Biology: Class 11 — Chapter 15: Body Fluids and Circulation

Plasma is a straw-colored, viscous fluid making up nearly 55% of the blood.

• Water: Constitutes 90-92% of plasma.
• Proteins: Contribute 6-8% of plasma. Major proteins include:
  • Fibrinogen: Essential for blood clotting/coagulation.
  • Globulins: Primarily involved in the body's defense mechanisms.
  • Albumins: Help maintain osmotic balance.
• Minerals & Nutrients: Contains small amounts of minerals like $Na^+$, $Ca^{2+}$, $Mg^{2+}$, $HCO_3^-$, $Cl^-$, along with glucose, amino acids, and lipids.
• Serum: Plasma without clotting factors.

Formed elements constitute about 45% of blood and include:

• Erythrocytes (Red Blood Cells - RBCs): Most abundant cells. Formed in red bone marrow. Biconcave, devoid of nucleus in most mammals. Contain the iron-bearing complex protein haemoglobin (12-16 g/100 mL). Average lifespan of 120 days; destroyed in the spleen (graveyard of RBCs).
• Leucocytes (White Blood Cells - WBCs): Colourless, nucleated, relatively fewer (6000-8000 $mm^{-3}$). Divided into:
  • Granulocytes: Neutrophils (most abundant, phagocytic), Eosinophils (resist infections, associated with allergic reactions), and Basophils (least abundant, secrete histamine, serotonin, heparin; involved in inflammatory reactions).
  • Agranulocytes: Lymphocytes (B and T forms, responsible for immune responses) and Monocytes (phagocytic).
• Thrombocytes (Platelets): Cell fragments produced from megakaryocytes in bone marrow. Play a critical role in blood coagulation. Reduction leads to clotting disorders and excessive blood loss.

• ABO Grouping: Based on the presence or absence of surface antigens A and B on RBCs, and natural antibodies in plasma.
  • Group A: Antigen A, antibody anti-B.
  • Group B: Antigen B, antibody anti-A.
  • Group AB: Antigens A and B, no antibodies. Universal recipient.
  • Group O: No antigens, antibodies anti-A and anti-B. Universal donor.
• Rh Grouping: Based on the presence (Rh+ve) or absence (Rh-ve) of the Rh antigen (similar to Rhesus monkeys). Critical in blood transfusion and pregnancy.
  • Erythroblastosis Foetalis: A condition occurring when an Rh-ve mother carries an Rh+ve foetus. During delivery, maternal exposure to fetal Rh+ve blood triggers anti-Rh antibody production. In subsequent pregnancies, these antibodies can cross the placenta, destroying fetal RBCs, causing severe anemia and jaundice. Prevented by administering anti-Rh antibodies to the mother immediately after the first delivery.

A protective mechanism to prevent excessive blood loss during injury.

1. Injury stimulates platelets and tissues to release coagulation factors.
2. An enzyme complex, thrombokinase, is formed via a cascade of linked enzymatic reactions.
3. Thrombokinase converts inactive prothrombin in plasma into active thrombin.
4. Thrombin converts soluble, inactive fibrinogen into insoluble fibrin threads.
5. Fibrin threads form a network trapping dead and damaged formed elements to form a clot (coagulum).
6. Calcium ions ($Ca^{2+}$) play a vital role in this process.

• Open Circulatory System: Blood pumped by the heart passes through large vessels into open body cavities (sinuses). Found in arthropods and molluscs.
• Closed Circulatory System: Blood is circulated through a closed network of blood vessels, allowing precise regulation. Found in annelids and chordates.
• Vertebrate Heart Evolution:
  • 2-chambered: 1 atrium, 1 ventricle (Fishes - single circulation).
  • 3-chambered: 2 atria, 1 ventricle (Amphibians and Reptiles, except crocodiles - incomplete double circulation).
  • 4-chambered: 2 atria, 2 ventricles (Crocodiles, Birds, and Mammals - double circulation).

Consists of a chambered heart, blood vessels, and blood.

• Heart Anatomy: Mesodermally derived, located in the thoracic cavity, protected by a double-walled pericardium containing pericardial fluid. Four chambers: two upper atria and two lower ventricles.
• Valves: Prevent backflow of blood.
  • Tricuspid valve: Guards the opening between right atrium and right ventricle.
  • Bicuspid (Mitral) valve: Guards the opening between left atrium and left ventricle.
  • Semilunar valves: Guard the openings of ventricles into the pulmonary artery and aorta.
• Nodal Tissue: Specialized auto-excitable cardiac musculature.
  • Sino-atrial node (SAN): Located in the right upper corner of the right atrium. Generates maximum action potentials (70-75/min), initiating and maintaining rhythmic contraction. Known as the pacemaker.
  • Atrio-ventricular node (AVN): Located in the lower left corner of the right atrium.
  • AV bundle & Purkinje fibres: Transmit action potentials throughout the ventricular musculature.

The sequential event of systole (contraction) and diastole (relaxation) of atria and ventricles.

1. Joint Diastole: All four chambers are relaxed; tricuspid and bicuspid valves are open, allowing blood to flow into ventricles.
2. Atrial Systole: SAN generates an action potential, causing atria to contract, increasing blood flow into ventricles by 30%.
3. Ventricular Systole: Action potential spreads to ventricles via AVN and Bundle of His. Ventricles contract, closing tricuspid and bicuspid valves (producing the first heart sound, lub) and forcing semilunar valves open.
4. Ventricular Diastole: Ventricles relax, ventricular pressure drops, closing semilunar valves (producing the second heart sound, dub). Tricuspid and bicuspid valves reopen as pressure falls further.

• Stroke Volume: Volume of blood pumped by each ventricle per beat (~70 mL).
• Cardiac Output: Stroke volume multiplied by heart rate (~5000 mL or 5 L/min in a healthy individual).

A graphical representation of the electrical activity of the heart during a cardiac cycle.

• P-wave: Represents atrial depolarization (excitation), leading to atrial contraction.
• QRS complex: Represents ventricular depolarization, initiating ventricular contraction.
• T-wave: Represents ventricular repolarization (return to relaxed state).