Chapter 5.2: Body Fluids and Circulation

1. Blood

Blood is a special connective tissue consisting of a fluid matrix, plasma, and formed elements.

Composition of Blood

a) Plasma

• Composition: A straw-coloured, viscous fluid constituting nearly 55% of the blood. 90-92% of plasma is water, and proteins contribute 6-8% of it.
• Plasma Proteins:
  • Fibrinogen: A major plasma protein required for clotting or coagulation of blood.
  • Globulins: Primarily involved in the defense mechanisms of the body (as antibodies or immunoglobulins).
  • Albumins: Help in maintaining the osmotic balance of the blood.
• Other components: Plasma also contains small amounts of minerals like Na+, Ca++, Mg++, HCO3-, Cl-, etc. Glucose, amino acids, lipids, etc., are also present in the plasma as they are always in transit in the body.

b) Formed Elements (Blood Corpuscles)

Erythrocytes, leucocytes, and platelets are collectively called formed elements and they constitute nearly 45% of the blood.

• Erythrocytes (Red Blood Cells - RBCs):

  • Structure: Biconcave discs, devoid of a nucleus in most mammals, which increases the surface area for oxygen binding.
  • Function: They have a red-coloured, iron-containing complex protein called haemoglobin, which plays a significant role in the transport of respiratory gases.
  • Lifespan: An average lifespan of 120 days, after which they are destroyed in the spleen (graveyard of RBCs).

• Leucocytes (White Blood Cells - WBCs):

  • Structure: Colourless due to the lack of haemoglobin. They are nucleated and are relatively lesser in number.
  • Types:
    • Granulocytes: (Contain granules in their cytoplasm)
      • Neutrophils: Most abundant (60-65%), phagocytic in nature.
      • Eosinophils: (2-3%) Resist infections and are also associated with allergic reactions.
      • Basophils: (0.5-1%) Secrete histamine, serotonin, heparin, etc., and are involved in inflammatory reactions.
    • Agranulocytes: (Do not contain granules)
      • Lymphocytes: (20-25%) Of two major types – B and T lymphocytes. Both are responsible for the immune responses of the body.
      • Monocytes: (6-8%) Phagocytic cells that destroy foreign organisms.

• Platelets (Thrombocytes):

  • Structure: Cell fragments produced from megakaryocytes (special cells in the bone marrow).
  • Function: Play a major role in blood coagulation.

Blood Groups

• ABO Grouping:
  • Based on the presence or absence of two surface antigens (chemicals that can induce an immune response) on the RBCs, namely A and B.
  • Similarly, the plasma of different individuals contains two natural antibodies (proteins produced in response to antigens).
  • Importance in Blood Transfusion: During blood transfusion, any blood cannot be used; the blood of a donor has to be carefully matched with the blood of a recipient before any blood transfusion to avoid severe problems of clumping (destruction of RBC). A compatible transfusion is essential.
    • Group O: Universal donors (no antigens).
    • Group AB: Universal recipients (no antibodies).

• Rh Factor:
  • Another antigen, the Rh antigen, similar to one present in Rhesus monkeys, is also observed on the surface of RBCs of a majority (nearly 80%) of humans. Such individuals are called Rh positive (Rh+ve) and those in whom this antigen is absent are called Rh negative (Rh-ve).
  • Importance in Transfusion: Rh group should also be matched before transfusions. An Rh-ve person, if exposed to Rh+ve blood, will form specific antibodies against the Rh antigens.
  • Importance in Pregnancy (Erythroblastosis Fetalis): A special case of Rh incompatibility has been observed between the Rh-ve blood of a pregnant mother with Rh+ve blood of the foetus. Rh antigens of the foetus do not get exposed to the Rh-ve blood of the mother in the first pregnancy. However, during the 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 foetus. In such cases, the mother starts preparing antibodies against the Rh antigen in her blood. In case of her subsequent pregnancies, the Rh antibodies from the mother (Rh-ve) can leak into the blood of the foetus (Rh+ve) and destroy the foetal RBCs. This could be fatal to the foetus or could cause severe anaemia and jaundice to the baby. This condition is called erythroblastosis fetalis. This can be avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Coagulation of Blood

• It is a mechanism to prevent excessive loss of blood from the body. It is a cascade process involving a number of factors.
• Brief Mechanism:
  1. At the site of an injury, platelets release certain factors which activate the enzyme thrombokinase.
  2. Thrombokinase converts inactive prothrombin in the plasma into active thrombin.
  3. Thrombin then converts inactive fibrinogen in the plasma into fibrin.
  4. Fibrin forms a network of threads in which dead and damaged formed elements of blood are trapped, forming a clot or coagulum.

2. Lymph (Tissue Fluid)

• Composition: As blood passes through the capillaries in tissues, some water along with many small water-soluble substances move out into the spaces between the cells of tissues leaving the larger proteins and most of the formed elements in the blood vessels. This fluid released out is called the interstitial fluid or tissue fluid. It has the same mineral distribution as that in plasma. It is a colourless fluid containing specialized lymphocytes.

• Function:

  1. Exchange of nutrients, gases, etc., between the blood and the cells always occurs through this fluid.
  2. It is also important for the immune response of the body.
  3. It carries proteins and fats from the intestine to the blood.

• Lymphatic System: An elaborate network of vessels called the lymphatic system collects this fluid and drains it back to the major veins. It consists of lymphatic capillaries, lymphatic vessels, lymph nodes, and lymphatic organs like the spleen and tonsils.

  • Lymphatic Capillaries: Blind-ended tubes that collect interstitial fluid.
  • Lymph Nodes: Small solid structures located at different points along the lymphatic system which filter the lymph and trap microorganisms.

3. Circulatory System

• Open Circulatory System: Blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses (e.g., arthropods and molluscs).
• Closed Circulatory System: Blood pumped by the heart is always circulated through a closed network of blood vessels (e.g., annelids and chordates).

Human Circulatory System

a) Structure of the Human Heart

• External Structure: A mesodermally derived organ, situated in the thoracic cavity, in between the two lungs, slightly tilted to the left. It is protected by a double-walled membranous bag, pericardium, enclosing the pericardial fluid.
• Internal Structure: Our heart has four chambers, two relatively small upper chambers called atria and two larger lower chambers called ventricles.
  • A thin, muscular wall called the inter-atrial septum separates the right and the left atria, whereas a thick-walled, the inter-ventricular septum, separates the left and the right ventricles.
  • The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum.
  • The opening between the right atrium and the right ventricle is guarded by the tricuspid valve (three muscular flaps).
  • The opening between the left atrium and the left ventricle is guarded by the bicuspid or mitral valve (two muscular flaps).
  • The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves.

b) Structure of Blood Vessels

• Artery: Thick, elastic, muscular walls. Carries oxygenated blood away from the heart (except pulmonary artery). The internal lumen is narrow.
• Vein: Thin walls. Carries deoxygenated blood towards the heart (except pulmonary vein). The internal lumen is wider. Valves are present to prevent the backflow of blood.
• Capillary: Extremely thin walls (one-cell thick, called endothelium). Site of exchange of materials between blood and tissues.

c) Cardiac Cycle

• The sequential event in the heart which is cyclically repeated is called the cardiac cycle and it consists of systole (contraction) and diastole (relaxation) of both the atria and ventricles.
• Phases of the Cardiac Cycle:
  1. Joint Diastole: All four chambers of the heart are in a relaxed state. Blood from the pulmonary veins and vena cava flows into the left and right atria, respectively. The semilunar valves are closed.
  2. Atrial Systole: The SA node generates an action potential which stimulates both the atria to contract simultaneously. This increases the flow of blood into the ventricles by about 30 percent.
  3. Ventricular Systole: The action potential is conducted to the ventricles, which contract. The atria relax (diastole). The tricuspid and bicuspid valves close to prevent backflow of blood into the atria, producing the first heart sound, 'lub'. The semilunar valves open, and blood is pumped into the pulmonary artery and aorta.
  4. Ventricular Diastole: The ventricles now relax and the ventricular pressure falls, causing the closure of the semilunar valves, which prevents the backflow of blood into the ventricles. This produces the second heart sound, 'dub'. As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by the veins. The heart is once again in joint diastole.

d) Cardiac Output

• Stroke Volume: The volume of blood pumped out by each ventricle during one cardiac cycle (approx. 70 mL).
• Cardiac Output: The volume of blood pumped out by each ventricle per minute. It is the product of stroke volume and heart rate (approx. 5 litres/minute).
  • Cardiac Output = Stroke Volume × Heart Rate

e) Electrocardiograph (ECG)

• A graphical representation of the electrical activity of the heart during a cardiac cycle.
• Waves of an ECG:
  • P-wave: Represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria.
  • QRS complex: Represents the depolarisation of the ventricles, which initiates the ventricular contraction.
  • T-wave: Represents the return of the ventricles from the excited to the normal state (repolarisation). The end of the T-wave marks the end of systole.

f) Double Circulation

• The blood flows twice through the heart in one complete cycle. This is called double circulation.
• Pulmonary Circulation: The path of blood from the right ventricle to the lungs and back to the left atrium.
• Systemic Circulation: The path of blood from the left ventricle to the rest of the body and back to the right atrium.

g) Regulation of Cardiac Activity

• Intrinsic Regulation: The heart is myogenic, meaning the normal activities of the heart are regulated intrinsically by specialized muscles (nodal tissue), hence the heart is called myogenic. A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system (ANS).
• Neural Regulation:
  • Sympathetic nerves: Can increase the rate of heartbeat, the strength of ventricular contraction and thereby the cardiac output.
  • Parasympathetic nerves: Decrease the rate of heartbeat, speed of conduction of action potential and thereby the cardiac output.
• Hormonal Regulation: Adrenal medullary hormones (adrenaline and noradrenaline) can increase the cardiac output.

h) Blood Pressure

• Arterial Blood Pressure: The pressure of blood flowing through the arteries.
  • Systolic Pressure: The maximum pressure in the arteries during ventricular systole (contraction). (Normal: 120 mm Hg)
  • Diastolic Pressure: The minimum pressure in the arteries during ventricular diastole (relaxation). (Normal: 80 mm Hg)

4. Disorders of the Circulatory System

• Hypertension (High Blood Pressure): A sustained blood pressure of 140/90 or above. It can lead to heart diseases and also affects vital organs like the brain and kidney.
• Coronary Artery Disease (CAD): Often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle. It is caused by deposits of calcium, fat, cholesterol, and fibrous tissues, which makes the lumen of arteries narrower.
• Angina Pectoris: A symptom of acute chest pain that appears when no enough oxygen is reaching the heart muscle. It can occur in men and women of any age but it is more common among the middle-aged and elderly.
• Heart Failure: The state of the heart when it is not pumping blood effectively enough to meet the needs of the body. It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease.