http://www.bupa.co.uk/individuals/health-information/directory/c/the-cardiovascular-system#textBlock262235

cardiovascular system is a network made up of blood vessels and your heart, which is responsible for pumping blood and oxygen around your body. It also transports carbon dioxide, a waste product, from your body to your lungs – breathing out removes carbon dioxide from your body.
When you breathe in air through your mouth and nose it travels to your lungs. Oxygen from the air is absorbed into your bloodstream through your lungs. Your heart then pumps oxygen-rich (oxygenated) blood through a network of blood vessels (arteries) to tissues including your organs, muscles and nerves, all around your body.When blood reaches the capillaries in your tissues it releases oxygen, which cells use to function. Cells release waste products, such as carbon dioxide and water, which your blood absorbs and carries away. The used (deoxygenated) blood then travels through your veins and back towards your heart. Your heart pumps the deoxygenated blood back to your lungs, where it absorbs fresh oxygen, releases the carbon dioxide and the cycle starts again.
Heart is a muscular organ, it lies just to the left side of the centre of your chest (thorax) and is surrounded by a protective membrane called the pericardium.

Your heart is a pump, divided into left and right sides. It has walls, made of muscle, which squeeze (contract) to pump blood into your blood vessels and around your body. You have around 8 pints of blood in your body, and in an average day your heart beats about 100,000 times to keep your blood circulating.
Your veins deliver deoxygenated blood from your body to the right side of your heart. Your heart pumps this blood back to your lungs, where it absorbs more oxygen. This oxygenated blood then returns to the left side of your heart, which pumps it out to the rest of your body through your arteries. The muscle on the left side of your heart is slightly larger because it has more work to do than the right – the right side only pumps blood to your lungs, the left side pumps blood around your whole body.

Each side of your heart is divided into an upper chamber called an atrium and a larger, lower chamber, called a ventricle. Blood flows from each atrium to the ventricle below, through a one-way valve.
When your blood pressure is measured, the result is expressed as two numbers or levels. Your blood pressure is measured in mmHg, or millimetres of mercury. A blood pressure reading shows one number on top of another, such as 120/80mmHg (one hundred and twenty over eighty millimetres of mercury).

The first figure is called the systolic blood pressure. This is a measure of the pressure when your heart muscle is contracted and pumping blood out of your heart, and is the maximum pressure in your blood vessels. The second figure is called the diastolic blood pressure. This is the pressure between heart beats when your heart is resting and filling with blood, and is the minimum pressure in your blood vessels.

The function of the heart is to pump blood around the body. The heart is a hollow, muscular organ divided by a vertical wall called the septum. These two chambers are further divided into the thin walled atrium above, and a thick walled ventricle below, making four chambers. Between each pair of chambers are valves preventing any back flow of blood. Blood vessels leaving the heart generally carry oxygenated blood through vessels known as arteries. These are large, hollow elastic tubes with thick muscular walls that are designed to withstand the high pressure with the blood leaving the heart. Their size gradually diminishes as they spread throughout the body, ultimately reaching fine, hair-like vessels known as capillaries. Blood vessels that return blood to the heart are known as veins which generally carry de-oxygenated blood to the heart. They are elastic tubes containing valves to help prevent back flow of blood. Blood is forced