Human Circulatory System Notes for SSC CGL, CHSL and Railways

Human Circulatory System Notes for SSC CGL, CHSL and Railways

Human Circulatory System

  1. Heart
  2. Arteries
  3. Veins
  4. Blood

Blood [Plasma (54%) + WBCs (0.7%) + RBCs (45%) + Platelets]

The main functions of blood are to transport oxygen, carbon dioxide, water, nutrients, hormones and waste around the body. Blood also fights infection and regulates temperature.

Haemopoiesis: The production of blood cells and platelets, which occurs in the bone marrow.

Plasma (Yellowish Liquid)

  • Mostly water (up to 95% by volume) and contains dissolved proteins (6–8%) (i.e.—serum albumins, globulins, and fibrinogen), glucose, clotting factors, electrolytes (Na+, Ca2+, Mg2+, HCO3−, Cl−, etc.), hormones, carbon dioxide and oxygen.
  • Density of approximately 1025 kg/m3 (equivalent to water)
  • Blood serum is plasma without clotting factors.

RBCs (Red Blood Cells) (Erythrocyte)

  • RBCs are red-coloured (due to haemoglobin 12-16gm in 100ml blood), non-nuclear having a biconcave shape (approx 4.5-5.5 million/mm3)
  • Involved in the transportation of oxygen by haemoglobin (haem: iron-containing)
  • They are made in the bone marrow, lasts approx 120 days.
  • The spleen contains specialised cells which break down the RBCs, which contain haemoglobin made up of protein and iron. The iron is stored as ferritin and transferin in the liver for future manufacture of haemoglobin. The protein is mostly excreted, with a bit of it being sent into the gall bladder to be stored as biliverdin and bilirubin. That’s why the spleen is called the graveyard of blood cells.

At higher altitudes, there’s low density of Oxygen in the blood. To compensate it, the amount of haemoglobin in blood increases (to circulate more oxygen), thereby increasing the size of the RBCs.

WBCs (White Blood Cells) (Leukocytes or Leucocytes)

  • Lasts 3-5 days.
  • All white blood cells have nuclei.
  • Approx 4k-11k/mm3
  • Involved in protecting the body against both infectious disease and foreign invaders
  • All white blood cells are produced and derived from multipotent cells in the bone marrow known as hematopoietic stem cells.
  • Leukocytes are found throughout the body, including the blood and lymphatic system
  • Two pairs of broadest categories classify them either by structure (granulocytes or agranulocytes) or by cell lineage (myeloid cells or lymphoid cells). These broadest categories can be further divided into the five main types: neutrophils, eosinophils (acidophiles), basophils, lymphocytes, and monocytes.
  • Largest WBCs: Monocytes, Smallest WBCs: Lymphocytes

Platelets (Thrombocytes)

  • Small colourless disc-shaped cell fragment without a nucleus
  • Approx 1.5-4 lakh/mm3
  • Lasts just 7 days.
  • They are essential in clotting blood.

Blood Groups (A, B, AB, O)

  • The ABO blood types were discovered by Karl Landsteiner in 1901, for which he received the Nobel Prize in Physiology or Medicine in 1930.
  • Universal Donor Blood Group O-
  • Universal Receptor Blood Group AB+

William Harvey explained about blood circulation for the first time.

Heart (4 valves + 4 chambers)

  • The heart pumps oxygenated blood to the body and deoxygenated blood to the lungs.
  • In one cycle heart pumps 70ml blood.
  • The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium.
  • In the human heart there is one atrium and one ventricle for each circulation, and with both a systemic and a pulmonary circulation there are four chambers in total: left atrium, left ventricle, right atrium and right ventricle.
  • There is a tricuspid valve between the right atrium and the right ventricle and a bicuspid valve between the left atrium and left ventricle.
  • The blood that is returned to the right atrium is deoxygenated and passed into the right ventricle to be pumped through the pulmonary artery to the lungs for re-oxygenation and removal of carbon dioxide. The left atrium receives newly oxygenated blood from the lungs as well as the pulmonary vein which is passed into the strong left ventricle to be pumped through the aorta to the different organs of the body.
  • An adult heart has a mass of 250–350 grams
  • Heart Beat of Adult: 72 beats/minute; Heart beat of infants: 130-150 beats per minute

The blue whale (Balaenoptera musculus) has the largest heart of any living creature (200 kg)

Cockroaches have 13 chambered heart


The septum that separates the two upper chambers (the right and left atria) of the heart is termed the atrial (or interatrial) septum while the portion of the septum that lies between the two lower chambers (the right and left ventricles) of the heart is called the ventricular (or interventricular) septum.


  • Transports oxygenated blood
  • Pulmonary artery carries deoxygenated blood.
  • Aorta: Largest artery of the body
  • Blood pressure is measured from Brachial artery.
  • Coronary Artery: An artery that supplies blood to the heart.


  • Transports deoxygenated blood
  • Pulmonary vein carries oxygenated blood.

Blood Pressure (Systolic + Diastolic)

Blood pressure is the force blood exerts on the walls of blood vessels.

  • Measured by Sphygmomanometer from Brachial artery.
  • Systolic Pressure: 120 mm Hg
  • Diastolic Pressure: 80 mm Hg
  • Hypertension (High Blood Pressure): 150/90 mm Hg
  • Hypotension (Low Blood Pressure): 100/50 mm Hg

World Hypertension Day is celebrated on 17 May.

Lymphatic System

  • The lymphatic system is part of the vascular system and an important part of the immune system, comprising a network of lymphatic vessels that carry a clear fluid called lymph (from Latin, lympha meaning “water”) directionally towards the heart
  • Lymph nodes are small spherical-shaped organs of the lymphatic system that contain many white blood cells.
The Functions of the lymphatic system:
  1. Filters lymph white blood cells in lymph nodes remove bacteria and viruses.
  2. Absorb fat from small intestine lymph vessels throughout the wall of the digestive system absorb lipids.
  3. Maturation of certain white blood cells lymphocytes mature and become fully active in the thymus.
  4. Fighting infection white blood cells produce antibodies to kill bacteria and viruses.


Cholesterol is a waxy substance in human body which it uses to protect nerves, make cell tissues, and produce certain hormones.

  • Total cholesterol levels less than 200 milligrams per deciliter (mg/dL) are considered desirable for adults.


When we eat food, the carbohydrate in food breaks down into sugar and goes into the bloodstream. The pancreas releases insulin when this happens.

  • Insulin is a hormone produced by the pancreas that “unlocks” the body’s cells, allowing the sugar go from the blood and into the cells. The cells in the body use this sugar for energy.
  • When the body does not make any or enough insulin, or when the cells are unable to use the insulin correctly, blood sugar levels go up.

Diseases and related terms to circulatory system

  • Thalassemia: A genetic blood disorder involving lower-than-normal amounts of an oxygen-carrying protein (haemoglobin).
  • Leukemia: A cancer of WBCs, hindering the body’s ability to fight infection.
  • Ischaemia: An inadequate blood supply to an organ or part of the body, especially the heart muscles.
  • Anaemia: A condition in which there is a deficiency of red cells or of haemoglobin in the blood, resulting in pallor and weariness.
  • Haemophilia: Mostly inherited genetic disorder that impairs the body’s ability to make blood clots. This results in people bleeding longer after an injury, easy bruising, and an increased risk of bleeding inside joints or the brain.
  • Hypoglycemia: Low blood sugar, the body’s main source of energy.
  • Hyperglycemia: A condition in which an excessive amount of glucose circulates in the blood plasma.
  • Diabetes: A group of diseases that result in too much sugar in the blood (high blood glucose).
  • Heart attack: A blockage of blood flow to the heart muscle.
  • Tachycardia: Heart rate that exceeds the normal resting rate.
  • Bradycardia: A condition wherein an individual has a very slow heart rate,
  • Christmas disease (hemophilia B or factor IX hemophilia): A rare genetic disorder in which blood doesn’t clot properly.
  • Erythropoiesis: The process which produces red blood cells (erythrocytes)
  • Apoptosis: A form of programmed cell death that occurs in multicellular organisms.
  • Atherosclerosis: The hardening of artery walls due to a build-up of fatty deposits.

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