Comprehensive Study Notes on Body Fluids and Circulation

Introduction to Body Fluids and Mechanisms of Transport

• Necessity of Transport: All living cells require a continuous supply of nutrients, oxygen ($O_2$), and other essential substances. Simultaneously, waste or harmful substances produced by cellular metabolism must be removed to ensure healthy tissue functioning.

• Evolution of Transport Mechanisms:

  • Simple Organisms: Sponges and coelenterates circulate water from their surroundings through body cavities to facilitate exchange between the environment and cells.

  • Complex Organisms: Use specialized internal fluids for transport.

  • Blood: The most common body fluid used for transport in higher organisms, including humans.

  • Lymph (Tissue Fluid): Another body fluid that assists in the transport of specific substances.

Blood: Composition and Properties

• Definition: Blood is a specialized connective tissue consisting of a fluid matrix called plasma and formed elements.

• Plasma:

  • Description: A straw-colored, viscous fluid making up nearly $55 \%$ of the total blood volume.

  • Composition: Contains $90-92 \%$ water and $6-8 \%$ proteins.

  • Major Proteins:

    • Fibrinogen: Essential for blood clotting (coagulation).

    • Globulins: Primarily involved in the body's defense mechanisms.

    • Albumins: Play a critical role in maintaining osmotic balance.

  • Other Constituents: Small amounts of minerals ($Na^+$, $Ca^{++}$, $Mg^{++}$, $HCO_3^-$, $Cl^-$, etc.), glucose, amino acids, and lipids (which are in transit).

  • Coagulation Factors: Present in plasma in an inactive state.

  • Serum: Defined as plasma without the clotting factors.

Formed Elements of Blood

Formed elements constitute approximately $45 \%$ of the blood and include erythrocytes, leucocytes, and platelets.

• Erythrocytes (Red Blood Cells - RBCs):

  • Abundance: The most numerous blood cells. A healthy adult male averages $5 \text{ millions}$ to $5.5 \text{ millions}$ of RBCs per $mm^{-3}$ of blood.

  • Formation: Produced in the red bone marrow in adults.

  • Structure: Biconcave in shape and lacks a nucleus in most mammals.

  • Haemoglobin: Contains a red-colored, iron-containing complex protein called haemoglobin ($Hb$). A healthy individual carries $12-16\,g$ of $Hb$ per $100\,ml$ of blood. It is vital for respiratory gas transport.

  • Life Span: Average of $120\,days$. They are destroyed in the spleen, often termed the "graveyard of RBCs."

• Leucocytes (White Blood Cells - WBCs):

  • Characteristics: Colourless (lack haemoglobin), nucleated, and relatively few in number ($6000-8000\,mm^{-3}$). Generally short-lived.

  • Granulocytes: Includes Neutrophils, Eosinophils, and Basophils.

  • Agranulocytes: Includes Lymphocytes and Monocytes.

  • Subtype Details:

    • Neutrophils: Most abundant ($60-65 \%$); phagocytic cells that destroy foreign organisms.

    • Monocytes: $6-8 \%$ of WBCs; also phagocytic.

    • Basophils: Least abundant ($0.5-1 \%$); secrete histamine, serotonin, and heparin; involved in inflammatory reactions.

    • Eosinophils: $2-3 \%$ of WBCs; resist infections and associated with allergic reactions.

    • Lymphocytes: $20-25 \%$ of WBCs; exist in 'B' and 'T' forms, both responsible for immune responses.

• Platelets (Thrombocytes):

  • Origin: Cell fragments produced from megakaryocytes (special cells in the bone marrow).

  • Count: Ranges from $1,500,00 - 3,500,00\,mm^{-3}$.

  • Function: Release substances involved in blood coagulation. A reduction in count results in clotting disorders and excessive blood loss.

Blood Grouping Systems

• ABO Grouping: Based on two surface antigens (A and B) on RBCs and natural antibodies in the plasma.

  • Group A: Antigen A; Anti-B antibody; Can receive from A, O.

  • Group B: Antigen B; Anti-A antibody; Can receive from B, O.

  • Group AB: Antigens A and B; No antibodies; Universal Recipient (can receive from AB, A, B, O).

  • Group O: No antigens; Anti-A and Anti-B antibodies; Universal Donor (can donate to any group).

  • Transfusion: Requires blood matching to avoid clumping (destruction of RBCs).

• Rh Grouping: Based on the Rh antigen (similar to Rhesus monkeys) found in $80 \%$ of humans ($Rh^{+ve}$).

  • Rh Incompatibility (Erythroblastosis Foetalis): Occurs when an $Rh^{-ve}$ mother carries an $Rh^{+ve}$ foetus.

    • First pregnancy is usually safe due to the placenta separating blood.

    • During delivery, maternal blood may be exposed to foetal $Rh^{+ve}$ blood, causing the mother to develop Rh antibodies.

    • In subsequent pregnancies, these antibodies can cross the placenta and destroy foetal RBCs, leading to severe anaemia, jaundice, or death.

    • Prevention: Administering anti-Rh antibodies to the mother immediately after the first delivery.

Coagulation of Blood

• Mechanism: Prevents excessive blood loss through a cascade of enzymatic reactions.

• The Clot (Coagulum): A dark reddish-brown scum formed of a network of threads called fibrins, trapping dead and damaged blood elements.

• The Cascade Process:

  1. Thrombokinase: An enzyme complex formed by linked reactions in response to injury.

  2. Prothrombin to Thrombin: Thrombokinase converts inactive prothrombin (in plasma) into active thrombin.

  3. Fibrinogen to Fibrin: Thrombin converts inactive fibrinogen into fibrins.

• Ionic Requirement: Calcium ions ($Ca^{++}$) play a crucial role in the clotting process.

Lymph (Tissue Fluid)

• Formation: As blood passes through capillaries, water and small water-soluble substances filter into intercellular spaces, leaving behind large proteins and formed elements. This is interstitial or tissue fluid.

• Lymphatic System: An elaborate network of vessels that collects tissue fluid (now called lymph) and drains it back into major veins.

• Characteristics of Lymph: Colourless fluid containing specialized lymphocytes.

• Functions:

  • Exchange of nutrients and gases between blood and cells.

  • Carrier for nutrients and hormones.

  • Absorption of fats through lacteals in intestinal villi.

Circulatory Pathways and Patterns

• Types of Systems:

  • Open Circulatory System: Blood pumped by the heart enters large vessels then open spaces/cavities called sinuses (e.g., arthropods, molluscs).

  • Closed Circulatory System: Blood is circulated within a closed network of vessels (e.g., annelids, chordates). This allows for more precise regulation of flow.

• Vertebrate Heart Evolution:

  • Fishes: 2-chambered (one atrium, one ventricle); Single circulation (deoxygenated blood to gills to body back to heart).

  • Amphibians/Reptiles: 3-chambered (two atria, one ventricle); Incomplete double circulation (mixing of oxygenated and deoxygenated blood in the ventricle).

  • Crocodiles, Birds, Mammals: 4-chambered (two atria, two ventricles); Double circulation (no mixing of blood).

Human Circulatory System Anatomy

• Heart: Mesodermally derived, located in the thoracic cavity between lungs, slightly tilted left. It is roughly the size of a clenched fist.

• Pericardium: A double-walled membranous bag protecting the heart, containing pericardial fluid.

• Chambers:

  • Atria: Two small upper chambers.

  • Ventricles: Two larger lower chambers with much thicker walls.

• Septa:

  • Inter-atrial septum: Thin muscular wall separating atria.

  • Inter-ventricular septum: Thick-walled barrier separating ventricles.

  • Atrio-ventricular septum: Thick fibrous tissue separating an atrium and ventricle on the same side.

• Valves:

  • Tricuspid valve: Three muscular flaps guarding the right atrio-ventricular opening.

  • Bicuspid (Mitral) valve: Two flaps guarding the left atrio-ventricular opening.

  • Semilunar valves: Guard the openings of ventricles into the pulmonary artery and aorta. These prevent backflow.

Nodal Tissue and Heart Rate

• Autoexcitability: The heart is myogenic; its specialized nodal tissue generates action potentials without external stimuli.

• Sino-atrial Node (SAN): Located in the right upper corner of the right atrium. Known as the Pacemaker, generating $70-75\,action\,potentials\,min^{-1}$.

• Atrio-ventricular Node (AVN): Located in the lower-left corner of the right atrium.

• Conduction Path: AVN → AV bundle → Inter-ventricular septum → Right and Left Bundles (Bundle of His) → Purkinje fibres (throughout ventricular walls).

• Heart Rate: Averages $72\,beats\,min^{-1}$.

The Cardiac Cycle and Output

• Phases:

  1. Joint Diastole: All four chambers are relaxed. Tricuspid/bicuspid valves open; blood flows from vena cava/pulmonary veins into ventricles.

  2. Atrial Systole: SAN generates action potential; atria contract, increasing ventricular blood flow by $30 \%$.

  3. Ventricular Systole: Action potential reaches ventricles via AVN/Bundle of His. Ventricles contract; atria relax (diastole). High pressure closes AV valves (yielding the 'lub' sound) and opens semilunar valves.

  4. Ventricular Diastole: Ventricles relax; pressure drops, closing semilunar valves (yielding the 'dub' sound) and eventually opening AV valves.

• Duration: One cycle lasts $0.8\,seconds$.

• Stroke Volume: Approximately $70\,ml$ of blood pumped by each ventricle per beat.

• Cardiac Output: Stroke volume $\times$ Heart rate = approx. $5000\,ml$ or $5\,litres\,min^{-1}$.

Electrocardiogram (ECG)

• Definition: Graphical representation of the electrical activity of the heart during a cardiac cycle.

• Technique: Standard ECG involves leads attached to each wrist and the left ankle.

• Waves:

  • P-wave: Depolarisation (excitation) of atria; leads to atrial contraction.

  • QRS complex: Depolarisation of ventricles; initiates ventricular contraction (systole starts shortly after Q).

  • T-wave: Repolarisation of ventricles (return to normal state). The end of the T-wave marks the end of systole.

Double Circulation and Blood Vessels

• Vessel Structure:

  • Tunica Intima: Inner lining of squamous endothelium.

  • Tunica Media: Middle layer of smooth muscle and elastic fibres (thinner in veins).

  • Tunica Externa: Outer layer of fibrous connective tissue with collagen.

• Circulatory Pathways:

  • Pulmonary Circulation: Right ventricle → Pulmonary artery → Lungs → Pulmonary veins → Left atrium.

  • Systemic Circulation: Left ventricle → Aorta → Tissues → Veins/Vena cava → Right atrium.

• Special Systems:

  • Hepatic Portal System: Connection between the digestive tract and liver; hepatic portal vein carries blood from the intestine to the liver.

  • Coronary System: Dedicated vessels for blood supply to the cardiac musculature.

Regulation and Disorders

• Regulation:

  • Intrinsic: Nodal tissue (myogenic).

  • Neural: Medulla oblongata via Autonomic Nervous System (ANS). Sympathetic nerves increase heart rate/output; Parasympathetic nerves decrease them.

  • Hormonal: Adrenal medullary hormones increase cardiac output.

• Disorders:

  • Hypertension (High BP): Pressure higher than $120/80\,mm\,Hg$. Repeated readings of $140/90$ or higher indicate hypertension, affecting the brain and kidneys.

  • Coronary Artery Disease (CAD/Atherosclerosis): Caused by deposits of calcium, fat, and cholesterol, narrowing the arterial lumen.

  • Angina (Angina Pectoris): Acute chest pain due to insufficient oxygen reaching the heart muscle.

  • Heart Failure: Heart fails to pump blood effectively to meet body needs. Often involves lung congestion (congestive heart failure). Distinct from cardiac arrest or heart attack.