question 1. Which is the largest blood vessel in the body?
Answer: The aorta is the largest blood vessel in the body. It is more than 12 inches long and its widest point has a diameter of about 1 inch.

Question 2. Do all arteries carry oxygenated blood?
Answer: All arteries carry oxygenated blood, except the pulmonary artery and umbilical artery. The pulmonary arteries carry deoxygenated blood from the right ventricle of the heart to the lungs and the umbilical arteries carry deoxygenated blood from the foetus to the placenta.

Question 3. Which is the longest vein in the body?
Answer: The great saphenous vein which carries blood from the foot, leg and thigh to the deep femoral vein at the femoral triangle is the longest vein in the body.

Question 4. What is the arterial system?
Answer: Arterial system is the blood vascular system which is composed of blood vessels which have thick muscular walls and carry blood away from the heart to the body parts. Two arterial trunks arise from the heart, the pulmonary aorta or arch from the right ventricle and the systemic aorta from the left ventricle.

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Related Topics

Human circulatory system 

Cardiac cycle and Cardiac output 

Blood : Overview and plasma - Definition, Physical characteristics of blood, Blood components, Blood plasma, Serum 

Tissue fluid and Lymph 

Blood: RBCs and its fate, variation in count, WBCs and platelets 

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