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In this article, we will discuss that the mammalian circulatory system is a closed double circulation consisting of a heart, blood, and blood vessels including arteries, arterioles, capillaries, venules, and veins. Moreover, we will also describe the functions of the primary blood vessels of the pulmonary and systemic circulations, limited to the pulmonary artery, pulmonary vein, aorta, and vena cava. So, let us get started with why mammals need a circulatory system.
Why Mammals Need a Circulatory System?
The cells in all the living organisms require an uninterrupted supply of reactants for metabolism, for instance, glucose and oxygen. Unicellular organisms, i.e., organisms made up of a single cell only can get oxygen and glucose directly from their surroundings. In these organisms, the molecules can diffuse to all components of the cell quickly because of the short diffusion distances.
As opposed to unicellular organisms, larger organisms are multicellular, i.e., they are composed of many cells. Since the larger organisms are made up of several layers of cells, hence the time taken by the substances like oxygen and glucose would be far too much to diffuse into every cell of the body. Not only the time, but the diffusion distances are also too long in multicellular organisms.
To solve this issue, the exchange surfaces of larger organisms are connected to a mass transport system such as:
- The digestive system and circulatory system are connected
- The lungs and circulatory system are connected
Now, you may be wondering what is meant by mass transport? Well, mass transport refers to the bulk movement of liquids or gases in a single direction usually through a system of tubes and vessels. The best and most well-studied example of a mass transport system is the circulatory system in mammals. In mammals, the circulatory system is the unidirectional (one-way) flow of blood within the blood vessels which transports important nutrients and gases to all cells of the body.
In the next section of the article, we will discuss open and closed systems.
Open and Closed Circulatory Systems
Circulatory systems can either be open or closed.
- Closed Circulatory System: In a closed circulatory system, the blood is pumped around the body. The blood in this circulatory system is always contained within a network of blood vessels. A closed circulatory system is present in all vertebrates and several invertebrates.
- Open Circulatory System: As opposed to the closed circulatory system, blood in the open circulatory system is not contained within the blood vessels, rather it is pumped directly into the body cavities. The open circulatory system is present in organisms such as arthropods and molluscs.
Human Circulatory System
A closed double circulatory system is present in human beings. It implies that in a single complete circuit of the body, the blood passes through the pumping organ, i.e., the heart two times. The right side of the human heart pumps deoxygenated blood to the lungs for the exchange of gas. This is known as the pulmonary circulatory system.
The blood is then returned to the left side of the heart so that the oxygenated blood can be efficiently pumped around the body. This is referred to as a systemic circulatory system.
In the next section of the article, we will discuss the structures present in the mammalian circulatory system and their respective functions.
Structures Present in Mammalian Circulatory System
The mammalian circulatory system has the following structures:
- Heart: It is a hollow, muscular organ that is present in the chest cavity. The main function of this organ is to pump blood. Blood provides essential nutrients and oxygen to the human body and also helps to eliminate metabolic waste. In mammals, the heart is located off-centre to the left side of the rib cage between the lungs. Cardiac muscle tissue contracts involuntarily without rest. The heart in mammals is a double pump which means that the blood passes through the heart two times in a single cardiac cycle. This kind of circulation is referred to as double circulation.
- Arteries: They refer to the blood vessels that carry blood away from the heart. There is a lot of muscle, elastic tissue, and a narrow lumen in the walls of the arteries so that high blood pressure can be maintained. The diameter of the arteries ranges from 0.4 to 2.5 cm.
- Arterioles: They refer to the small arteries which branch from the larger arteries and are connected to the capillaries. The diameter of the arterioles is approximately 30 micrometres.
- Capillaries: They refer to the small blood vessels having a diameter ranging from 5 to 10 micrometres that connect arterioles and venules. Their size implies that they directly pass through cells and tissues and perform not only gas exchange but also exchange of substances such as glucose.
- Venules: They are smaller veins that connect capillaries to larger veins. Their diameter ranges from 7 micrometres to 1 millimetre.
- Veins: Veins are the blood vessels that carry blood back towards the pumping organ, i.e., the heart. As compared to arteries, the walls of the veins are thin. They also have less muscle and elastic tissue, but a wider lumen. Valves help in maintaining the flow of blood back towards the heart.
In the next section of the article, we will discuss the main blood vessels such as the pulmonary artery, pulmonary vein, coronary arteries, vena cava, renal artery, and renal vein.
Primary Blood Vessels
- Pulmonary artery: It carries deoxygenated blood, i.e., the blood with low oxygen saturation away from the heart, towards the lungs
- Pulmonary vein: It transports oxygenated blood, i.e., the blood with high oxygen saturation away from the lungs, towards the heart
- Aorta: Aorta is the primary artery in the body that has a thick muscular wall and is connected to the left side of the heart. It takes oxygenated blood out of the heart and to the rest of our body
- Coronary arteries: Coronary arteries supply the heart with oxygenated blood.
- Vena cava: Vena cava takes deoxygenated blood into the heart
- Renal artery: Renal artery supplies oxygenated blood to the kidneys
- Renal vein: takes deoxygenated blood away from the kidneys, toward the heart
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