Biology - 5090

Transport in Humans


Transport in Humans
The circulatory system is a system of blood vessels with a pump and a valve to ensure a one-way flow of blood. It transports all the nutrients e.g. amino acids, glucose, oxygen, hormones, etc. It is composed of a pump, a network of closed tubes, and a medium. Pump refers to the heart which circulates the blood through vessels, gives it pressure and maintains flow. The tubes refer to the blood vessels which transport blood throughout the body, while the medium is the blood or lymph. There are two types of circulatory systems:

Single Circulation of Fish

Blood passes through the heart only once in a single circulation. The heart will receive the deoxygenated blood and pump it to the gills. The gaseous exchange takes place in the gills, making the blood oxygenated. The gills will supply oxygenated blood to the whole body. 

Double Circulation of Mammals/Birds

Blood passes through the heart twice in a double circulation. Deoxygenated blood from the cells goes to the heart, which passes it to the lungs. The lungs oxygenate the blood through the gaseous exchange. The oxygenated blood then goes back to the heart which pumps it to all the organs and tissues in the body.

Double circulation provides greater blood flow, and oxygen delivery is more efficient because the heart pumps the blood. This circulation also reduces the pressure in the lungs, and it ensures that the oxygenated blood and deoxygenated blood do not mix.


The heart is present in the chest cavity towards the left side. It is made of cardiac muscle that contracts and relaxes.

The heart has four chambers. The two chambers located at the top are called atria. On the other hand, the lower two chambers are called ventricles.

The chambers on the right-hand side are completely separated from those on the left-hand side by a wall, known as the septum. It ensures that the oxygenated blood does not mix with the deoxygenated blood and vice versa.                                                                             

Figure (i) Heart, Credit: Wikipedia
A valve ensures the one-way flow of the blood. The valve between the right atrium and the right ventricle has three flaps, thus known as the tricuspid valve. This valve opens to allow blood to be pumped from the right atrium to the right ventricle. Once the blood has passed through, the valve closes so blood cannot go back. The valve joining the left atrium to the left ventricle is bicuspid valve. It has two flaps. 

Veins are blood vessels that come towards the heart. They always carry oxygenated blood. Arteries are blood vessels that carry oxygenated blood, going away from the heart. Capillaries are responsible for the exchange of gases i.e. oxygen and carbon dioxide.

The vein that carries deoxygenated blood from the lower parts of the body going towards your heart is known as inferior vena cava. They open into the right atrium. On the other hand, the vein that carries deoxygenated blood from the upper parts of the body going towards your heart is known as superior vena cava. It also opens in the right atrium.

The deoxygenated blood is in the right atrium. The tricuspid valve opens and lets the blood flow into the right ventricle. The pulmonary artery is responsible for carrying deoxygenated blood. In the lungs, the mixing of oxygen takes place. The blood gets oxygenated there and then goes to the heart. The pulmonary vein then carry the oxygenated blood. All the other veins in the body carry deoxygenated blood. Now the oxygenated blood is in the left atrium. The left atrium gets blood at low pressure from the veins. Through the bicuspid valve, it flows to the left ventricle. The left ventricle pumps blood at high pressure out to the arteries. The ventricles then pump it out of the heart by contracting the muscle in their walls. The cardiac muscle contracts with a lot of force, squeezing inwards on the blood inside the heart and pushing it out. The biggest vessel comes, known as Aorta, comes out of the heart. It takes blood around the body. The blood pressure is at its highest in the aorta, and the strongest pulse is felt here. When aorta comes out of the heart, it makes two branches. One goes above the heart into the upper body, while the other goes below the heart to the lower body. Those two branches give smaller branches and supply blood to each body cell. 

The atrioventricular valves (Tricuspid and Bicuspid) separate atria from the ventricles. The semilunar valves are one-way valves that separate ventricles from major arteries. 

Importance of Septum

It is a muscular wall that separates the left and the right side of the heart. It prevents blood from flowing from the right to the left ventricle during contraction, and vice versa. It also helps in the prevention of mixing of deoxygenated and oxygenated blood. Other than those, it maintains the shape and rigidity of the heart, providing strength to the walls of the heart.

Relative Thickness of the Right and Left Ventricles

The right ventricles of the heart have thinner muscle walls compared to the left ventricle. The right ventricle needs less pressure because they send blood to the lungs only, and the distance between the heart and the lungs is short. The left ventricle needs more pressure as it pumps blood all around the body.

Relative Thickness of Atria and Ventricles

The atrial walls are not as thick as the ventricular walls. This is because the atria simply receive blood from the lungs and supply it to the ventricles while the ventricles pump blood out of the heart and all around the body. They have thicker and more muscular walls to help them accomplish this.

Pulmonary circulation is the part of the blood circulation which carries deoxygenated blood away from the heart to the lungs and returns to the pulmonary vein. The oxygenated blood goes from the left ventricle to the aorta.

Systemic circulation is the part of the blood circulation that receives deoxygenated blood from the body tissues, and it provides the oxygenated blood to the body tissues.


These are vessels carrying blood away from the heart to the tissues. They have a thick, muscular wall, thus a short lumen to produce higher pressure for the delivery of blood to all parts of the body. Elastic fibers are present in the arteries to stretch and recoil them to withstand high pressure.


These are vessels carrying blood from the tissues towards the heart. They have a thin, muscular wall thus a wider lumen because it needs less resistance to blood flow. The thin walls provide low pressure to fill the blood. Only a few elastic fibers are present in veins due to low blood pressure. They also have semilunar valves to prevent the backflow of deoxygenated blood. 


These are vessels that carry blood from the arteries to the veins. They are one cell thick, made of endothelium cells. Capillaries are responsible for the exchange of gases, nutrients, and waste materials. They have a large surface area for the exchange of substances.

Coronary Heart Disease

Aorta gives branches to all the cells of the body. These branches are known as coronary arteries. They supply oxygen, glucose and amino acids to cells of the body. If there is some blockage in coronary arteries due to a blood clot, the heart muscles will not receive oxygen and nutrients. This can result in a minor or major heart attack. 

As you get older, your blood gets thicker, increasing the formation of a blood clot in the coronary arteries. You also have higher chances of getting heart disease if it runs in your family. Thus, you should eat a balanced diet and exercise regularly. 

There are four methods for its treatment, as stated below:

Aspirin (medicine): Aspirin helps in thinning the blood which prevents the formation of clots. 

Stents (heart surgery): This requires the insertion of a metallic ring in a few coronary arteries to make them wider. It will provide a wider passage for blood flow. 

Angioplasty (heart surgery): A small balloon is inserted into the coronary arteries. It will then be inflated to stretch the walls of the artery causing the fats to blow. This will provide easy blood flow. 

By-pass (heart surgery): An extra vein from the leg is inserted with aorta so it gives blood to the vein which supplies oxygen to the heart. 


Red blood cells

They transport oxygen to body tissues as they have a special protein, known as hemoglobin. Hemoglobin is an iron-containing pigment that picks up oxygen in the lungs and lets go of it at the tissues. Red blood cells are biconcave shaped. They have a dent in the middle, so they can squeeze through Capillaries. The nucleus is absent to increase the surface area for the transport of oxygen. 

White blood cells 

WBCs are nucleated cells of two types. Phagocytes have a lobed nucleus, and they are responsible for the cell-eating action known as phagocytosis. Phagocytes identify the foreign antigen and engulf it by making an invagination in the cell membrane. When the bacteria go inside the cell, the white blood cells have their digestive enzymes that digest the bacteria and destroy it. On the other hand, lymphocytes do not have a lobed nucleus. Their function is to produce antibodies to kill the pathogens. The antibodies have a shape complementary to that of the antigen, aiding them in their destruction. 


It is a fluid that carries all the blood components. It comprises of blood cells, soluble nutrients, hormones, glucose, carbon dioxide, and mineral ions. 


Platelets are remnants of cytoplasm. These are involved in the blood clotting process. 

Blood clotting

Blood comes out immediately after a blood vessel is cut. This stimulates the platelets and a protein, known as fibrinogen. Fibrinogen is an inactive, soluble protein that is converted into fibrin, which is insoluble, thread-like structures. Fibrin, along with blood cells make a mesh, forming a scab. This seals the wound, known as a clot.

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