Cardiovascular System

The cardiovascular system, also known as the circulatory system, is responsible for keeping the blood moving around the body. It consists of two primary circuits: the pulmonary circuit and the systemic circuit.

The Pulmonary Circuit

The pulmonary circuit transports deoxygenated blood from the right side of the heart to the lungs. In the lungs the blood picks up oxygen and returns to the left side of the heart. The chambers of the heart that pump blood through to the pulmonary circuit are the right atrium and right ventricle.

The Systemic Circuit

The systemic circuit transports oxygenated blood from the left side of the heart to the rest of the

body. The chambers of the heart that pump blood through to the systemic circuit are the left atrium and left ventricle.

Blood is pumped out of the left ventricle via the aorta, the largest artery in the body. All other arteries branch off the aorta, carrying oxygenated blood to all parts of the body. Deoxygenated blood returns to the heart via the veins of the body. All veins join either the superior or inferior vena cava which return the blood to the right atrium.

The main organs of the cardiovascular

system are the:

The Heart

Blood Vessels

Blood

The main functions of the circulatory system are to:

Transport materials to and from cells e.g. oxygen, carbon dixide and glucose
Distribute heat around the body and assists in temperature regulation
Assist water regulation
Help provide defense of the body

Blood

Blood makes up about 8% of body weight and is made up of two parts, the liquid part (55%) and the solid part (45%).

The liquid part (PLASMA) is a yellow liquid made up of 90% water and 10% dissolved substances. The majority of the dissolved substances consist of proteins that are manufactured in the liver. These proteins function to maintain the volume of plasma inside the cell by osmotically drawing water across the cell membrane. This balances out the loss of water from the capillaries due to the blood pressure forcing it out.

The main functions of plasma are:

To carry materials such as fat, glucose and amino acids around the body
Transport enzymes, hormones and antibodies around the body
Contains fibrinogen which is required for blood clotting
Carry waste products around the body for excretion

The solid part of blood consists of the following components:

Red blood cells
White blood cells
Platelets

Red blood cells

Red blood cells are the second smallest cell in the body after sperm. In an adult, they are formed in the bone marrow of the sternum (breastbone), ribs, spine and pelvis. They only live for four months because they have no nucleus, hence no genetic material to repair themselves.

Structure

Red blood cells are biconcave and have no nucleus.

(This helps increase their surface area, allowing more room for the oxygen)

They contain haemoglobin, which the oxygen attaches to when it

enters the cell

Function

Transports oxygen around the body by binding to haemoglobin.

White blood cells

White blood cells are the cells that help the body fight infection. There are a number of different types and sub-types of white blood cells and each has different roles to play.

There are three main types of white blood cell - Granulocytes, Monocytes, Lymphocytes

Granulocytes are phagocytes. They fight infection by surrounding and digesting any pathogens e.g. bacteria and viruses
Monocytes are the largest of the blood cells. There are two main types. Dendritic cells identify cells that are antigens (foreign bodies) that need to be destroyed by lymphocytes, and Macrophages that are phagocyte cells (fight infection by surrounding and digesting any pathogens)

Lymphocytes are small white blood cells that fight infection by either producing antibodies which destroy bacteria and viruses outside the cell (B cells), or by attacking the body cells that have been invaded by viruses from within (T cells)

Platelets

Structure

Consist of small colourless disk-shaped cell fragments from bone marrow cells
No nucleus

Function

Play an important role in blood clotting

Blood vessels

Blood vessels are hollow tubes that form the transport system, which carries the blood around the body. There are three main types of blood vessel:

Arteries
Veins
Capillaries

The arteries and veins are made up of three layers. The innermost layer consists of epithelial cells and its role is to line the vessel and protect the blood from the rest of the vessel wall. It also secretes factors that regulate blood pressure.

The middle layer is made up of smooth muscle cells, which can contract and relax; changing the diameter of the vessel. This allows the blood to be directed to different parts of the body. If we need more blood in a particular part of the body, the blood vessels will relax; causing them to dilate allowing more blood to reach that area.

This layer also plays a part in regulating blood pressure. If blood pressure is too high the blood vessels will relax; causing them to dilate. This reduces the resistance in the blood vessels and lowers the blood pressure. If blood pressure is too low, the blood vessels contract; causing them to narrow. This increases the resistance in the vessels and raises the blood pressure.

The outer layer is made up of connective tissue. This protects the blood vessel and helps anchor it to the surrounding tissues.

The thickness of these layers varies depending on the type of blood vessel and its function.

Arteries and Arterioles

Structure

Have thick muscular walls to withstand pressure from the pumping heart
Contain no valves
Impermeable (i.e nothing can pass through their walls)
The biggest artery in the body is the aorta. It is the first vessel blood reaches when it leaves the heart and therefore has to cope with the highest forces; hence its size
Other arteries branch off of the aorta, and others off of these. As they continue to branch they get smaller and smaller until they become arterioles the smallest type of arteries

Function

Take blood away from the heart
Mainly carry oxygenated blood (except for the pulmonary artery, which carries deoxygenated blood away from the heart to the lungs)
Arterioles are the smaller branches of arteries that control the amount of blood flowing into the capillaries in specific areas by the degree of contraction of the smooth muscle in the vessel walls
If more blood is required the arterioles will dilate (widen) and if less blood is required the arterioles will narrow (vasoconstriction). This is controlled by the nervous system.
It is the amount of resistance to the flow of blood through the arterioles, that increases the blood pressure in the major arteries

Veins and Venules

Structure

Have thinner walls of smooth muscle with little elasticity as under low pressure
Contain valves to prevent backflow of blood, and ensure it flows in the right direct
Impermeable (i.e nothing can pass through their walls)
The smallest veins are known as venules
The largest vein is the vena cava which drains blood back into the heart from the body

Function

Take blood to the heart
Mainly carry deoxygenated blood (except for the pulmonary vein, which carries oxygenated blood from the lungs back to the heart)
Venules (small veins) carry blood from the capillary beds back to the main veins towards the heart
Blood from the venules drain into the larger veins which all eventually join the inferior or superior vena cava, that returns the blood to the heart
Because blood pressure is very low in the veins, blood needs help to return to the heart. This is achieved by the valves that prevent backflow of the blood, the suction action of inhaling (Boyle's law), contracting muscles (squeeze all the little vessels in the muscle, causing the blood to move forwards) and by vasoconstriction of the vessels (squeezes the blood towards the heart)