circulatory system

The Circulatory System

The circulatory system performs an imperative function of supplying oxygen to all the tissues of the body. The heart, the vascular system and blood make up the circulatory system.  Our body cells receive nutrition from the external environment. They are however, not directly connected to the external environment. The circulatory system with the help of blood, supplies the necessary nutrients and helps to eliminate waste products from the body. Our heart, a pump, beats about 115,200 times per day, and assists in this function.

Parts of the human circulatory system

Our circulatory system is composed of the heart, blood vessels and blood.

Blood vessels
In humans, the blood vessels span over 60,000 miles. There are mainly three types of blood vessels – arteries, veins and capillaries. Arteries mainly carry oxygenated blood (pure) to all parts of the body. They do not have valves. Veins are the blood vessels, which contain valves and carry deoxygenated blood (impure) towards the heart. Arteries and veins further divide to become smaller vessels known as arterioles and venules. Capillaries are very fine blood vessels connecting the arterioles and venules. As the capillaries possess a very thin wall, it is allows effortless passage of blood cells as well as exchange of materials and gas between the blood and body tissues.


Diagram of the human heart (cropped)
The heart is located safely between two lungs and the diaphragm within the mediastinum. It is a hollow, muscular organ having four chambers i.e. two atria and two ventricles. Atria are the two upper compartments while ventricles are the lower compartments. There are walls separating these chambers known as the septa. The heart has a protective covering, the pericardium.

Function: Heart performs the function of pumping blood throughout the body through the vascular system. It thus carries the necessary nutrients to the body tissues and removes waste products (mainly carbon dioxide) from the cells.

Blood Flow through the Heart – Deoxygenated / impure blood from the body enters the heart, which is then carried to the lungs to be oxygenated through the pulmonary arteries. The lungs help in oxygenation/ purification of blood. Oxygenated blood from the lungs is carried back to the heart via the pulmonary veins and then this oxygenated blood is comes out through the aorta and supplies the entire body.

Sound of the Heartbeat – Heartbeats make the sound of  “LubDub”, which is caused by the closing of the heart’s valves.

Control of the cardiac activity/ heartbeat – The Sinoatrial Node (SA node) i.e. the pacemaker of the heart regulates the heartbeat. The brain sends an electrical impulse which is received by the SA node, which is located in the right atrium. The SA node sends a signal to the atrioventricular node (AV node) in the right ventricle, thus making the heart contract. Also, a neural center in the medulla oblangata moderates the cardiac activity through autonomic nervous system (ANS). Sympathetic neural signals can increase the heart rate by causing the heart to contract, thereby increasing the cardiac output. Alternately, the parasympathetic neural signals decrease the rate of heart beat and thereby the cardiac output.  The vagus nerve is a cranial nerve that leads to decreased cardiac activity.

Blood is a special connective tissue composed of plasma and the formed elements. It supplies nutrients and oxygen to the cells, and carries waste products away from the cells. It also helps regulate body temperature.

Components of blood:

Plasma is a straw colored fluid that constitutes about 55% of blood volume. 92% water, 7% proteins and 1% dissolved solutes. Plasma contains proteins namely albumin, globulin, fibrinogen and antibodies. It also has minerals, glucose, fats, etc. in small amounts. Albumin helps maintain the osmotic balance by preventing the loss of water from blood. Fibrinogens are essential for blood clotting. Globulins are a part of the defence mechanisms of the body. Plasma without the clotting factors is called serum.

Formed elements (Blood cells): Red blood cells, white blood cells and platelets are together known as the formed elements. They occupy nearly 45% of the blood volume.

  1. Red blood cells (RBCs) are the most abundant blood cells and are normally found in the range of 5 – 5.5 millions/mm3 of blood, so our blood contains about 30 trillion RBCs. RBCs are produced in the red bone marrow. They have a complex protein hemoglobin, which is red in color because it made up of iron. RBCs are thus named according to their color. Normal level of hemoglobin (Hb) in a healthy adult should be 12-16 gms/100 ml of blood. Hb plays a vital role in transport of respiratory gases, oxygen and carbon dioxide. RBCs have an average life span of 120 days.
  2. White blood cells (WBCs) are named so because they are colorless (as they do not contain hemoglobin). They are larger in size than the RBCs, but are relatively lesser in number.  Normal range of WBCs is 6000-8000/mm3 of blood. They also have a shorter life span. WBCs are mainly of two types, granulocytes and agranulocytes, depending upon the presence or absence of granules. Granulocytes are inclusive of the neutrophils, eosinophils and basophils, while agranulocytes include the lymphocytes and monocytes. Of the total number of WBCs, neutrophils are the most abundant cells (60-65%) and basophils are the least (0.5-1%). Neutrophils and monocytes (6-8%) are phagocytic cells i.e. they cause the destruction of pathogens or foreign organisms that enter our body. Basophils are associated with inflammatory reactions by secretion histamine, serotonin, heparin, etc. Eosinophils (2-3%) are involved in allergic reactions. Lymphocytes (20-25%) are of two main types, B and T lymphocytes. Both B and T lymphocytes play an important role in immune responses of the body.
  3. Platelets are cell fragments found in the blood. They are produced from megakaryocytes (special cells in the bone marrow). Normal range of platelets is 1.5-3.5 lac /mm3. Platelets are mainly involved in blood clotting. Reduction in the number of platelets can lead to clotting disorders wherein there is excessive loss of blood from the body. Their average life span is about 7 days.

Divisions of human circulation

In humans there are two types of circulation, pulmonary and systemic circulation. Pulmonary circulation is the transport of blood from the heart and to the lungs and vice versa. In lungs the blood is oxygenated or purified. This type of circulation adds oxygen and removes carbon dioxide from the blood. Systemic circulation in itself is composed of coronary circulation, hepatic-portal circulation and renal circulation. Here blood is circulated from the heart to the body and vice versa. In coronary circulation blood and nutrients are directly supplied to the heart muscles. In hepatic – portal circulation, blood from the digestive system is transported to the liver so that glucose levels in the body are adequately maintained. In renal circulation blood is carried to and from the kidneys. This helps in removal and excretion of nitrogenous waste products by the kidneys.


Circulatory system performs the basic functions of transport of nutrients and waste products, homeostasis as well as protective function. These are described in detail below.

  1. Transportation: All tissues of our body are dependent on nutrition for normal functioning. The circulatory system performs this vital function of supplying adequate amounts of nutrients as well as oxygen to all the cells of our body. These are carried from the digestive and respiratory systems and then delivered to the cells. Simultaneously wastes and carbon dioxide from cells are taken and delivered to kidneys and lungs for elimination from our body. The circulatory system also helps in the transport of various enzymes, hormones & other chemicals etc. throughout the body.
  2. Homeostasis: The circulatory system through the specific volume of blood in our body helps in maintaining fluid and electrolyte balance in the tissues as well as cells. It functions to control the pH balance/ acid-base balance in tissues and cells. By transfer of excess heat from the body through the skin, it also helps in regulation of the body temperature.
  3. Protective Functions: Whenever there is injury or trauma to the body the bleeding is arrested by the clotting mechanism of blood. This helps in preventing excessive fluid loss from the body. Also the spread of infection is limited due to inflammation, which is a protective response of our body. The blood cells (White blood cells) also contribute to the immune functions of our body by removing the pathogens.

Clinical significance

Blood Groups
Though blood from different human beings appears to be similar, it is different at the molecular level. Thus if a person requires blood transfusion he has to be given a specific ABO and Rh blood group after careful matching. Otherwise RBC destruction takes place leading to the clumping of the RBCs which can be fatal.  ABO and Rh are the two important and universally used blood groups. RBCs have certain surface antigen, which can induce immune response. ABO grouping is based on the presence or absence of two such surface antigens namely A and B.

Natural antibodies are produced in response to these antigens and are present in the plasma. A specific combination of these antigen and antibodies gives rise to a specific blood group. Various blood groups seen are – A, B, AB and O. Persons with blood group O can donate blood to persons with any other blood group, thus ‘O’ group donors are known as ‘universal donors’. On the other hand persons with ‘AB’ group can accept blood from persons with AB as well as the other blood groups and are thus known as ‘universal recipients’.

Another antigen known as ‘Rh’ is also present on the surface of RBCs. It is named so because it is similar to one present in Rhesus monkeys. If the antigen is present on the RBC the person’s blood group is Rh positive (Rh+ve) and if absent blood group is Rh negative (Rh-ve). If an Rh-ve person is transfused with Rh+ve blood specific antibodies against the Rh antigens are formed. Erythroblatosis fetalis is a special condition of Rh incompatibility seen when a mother mother having Rh-ve blood is pregnant with Rh+ve foetus. During first pregnancy mother is not exposed to the Rh antigens of the fetus. However, at the time of delivery of the first child, there is a possibility of exposure of the maternal blood to small amounts of the Rh+ve blood from the fetus. That’s when the mother starts preparing antibodies against Rh antigen in her blood. In case of second pregnancy, the Rh antibodies from the mother (Rh-ve) can leak into the blood of the fetus (Rh+ve) and destroy the fetal RBCs. This could be fatal to the fetus or, could cause severe anemia and jaundice to the baby. This condition is called as the erythroblastosis foetalis. This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Blood Pressure
Blood pressure is a measure of the lateral pressure exerted by blood on the walls of arteries. Blood pressure is measured using an instrument known as the Sphygmomanometer. It measures the pressure in an artery while the heart is contracting and relaxing. Blood pressure is calculated using the formula: Blood Pressure = Systolic Pressure/ Diastolic Pressure. Systolic pressure is the pressure while contraction of heart while diastolic pressure is the pressure while relaxation of heart. Thus if a person has a blood pressure 140/90, it means that the person has a pressure of 140 when the heart is contracting and 90 when the heart is relaxing. Normal blood pressure is different for every individual but usually 120/80 is considered to be normal.

Circulatory System Disorders

  1. High Blood Pressure / Hypertension: Hypertension is the term used for blood pressure measuring higher than normal (120/80) for that age group. In this measurement 120 mm Hg (millimetres of mercury pressure) is the systolic blood pressure and 80 mm Hg is the diastolic pressure. If repeated checks of blood pressure of an individual is 140/90 (140 over 90) or higher, it shows hypertension. High blood pressure requires medical attention as it can lead to heart diseases and also affect vital organs like brain and kidney. Unhealthy diet and lifestyle are the main cause of hypertension.
  2. Arteriosclerosis: In arteriosclerosis there is hardening of the arteries which reduces the blood flow.  The reduced blood supply increases the load on heart giving rise to a heart attack or stroke.
  3. Atherosclerosis; It is a condition in which the arteries become narrow due to deposits of calcium, fat, cholesterol and fibrous tissues within walls of the arteries. There is reduced blood flow to heart and brain, which may lead to heart attack or stroke. It is also known as Coronary Artery Disease.
  4. Cerebrovascular accident or Stroke: Due to the above mentioned causes the blood supply to the brain is hampered. This leads to death of the brain cells and thus loss of brain function and/or motor control  or even death in some cases.
  5. Heart Failure: It is a state when the heart is not able to pump blood efficiently. Thus it cannot supply enough blood for normal functioning of the body. It is also known as congestive heart failure as congestion of lungs is one of the main symptoms of this disease. Heart failure is not cardiac arrest wherein the heart completely stops beating or, a heart attack wherein when the heart muscle is damaged due to inadequate blood supply.
  6. Angina: When the heart muscles do not receive an adequate amount of oxygen an acute chest pain arises. This is known as angina or ‘angina pectoris’. Angina can occur in men and women of any age but it is prevalent among the middle-aged and elderly. It occurs due to conditions that affect the blood flow.


Heart and the associated parts of the circulatory system perform vital functions of our body and by having a healthy diet and stress free lifestyle we can help them in performing their functions efficiently.