Body Fluids and Circulation


Cells depend on efficient nutrient delivery, oxygen supply, and waste elimination in the complex symphony of life. Unique techniques have evolved for effective material transport in creatures ranging in complexity from sponges to more complex ones. The life force of higher creatures, such as humans, is blood, which ensures nutrient delivery and waste elimination. Lymph assists in the transportation of substances alongside blood. Humans have an advanced network of blood vessels, blood, and the heart that performs this function. The lymphatic system runs alongside the circulatory system.

The Blood

Blood is the life-giving substance that flows through our blood vessels. Blood is made up of a complex mixture of formed elements (blood cells) in the fluid matrix plasma.


  • The fluid part of blood constitutes 55% of total blood.
  • 93% of plasma weight is water.

Composition of the plasma

  • Electrolytes (<1% of plasma Wt.) like Na+, K+, Ca2+, Mg2+, H+, Cl, HCO3, HPO42-, SO42-.
  • Proteins: albumin, globulins, fibrinogen.
  • Gases: Oxygen, carbon dioxide, nitrogen.
  • Nutrients: glucose and other carbohydrates, amino acids, lipids, and cholesterol.
  • Waste products: urea, creatinine, uric acid, bilirubin (it provides a yellowish color to the plasma).

Serum = Plasms – blood clotting factors (fibrinogens and other proteins involved in blood clotting.

Formed elements

All blood cells together are called formed elements. It includes erythrocytes (RBCs), leucocytes (WBCs), and thrombocytes (platelets).


  • Number = 5 – 5.5 million/mm3 of blood.
  • Mature RBCs (and platelets) are cells without any organelle.
  • They are biconcave in shape with a diameter of 6 – 8micron, and a thickness of 1–1.4 microns.
  • Life span = 120 days.
  • Produced in (red) bone marrow (iron, folic acid and vitamin B12 are essential for its production).
  • They contain iron-containing protein Hb which carries oxygen.
  • The average concentration of Hb is 14 g/100 ml blood in women and 16 g/100 ml in men.
  • It imparts a high surface-to-volume ratio for efficient O2/CO2
  • Erythrocyte plasma membranes hold distinct polysaccharides and proteins, varying among individuals, determining the blood type or group.
  • Erythrocyte destruction normally occurs in the spleen and the liver.
  • A deficiency of Hb is called Anaemia.
  • Hematocrit refers to the proportion of a person’s blood volume that is taken up by red blood cells.

Leucocytes (WBCs)

  • Total number = 6000-8000/mm3 of blood.
  • Involved in immune functions.

A. Polymorphonuclear granulocytes or granulocytes

  • They have multilobed nuclei and abundant membrane-surrounded granules.
  1. Basophils (affinity for a blue dye termed a “basic” dye)
  • 5-1 per cent of all WBCs
  • Secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions.
  • Heparin is anti-coagulant.
  1. Eosinophils (take up the red dye eosin)
  • 1-4%
  • Produces allergic response, defence against certain parasites like helminths,
  1. Neutrophils (have little affinity for either dye).
  • 50-70%
  • Phagocytic cells.

B. Agranulocytes

  • They have little membrane-bound granules.
  1. Monocytes
  • 28%
  • Single oval or horseshoe-shaped nucleus.
  • Phagocytic cells
  1. Lymphocytes (B and T- cells)
  • 20-25 %
  • A single relatively large nucleus.
  • Produces humoral (B-cells) and cell-mediated (T-cells) immunity.
  • Provides memory to the immune system.

Thrombocytes (platelets)

  • Colorless cell fragments that contain numerous granules.
  • 1,500,00-3,500,00 platelets mm3 of blood.
  • Involved in blood clotting (simplified pathway shown in the following flow-chart).

Blood clotting pathway

  • Calcium ions (factor IV) play a crucial role in blood clotting.
  • There are 13 blood clotting factors.
  • A clot or coagulum is primarily composed of a meshwork of threads known as fibrins. Within this structure, deceased and impaired blood components get stuck, resulting in the formation of a clot. This process serves as the body’s natural mechanism to prevent excessive bleeding.
Flow chart representing simplified blood clotting pathway

Blood groups

  • Blood groups, also known as blood types, are a classification system that categorizes blood based on specific antigens present on the surface of red blood cells.
  • Among the various blood groups in humans, two are of clinical significance. These blood groups are ABO and Rh blood groups.

ABO blood group

ABO blood type

Antigen on RBCs

Antibodies in serum











Both A and B





Both anti-A and anti-B


Rh blood group

  • ABO blood group and Rh blood group are clinically significant therefore, they are represented together.
  • Rh blood type is determined by the presence or absence of D-antigen.
  • The presence of D-antigen (rhesus factor) in the blood is assigned as Rh-positive and its absence is assigned as Rh-negative.
  • ABO blood group and Rh-group taken together can produce eight blood types.

Blood donor/recipient list

Blood Type

Can be transfused to

Can Receive From


A-Positive, AB-Positive

A-Positive, A-Negative,

O-Positive, O- Negative


A-Positive, A-Negative,

AB-Positive, AB-Negative

A-Negative, O- Negative


B-Positive, AB-Positive

B-Positive, B-Negative,

O-Positive, O- Negative


B-Positive, B-Negative,

AB-Positive, AB-Negative

B-Negative, O- Negative



ALL (Universal Acceptor)


AB-Positive, AB-Negative

A-Negative, B-Negative,

AB-Negative, O- Negative


A-Positive, B-Positive,

AB-Positive, O-Positive

O-Positive, O- Negative


ALL (Universal Doner)

O- Negative

  • O(-ve) is the universal donor and AB(+ve) is the universal acceptor.

For more details on blood groups, please visit

Erythroblastosis fetalis or haemolytic disease of the newborn (HDN)

Levine et al. confirmed that incompatibility between mother and foetus was the cause of erythroblastosis fetalis or haemolytic disease of the newborn (HDN). HDN is caused if the pregnant mother is Rh negative and the developing foetus is Rh positive. During childbirth, the accidental mixing of blood from a child with the mother’s blood causes anti-Rh antibodies in the mother’s blood. During the subsequent pregnancy, if the foetus is Rh positive, an anti-Rh antibody developed in the mother’s blood during the previous pregnancy leak into the foetal blood, causing loss of RBCs which leads to severe anaemia and jaundice or even death of the young one.

Pathway leading to HDN

Circulatory Patterns:

• Two types: open and closed.
• Arthropods and molluscs: Open circulatory system.
The heart pumps blood into the sinuses or body cavities.
• Annelids and chordates: Closed circulatory system.
Blood circulated through a closed network of vessels.
Precise fluid regulation due to closed vessels.

Vertebrate Heart Types:

• All vertebrates have muscular, chambered hearts.
• Fish: 2-chambered heart (atrium, ventricle).
• Amphibians and reptiles (except crocodiles): 3-chambered heart, 2 atria and 1 ventricle.
• Crocodiles, birds, and mammals: 4-chambered heart. Two atria and two ventricles.

Blood Circulation in Fish:

• Deoxygenated blood is pumped out, and oxygenated by gills. Oxygenated blood is supplied to body parts. Deoxygenated blood returned to the heart.
• Single circulation.

Blood Circulation in Amphibians and Reptiles:

• Oxygenated blood (gills/lungs/skin) to the left atrium.
• Deoxygenated blood (body parts) to the right atrium.
• Mixed in a single ventricle.
• Incomplete double circulation.

Blood Circulation in Birds and Mammals:

• Oxygenated and deoxygenated blood are kept separate.
• Oxygenated blood to the left atrium, deoxygenated to the right.
• Two separate circulatory pathways.
• Complete double circulation.

Human Circulatory System:

• Follows the double circulation pattern (systemic and pulmonary).
• Oxygenated blood from the lungs to the left atrium.
• Deoxygenated blood from the body to the right atrium.
• Separate circulation through left and right sides.

Artery and vein: a comparison



Carry blood away from the heart

Carry blood towards the heart

Carry oxygenated blood (except pulmonary artery)

Carry deoxygenated blood (except pulmonary vein)

No valves to prevent backflow of blood (except at the beginning of the aorta, aortic valve)

Valves present to prevent backflow of blood

Large diameter

Small diameter

The arteriolar wall is thicker containing a large number of elastic muscles

Wall is thinner

High blood pressure

Low blood pressure

Artery and vein: cross-section

Innermost TUNICA INTIMA (squamous epithelium), middle TUNICA MEDIA (smooth muscle and elastic fibres), and outermost TUNICA EXTERNA (collagenous, fibrous connective tissue). The cross-section through the blood vessel is shown below.

Cross section of blood vessel

Blood circulation in human

  • The human heart receives oxygen-rich blood from the lungs and carbon dioxide-rich blood from the body.
  • The right atrium receives dioxide-rich blood.
  • The left atrium receives carbon oxygen-rich blood.
  • Blood passes from the atrium to the respective ventricles.
  • Atrioventricular (AV) valves guard the blood flow from the atrium to the ventricles.
  • The right AV valve has three muscular flaps hence called the tricuspid valve.
  • The left AV valve has two muscular flaps hence called a bicuspid or mitral valve.
  • AV valves prevent the backflow of blood during ventricular systole.
Blood Flow Pathway
  • The opening of AV valves in the opposite direction (towards the atrium) is prevented by fibrous strands (chordae tendinae) attached through muscular projections (papillary muscles).
  • The blood vessels taking blood away from the heart (aorta and pulmonary trunk) are also guarded by the semilunar valves to prevent the backflow of blood during ventricular diastole.
  • The part of circulation from the heart to the lungs back to the heart is called pulmonary circulation.
  • The part of circulation from the heart to body parts back to the heart is called systemic circulation.
  • This physiological phenomenon involves two distinct circulatory pathways: the pulmonary circulation and the systemic circulation, it is called double circulation.
  • Heartbeats 70-75 (average 72) times/minute. This is called heart rate.
  • In each heartbeat, it pushes out 70mL of blood. This is called stroke volume.
  • The amount of blood pushed out every minute by the heart is called cardiac output.


Cardiac output = Stroke volume x Heart rate

= 70mL/stroke x 72 strokes/min

= 5040 mL/min

= 5.04L/min

  • The blood in the heart chambers does not exchange nutrients/O2-CO2/metabolic wastes with the heart wall.
  • The coronary artery supplies blood to the heart.
  • The coronary artery supply oxygenated blood to the heart and coronary veins drains the deoxygenated blood in the heart through a large blood vessel called the coronary sinus.
  • Blood pressure refers to the force exerted by the blood against the walls of the blood vessels. The higher value represents the pressure in the arteries when the heart contracts is called systolic pressure, and the lower value represents the pressure in the arteries when the heart is at rest. The lower value is called diastolic pressure.
  • The blood pressure is represented as systolic/diastolic. Normal blood pressure is 120/80 mmHg.
Schematic representation of human heart showing the major blood vessels and direction of blood flow. The blue parts represent oxygen-poor blood, while the red parts represent oxygen-rich blood.

Conducting system in heart and conduction of electrical impulse

  • The electrical impulse is generated in the Sinoatrial node (SAN). Hence called the pacemaker of the heart.
  • In atria, impulse spread without the conducting system.
  • The Spread of impulse in ventricles involves the conducting system.
  • The transmission of impulse from SAN to Atrio-ventricular node (AVN) is delayed by 0.1 sec during which atrial contraction is completed. Ventricular contraction starts only after that.
  • SAN fires at a rate of ~100 depolarizations/min.
The conduction of electrical impulses through the heart's conducting system.

The Electrocardiography

  • Electrocardiography (ECG or EKG) is a non-invasive medical test that measures and records the electrical activity of the heart over a period of time.
  • The graphical representation of the electrical activity of the heart is called Electrocardiograph.
  • The are three prominent waves in the Electrocardiograph and their interpretation is shown below.
Heart Rhythms on Display, a normal ECG (not on scale).

Electrical waves and heart activity

Electrocardiograph (ECG)


Atrial excitation (or depolarisation)

Contraction of atria


Beginning of systole


Ventricular excitation (or depolarization)

Contraction of ventricles (ventricular systole)

0.15 sec after P-wave


Ventricular repolarization

End of ventricular systole

  • Atrial repolarization wave does not observe in ECG because it occurs simultaneously with QRS-complex.
  • The duration of diastole is 0.3 sec and that of systole is 0.5 sec.

Heart sounds

  • Closure of the AV valves: lub
  • Closure of the pulmonary and aortic valves: dub

Regulation of heartbeat

• In the absence of any hormonal or neural regulation, it will beat at the rate of 100/min.
• The human heart is myogenic, which means it can initiate its heartbeat without relying on external nerve signals. The SAN is the impulse generator (called the pacemaker of the heart).
• The cardiovascular centre in the medulla oblongata can moderate the heart rate through ANS.
• Signals through the sympathetic nervous system (ANS) increase heart rate.
• Signals through the parasympathetic nervous system (ANS) decrease heart rate.
• Epinephrine and norepinephrine from the adrenal medulla bind beta-adrenergic receptors and cause an increase in the heart rate.

Diseases related to the circulatory system

Atherosclerosis (Coronary Artery Disease, CAD)

  • Atherosclerosis is a progressive condition that involves the gradual build-up of plaque within the walls of arteries. Plaque is a mixture of cholesterol, fat, calcium, and other substances that accumulate over time. A high ratio of plasma LDL cholesterol to plasma HDL cholesterol increases the risk of Atherosclerosis.

Angina pectoris

  • A condition in which acute chest pain occurs when the heart muscle doesn’t receive enough oxygen-rich blood.
  • It can arise due to reduced blood supply to heart muscles.

Heart failure

  • The condition in which the heart cannot supply enough blood to meet the needs.

Cardiac arrhythmias

  • Abnormal heart rhythm.

Lymphatic system

• The lymphatic system is a part of the body’s immune and circulatory systems. It consists of a network of lymphatic vessels, lymph nodes, and various lymphoid organs.
• The fluid in the lymphatic vessels is called lymph. It is a colourless fluid.
• The lymphatic system helps maintain fluid balance by collecting excess fluid, proteins, and other substances that leak from the blood vessels into the surrounding tissues. This excess fluid is then returned to the bloodstream.
• The lymphatic system is involved in the absorption and transport of dietary fats and fat-soluble vitamins from the digestive system to the bloodstream.
• Lymph nodes are small, round structures located along the lymphatic vessels. They contain immune cells that help filter and fight infections.
• Lymph serves as a vital conduit for waste removal and a carrier for essential nutrients and hormones. It is involved in the absorption of dietary fats. This process occurs through specialized structures called lacteals, which are found within the intestinal villi.


  • Biology: Text Book for Class Xi. (2006).
  • Hall, J. E., PhD. (2015). Guyton and Hall Textbook of Medical Physiology. Elsevier Health Sciences.
  • Widmaier, E. P., Raff, H., & Strang, K. T. (2003). Vander et Al’s Human Physiology: With OLC Bind-In Card. McGraw-Hill Science, Engineering & Mathematics

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