Circulatory System

The main functions of this system are to supply oxygenated blood throughout the body, and to remove waste products, such as carbon dioxide. It is able to carry out this task by using three organs; the blood, vessels and the heart.


If we think of the circulatory system as a transportation service, then the blood would be the bus. Carrying and distributing oxygen, nutrients, antibodies, heat and hormones, it travels through the body, whilst also collecting waste products, such as carbon dioxide, which need to be removed. Its main functions are therefore protection, heat regulation, clotting and transportation. The blood is made up of 4 components, plasma, erythrocytes, leucocytes and thrombocytes, and an adult has 10.6 pints. It is one of the major types of connective tissue.

Plasma is a straw coloured fluid and accounts for about half of the total volume of blood. It is necessary for the suspension of blood cells and is made up of 90% water. The major protein in plasma is albumin which prevents fluid from leaking out of the blood vessels into tissues. Plasma also supplies water when additional liquids are needed in the tissues of the body, as well as play a crucial role in regulating the body temperature by carrying heat around the body. The Plasma contains dissolved substances, most of these are useful and are carried to places where they are to be stored or used. The products of digestion including glucose, amino acids, mineral salts and vitamins are carried from the small intestines (ileum) to other organs. Without plasma, the life-giving blood cells would be left without transportation.

Red blood cells (erythrocytes) carry oxygen, which is needed by the cells to produce energy, and are formed in the bone marrow of long bones. They are the most common type of blood cell and live for around 12o days and make up around 40% of the bloods volume. These blood cells contain protein chemical called haemoglobin which is bright red in colour. Haemoglobin allows the oxygen to be collected in the lungs by binding its molecules with the oxygen and then distributes it around the body. Carbon dioxide is then collected to allow it to be removed. If you have a lack of haemoglobin, you may develop a condition called anaemia.

White blood cells (leucocytes) are involved in the protection of the body and are on the continual look out for any sign of bacteria. There are five main types of white blood cells which all have a differing role. The white blood cells that are most numerous are Neutrophils which kill and ingest foreign material. Lymphocytes help protect against viral infections and produce antibodies. Monocytes ingest dead and damaged cells, Eosinophils protect by killing parasites and destroying some cancer cells as well as being involved in the allergic response, as well as the basophils.

White blood cells have a shorter life expectancy than red, only surviving for about 3 weeks. A drop of blood can contain anywhere from 7,000 to 25,000 white blood cells at a time. If an invading infection fights back and persists, that number will significantly increase.

Platelets, also called thrombocytes are necessary for the blood clotting process to take place. They are irregularly-shaped and colourless and have a sticky surface that lets them form clots to stop bleeding. When you cut yourself, platelets in the blood react to the air and calcium, vitamin K, and a protein called fibrinogen are released. This forms a blood clot, which seals or plug’s the hole and later on becomes a scab. A scab is an external blood clot that we can easily see, but there are also internal blood clots. A bruise, or black-and-blue mark, is the result of a blood clot. Clotting is necessary, but sometimes it can be extremely dangerous as if a blood clot forms inside of a blood vessel, it can block the flow of blood, cutting off the supply of oxygen.

Blood Vessels

If the blood acts as a bus, then the blood vessels are road networks that it travels along. There are three main vessels, and the blood follows two pathways known as pulmonary and systemic.

Arteries always carry blood away from the heart, apart from the pulmonary artery (we will look at that later). They are the biggest of the vessels and carry oxygenated blood. The walls of the artery are muscular and elastic which helps allow the blood to travel the body. The largest artery of the body is the aorta which originates from the heart and branches out into smaller arteries. The smallest arteries are called arterioles which branch into capillaries. An artery has three layers. An outer layer of tissue a muscular middle and an inner layer of epithelial cells. There are two types of arteries. Pulmonary arteries carry blood from the heart to the lungs and systemic arteries carry blood to the rest of the body. The smallest arteries are called arterioles and deal with delivering blood from the arteries to the capillaries. Sometimes, pulmonary circulation is referred to. This means blood is circulated from the heart to the lungs and back to the heart. Arteries are found deep in the tissues to prevent damage.

Veins carry deoxygenated blood to the heart, under low pressure, for it to get sent to the lungs. Veins contain valves, which act like doors – preventing the blood from flowing in the wrong direction. The largest vein is the vena cava which leads to the right atrium of the heart. Veins also have three layers: an outer layer of tissue, muscle in the middle, and a smooth inner layer of epithelial cells, but the layers are thinner and contain less tissue. Because it lacks oxygen, the blood that flows through the veins has a deep red colour. The walls of the veins are rather thin which makes the blood visible through the skin on some parts of the body, such as the hands, wrists and ankles. As the skin refracts light, the deep red colour actually appears a little blue from outside the skin. Veins can be classified into four different types. Pulmonary veins carry blood from the lungs to the left atrium of the heart. Systemic veins carry deoxygenated blood from the remainder of the body to the right atrium of the heart. Superficial veins are to be found close to the surface of the skin and deep veins are located deep within muscle tissues.

Capillaries are small vessels that transport blood from the arteries to the veins. They have thin walls, made up of endothelium (single layer of overlapping flat cells) that allows substances such as nutrients to exchange. The capillaries are so small that red blood cells must travel through them in single file. The flow of blood through the capillaries is controlled by structures called precapillary sphincters, which are located between arterioles and capillaries. They contain muscle fibres that allow them to contract. Blood flows freely to the capillary beds of body tissue when the sphincters are open, but when the sphincters are closed blood is not allowed to flow. Plasma moves out of the capillaries and becomes tissue fluid. This fluid bathes the cells in nutrients and oxygen, some waste and excess fluids move into the lymphatic vessels, with the carbon dioxide and waste returning to the capillaries.

Diagram of a vein and an artery

Arteries of The Neck

The Heart

The heart is a muscular organ that is primarily a shell containing 4 chambers, which are the right and left atrium and the right and left ventricle. Its main function is to act as a pump and maintain a constant circulation of blood around the body.

The Right Atrium

This chamber receives de-oxygenated blood from the body through the superior vena cava (head and upper body) and inferior vena cava (legs and lower torso). An impulse is sent via the sinoatrial node, which causes the cardiac muscle tissue of the atrium to contract, allowing the tricuspid valve, which separates the right atrium from the right ventricle to open. This allows the de-oxygenated blood which has collected in the right atrium to flow into the right ventricle.

The Right Ventricle

This chamber receives de-oxygenated blood from the atrium as it contracts. The pulmonary valve leading into the pulmonary artery is closed which allows the ventricle to fill with blood, then to contract. As this contraction occurs, the tricuspid valve closes, and the pulmonary valve opens. The closure of the tricuspid valve prevents blood from backing into the right atrium and the opening of the pulmonary valve allows the blood to flow into the pulmonary artery toward the lungs.

The Left Atrium

This chamber receives the newly oxygenated blood from the lungs through the pulmonary vein. A contraction triggered by the sinoatrial node progresses through the atrium and the blood passes through the mitral valve into the left ventricle.

The Left Ventricle

This chamber receives the oxygenated blood as the left atrium contracts, and the blood passes through the mitral valve into the left ventricle. The ventricle is able to fill with blood as the aortic valve leading into the aorta is closed. Once the ventricle is full it contracts, the mitral valve closes, and the aortic valve opens. The closure of the mitral valve prevents blood from backing into the left atrium and the opening of the aortic valve allows the blood to flow into the aorta and flow throughout the body.

The right side of the heart is completely separate from the left side by the septum to prevent blood flowing into the opposite side.

The function of the heart is to pump blood around the body and is approximately the size of a fist. The heart walls are made up of a special type of muscle called cardiac muscle which allows it to contract and relax. The heart is centrally located but is tilted so that most of the heart muscle is to the left. The left ventricle contracts most forcefully, so you can feel your heart beating stronger on the left side of your chest.

  • Deoxygenated blood enters the right side of the heart via the inferior and superior vena cava into the right atrium.
  • From here it travels through the tricuspid valve, which shuts off once the blood fills the right ventricle.
  • The blood then passes through the pulmonary valve into the pulmonary artery to the lungs to allow the carbon dioxide to be removed and to collect oxygen.
  • Oxygenated blood then enters the left side of the heart via the pulmonary vein and enters the left atrium.
  • It passes through the mitral valve that closes once the left ventricle is full.
  • The ventricle now contracts and forces the blood through the aortic valve into the aorta so that blood is pumped to the head and rest of the body.

The function of the valves is to prevent the blood from flowing back the wrong way. The bodies’ blood is circulated through the heart more than 1,000 times per day and beats an average of 70 to 80 times per minute. Many factors can affect the pulse, such as exercise, age, gender, emotion and drugs.

Coronary Arteries

The heart tissue must have a constant supply of oxygen to allow the heart to contract and relax, so there is a network of vessels that deliver oxygenated blood to the tissues.

The aorta is supplied with the left and right coronary arteries, which gradually branch off into smaller vessels. The larger vessels are situated on the surface of the heart, with the smaller vessels penetrating the heart muscle. Over time, and in a diet that is rich in cholesterol, plaques can build up and eventually block the flow of blood through the coronary artery. When this happens, the heart tissue becomes starved of oxygen and stops functioning as it should. This results in a heart attack.

Blood Pressure

Blood pressure is the force applied against the walls of the arteries as the heart pumps blood through the body. The pressure is determined by the force and amount of blood pumped and the size and flexibility of the arteries.

Each time the heart beats (about 60–70 times a minute at rest); it pumps out blood into the arteries.

  • our blood pressure is at its highest when the heart beats, pumping the blood. This is called systolic pressure.
  • When the heart is at rest, between beats, your blood pressure falls. This is the diastolic pressure

If the blood pressure is too high, the heart may get larger, which could lead to heart failure. Small bulges (aneurysms) form in blood vessels. Common locations are the main artery from the heart (aorta); arteries in the brain, legs, and intestines; and the artery leading to the spleen.

Blood vessels in the kidney narrow, which may cause kidney failure. Arteries throughout the body “harden” faster, especially those in the heart, brain, kidneys, and legs. This can cause a heart attack, stroke, kidney failure, or amputation of part of the leg. Blood vessels in the eyes can burst or bleed which may cause vision changes and can result in blindness

In 90 to 95% of high blood pressure cases, the cause is unknown. In fact, you can have high blood pressure for years without knowing it. When the cause is unknown, you have what’s called essential or primary hypertension. Factors that may lead to high blood pressure in the remaining 5–10 percent of cases, which are known as secondary hypertension, include:

Kidney abnormality, a structural abnormality of the aorta (large blood vessel leaving the heart) existing since birth, narrowing of certain arteries, lifestyle factors such as diet and smoking.

Pathologies of the Circulatory System

AneurysmA bulge in a blood vessel, which can split open.
GangreneBody’s tissues begin to decay due to an interruption of blood flow.
ArteriosclerosisWhere the arteries lose their elasticity and is a form of atherosclerosis’
AtherosclerosisHardening of the arteries, usually caused by cholesterol.
PalpitationsNoticeable heartbeat often felt in the throat or neck.
Deep Vein ThrombosisBlood clot within a blood vessel.
StrokeA blockage of the blood supply to the brain due to a bleed of a blood clot.
PhlebitisInflammation of a vein usually caused by local trauma.
Varicose VeinsSwollen or enlarged veins, caused when valves within the veins become weakened.