(a) The circulatory system and its components


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As you know, The circulatory system is mainly about “blood”. Then, What is blood??
· Blood is thicker than water and has a little bit salty taste. In an adults body there is 10.6 pints of blood circulating around.
· In their blood there is billions of living blood cells floating in a liquid called plasma. If you took a small sample of this blood and poured it into a test tube and then put it in a machine called a centrifuge, you would be able to see the layers of this blood. This machine spins the blood around so fast that it separates the red blood cells, from the white blood cells, from the platelets.
· The red blood cells sink to the bottom because they are the heavier, more solid parts, but the plasma remains at the top because it is lighter. The plasma is 95% water and the other 5% is made up of dissolved substances including salts.


Main components of blood
- Red Blood cell: It carries the oxygen around the body.
- White blood cell: It defences the bacterial infections.
- Platelets: It circulates around the blood of mammal that brings the formation of blood clot when it gets hurt or bleeding.
- Plasma: It is a liquid that consists of 95 % of water and 5% of slat in the blood.

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As blood begins to circulate, it leaves the heart from the left ventricle and goes into the aorta. The aorta is the largest artery in the body. The blood leaving the aorta is full of oxygen. This is important for the cells in the brain and the body to do their work. The oxygen rich blood travels throughout the body in its system of arteries into the smallest arterioles. On its way back to the heart, the blood travels through a system of veins. As it reaches the lungs, (a waste product) is removed from the blood and replace with fresh oxygen that we have inhaled through the lungs.

Parts & functions of the human transport system

Part
Role /function
1
A medium
A fluid that flows around our body, which is blood.
2
System of tubes
The place where fluids are which are the veins and arteries.
3
Pump
That supply pressure so the blood moves around which is the heart
4
Site of exchange
It allows materials to get send in to it. This is the Capillaries

The circulatory system is comprised of five main parts:
1. The Heart
2. Arteries
3. Arterioles
4. Capillaries
5. Veins
Each of the components has a specific job to do in order for the circulatory system to function properly.

Heart



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The heart is hollow, and the inside of the heart is divided into four separate chambers by thin, muscular walls. If you picture the heart as a simple square, there are two upper chambers and two lower chambers. The top chambers of the heart are collectively called atria and the bottom chambers are called ventricles. The four chambers of the heart are the
· Right Atrium
· Right Ventricle
· Left Atrium
· Left Ventricle

Function:
The heart is the pump which drives circulation by contracting and squeezing blood out into the body.

Special Properties:
The heart is a muscle, and shares many properties of the other muscles in the body. Just like any other muscle, the harder the heart works, the bigger and thicker it will get.

Artery

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Arteries are a type of blood vessel. Blood vessels are the tubes in your body which carry blood from one place to another.

Structure:
Arteries are large, and contain a high percentage of a special type of muscle, called smooth muscle, that can be controlled by hormones and special signals from the nervous system.

Function:
Arteries are the blood vessels that carry blood away from the heart. They are responsible for carrying oxygen and other important nutrients out to the body. In this sense, arteries can be thought of as the supply train of the body, bringing in the raw materials that your organs need in order to do their jobs.


Special Properties:
The thick, strong walls of arteries make them able to resist the high pressures that exist near the heart. All of the major organs in the body have their own special kind of arteries which are uniquely structured to supplying that organ with the supplies it needs. Even the heart has its own special arteries, called the coronary arteries.


Arterioles
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In general, arteries get smaller as they get further away from the heart. When they have decreased in size to a certain point, they are then referred to as arterioles.
Structure:
Arterioles share many of the properties of arteries – they are strong, have a relatively thick wall for their size, and contain a high percentage of smooth muscle.
Function:
Just like arteries, arterioles carry blood away from the heart and out to the tissues of the body. In addition to this "supply train" function, arterioles are very important in blood pressure regulation.
Special Properties:
Arterioles are very important because, as a group, they are the most highly regulated blood vessels in the body, and contribute the most to overall blood pressure. Arterioles respond to a wide variety of chemical and electrical messages and are constantly changing size to speed up or slow down blood flow.


Capillary
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Capillaries are the smallest blood vessels in the body, and are the blood vessels responsible for actually delivering oxygen and other nutrients to the tissues.

Structure:
Capillaries are very small. They are so small that you cannot see them without a microscope. In most cases, the walls of capillaries are only one or two cells thick, and they are so narrow that blood cells have to line up in single file to pass through them.
Depending on where they are in the body, some capillaries actually have microscopic holes. These holes are too small to allow blood to pass through, but large enough to allow passage of other molecules, like proteins, to feed tissues.


Function:
Capillaries are the site where oxygen and other nutrients in the blood are actually delivered to the tissues of the body. Capillaries are so small that these substances actually pass right through them via a process known as diffusion.
While diffusion might seem like a new concept, it is a concept that everyone is already familiar with. If you spray perfume on one side of a room, in few minutes you can smell it on the other side of the room, too. That's diffusion, and it's how cells get food.

Special Properties:
The properties of capillaries are highly varied depending on where they are.
· Kidney capillaries have lots of microscopic holes so that substances may easily pass through them and be excreted

· Liver capillaries look almost like coiled snakes, making many loops back and forth in liver tissue to allow ample time to filter harmful materials from the blood

· Brain capillaries are very tight with almost no holes to protect fragile brain tissue
This variability in structure is because capillaries play many roles besides delivery of oxygen, and these roles are differ from organ to organ.


Vein
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Veins are the blood vessels that carry blood towards the heart.

Structure:
While arteries are thick and muscular, veins are very thin and contain almost no muscle. The walls of veins are so thin that you can see the blood inside of them.

Function:
The main function of veins is to carry blood from the body back to the heart. This blood is low in oxygen, depleted of nutrients, and loaded with waste products. If arteries are the supply train, veins can be thought of as the garbage trucks.



Special Properties:
Veins are very stretchy, and can distend to several times their normal diameter. This ability lets them act as blood warehouses if the flow of blood slows down because of lowered heart rate or contracted arterioles. Because of this Property, they can help lower the impact of increasing blood pressure.