Body Fluids and Circulation: Comprehensive Study Guide

Necessity of Circulation and Body Fluids

Fundamental Requirement: All living cells require a continuous supply of nutrients, $O_2$ , and other essential substances to maintain healthy functioning.
Waste Removal: Harmful substances or metabolic wastes produced by cells must be removed continuously.
Evolutionary Mechanisms: - Simple Organisms: Sponges and coelenterates circulate water from their surroundings through body cavities to facilitate substance exchange. - Complex Organisms: Utilize special fluids within their bodies for transport. - Blood: The most commonly used body fluid in higher organisms, including humans. - Lymph (Tissue Fluid): Another body fluid that assists in the transport of specific substances.

Blood: Composition and Plasma

Definition: Blood is a special connective tissue composed of a fluid matrix, plasma, and formed elements.
Plasma Characteristics: - It is a straw-coloured, viscous fluid. - It constitutes nearly $55 \%$ of the total blood volume.
Chemical Composition of Plasma: - Water: Comprises $90-92 \%$ of plasma. - Proteins: Contribute $6-8 \%$ of plasma. Major proteins include: - Fibrinogen: Essential for blood clotting or coagulation. - Globulins: Primarily involved in the body's defense mechanisms. - Albumins: Aid in maintaining osmotic balance. - Minerals: Contains small amounts of $Na^+$ , $Ca^{++}$ , $Mg^{++}$ , $HCO_3^-$ , and $Cl^-$ . - Nutrients and Organic Compounds: Glucose, amino acids, and lipids are present as they are constantly in transit. - Coagulation Factors: Present in an inactive form.
Serum: Defined as plasma without the clotting factors.

Formed Elements of Blood

Overview: Formed elements constitute nearly $45 \%$ of the blood and include erythrocytes, leucocytes, and platelets.
Erythrocytes (Red Blood Cells - RBCs): - Abundance: The most numerous cells in the blood. - Count: A healthy adult man averages $5 \times 10^6$ to $5.5 \times 10^6 \, \text{RBCs} \, mm^{-3}$ of blood. - Formation: Produced in the red bone marrow in adults. - Structure: Devoid of a nucleus in most mammals and possess a biconcave shape. - Haemoglobin: Contains a red-coloured, iron-containing complex protein called haemoglobin. A healthy individual has $12-16 \, g$ of haemoglobin per $100 \, ml$ of blood. - Function: Play a significant role in the transport of respiratory gases. - Life Span: Average of $120 \, days$ . They are destroyed in the spleen, often referred to as the "graveyard of RBCs."
Leucocytes (White Blood Cells - WBCs): - Characteristics: Colourless due to lack of haemoglobin, nucleated, and relatively fewer in number ( $6000-8000 \, mm^{-3}$ ). - Categories: - Granulocytes: Include Neutrophils, Eosinophils, and Basophils. - Agranulocytes: Include Lymphocytes and Monocytes. - Types and Functions: - Neutrophils: Most abundant ( $60-65 \%$ of total WBCs); phagocytic cells that destroy foreign organisms. - Monocytes: ( $6-8 \%$ ); also phagocytic. - Basophils: Least abundant ( $0.5-1 \%$ ); secrete histamine, serotonin, and heparin; involved in inflammatory reactions. - Eosinophils: ( $2-3 \%$ ); resist infections and are associated with allergic reactions. - Lymphocytes: ( $20-25 \%$ ); exist in 'B' and 'T' forms, responsible for immune responses.
Platelets (Thrombocytes): - Origin: Cell fragments produced from megakaryocytes (special cells in the bone marrow). - Count: Normally ranges from $1,500,00$ to $3,500,00 \, platelets \, mm^{-3}$ . - Function: Release substances involved in blood coagulation. A reduction leads to clotting disorders and excessive blood loss.

Blood Groups: ABO and Rh

ABO Grouping: Based on the presence or absence of two surface antigens ( $A$ and $B$ ) on RBCs and two natural antibodies in the plasma. - Group A: Antigen A, Anti-B antibodies; can receive from A, O. - Group B: Antigen B, Anti-A antibodies; can receive from B, O. - Group AB: Antigens A and B, no antibodies; can receive from AB, A, B, O (Universal Recipient). - Group O: No antigens, Anti-A and Anti-B antibodies; can receive only from O (Universal Donor). - Transfusion Principle: Donor blood must match the recipient's to avoid clumping (destruction of RBCs).
Rh Grouping: - Based on the Rh antigen (similar to Rhesus monkeys) found on the RBC surface of nearly $80 \%$ of humans ( $Rh+ve$ ). - Rh Incompatibility (Erythroblastosis Foetalis): - Occurs when an $Rh-ve$ mother carries an $Rh+ve$ foetus. - First pregnancy is usually safe as blood is separated by the placenta. - During delivery of the first child, maternal blood may be exposed to foetal $Rh+ve$ blood, leading to the formation of Rh antibodies. - In subsequent pregnancies, Rh antibodies can leak into the foetal blood, destroying foetal RBCs. - Results in severe anaemia, jaundice, or death for the foetus. - Prevention: Administering anti-Rh antibodies to the mother immediately after the first delivery.

Coagulation of Blood

Mechanism: A defense process to prevent excessive blood loss following injury or trauma.
Clot/Coagulum: A dark reddish-brown scum consisting of a network of threads called fibrins trapping dead and damaged formed elements.
The Cascade Process: - Fibrin Formation: Inactive fibrinogens in plasma are converted to fibrins by the enzyme thrombin. - Thrombin Formation: Formed from inactive prothrombin in plasma. - Thrombokinase: An enzyme complex required for these reactions, formed through a series of linked enzymic reactions (cascade) involving various plasma factors.
Stimulation: Injury stimulates platelets and tissues to release specific factors that activate coagulation.
Calcium Role: $Ca^{++}$ ions play a crucial role in the clotting process.

Lymph (Tissue Fluid)

Formation: As blood passes through capillaries, water and small water-soluble substances move into spaces between tissue cells, leaving large proteins and most formed elements in the vessels.
Characteristics: Colourless fluid with the same mineral distribution as plasma.
Functions: - Facilitates exchange of nutrients and gases between blood and cells. - Lymphatic System: A network of vessels that collects interstitial fluid and drains it back to major veins. - Specialized Content: Contains lymphocytes for immune responses. - Fat Absorption: Fats are absorbed through lymph in the lacteals located in the intestinal villi.

Circulatory Pathways and Heart Evolution

Types of Circulatory Systems: - Open: Blood pumped by the heart passes through large vessels into open spaces or body cavities (sinuses). Found in arthropods and molluscs. - Closed: Blood is circulated through a closed network of vessels, allowing for precise regulation. Found in annelids and chordates.
Heart Evolution in Vertebrates: - Fishes: 2-chambered (1 atrium, 1 ventricle). Single circulation (heart pumps deoxygenated blood to gills). - Amphibians/Reptiles (except crocodiles): 3-chambered (2 atria, 1 ventricle). Incomplete double circulation (oxygenated and deoxygenated blood mix in the ventricle). - Crocodiles, Birds, Mammals: 4-chambered (2 atria, 2 ventricles). Double circulation (no mixing of blood).

Human Circulatory System: Anatomy and Nodal Tissue

Structure: Consists of a muscular chambered heart, closed branching blood vessels, and blood.
Heart Location: Mesodermally derived, situated in the thoracic cavity between the lungs, slightly tilted left, roughly the size of a clenched fist.
Protective Layers: Protected by a double-walled pericardium containing pericardial fluid.
Chambers and Septa: - Two upper atria and two larger lower ventricles. - Inter-atrial septum: Thin muscular wall between atria. - Inter-ventricular septum: Thick-walled partition between ventricles. - Atrio-ventricular septum: Thick fibrous tissue separating an atrium and ventricle on the same side.
Valves: - Tricuspid valve: Three muscular flaps between the right atrium and right ventricle. - Bicuspid/Mitral valve: Between the left atrium and left ventricle. - Semilunar valves: Guard the openings of the right ventricle into the pulmonary artery and the left ventricle into the aorta. - Function: Ensure unidirectional blood flow and prevent backflow.
Nodal Tissue (Autoexcitable Musculature): - Sino-atrial node (SAN): Located in the right upper corner of the right atrium. Known as the pacemaker because it generates the maximum number of action potentials ( $70-75 \, min^{-1}$ ). - Atrio-ventricular node (AVN): Located in the lower left corner of the right atrium. - AV Bundle/Bundle of His: Continues from AVN, passes through the interventricular septum, and divides into right and left bundles. - Purkinje Fibres: Minute fibres throughout the ventricular musculature.

The Cardiac Cycle

Sequence of Events: 1. Joint Diastole: All four chambers are relaxed; tricuspid and bicuspid valves are open; semilunar valves are closed. Blood flows from vena cava/pulmonary veins into ventricles. 2. Atrial Systole: SAN generates action potential; both atria contract, increasing ventricular blood flow by about $30 \%$ . 3. Ventricular Systole: Action potential passes from AVN to Bundle of His and Purkinje fibres. Ventricles contract; atria relax (diastole). Ventricular pressure rises, closing AV valves (first sound 'lub') and opening semilunar valves. 4. Ventricular Diastole: Ventricles relax; pressure falls, closing semilunar valves (second sound 'dub') to prevent backflow. Ventricular pressure drops further, reopening AV valves as atrial pressure increases.
Cycle Duration: Since the heart beats $72 \, times/min$ , one cycle lasts $0.8 \, seconds$ .
Stroke Volume: Amount of blood pumped by each ventricle per beat (approx. $70 \, mL$ ).
Cardiac Output: Stroke volume $\times$ Heart rate. Average is $5000 \, mL$ or $5 \, litres/min$ .
Heart Sounds: 'Lub' (closure of AV valves) and 'Dub' (closure of semilunar valves).

Electrocardiogram (ECG)

Definition: Graphical representation of the electrical activity of the heart during a cardiac cycle.
Standard Leads: Three electrical leads (one for each wrist and one for the left ankle).
ECG Peaks: - P-wave: Represents depolarisation (excitation) of atria, leading to atrial contraction. - QRS complex: Represents depolarisation of ventricles; marks the beginning of ventricular systole. - T-wave: Represents repolarisation (return to normal state) of ventricles; end of T-wave marks the end of systole.
Clinical Significance: Helps determine heart rate and detect abnormalities or diseases.

Double Circulation

Blood Vessel Structure: Inner tunica intima (endothelium), middle tunica media (smooth muscle/elastic fibres), and outer tunica externa (fibrous connective tissue).
Two Pathways: 1. Pulmonary Circulation: Right ventricle $\rightarrow$ Pulmonary artery $\rightarrow$ Lungs $\rightarrow$ Pulmonary veins $\rightarrow$ Left atrium. 2. Systemic Circulation: Left ventricle $\rightarrow$ Aorta $\rightarrow$ Tissues $\rightarrow$ Vena cava $\rightarrow$ Right atrium.
Hepatic Portal System: Vascular connection from the intestine to the liver via the hepatic portal vein before reaching systemic circulation.
Coronary System: Specialized vessels for blood supply to and from the cardiac musculature.

Regulation and Disorders

Regulation: - Myogenic: Auto-regulated by nodal tissue. - Neural: Medulla oblongata moderates function via the Autonomic Nervous System (ANS). Sympathetic nerves increase heart rate/output; Parasympathetic nerves decrease them. - Hormonal: Adrenal medullary hormones increase cardiac output.
Disorders: - Hypertension: High blood pressure ( $140/90 \, mmHg$ or higher vs. normal $120/80 \, mmHg$ ). - Coronary Artery Disease (CAD/Atherosclerosis): Narrowing of heart arteries due to deposits of calcium, fat, and cholesterol. - Angina (Angina Pectoris): Acute chest pain due to insufficient oxygen reaching the heart muscle. - Heart Failure: Heart fails to pump blood effectively; often called congestive heart failure due to lung congestion.