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Biochemistry is a field within Biology Topics that explores the chemical processes in living organisms.
Human Circulatory System – Organs, Diagram and Functions
In everyday language ‘transport’ means ‘to carry things from one place to another’. In
biology, transport is a life process in which a material absorbed (or made) in one part of the body of an organism is carried to other parts in its body. This point will become clear from the following discussion. We have learnt earlier that all the living organisms (animals and plants) need a continuous supply of different materials such as food, water and oxygen for their survival.
These materials must reach the various parts of an organism where they are needed. All the living organisms also produce some waste materials (like carbon dioxide, etc.) as a result of chemical processes occurring in their body cells. These waste materials are harmful and have to be removed from the body. So, it is necessary that all the living organisms have some means of transporting (or carrying) materials from one part to another part within their body.
We can now say that: All the living organisms (animals and plants) need a transport system to supply them with food, water and oxygen, and to carry away the harmful waste materials produced in their bodies. Both, animals and plants have their own transport systems for transporting materials within their body. In this Chapter, we will learn about the transport system in animals and plants. Let us discuss the transport system in animals first.
In most of the animals, the blood that circulates in their body transports (or carries) food, water and oxygen to cells in all the parts of their body. The blood also transports the waste materials made in the body cells to different parts of the body for excretion (or removal). We will now describe the transport system in animals by taking the example of human beings.
The main transport system in human beings is the ‘blood circulatory system’ (which is commonly known as just ‘circulatory system’). In the circulatory system, blood carries digested food, water and oxygen to all the parts of the body. It also takes away the waste products like carbon dioxide made in the body cells. Thus, the blood circulatory system makes food, water and oxygen available to every part of the body, and helps in removing waste materials of the body like carbon dioxide, etc.
The various organs of the circulatory system in humans are : Heart and Blood vessels (Blood vessels are of three types : arteries, veins and capillaries). Blood is also considered a part of the circulatory system. So, we can now say that: The human circulatory system consists of the heart, blood vessels (arteries, veins, capillaries) and blood. In the circulatory system, the heart acts as a pump to push out blood. The blood vessels (arteries, veins and capillaries) act as tubes or pipes through which blood flows in the whole body (see Figure). We will now describe all the parts of the circulatory system in detail, one by one. Let us start with blood.
Blood
Blood is a red coloured liquid which flows in blood vessels and circulates in our body. Blood is red because it contains a red pigment called haemoglobin. Blood has many different cells which perform different functions. Blood consists of four components : plasma, red blood cells, white blood cells and platelets. Plasma is a liquid and red blood cells, white blood cells and platelets keep floating in it. We can now define blood as follows : Blood is a liquid called plasma with red cells, white cells and platelets floating in it (see Figure). The blood is able to transport (or carry) various substances in the body due to the presence of different components in it. We will now describe all the four components of blood and their functions in somewhat detail.
1. PLASMA. The liquid part of blood is called plasma. Plasma is a pale yellow, sticky liquid. It is 90 per cent water and 3.5 per cent common salt. Plasma contains dissolved substances such as digested food and waste products (like carbon dioxide and urea). Plasma carries water and dissolved substances such as digested food and waste products from one part to another part in the body. Red blood cells, white blood cells and platelets are immersed in this liquid called plasma.
2. RED BLOOD CELLS (RBC). Red blood cells are red in colour due to the presence of a red pigment called haemoglobin inside them. Red blood cells are carriers of oxygen. Red blood cells carry oxygen from the lungs to all the cells of the body. It is actually the haemoglobin present in red blood cells which carries oxygen in the body. Haemoglobin binds with oxygen and transports it to all the parts of the body and ultimately to all the cells. If there is a deficiency of haemoglobin in the blood of a person, it becomes difficult to provide oxygen efficiently to all the cells of his body. It is the presence of haemoglobin which makes the blood appear red. Please note that most of the cells in blood are red blood cells.
3. WHITE BLOOD CELLS (WBC). The white blood cells fight infection and protect us from diseases. This is because white blood cells help to fight against germs which may enter our bodies and cause diseases. Some white blood cells can eat up the germs (like bacteria) which cause diseases. Other white blood cells make chemicals known as ‘antibodies’ to fight against infection. White blood cells are much smaller in number than red blood cells.
4. PLATELETS. Platelets are the tiny fragments of special cells formed in the bone marrow. Platelets help in the clotting of blood in a cut or wound. When a cut or wound starts bleeding, then platelets clot the blood (make the blood semi-solid) due to which further bleeding stops. If a child falls down while playing a game and his knee (or any other body part) gets injured, then blood starts flowing from the cut made by the injury. After some time, however, a dark red clot is formed which plugs the cut and bleeding stops. The blood clot (which stops bleeding) is formed due to the presence of the cells called ‘platelets’ in the blood. If, however, there were no platelets in the blood, then bleeding caused by a cut from an injury would not stop. This may cause loss of too much blood from the body of a person leading to death.
Blood is needed by all the parts of our body because it performs the following important functions in our body:
- Blood carries digested food from the small intestine to all the parts of the body.
- Blood carries water to all the parts of the body.
- Blood carries oxygen from the lungs to different parts of the body.
- Blood carries the waste product carbon dioxide from the body cells to the lungs for breathing out.
- Blood carries a waste product called urea from the liver to the kidneys for excretion in urine.
- Blood protects the body from diseases. This is because white blood cells kill the bacteria and other germs which cause diseases.
Blood circulates in our body in two forms : oxygenated blood and deoxygenated blood. The blood carrying oxygen in it is called oxygenated blood. Thus, oxygenated blood is rich in oxygen. We get oxygenated blood in the lungs where the fresh oxygen of air passes into the blood. The blood having no oxygen in it is called deoxygenated blood. The deoxygenated blood is formed in all the organs of the body (except the lungs). This is because when the oxygenated blood passes through the organs of the body, the body cells use up its oxygen and make it deoxygenated. The deoxygenated blood, however, carries carbon dioxide in it (which is produced during respiration in body cells). Thus, deoxygenated blood is rich in carbon dioxide. Oxygenated blood is bright red whereas deoxygenated blood is darker.
Heart
The heart is an organ which pumps blood to all the parts of our body through a network of tubes called blood vessels. Our heart ‘beats’ continuously to circulate blood in the body. The heart works like a pump non-stop throughout our life. Our heart is located in the chest cavity slightly towards the left side. The heart lies between the two lungs and above the diaphragm. The size of our heart is roughly equal to our fist (mutthi). The heart is made of special muscle called cardiac muscle ’ but it is not solid muscle.
The heart is hollow inside. The heart has four compartments called ‘chambers’ (see Figure). The upper two chambers of heart are called atria (singular of atria is atrium), and the lower two chambers of heart are called ventricles. On the left side of the heart are left atrium and left ventricle. On the right side of the heart are right atrium and right ventricle.
(i) The top side of left atrium is connected to pulmonary vein which brings oxygenated blood into it from the lungs (see Figure) (The term pulmonary refers to lungs). The lower side of left atrium is connected to left ventricle. Oxygenated blood from left atrium goes into left ventricle. The other side of left ventricle is connected to main artery (called aorta). The left ventricle pumps oxygenated blood into main artery for taking it to all the body organs (except lungs).
(ii) The top side of right atrium is connected to main vein (called vena cava) which brings deoxygenated blood into it from all the body organs (except lungs) (see Figure). The lower side of right atrium is connected to right ventricle. Deoxygenated blood from right atrium goes into right ventricle. The other side of right ventricle is connected to pulmonary artery. The right ventricle pumps deoxygenated blood into pulmonary artery for taking it to lungs.
The left side of heart is completely separated from its right side by means of a partition wall called ‘septum’ which is made of a thick muscle (see Figure). The partition wall (or septum) in the heart prevents the mixing of oxygenated blood on its left side with the deoxygenated blood on its right side. A sheath of tissue called pericardium’ protects the heart. The job of heart is to pump blood around our body continuously. All the atria and ventricles of the heart contract and relax (expand) at appropriate times and make the heart behave like a pump.
The heart is not a ‘single pump’. Actually, the heart is a ‘double pump’. The left side of heart and right side of heart act as two separate pumps. The left side of heart acts as one pump which pumps oxygenated blood into the whole body (except the lungs). The right side of heart acts as another pump which pumps deoxygenated blood only to the lungs.
The left side of heart has oxygen rich blood (oxygenated blood) whereas the right side of heart has carbon dioxide rich blood (deoxygenated blood). It is necessary to keep the oxygen rich blood separate so that it may supply good amount of oxygen to the body cells for respiration and release of energy. If, however, the oxygen rich blood mixes with carbon dioxide rich blood, then the body cells will not be able to get enough oxygen for respiration due to which less energy will be released in the body.
Blood Vessels
Blood vessels are a kind of tubes (or pipes) which carry blood throughout our body. The blood vessels run between the heart and the rest of the body. There are three types of blood vessels in the body: arteries, veins and capillaries. We will now describe arteries, veins and capillaries in detail.
(i) Arteries are the blood vessels which carry blood from the heart to all the parts of the body. In other words, arteries are the blood vessels which carry blood away from the heart. Arteries are found in the whole of our body. The arteries lie quite deep under our skin and hence they are not seen easily. Blood leaves the heart at high pressure through arteries. The arteries have thick and elastic walls. This is because blood flows through them at high pressure. The thick and elastic walls of arteries can withstand the high pressure of blood flowing through them.
The main artery (called aorta) is connected to the left ventricle of the heart (see Figure). The main artery carries oxygenated blood from the left ventricle to all the parts of the body (except the lungs ). Another artery called pulmonary artery is connected to the right ventricle of the heart (see Figure). The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs.
Please note that arteries normally carry oxygenated blood (rich in oxygen) from the heart. But there is one exception to this rule. One artery called ‘pulmonary artery’ carries deoxygenated blood (rich in carbon dioxide) from the heart to the lungs. The pulmonary artery carrying deoxygenated blood from the heart to the lungs is called an artery (and not a vein) because it carries blood away from the heart.
(ii) Veins are the blood vessels which carry blood from all the parts of the body back to the heart. In other words, veins are the blood vessels which carry blood towards the heart. Veins are also found in the whole of our body. The veins are less deep than arteries and hence can be seen easily. The greenish- blue lines which we see just below the skin on our hands and legs are the veins. The deoxygenated blood returns to the heart at low pressure through the veins. Since the blood flows at low pressure through the veins, so the veins have thin walls.
Veins have valves in them which allow the blood in them to flow in one direction only : towards the heart. The valves prevent the backflow of blood in veins. The main vein is connected to the right atrium of the heart (see Figure). The main vein carries deoxygenated blood (rich in carbon dioxide) from all the parts of the body (except lungs) back to the heart. Another vein called pulmonary vein is connected to the left atrium of the heart (see Figure). The pulmonary vein carries oxygenated blood (rich in oxygen) from lungs back to the heart.
Please note that veins normally carry deoxygenated blood (rich in carbon dioxide) from the body organs. But there is one exception to this rule. One vein called ‘pulmonary vein’ carries oxygenated blood (rich in oxygen) from the lungs to the heart. The pulmonary vein carrying oxygenated blood from the lungs is called a vein (and not an artery) because it carries blood back to the heart.
The main difference between an artery and a vein is that an artery normally carries blood from the heart to the body organs whereas a vein normally carries blood from the body organs back to the heart.
(iii) Capillaries are the extremely thin blood vessels which connect arteries to veins (see Figure). Thus, capillaries are always present in-between the arteries and veins in our body. Capillaries (also called ‘blood capillaries’) are present throughout our body. Every cell of the body is near a capillary.
The capillaries have extremely thin walls which allow substances to pass from blood into the body cells, and also from body cells into the blood. The oxygenated blood from arteries enters into the capillaries in all the parts of the body. The various dissolved substances present in the blood (like food and oxygen) pass into body cells through the thin walls of the capillaries (see Figure). At the same time, the waste products (like carbon dioxide) formed in the body cells enter into blood through the thin walls of capillaries (see Figure). Thus, the exchange of substances (like food, oxygen, carbon dioxide, etc.) between the blood and the body cells takes place through capillaries. The other ends of capillaries are joined to veins (see Figure). The deoxygenated blood (containing waste products like carbon dioxide) coming from the capillaries enters into veins and carried back to the heart.
Working of the Circulatory System
The blood is circulated in the human body by the rhythmic (regularly occurring) contractions and relaxations of the heart muscles which make up the atria and ventricles. The two atria (left atrium and right atrium) contract together and relax together. Similarly, the two ventricles (left ventricle and right ventricle) contract together and relax together. The contraction of two atria is immediately followed by the contraction of two ventricles and leads to the continuous flow of blood in the human body. We will now describe the circulation of blood in the human body with the help of a simplified diagram.
1. When blood passes through the capillaries of the lungs, then oxygen from air enters into blood and we get oxygenated blood (see Figure).
2. Pulmonary vein brings oxygenated blood from the lungs into the left atrium. When the left atrium contracts, the oxygenated blood is pushed into left ventricle.
3. When the left ventricle contracts, it pumps oxygenated blood into the main artery (see Figure). This main artery then branches into many smaller arteries which go into different body organs (These smaller arteries have not been shown in Figure to keep the diagram simple).
4. The arteries carry oxygenated blood to all the organs (or parts) of the body like head, chest, arms, stomach, intestines, liver, kidneys, trunk and legs (except lungs).
5. All the body organs have capillaries in them which are joined to arteries. When oxygenated blood from arteries passes through the capillaries of the body organs, then it gives food and oxygen to the body cells. Since the blood loses oxygen here we say that the blood has been deoxygenated. At the same time, carbon dioxide produced as a waste product in the body cells during respiration, enters into blood through capillaries.
6. The deoxygenated blood (rich in carbon dioxide) coming from the body organs enters from the capillaries first into smaller veins and then into main vein. The main vein carries deoxygenated blood to the right atrium (see Figure).
7. When the right atrium contracts, the deoxygenated blood is pushed into right ventricle.
8. When the right ventricle contracts, it pumps deoxygenated blood (rich in carbon dioxide) into the lungs (see Figure).
9. In the lungs, deoxygenated blood gives out the waste product carbon dioxide and absorbs fresh oxygen from the air being breathed in. So, the blood becomes oxygenated again.
The oxygenated blood is again sent by the lungs to the left side of the heart for circulation in the body once again. This process is repeated continuously, all day and all night, even when we are asleep. In this way, the blood keeps on circulating in the body without stopping due to which all the parts of the body keep on getting digested food, oxygen and other materials all the time. At the same time, blood circulation keeps on removing waste products like carbon dioxide formed in the body cells.
The circulation of blood was discovered by an English physician named William Harvey. Please note that the animals such as cow, dog, deer, lion, tiger, birds, and fish, etc., all have circulatory systems in their bodies consisting of heart, blood vessels and blood. We will now discuss heartbeats and pulse.