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In this article, you will learn all about human circulatory system. You will especially learn about the functions of arteries veins and capillaries, the structure of the heart, and components of blood (RBC, WBC, platelets, and plasma). So, let us get started.
Circulatory System – Introduction
It is critical for every organism to exchange materials with its environment. Smaller animals that have a large surface area to volume ratio can merely depend on diffusion for the exchange of gases such as oxygen and carbon dioxide, and nutrients with their environment.
Larger animals that have the smaller surface area to volume ratios cannot just rely on diffusion. This is because the exchange of materials between cells that are far away from the surface of the organism with the environment cannot occur through diffusion in these larger organisms. These organisms have specialized circulatory systems that are responsible for carrying around fluids containing materials required by the organism and waste materials that need to be eliminated.
There are three main components of a circulatory system:
- Blood
- Blood vessels
- Heart
There are two primary functions of a circulatory system:
- It transports substances around the body needed by an organism
- It protects against diseases
Human Circulatory System
The human circulatory system is referred to as a double circulatory system. It has two distinct circuits and blood passes through the heart two times:
- Pulmonary circuit: It is between the heart and lungs
- Systemic circuit: It is between the heart and other organs
Pulmonary Circuit
The pulmonary circuit is responsible for carrying blood to the lungs. The blood gets oxygenated there which is then transported back to the heart. Gaseous exchange occurs in the lungs:
- There is a diffusion of carbon dioxide from the blood into the air in the alveoli
- There is a diffusion of oxygen from the air in the alveoli into the blood. It is absorbed by hemoglobin present in the red blood cells
In contrast to other veins and arteries, the pulmonary artery carries deoxygenated blood and the pulmonary vein transports oxygenated blood.
Systemic Circuit
The systemic circuit is responsible for transporting blood around the body. It carries oxygen and nutrients to tissues of the body and carries away deoxygenated blood that contains carbon dioxide and other waster products.
In the next section of the article, we will discuss arteries, veins, and capillaries in detail.
Arteries vs Veins
Arteries are responsible for carrying the blood away from the heart towards an organ, whereas veins transport blood from an organ to the heart.
The arteries and veins connected with the lungs, liver, and kidneys are mentioned below:
- Lung: Pulmonary artery carries blood towards the lung, whereas the pulmonary vein carries blood away from it.
- Liver: Hepatic artery carries blood towards the liver, whereas the hepatic vein carries blood away from it.
- Kidney: Renal artery carries blood towards the kidney, whereas the renal vein carries away the blood from the kidney.
Now, we will discuss the functions and structure of arteries, veins, and capillaries one by one.
Arteries
- The arteries are responsible for carrying blood away from the heart. This blood is always oxygenated except the blood carries by the pulmonary artery which transports blood from the heart to the lungs.
- They have thick muscular walls and contain tiny passageways for blood (internal lumen).
- They have blood under high pressure
Veins
- All veins carry blood to the heart. This blood is always deoxygenated except for the pulmonary vein that carries blood from the lungs to the heart.
- The veins have thin walls and contain larger passageways for blood (internal lumen).
- They have blood under low pressure and contain valves to prevent the backward flow of blood.
Capillaries
- Capillaries are microscopic, i.e. one cell thick, and are present in lungs and muscles.
- They have extremely low blood pressure
- Gaseous exchange occurs in capillaries. It means that the oxygen passes through the capillary wall and into the tissues, whereas the carbon dioxide passes from tissues into the blood.
In the next section of the article, we will discuss the structure of the heart and its various functions in detail.
Heart
The heart is a muscular organ whose primary function is to pump the blood. The right side of the heart pumps blood through the pulmonary circuit, whereas the left side pumps it through the systemic circuit. The right and left sides of the heart are segregated through a septum.
The left side of the heart has thick walls which allow it to put the blood under high pressure as compared to the right side. Remember that the heart is a unidirectional pump. It has valves that prevent the backflow of the flood. The right side of the heart pumps deoxygenated blood to the lungs, whereas the left side pumps oxygenated blood to the body organs.
How blood flows into the heart?
Generally, blood flows into the heart from a vein. Then it enters into an atrium, then a ventricle, and out through an artery. The following valves are present in the heart to prevent the backward flow of blood:
- Tricuspid valve: Present on the right side. As the name implies, it is a valve with three flaps.
- Bicuspid valve: Present on the left side. As the name indicates, it is a valve with two flaps.
Both sides of the heart contain semi-lunar valves. These valves are present at the entrances to the pulmonary artery and aorta.
The points below explain how blood enters the heart and is carried around the body.
- Deoxygenated blood (the blood has low oxygen saturation) enters the right atrium from the vena cava.
- Blood then enters into the right ventricle.
- Blood is then pumped into the pulmonary artery which carries deoxygenated blood to the lungs.
- The blood becomes oxygenated in the lungs which then leaves the organ through a pulmonary vein
- Blood is then passed into the left atrium and then it further enters into the left ventricle.
- After that, the blood is pumped into the aorta, which transports oxygenated blood around the body.
There is a unidirectional flow of blood through the heart which indicates that mammals have a double circulatory system. It means that the blood travels through the heart two times in a single circulation of the body.
In the next section of the article, we will discuss the components of blood, their structure, and their functions.
Components of Blood
Blood has the following four components:
Red blood cells
These cells contain hemoglobin which carries oxygen. They have a biconcave shape that provides a large surface area for the oxygen to diffuse. Red blood cells do not contain a nucleus which gives them more space for hemoglobin.
White blood cells
They are large cells that have a nucleus. Their main function is to defend the body against diseases. There are two types of white blood cells:
- Lymphocytes: They create antibodies
- Phagocytes: They swallow up and digest microorganisms
Platelets
They have extremely small cell fragments and are responsible for converting fibrinogen to fibrin. Fibrin creates a mesh that traps blood. They play a critical role in blood clotting and scab formation.
Plasma
Plasma is a straw-colored liquid that is responsible for transporting blood cells, and digested food molecules such as glucose, urea, carbon dioxide, and hormones.
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