Transport in Humans

Circulatory System

Circulatory system is an organ system which consists of following organs:

Heart
Lungs
Arteries
Veins

Reason why arteries and veins are considered as organs where capillaries are considered as tissues

Arteries and veins are made up of different types of tissues such as muscle tissue, elastic fibre, collagen fibre etc. That is why they are considered as organs.
Capillaries are made up of only one type of cell, that is why they are considered as tissues.

Heart: The heart is a muscular organ that pumps blood throughout the body. The heart is made up of cardiac muscle.

The heart has 4 chambers: The upper two chambers are known as the right and left atria and the lower two chambers are known as the right and left ventricles.
The right side and left side of the heart are separated by a muscle known septum.
The right side of the heart always deals with deoxygenated blood.
The left side of the heart always deals with oxygenated blood.
The muscles of the heart are supplied with oxygen and nutrients such as glucose by coronary arteries which are branches of the aorta.

Arteries and veins are made up of different types of tissues such as muscle tissue, elastic fibre, collagen fibre etc. That is why they are considered as organs.

Capillaries are made up of only one type of cell, that is why they are considered as tissues.

Valves within the heart: -

Heart contains 2 sets of values:

1. Atrio-ventricular valves:

Situated between atria and ventricles.
Right atrio-ventricular valve is tricuspid valve.
Left atrio-ventricular valve is bicuspid valve

2. Semi lunar valves:

Situated at base of pulmonary artery and at the base of aorta.
Semi lunar valves are also found in the veins.

Function:

To prevent backflow of blood and to ensure one-way flow of blood.

Relative chamber Thickness of Heart:

The wall of both atria is thin because they only receive blood and pump it to the ventricles just beneath them. That is why they do not need to create much pressure.
The wall of the right ventricle is thicker than the wall of the atria but is thinner than the wall of the left ventricle because the right ventricle needs to pump the blood to a relatively shorter distance toward the lungs. If the wall of the right ventricle were thicker, it would generate more pressure which would eventually burst out pulmonary capillaries.
The wall of the left ventricle is the thickest of all the chambers because it pumps the blood throughout the whole body which is why it needs to generate more pressure.

Cardiac Events:

Right atrium receives blood from the superior and inferior vena cava, left atrium receives blood from the pulmonary vein. After both atria are filled up with blood, they start to contract, pumping the blood towards the ventricles, this contraction of the atria is called atrial systole. Atrio ventricular valves remain open during atrial systole.
0.1 second after atrial systole, when both ventricles are filled with blood, both of them start to contract. The right ventricle pumps blood through the pulmonary artery and the left ventricle pumps blood through the aorta. The contraction of ventricles is known as ventricular systole. Atrio-ventricular valves remain close and semilunar values remain open during the ventricular system.
Just after ventricular systole all the chambers of the heart relax. This is known as diastole. During diastole, semilunar valves remain closed.

The Double Circulatory System

The double circulatory system is composed of two circuits:

From heart to full body under high pressure and then full body to the heart under relatively low pressure.
Pulmonary circulation: In pulmonary circulation blood flows from the heart to the lungs under low pressure and then back to the heart.

The human circulatory system is known as closed circulatory system as blood flows through closed tubes or blood vessels such as arteries and veins.
It is known as double circulatory system as blood flow through the heart twice during each single beat.

Blood and blood Vessels

Blood and its constituents

Blood: Blood is an aqueous suspension of connective tissue. Blood is the only liquid tissue in our body. Blood has a liquid portion known as plasma and a solid portion known as blood cells.

Functions of plasma in transport:

Soluble proteins, such as fibrinogen, prothrombin and antibodies. Fibrinogen and prothrombin (Plasma protein) play an important part in blood clotting. These proteins are made in liver.
Dissolved mineral salts such as hydrogen carbonates, chlorides, sulphates and phosphates of calcium, sodium and calcium.
Food substances such as glucose, amino acids and vitamins.
Excretory products such as urea, uric acid and creatine. It also carries CO2 in form hydrogen carbonate ions and also as dissolved gass.
Carries hormones such as insulin etc.
Carries heat that is generated by metabolic reaction throughout the body to keep all body parts warm.

Blood cells: Blood cells are formed within the bone marrow stem cells of long bones.

There are 3 types of blood cells:

Red blood cell
White blood cell
Platelets

Red blood cells: Function of red blood cell is to transport oxygen and carbon dioxide throughout the body.

They are adopted to carry its function by the following ways:

Red blood cells have a biconcave disc shape which provides them with a large surface area for faster diffusion of gases.
Red blood cells contain the red pigment called hemoglobin which can combine with oxygen and carbon dioxide to transport them.
Red blood cells contain no nucleus or other organelles, so that there is more space for oxygen and carbon dioxide to be transported.
Red blood cells have thin membrane through which diffusion occur faster.
Red blood cells are very tiny and flexible, so can easily squeeze through capillaries.

White blood cells: White blood cells are the cells that form our immune system.

Some properties include:

White blood cells are irregular in shape.
White blood cells contain large nucleus.
White blood cells don’t contain any pigment such as hemoglobin.

White blood cells can also be divided into two types:

Phagocytes: The function of phagocytes is to engulf and digest pathogen by the process known as phagocytosis.
Lymphocytes: The function of lymphocytes is to produce specific antibodies to destroy specific pathogen.

Blood clotting mechanism

When blood vessels are damaged, damaged blood vessels are platelets secrete an enzyme thrombokinase.
Thrombokinase converts plasma protein prothrombin into thrombin Ca2+ is required for this step.
Thrombin also acts as an enzyme and converts soluble fibrinogen into insoluble fibrin threads which forms a meshwork and traps the blood and forms scab which prevent entry of pathogen into blood from outside.

Blood vessels

Artery:

Arteries are blood vessels that carry blood away from the heart.
Have thick tough walls with more muscles and elastic fibers to withstand the high pressure the blood.
Have a narrow lumen which can maintain the blood pressure and regulate blood flow.
Don’t contain any valves.

Vein:

Vein is the blood vessel that carry blood towards the heart, under low pressure.
Veins have thin walls with less muscles.
As blood flows under low pressure, within the veins, some back flow can occur.
To prevent back flow of blood in veins, they contain valves (semilunar valves).
Veins have wider lumen which allow more volume of blood to pass through from one set of valves to the next.

Capillaries:

Tiny blood vessels which connect artery with the vein.
Have very thin walls which are only one cell thick, allowing quicker diffusion of substances between capillaries and surrounding tissues.
Have very narrow lumen which allow blood to pass through slowly around the tissues so that nutrients have enough time to diffuse into the tissues.
Walls are porous, so plasma can leak from the capillaries in the tissues forming tissue fluid that acts as exchange medium of substances.

The immune system

Production of antibody

All cells have proteins on their surface by which white blood cells recognizes the antigens.
Lymphocytes recognize foreign antigens from foreign cells (such as bacteria) and make antibodies to destroy them.
A different antibody is produced for each antigen.
Antibodies make bacteria clump together in preparation for action by phagocytes or neutralizes the toxins produced by bacteria. Antibodies can also destroy the pathogen activity

Phagocytosis

Phagocytes have the ability to move out of capillaries to the site of infection.
Phagocytes then engulf (ingest) the infection pathogen and kill them by digesting them in a process called phagocytosis.

Tissue rejection

Transplants involve replacing a damaged organ with a doner organ.
Lymphocytes detect the foreign antigens of the donor organ.
The donated organ is rejected as antibodies “fight” the foreign tissue.

To prevent such phenomenon:

The donated organ needs to be a similar tissue type to patient e.g., from a close relative.
Immuno suppressive drugs are used, which switch off body’s immune response, However the drawback of immuno, suppressive drug is that the patient needs to be kept at isolation as they are at the risk of dying from any disease they are exposed to.

Coronary heart disease

Coronary heart diseases occur when coronary arteries get blocked due to deposition of cholesterol.

Common causes include:

Diet with saturated fat
Smoking
Lack of exercising
Stress
Inherited condition

How nicotine can play a role in causing coronary heart disease:

Nicotine stimulates the release of Adrenaline which increases the heart rate and blood pressure.
Nicotine constricts the arterioles, increasing blood pressure.
Blood flows under very high pressure through the artery, which increases the friction between blood and inner layer of the artery (endothelium).
Due to this increased friction, the inner wall of artery gets damaged, which results in deposition of cholesterol in the coronary artery.
The coronary arteries become narrow and less blood flows through it.
If the coronary arteries become completely blocked, heart muscles will not receive any nutrients and oxygen.
Heart becomes fatigued and cardiac arrest might occur.
Nicotine makes the blood platelets sticky, thus increasing the risk of blood clotting within the arteries, thus creating a block.

Tissue Fluid

At the arteriole end of capillaries, as there is high pressure with the the walls of the capillaries being porous, plasma tends to leak out from capillaries into tissues forming tissue fluid.

Dissolved food substances and oxygen diffuse from the blood in the capillaries into the tissue fluid and then into the cells.
Metabolic waste products diffuse from the cells into the tissue fluid and then through the blood capillary walls into the blood to be excreted.

Composition:

Tissue fluid is virtually plasma except for large plasma protein and red blood cell. White blood cell along with small proteins such as antibodies can be found in tissue fluid.