Title: The Circulatory System

Key words: heart, blood vessels, lymph vessels, spleen, blood, hypertension, hypotension, nutrients, gases, plasma, proteins, erythrocytes, leucocytes, thrombocytes, capillaries,

Date: July 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish


The Circulatory System


The circulatory system consists of the heart, blood vessels, lymph vessels, the spleen and, of course, the blood. This system transports nutrients, gases and chemical messages to cells and tissues throughout the body. It removes the waste products of respiration and helps to maintain the normal balance of water and salts in body tissues. Blood vessels are also involved in maintaining normal body temperature.

The heart

The heart is a muscular pump the size of a clenched fist. The average heart pumps around 9000 litres of blood per day, beating at a rate between 60 and 80 beats per minute. The right side of the heart contains two chambers, the atrium and the ventricle. These pump blood to the lungs (pulmonary circulation), where carbon dioxide is removed and oxygen picked up by the red blood cells.

The left side of the heart, which is thicker and more muscular, also consists of an atrium and a ventricle. These pump the oxygenated blood around the rest of the body (systemic circulation). The atria are receiving vessels, while the ventricles provide the pumping force. The mitral and tricuspid valves, which are one-way valves, ensure that blood can only flow in one direction through the heart.

The walls of the heart have three layers. The outermost is the pericardium, which simply prevents friction between the heart and the surrounding tissues. The next layer inwards is the myocardium, composed of the cardiac muscle tissue described earlier. The lining of the inner chambers is the endocardium, which is a continuation of the lining of the blood vessels, composed of endothelial cells.

Blood returning from the tissues through the superior and inferior vena cavae collects in the right atrium. When this is full, the blood is pumped into the right ventricle, which forces it through the right and left pulmonary arteries to the lungs, for oxygenation. Blood carrying oxygen returns to the heart from the lungs, via the pulmonary veins. It enters the left atrium, then is passed into the left ventricle. When full, the left ventricle pumps the blood through the aorta to the rest of the body, via the systemic circulation.


Blood pressure is created by the muscular activity of the left ventricle as it pumps. The pressure is highest at the moment when the ventricle forces the blood into the aorta and lowest when the atrium is refilling. Pressure is regulated by flow, so the resistance to flow of the blood vessels also determines blood pressure. Measuring blood pressure is very important as hypertension (persistently raised blood pressure) and hypotension (low blood pressure can lead to or can indicate various medical problems.

The blood vessels consist of arteries and arterioles, which carry oxygenated blood to the tissues, veins and venules, which return deoxygenated blood to the heart and capillaries, which supply cells with blood. (Note: the pulmonary arteries and veins are an exception, as they carry deoxygenated blood to the lungs and oxygenated blood back to the heart) The arteries have thick, muscular walls that help to even out peaks and troughs in blood pressure. Arterioles are small arteries which can expand (relax) and contract more easily than the arteries. They are important in controlling local blood flow.

Veins have thinner, less muscular walls as the blood returning to the heart is at a lower pressure. Many veins have one way valves to prevent backflow due to this lower pressure, especially in the lower limbs, where gravity resists the flow back up to the heart. Small veins are called venules. Within the tissues, venules are connected to the arterioles by microscopic capillaries in a capillary bed. Here there is a constant exchange of gases, nutrients and waste products between the blood, intercellular fluid and cells of the tissues. (Intercellular fluid fills the spaces between cells and tissues).

The lymphatic system collects excess intercellular fluid from the tissues. It returns the fluid or lymph to the bloodstream through a series of lymphatic vessels that eventually lead into ducts draining into the veins of the neck. The lymphatic system also contains lymph nodes, small masses of lymphatic tissue along the major lymphatic vessels. As lymph filters through these nodes, bacteria and other micro-organisms are removed and destroyed by white blood cells. The lymph nodes swell when fighting off infection, giving rise to the phrase 'swollen glands'. Another function of the lymphatic system is to supply additional white blood cells to the bloodstream.

The blood is the major fluid transport system within the body. Typically, blood makes up 7-8% of our body weight, or around 5 litres by volume. Around 60% of the blood is plasma. The remaining 40% of the blood is made up of cells. Approximately 91.5% of plasma is water, 7% proteins and 0.4% a mixture of dissolved proteins, sugars, fats, hormones, antibodies, waste products and enzymes.

Red blood cells or erythrocytes carry oxygen to the tissues and remove carbon dioxide, which they carry back to the lungs. These cells are produced in red bone marrow. White blood cells or leucocytes protect the body in various ways against infection and cancer. They also remove dead and damaged tissue. These cells are produced in bone marrow and the lymphatic system. Other cells called platelets or thrombocytes play a role in the clotting mechanisms of the blood.

plasma 60 %

cellular elements 40%

91.5% water

7% proteins

0.4% hormones



waste products


red blood cells

white blood cells

platelets (thrombocytes)


Body fluids

(60% of body weight is water)

75% is intra-cellular fluid (within cells)

17% is intercellular fluid (between cells)

8% is plasma