The Circulatory System

The main transport system of human is the circulatory system.

The circulatory system consists of:

  • Blood vessels– a network of tubes
  • The heart– a pump
  • Valves– that ensures the flow of blood is in the right direction.

Functions of the circulatory system:

  • To transport nutrients and oxygen to the cells
  • To remove waste and carbon dioxide from the cells
  • To provide for efficient gas exchange

Oxygenated and deoxygenated blood

  1. The blood from the left side of the heart comes from the lungs

2. The blood capillaries surrounding the alveoli get oxygen that diffuses into the blood

3. This blood now contains oxygen and thus it is called as oxygenated blood

4. The oxygenated blood is transported all around the body

5. The oxygen in the blood is used up by body cells in metabolic reactions

6. Now, the blood that remains is called as deoxygenated blood

7. The deoxygenated blood is returned back to the right hand side of the heart, and is sent to the lungs to get oxygenated once again.

Double circulatory system

The human circulatory system is a double circulatory system.

A double circulatory system is one where blood is transported through the heart twice in one complete cycle.

Beginning at the lungs, blood flows into the left-hand side of the heart, and then out to the rest of the body. It is brought back to the right-side of the heart, before going back to the lungs again.

The importance of a double circulatory system

The pressure applied to pump the blood all over the body is not lost; it is returned to the heart to raise the pressure again.

In a double circulatory system, the oxygenated blood is transported at a faster rate through the body’s organs.

Transporting blood at a faster rate is particularly important as tissues that are metabolically active will require oxygen in abundance. A double circulatory system ensures that the oxygenated blood reaches the tissues on priority.

The Heart

Is made up of the cardiac muscle which contracts and relaxes throughout life

The heart is divided into 4 chambers:
Left Atrium Receives oxygenated blood from the lungs and passes it to the left ventricle
Left Ventricle Receives oxygenated blood from the Left Atrium and pumps it all over the body
Right Atrium Receives deoxygenated blood from the body and passes it to the Right Ventricle
Right Ventricle Receives deoxygenated blood from the right atrium and pumps it over to the lungs to get oxygenated
The Heart also has 4 associated blood vessels:
Vein Function
Pulmonary vein Brings oxygenated blood to the left atrium from the lungs
Aorta Receives oxygenated blood from the left ventricle and pumps it all over the body
Vena cava Brings deoxygenated blood to the right atrium from the body
Pulmonary artery Receives deoxygenated blood from the right ventricle and pumps it over to the lungs to get oxygenated
Important: the reason why the walls of the ventricles are thicker than those of the atria is due to the fact that the atria just receive the blood; the actual task of pumping it out of the heart is done by the ventricles.
Important: the reason why the left ventricle’s walls are thicker than those of the right ventricle is due to the fact that the right ventricle pumps the blood to the lungs, which are in close proximity to the heart. The left ventricle has the job of transporting the blood all over the body.

Other Parts of the Heart

The Pacemaker: is a patch of muscle in the right atrium which controls the rate at which the heart beats according to the needs of the body.

  • If you are exercising, then the body will need a lot of oxygen; you soon take up an oxygen debt which causes a drop in the ph of blood (due to the production of lactic acid)
  • The brain senses the drop in pH and sends electrical impulses to the pacemaker to make the heart beat faster.

Systole: the stage of a heart beat in which the muscles in the walls of the heart chambers contract

Diastole: the stage of a heart beat in which the muscles in the walls of the heart relax

Atrioventricular valves: are valves between the atria and ventricles in the heart that prevent the blood from flowing from the ventricles, into the atria.

  • The valve on the left hand side of the heart is made of 2 parts and thus is called the bicuspid valve
  • The valve on the right hand side of the heart is made of 3 parts and thus is called the tricuspid valve

Coronary Arteries

The muscles of the heart are so thick that the nutrients and oxygen in the blood inside the heart would not be able to diffuse to all the muscles quickly enough.

The heart muscles need a constant supply of oxygen and nutrients so that it can keep transporting and pumping blood. The coronary arteries are responsible for it.

If a coronary artery gets blocked (e.g. by a blood clot), the cardiac muscles run short of oxygen and they cannot respire to obtain energy to contract causing the heart to stops beating. This is called a heart attack or cardiac arrest.

Image depicting Coronary Heart Disease in development

Causes of Coronary Heart Disease

Cause Description
Smoking Nicotine damages the circulatory system by narrowing and stiffening blood vessels
Blood Cholesterol levels Diets rich in animal fats containing Low Density Lipids (LDL) cause CHD to develop
Age As you grow older, the risk of developing CHD increases
Stress Unmanageable and long term stress leads to the development of CHD
High Blood pressure Is caused due to heavy amounts of stress and again leads to the development of CHD
Gender CHD often develops in males than in females. (It may be due to sex-linked genes)

Preventing Coronary Heart Disease

  • Stop smoking
  • Keep the diet based on saturated fatty food in control
  • Have a diet based on fish and vegetable oils
  • Exercise Regularly
  • Take drugs such as ‘statin’ under the guidance of a physician

Treating Coronary Heart Disease

Treatment Function
Statins Helps lower blood pressure
Lower the chances of a blood clot forming
Coronary Bypass Operation A blocked or severely damaged coronary artery is replaced by another length of blood vessel taken from other parts of the body
Angioplasty A balloon is inserted in the damaged coronary artery and is inflated using water
Lower the chances of a blood clot forming
Heart Transplant Operation In the rarest and the worst cases of CHD, a heart transplant operation may be undertaken.
Lower the chances of a blood clot forming

Blood Vessels

Blood vessels are an important part of human transport system.

There are 3 major types of blood vessels in the human transport system:

Blood vessel Function Structure of wall Width of lumen
Arteries Carry blood away from the heart Thick and strong Relatively narrow
Contains muscles and elastic tissues Varies with heart beat due to recoiling and stretching capacity
Capillaries Supply all cells with their requirements Very thin Extremely narrow
Take away their waste products Only one cell thick Wide enough for red blood cells to pass through
Veins Carry blood towards the heart Quite thin Wide
Contain lesser amounts of muscles and elastic tissues than arteries Contains valves
Blood Vessel How structure fits function
Arteries Strength and elasticity needed to withstand the pulsing of the blood as it is pumped through the heart
Capillaries No need for strong walls as most of the blood pressure has been lost.
Thin walls and narrow lumen bring blood into close contact with body tissues
Veins No need for strong walls as most of the blood pressure has been lost.
Thin walls and narrow lumen bring blood into close contact with body tissues
Thin walls and narrow lumen bring blood into close contact with body tissues

Components of blood plasma

Component Source Destination
Water Absorbed from small intestine and colon All cells
Fibrinogen Liver Remains in the blood
Antibodies Lymphocytes Remains in the blood
Lipids Absorbed in the ileum To the liver- for breakdown
Derived from fat reserves in the body To adipose tissue- for storage
To respiring cells- as an energy source
Carbohydrates Absorbed in the ileum To all cells for energy release by respiration
Derived by breakdown of glycogen in the liver
Urea Liver- by deamination Kidneys- for excretion
Mineral ions Absorbed in the ileum and colon To all cells
Hormones Endocrine glands Target hormones
Carbon dioxide Released by all cells as a waste product of respiration To the lungs for excretion
Oxygen Lungs Whole body
Heat Abdomen and muscles Whole body

Blood cells – structure and functions

There are 3 types of blood cells:

Blood Cell Function
Red blood cells (RBC) Transport oxygen
White blood cells (WBC) Protect the body against disease
Platelets help the blood to clot

Red Blood Cells (RBC)

Image of Red Blood Cells
  • Made in the bone marrow
  • Transport oxygen from lungs to all respiring tissues.
  • Transport CO2 from all respiring cells to lungs.
  • Contain a red pigment- Haemoglobin which contains iron
  • Haemoglobin carries oxygen by combining it with iron, to cells that are actively respiring
  • Are biconcave disc shaped (this increases the surface area and thus diffusion of oxygen and carbon dioxide)
  • Have no nucleus (hence live up to only 4 months)
  • Broken down in the liver, spleen and bone marrow
  • Some of the iron from the Haemoglobin is stored, and used for making new haemoglobin; some of it is turned into bile pigment and excreted.

White Blood Cells (WBC)

  • White blood cells are made in the bone marrow and in the lymph nodes.
  • Have a nucleus, often large and lobed.
  • Can move around and squeeze out through the walls of blood capillaries.
  • They have the function of fighting pathogens
Image of Lymphocytes

White blood cells are of two major types:

Phagocytes

  • Have lobed nuclei and granular cytoplasm.
  • Can move out of capillaries, to the site of an infection.
  • Remove any microorganisms that invade the body and might cause infection by engulfing and digesting

Lymphocytes

  • produce antibodies to fight antigens
  • Have large nuclei

There are two different types of lymphocytes:

  • B-lymphocytes: secrete antibodies in response to contact with their particular antigen, which may be an invading pathogen or a foreign tissue that has been transplanted.
  • T-lymphocytes attack foreign or infected cells and kill them by binding onto their surfaces.

Platelets

  • Small fragments of cells, with no nucleus.
  • Made in the bone marrow.
  • Involved in blood clotting: form blood clot, which stop blood loss and the entrance of pathogens.

Substances transported in the blood

Substance Source Destination
Oxygen Lungs Whole body
Carbon dioxide Whole body Lungs
Urea Liver Kidneys
Hormones Endocrine glands Target organs
Digested food Intestine Whole body
Heat Muscles and abdomen Whole body

Blood clotting

Till now, we have learnt that platelets help in the clotting of blood. Let’s see how this happens now!

  1. There is cut in the skin
  2. Blood vessels are damaged
  3. Damaged blood vessels and tissues begin secreting chemicals
  4. This activates blood clotting factors
  5. The soluble plasma protein- fibrinogen changes to an insoluble substance called fibrin.
  6. Fibrin causes fibres to be made in the damaged blood vessel and tissue
  7. Red blood cells and platelets get trapped in the fibres
  8. This forms a blood clot!

Importance of Blood Clotting

Prevent excessive blood loss

  • Maintain the blood pressure.
  • Prevent the entry of pathogens
  • Help in healing

Genetic disease where blood does not clot– Haemophilia

The lymphatic system and tissue fluid

Capillaries leak(!). Their cell walls don’t fit together properly and thus there are small gaps between them.

Substances that leak out from the capillaries:

  • White Blood Cells (WBCs)- can easily change their shape unlike red blood cells.
  • Blood Plasma

So the substances that leak out from the blood capillaries are known as tissue fluid.

The tissue fluid simply surrounds the body cells.

Importance and functions of tissue fluid

  • Supply cells with all their requirements (such as oxygen and nutrients that diffuse)
  • Take away the waste products of metabolism out from the cells
  • Immediate environment of every cell in the human body

Lymph

  • The tissue fluid surrounding the body cells ought to be eventually returned to the blood.
  • To make sure this happens, there are another set of capillaries in our body called as lymphatic capillaries.
  • The tissue fluid slowly drains into the lymphatic capillaries.
  • It is now called lymph
  • The lymphatic capillaries eventually join up to form larger lymphatic vessels which empty themselves into the subclavian veins.
  • Here the lymph enters the blood.

Features of the Lymphatic system and lymph nodes

  • Have valves to ensure the flow of lymph is in one direction
  • Run close to the muscles so that muscular contractions squeeze the lymph and for it to move along the vessels.
  • Have structures called lymph nodes where new white blood cells are produced
  • The white blood cells help in destroying most toxins in the lymph before entering the blood from the subclavian vein.

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