The Circulatory System
Cardiovascular System: How Blood Is Pumped Around The Body

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Veins in the body

Circulatory System


What Is The Circulatory System?
What Does The Heart Do?
How Blood Is Pumped Into/Out Of Heart
How Blood Is Pumped Around The Body
Blood Pressure Explained
Diseases Of The Cardiovascular System
Other Systems It Links To

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Human Body Diagram
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Vascular Screening
Heart Tests

What Is The Circulatory System?

Terminology: Circulatory system is also called the cardiovascular system and circulation system.

The circulatory system is a network of veins and arteries that transport blood around the body. It consists of:
- blood
- the heart
- veins and arteries.

How Does Blood Circulate?

Blood is pumped from the heart (a muscular organ) around the body through a transport system of arteries, veins and capillaries. Our body actually has two closed circulatory systems:

Pulmonary circulation transports blood from the heart to the lungs and back again.
Systemic circulation transports blood from the heart to the rest of the body and back. This is the system most people think of when they talk about the circulatory system.

What Is Blood?

Blood is a liquid that circulates the body. It carries oxygen and food to the cells in the body and takes away carbon dioxide and other waste products. It fights infection, keeps you warm and distributes chemicals. See what is blood?, expanded article.


What Is The Heart?

The heart is the centre of the circulatory system (hence the use of the word heart to mean the centre of something in English). If blood is the body's fuel, the heart is its engine.

What Does The Heart Do?

The heart is the pump that drives the whole circulatory system. Its job is to pump blood that is high in oxygen from the lungs to the body and to return blood low in oxygen to the lungs for a refill. This has to be achieved by keeping the low oxygenated blood and high oxygenated blood apart. It they got mixed up, our whole system would fail. This is why the heart is divided into 4 chambers. The heart's action is controlled by the autonomic nervous system.

How the heart works

What Does The Heart Look Like?

It is a hollow red organ, about the size of your fist. It is positioned in the centre of the thorax (breast plate). The heart is composed almost entirely of muscle and is very similar to all the other muscles in the body (like the leg or arm muscles). However, it is specially designed to contract (squeeze) again and again without us having to remember to ‘tell’ it to do so. It has special fibers that act as an electrical system that co-ordinate how often it beats. The heart is divided into four chambers, two on the left and two on the right. Each side has an atrium (plural is atria or auricles) in the upper part, where blood is received and a ventricle in the lower part, where it is pumped out. Atria and ventricles are connected by the atrio-ventricular opening. The septum, a muscular wall, separates the right and left sides of the heart. This prevents deoxygenated blood from the veins on the right coming into contact with oxygenated blood going to the arteries on the left. The heart makes sure that blood flows in the correct direction by the use of one-way heart valves.

The heart has a muscular wall with membranes covering and lining it. The wall is divided into three layers:

Endocardium: the inner layer, is the thin serous membrane, composed of endothelial tissue, that lines the interior of the heart.
Myocardium: the middle layer. This is the thickest layer and it is made of cardiac muscle.
Pericardium: the outer layer, is a double-walled sac that contains the heart and the roots of the great vessels. The inner layer is serous pericardium, while the outer layer is fibrous pericardium, a structure which helps to keep the heart in the right position in the chest.

What Is A Heartbeat?

A heartbeat (also called cardiac cycle), is one contraction (squeezing) of the heart muscle. For a heartbeat to take place, the following pattern of muscular contraction of the heart wall must take place:

• Both atria contract, forcing their contents of blood into the ventricles.
• Atria relax but the ventricles contract, emptying their contents into the arteries.
• Ventricles relax and the heart is at rest.
• The atria fill with blood again.

Every heartbeat has two phases - systole (contraction) and diastole (resting). While resting the heart dilates and fills with blood. The period of rest (diastole) takes the same time as the period of contraction (systole). The heartbeat (or cycle) starts at a point in the right atrium called the pacemaker (sino-atrial node). This consists of specialized neuromuscular tissue which is supplied by the autonomic nervous system. From here, the contraction of the heart muscle spreads through the atria and then down the septum to the walls of the ventricles. In some people the sino-atrial node does not function correctly, leading to an irregular heartbeat. This condition can be dangerous and may require insertion of an artificial pacemaker to correct.

Facts And Statistics

• The heart rate of most adults is 72 to 80 beats per minute whereas most babies have a rate of 130 times a minute.
• In one lifetime, a heart will beat approximately 2,700,000,000 times.
• If you take a heart out of a human body it will continue to beat, even if it is cut into pieces...!
• During a 24-hour period an adult human heart pumps 36,000 liters of blood through 20,000 km of blood vessels.

Heartbeat, Heart Rate and Pulse. What's The Difference?

Your heartbeat is when the heart expands and contracts and blood is pumped through its chambers. The average heart beats about 100,000 times in a day. If you place your hand over your heart you can feel your heartbeat. Your heart rate is how many times your heart beats in a minute. You can measure this by taking your pulse. Your pulse is a measurement tool for checking your heart rate.

Is A Heart Rate Always The Same?

The heart rate changes in both healthy and unhealthy bodies, for a variety of reasons. The following all affect it:
Exercise: increases the rate of the heartbeat (and rest slows it down again).
Age: heart rate is faster in infants and slows gradually as age increases.
Size of the heart: a smaller heart may have a faster heart rate and a larger heart a slower heart rate.
Emotions and excitement: increase the heart rate, first through nervous stimuli and then through an increase in the level of adrenaline.
Temperament: a placid, slow heart rate is not easily varied whereas an excitable person will have a quicker heart rate which changes easily.
Disease: the heart rate is quickened by fever, hemorrhage, and hyperthyroidism and slowed by jaundice, heart blockages and pressure on the brain.


What Is Pulmonary Circulation?

The circulation of blood from the heart to the lungs and back. Deoxygenated blood travels from the heart to the lungs in the pulmonary artery. The blood gets rid of its carbon dioxide (CO2) and replaces it with oxygen (O2). It then returns to the heart via the pulmonary veins (from lungs to heart) ready to be pumped around the body.

Pulmonary and Systemic Circuit

pulmonary and systemic circulation system

How Does This Happen?

The right atrium receives deoxygenated blood from the superior vena cava (the vein from the upper body) and the inferior vena cava (the vein from the lower body). The blood then flows into the right ventricle from where it is pumped into the pulmonary artery which divides into the right and left pulmonary arteries (which go to the right and left lungs). Blood reaches the lungs via tiny vessels called capillaries which are porous to gases (see Respiratory System). The lungs remove the carbon dioxide (CO2) from the blood in the capillaries, replace it with oxygen and return the oxygenated blood to the left atrium of the heart through the four pulmonary veins. The blood is pushed by the contraction of the left atrium through the bicuspid valve into the left ventricle. The left ventricle then contracts and pumps the blood through the aorta, which branches to form the ascending and descending aorta, for distribution around the body.

How Is The Direction Of The Blood Correctly Maintained?

The direction of blood is maintained by valves. The atrio-ventricular openings each have a valve: the tricuspid valve on the right and the bicuspid valve on the left. Both these valves allow blood to flow from the atria into the ventricles, but block the atria when the ventricles contract, ensuring that blood continues to circulate in the correct direction. The semi-lunar valves (three pocket-shaped flaps at the vessel's entrance) in the aorta and the pulmonary artery, ensure that there is no back flow from the aorta to the left ventricle or from the pulmonary artery into the right ventricle.

What Is Coronary Circulation?

coronary circulation

The heart is, of course, a muscle which needs the benefits of circulation like every other muscle and organ in the body. It has its own circulatory system called coronary circulation. Right and left coronary arteries leave the beginning of the aorta and branch into the heart wall to form a network of capillaries to feed the tissue cells. The blood is then collected back into the coronary veins which empty into the right atrium of the heart. Blockages in these arteries and veins can lead to the heart muscle being starved of nutrients and oxygen - symptoms of which include chest pain, angina and ulimately a heart attack.


You now know how blood is pumped into and out of the heart. The following section explains how it travels from the heart around the body.

What Is Systemic Circulation?

Systemic circulation is the circulation of blood from the heart to the body. Blood leaves the heart by the aorta, the largest artery in the body, travels throughout the body and returns to the heart through the inferior and superior vena cava (two of the largest veins). An extensive network of arteries, veins and capillaries transports blood to every cell in the body.

What Do Arteries And Veins Do?

Arteries carry oxygenated blood from the heart and veins carry deoxygenated blood to the heart, except in the pulmonary system.


Structure: arteries are thick-walled, hollow tubes. They all have the same basic construction:
• Fibrous outer covering
• Middle layer of muscle and elastic tissue
• Endothelial layer made of squamous epithelial tissue.
The quantity of muscle and elastic tissue in the middle layer depends on the size of the artery and its distance from the heart because arteries need to expand in order to propel blood along. Small arteries further from the heart have more muscle - to maintain blood pressure and keep the blood moving around the body, and less elastic tissue - because blood has been distributed to organs and so the flow has decreased and there is less stretching force placed on the vessels. The movement of the blood maintains potency (the openness of the vessel). Large arteries branch into small arteries which branch into arterioles which branch into capillaries.
: systemic arteries carry oxygenated blood from the heart to the body. The pulmonary artery carries deoxygenated blood to the lungs.


Structure: arterioles are a smaller version of arteries. They have a similar structure, though the middle layer of the walls is mainly muscle tissue with less elastic tissue than arteries. Under normal conditions all the arterioles are slightly contracted which helps to maintain blood pressure.
Functions: when more oxygen and nutrients are required by an active organ, the arterioles relax and dilate to increase blood supply to it (e.g. muscles during exercise, the stomach and intestines after eating and the skin when the body temperature rises). They contract when an organ is at rest.
General characteristics:
• The hormones adrenaline, noradrenaline and vasopressin (anti diuretic hormone) may cause the arterioles to contract
• In cases of shock, all the arterioles relax and blood pressure is very low. This is a dangerous condition.

Structure: capillaries are the smallest blood vessels. Their walls are one cell thick (i.e. microscopic) and porous, thus allowing the passage of gases (like oxygen and carbon dioxide) and nutrients. A large amount of water, plus the solutions dissolved in it, filters out through the capillary walls and bathes the body tissues. This liquid is called interstitial fluid. It carries food, vitamins, mineral salts and hormones out to the tissues and collects waste products, especially carbon dioxide and urea, from them. Most of the fluid then returns to the capillaries before they join up to become venules.
Function: to distribute essential oxygen and nutrients to most parts of the body. Capillaries supply every part of the body except the deep brain, the hyaline cartilage and the epidermis.

Structure: venules are small veins. These have a thin wall with a large lumen (the passage in the centre in which the blood travels). They are easily collapsed under pressure.
Function: they carry deoxygenated blood from the capillaries to the larger veins.

Structure: veins have three-layered walls and though the basic structure is similar to that of arteries, their walls are much thinner and the lumen is much larger. They vary in size, the largest being the vena cava (from the body into the heart) and the pulmonary vein (from the lungs to the heart). The action of skeletal muscles pushes blood through the vessels. Valves in the endothelial layer of the veins prevent a back flow of blood. Blood pressure in veins is very low so these valves are essential.
Function: systemic veins carry deoxygenated blood back to the heart. Pulmonary veins carry oxygenated blood to the heart.

Difference Between Arteries And Veins

Characteristics of arteries Characteristics of veins
Transport blood from heart Transport blood to the heart
Oxygenated blood (not pulmonary) Deoxygenated blood (not pulmonary)
Lumen (internal width/space) is small Lumen is large
Pumped by heart and muscle tissue Pumped by skeletal muscle only
Thick, muscular and elastic walls Thin walls, not muscular or elastic
Oxygenated blood contains a high concentration of nutrients Deoxygenated blood contains a high concentration of waste products

Main Veins And Arteries Of The Body

Circulations begins at the heart. The inferior and superior vena cava bring deoxygenated blood into the right atrium, the pulmonary veins bring oxygenated blood into the left atrium. The pulmonary arteries take blood to the lungs. The aorta, the main artery in the body, carries oxygenated blood to the body. It branches upwards to form the ascending aorta, which takes blood to the upper body (arms and head) and downwards, to form the descending aorta, taking blood to the rest of the body. Usually the names of veins correspond to the names of the arteries and they generally follow the same course, albeit in a different direction. When the blood reaches the various branches it is distributed through a network of arteries, arterioles and capillaries. The capillaries, the last vessels to distribute oxygenated blood, join the first vessels to collect deoxygenated blood, also called capillaries, which link up to form venules which feed into a network of veins taking the blood back to the heart where it travels to the lungs for reoxygenation.


If you have ever visited the doctor, you will probably have had your blood pressure checked. But what is it and how does it affect circulation?

What Is Blood Pressure?

It is the force that blood exerts on the walls of the blood vessels as it is transmitted from the heart. Without pressure blood would not move at all. Blood is always under pressure but the amount of pressure varies in different types of blood vessels: high blood pressure in the arteries gradually becomes lower in the capillaries and veins. In the large veins approaching the heart there is negative pressure. The heartbeat also affects blood pressure: when the ventricle is contracting it is high, when the ventricle is dilating it is low (see also, what is blood pressure?).

Blood pressure is given as two readings:
• systolic: when the heart is contracting pressure reaches its peak level.
• diastolic: when the heart is relaxing (dilating) pressure reaches its lowest level.

A blood pressure reading of 100/70 means that systolic pressure is 100mmHg and diastolic pressure is 70mmHg. See, blood pressure readings to understand what these numbers mean.

Causes And Effects Of High And Low Blood Pressure

Hypertension (high blood pressure)
Causes: stress, medication, kidney disease, narrowing or hardening of the arteries, smoking, alcohol, diet and hereditary factors.
Effects: angina, heart attack, strokes, kidney complaints.

Hypotension (low blood pressure)
Causes: underactive adrenal glands, hereditary factors; shock may cause short term hypotension.
Effects: dizziness and/or fainting.

Related Questions
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Blood Clotting

How Does Blood Clot?

If a blood vessel (a capillary, vein or artery) is damaged (internally or externally) bleeding occurs until a clot forms. This clot stops excessive loss of blood from the system. If no blood clot forms it is called a hemorrhage. Expanded article: How does blood clot?

Diseases and Disorders Of The Circulatory System

Blood test: How a blood test is performed.

Varicose veins
Deoxygenated blood in the lower body has to move uphill in order to return to the heart. Valves prevent the blood flowing backwards but sometimes these valves, especially those in the superficial veins of the legs, no longer work effectively. Consequently the veins become dilated and blood collects in the veins instead of returning to the heart. The veins become distended and knobbly, showing through the skin.
Varicose veins are often caused by:
• heredity
• excessive periods of sitting and standing
• pregnancy (read about varicose veins in pregnancy)
• obesity.
Related Articles
How are varicose veins treated?

Anemia is a reduction in the blood's ability to carry oxygen, caused either by a decrease in red blood cells, or the hemoglobin they carry, or both. It may be caused by extensive loss of blood, lack of iron in the diet, the failure of bone marrow to produce the normal level of cells or it may be inherited. Read about anemia in pregnancy.

Angina is a heart condition characterized by chest pain due to reduced oxygen to the heart.

A cardiovascular disease characterized by a balloon-like widening of an artery resulting from weakening of the artery wall. If the artery wall bursts (ruptures), it can lead to internal bleeding and death.

A degenerative disease of the arteries, in which the walls of the vessels harden and lose elasticity. The loss of elasticity causes an increase in blood pressure. This condition mainly affects the elderly.

Atherosclerosis (a type of arteriosclerosis) is the build-up of fats, including cholesterol, inside the arteries which causes a narrowing of the artery passage, hardening of the vessel walls and a loss of elasticity.

Blue baby
A baby born with a congenital heart abnormality.

Cardiac failure
Also known as congestive heart failure. It is a condition in which a problem with the structure or function of the heart impairs its ability to supply sufficient blood flow to meet the body's needs. If the heart suddenly stop beating, this is called sudden cardiac arrest (SCA). Only about 5 percent of victims of SCA survive.

Coronary heart disease (CHD)
Coronary heart disease (CHD) is a narrowing of the arteries which supply blood and oxygen to the heart. When blood flow is restricted, the heart gradually becomes damaged and cannot function efficiently. Angina is a typical symptom and if the arteries become too blocked it can lead to a heart attack.

Coronary thrombosis
A blood clot in the coronary artery.

Chest Pain
Chest pain is an umbrella term to describe any pain or discomfort in the chest area. The cause usually falls into two categories: heart related and non-heart related. Read about chest pain in women.

Cardiac arrhythmia
Cardiac arrhythmia is a term for any of a large group of conditions in which there is abnormal electrical activity in the heart. The heart beat may be too fast or too slow, and may be regular or irregular.

DVT (deep vein thrombosis)
Is the formation of a blood clot ("thrombus") in a deep vein. Can be highly dangerous if it travels to the heart or lungs.

A blood clot in the heart or in the blood vessels.

Acquired Immune Deficiency Syndrome (AIDS) is a complex disease that follows infection with the Human Immunodeficiency Virus (HIV). The virus attacks T-lymphocytes, making the immune system incapable of fighting disease. Considered a sexually transmitted disease (STD).

High blood pressure
Also known as hypertension, this is blood pressure which consistently remains above the normal level.

Low blood pressure
Also known as hypotension, this is blood pressure which consistently remains below the normal level.

High cholesterol
High cholesterol is an excessive build-up of a fatty substance called cholesterol, which can cause a reduction in arterial capacity (atherosclerosis) and thus high blood pressure.

The blood's inability to clot. This is an inherited disease which affects mainly men but which can be carried by women.

A collection of blood outside of a blood vessel. Blood seeps into surrounding tissue giving a bruised appearance.

Also known as piles, these are enlarged veins in the rectum or anus which may collapse or contain blood clots. See hemorrhoids in pregnancy and how to treat piles.

Hepatitis ABC
Inflammations of the liver, caused by viruses, toxic substances or immunological abnormalities. Type A is spread by contaminated food. Types B and C are transmitted by infected body fluids including blood. Contagious.

Hole in the heart (septal defects)
Are small holes in the septa between the atria and ventricles. Usually a congenital disorder (person is born with the defect).

Leukemia is a cancer of the blood, caused by over-production of white blood cells.

Inflammation of a vein. Thrombophlebitis is the inflammation of a vein where a blood clot has formed.

Stress can be defined as any factor which affects mental or physical health. When a person is stressed, the heart beats faster, thus pumping blood more quickly. Excessive and unresolved stress can lead to high blood pressure, coronary thrombosis and heart attacks. Read about the dangers of stress.

An abnormally rapid heartbeat (over 100 beats per minute).

An abnormally slow heartbeat.

Epistaxis (nose bleeds)
Is the relatively common occurrence of hemorrhage from the nose, usually noticed when the blood drains out through the nostrils.

Is a complication of cell death characterized by the decay of body tissues, which become black (and/or green) and malodorous (smells like rotting flesh).

Intermittent claudication
Lameness due to pain in leg muscles because the blood supply is inadequate; pain subsides with rest.

Myocardial infarction (Heart Attack)
The destruction of heart tissue resulting from obstruction of the blood supply to the heart muscle. Results in a heart attack. Read about heart attacks in women.

Heart palpitations: an abnormal awareness of the beating of the heart, whether it is too slow, too fast, irregular, or at its normal frequency.

Pulmonary embolism
Is a blockage of the pulmonary artery by foreign matter or by a blood clot.

Raynaud's disease
Raynaud's disease is a vascular disorder that affects blood flow to the extremities (the fingers, toes, nose and ears) when exposed to cold temperatures or in response to psychological stress.

Also known as blood poisoning, this is a generalized disease associated with the circulation and multiplication of toxic bacteria in the blood.

Sickle cell anemia
A congenital (inherited) form of anemia occurring mostly in people with black skin; characterized by abnormal blood cells having a crescent shape.

Silent Heart Attack
Up to 4 million Americans have a heart attack every year and never know it. A silent heart attack is a heart attack that produces no obvious symptoms. These types of heart attacks are not less serious than the regular type (myocardial infarction). They have the same cause and consequence as a regular heart attack and carry the same life expectancy. The only difference is, you just do not know you had it.

A type of anemia that is inherited. It is a blood disorder that causes the body to produce an abnormal form of hemoglobin. There are many forms of thalassemia, some can be fatal before or just after birth.

Varicose ulcers
Ulcers (also called venous ulcers) which appear just above the ankles or on the legs. They are caused by the presence of varicose veins which interfere with normal blood circulation. Painful.

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Interrelationships: Working With Other Systems

The Circulatory system links to:

Respiratory system: carries oxygen to every cell and system of the body (internal respiration); removes waste gas from the body through diffusion between capillary/alveoli (external respiration).

Lymphatic system: linked to the lymphatic system at tissue level - the circulatory system transports some waste products away from the tissues (mainly carbon dioxide) and any additional waste products are carried away by the lymphatic system. The circulatory and lymphatic systems also work together to protect the body (immunity). The lymphatic system empties back into the blood system.

Endocrine system: hormones carried in blood to various target organs.

Digestive system: nutrients broken down in the digestive process are transported by blood from the small intestines to the liver then around the body.

Muscular system: blood transports glucose for energy conversion to the muscles.

Skeletal system: erythrocytes (red blood cells) and leucocytes (white blood cells) are manufactured in the bone marrow of long bones.

Urinary system: blood passes through the kidneys for purification of toxins.

Skin system: circulation transports oxygen and nutrition to skin, hair and nails.


1. Blood is the body's fuel, delivered by the circulatory system: it carries nutrients and oxygen to the body and collects waste and carbon dioxide from it
2. The heart is the circulatory system's engine: it pumps blood around the body
3. Arteries and veins are the circulatory system's pipes: they transport oxygenated blood from the heart (except the pulmonary artery) and deoxygenated blood to the heart (except the pulmonary vein).

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Recommended Health Screenings For Women: Including Pap smear and mammograms.
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Latest Health Statistics: Life expectancy, diseases and other interesting stats.

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