Comprehensive Study Notes on Body Fluids and Circulation

Overview of Body Fluids and Circulation

• Requirement for Transport Mechanisms:

  • All living cells require nutrients, $\text{O}_2$, and essential substances for survival.

  • Metabolic waste products and harmful substances produced by cells must be continuously removed to ensure optimal tissue function.

• Evolutionary Strategies for Internal Transport:

  • Simple organisms (e.g., sponges and coelenterates) circulate water from their surroundings through their body cavities to exchange substances directly with cells.

  • Complex organisms utilize specialized internal body fluids for substance transportation:

  • Blood: The primary body fluid used by higher organisms, including humans.

  • Lymph (Tissue Fluid): A secondary body fluid assisting in substance transport.

Composition and Properties of Blood

• Definition: Blood is a specialized connective tissue comprising a fluid matrix, plasma, and formed elements.

• Plasma Composition and Functions:

  • Represents approximately $55\%$ of total blood volume.

  • Appearance: A straw-colored, viscous fluid.

  • Constituents:

  • Water: Accounts for $90\text{--}92\%$ of plasma volume.

  • Proteins: Account for $6\text{--}8\%$ of plasma volume. Major proteins include:

    • Fibrinogen: Crucial for blood clotting and coagulation.

    • Globulins: Primary contributors to the body's defense mechanisms.

    • Albumins: Essential for maintaining osmotic balance.

  • Minerals: Present in small quantities ($\text{Na}^+$, $\text{Ca}^{++}$, $\text{Mg}^{++}$, $\text{HCO}_3^-$, $\text{Cl}^-$, etc.).

  • Organic Nutrients: Glucose, amino acids, lipids, and other substances in transit.

  • Clotting Factors: Present in an inactive state.

  • Serum: Plasma stripped of its clotting factors ($\text{Serum} = \text{Plasma} - \text{Clotting Factors}$).

• Formed Elements:

  • Account for approximately $45\%$ of blood volume.

• 1. Erythrocytes (Red Blood Cells / RBCs):

  • Most abundant cell type in blood.

  • Concentration: $5 \text{ million to } 5.5 \text{ million per mm}^{-3}$ of blood in a healthy adult male.

  • Formation site: Red bone marrow in adults.

  • Structural features: Enucleated (devoid of nucleus) in most mammals; biconcave shape.

  • Pigment content: Red, iron-containing complex protein called haemoglobin.

  • Haemoglobin concentration: $12\text{--}16\,\text{g}$ per $100\,\text{mL}$ of blood in healthy individuals.

  • Primary function: Transport of respiratory gases.

  • Lifespan: Average of $120\,\text{days}$.

  • Destruction site: Spleen (known as the "graveyard of RBCs").

• 2. Leucocytes (White Blood Cells / WBCs):

  • Features: Colorless due to the lack of haemoglobin; nucleated; generally short-lived.

  • Concentration: $6000\text{--}8000\,\text{mm}^{-3}$ of blood.

  • Granulocytes:

    • Neutrophils: Most abundant WBC ($60\text{--}65\%$ of total WBCs); phagocytic cells that engulf foreign organisms.

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

    • Basophils: Least abundant WBC ($0.5\text{--}1\%$); secrete histamine, serotonin, heparin, etc.; involved in inflammatory responses.

  • Agranulocytes:

    • Monocytes: Constitute $6\text{--}8\%$ of WBCs; phagocytic cells.

    • Lymphocytes: Constitute $20\text{--}25\%$ of WBCs; exist as 'B' and 'T' forms; responsible for immune responses.

• 3. Platelets (Thrombocytes):

  • Cell fragments derived from megakaryocytes (specialized bone marrow cells).

  • Concentration: $1,50,000\text{--}3,50,000\,\text{mm}^{-3}$ of blood.

  • Function: Release substances involved in blood coagulation.

  • Clinical impact: Reduction in count causes clotting disorders and severe blood loss.

Blood Groups and Compatibility

• ABO Grouping System:

  • Based on two surface antigens (surface chemicals capable of eliciting an immune response) on RBCs: A and B.

  • Natural antibodies (proteins produced in response to foreign antigens) present in plasma: anti-A and anti-B.

• Universal Donor: Group 'O' blood can be donated to individuals of any blood group because its RBCs lack A and B surface antigens.

• Universal Recipient: Group 'AB' individuals can accept blood from any group because their plasma lacks anti-A and anti-B antibodies.

• Transfusion Reaction: Mismatched blood transfusions cause severe clumping (agglutination and destruction) of RBCs.

  • Rh Grouping System:

• Based on the Rh antigen (first identified in Rhesus monkeys), present on RBC surfaces in approximately $80\%$ of humans.

• Classification:

  • Rh-positive ($\text{Rh}^{+\text{ve}}$): Rh antigen present.

  • Rh-negative ($\text{Rh}^{-\text{ve}}$): Rh antigen absent.

• Exposure of an $\text{Rh}^{-\text{ve}}$ person to $\text{Rh}^{+\text{ve}}$ blood stimulates specific anti-Rh antibody production.

• Erythroblastosis Foetalis (Rh Incompatibility Disorder):

  • Occurs when an $\text{Rh}^{-\text{ve}}$ mother carries an $\text{Rh}^{+\text{ve}}$ fetus.

  • First Pregnancy: Fetal and maternal blood remain separated by the placenta; minimal exposure occurs until delivery when tiny amounts of fetal blood enter maternal circulation, triggering anti-Rh antibody production in the mother.

  • Subsequent Pregnancies: Anti-Rh antibodies from the $\text{Rh}^{-\text{ve}}$ mother cross the placenta into $\text{Rh}^{+\text{ve}}$ fetal circulation, destroying fetal RBCs.

  • Consequences: Severe fetal anemia, jaundice, or fetal death.

  • Prevention: Administering anti-Rh antibodies (RhoGAM) to the mother immediately after delivering the first $\text{Rh}^{+\text{ve}}$ child.

Coagulation of Blood

• Definition & Purpose: A protective mechanism to prevent excess blood loss following vascular injury or trauma.

• Clot Structure: A dark reddish-brown scum (coagulum) composed of a network of fibrin threads trapping dead and damaged blood cells.

• Cascade Mechanism of Clotting:

  1. Enzyme Activation Cascade: Injury or trauma triggers platelets and injured tissues to release factors that initiate a chain reaction of linked enzymatic steps (cascade process).

  2. Thrombokinase Complex: The multi-factor cascade forms the enzyme complex thrombokinase.

  3. Thrombin Formation: Thrombokinase converts inactive prothrombin in plasma into active thrombin.

  4. Fibrin Formation: Thrombin converts soluble, inactive fibrinogen into insoluble fibrin threads.

  5. Role of Calcium ($\text{Ca}^{++}$): Calcium ions are essential for multiple steps throughout the coagulation cascade.

Lymph (Tissue Fluid) and Systemic Function

• Formation of Interstitial/Tissue Fluid:

  • As blood flows through tissue capillaries, water and small water-soluble solutes pass into intercellular spaces.

  • Plasma proteins and larger formed elements are retained within the capillary lumen.

  • Tissue fluid shares the same mineral distribution as plasma.

  • Serves as the exchange medium for gases, nutrients, and waste products between blood vessels and cells.

• Lymphatic System & Lymph Composition:

  • The network of lymphatic vessels collects interstitial fluid and drains it back into major systemic veins.

  • Lymph: The fluid inside lymphatic vessels; colorless, containing specialized lymphocytes.

  • Functions of Lymph:

  • Mediates immune responses via specialized lymphocytes.

  • Acts as a carrier for nutrients and hormones.

  • Absorbs digested dietary fats via lacteals in intestinal villi.

Evolutionary Circulatory Pathways and Patterns

• Open vs. Closed Systems:

  • Open Circulatory System: Heart pumps blood through large vessels into body cavities/open spaces called sinuses (found in arthropods and molluscs).

  • Closed Circulatory System: Heart pumps blood through a continuous network of closed blood vessels (found in annelids and chordates); allows precise regulation of fluid volume and pressure.

• Vertebrate Heart Chamber Diversity:

  • 2-Chambered Heart (1 Atrium, 1 Ventricle):

  • Present in fishes.

  • Path: Deoxygenated blood pumped to gills $\rightarrow$ oxygenated by gills $\rightarrow$ supplied to tissues $\rightarrow$ returned to heart (Single Circulation).

  • 3-Chambered Heart (2 Atria, 1 Ventricle):

  • Present in amphibians and reptiles (except crocodiles).

  • Path: Left atrium receives oxygenated blood (gills/lungs/skin); right atrium receives deoxygenated blood (body tissues).

  • Blood mixes inside the single ventricle before being pumped out (Incomplete Double Circulation).

  • 4-Chambered Heart (2 Atria, 2 Ventricles):

  • Present in crocodiles, birds, and mammals.

  • Path: Oxygenated and deoxygenated blood streams are completely separated into left and right sides, preventing mixing (Complete Double Circulation).

Human Circulatory System

• General Architecture:

  • Comprises a muscular chambered heart, a network of closed branching blood vessels, and blood.

  • Heart Location & Protection:

  • Mesodermally derived organ located in the thoracic cavity between the lungs, slightly tilted left.

  • Size: Roughly that of a clenched fist.

  • Enclosed by a double-walled membranous bag called the pericardium, filled with pericardial fluid.

• Internal Heart Anatomy:

  • Chambers: Two upper smaller atria (right and left) and two lower larger ventricles (right and left).

  • Septa:

  • Inter-atrial Septum: Thin muscular wall separating right and left atria.

  • Inter-ventricular Septum: Thick wall separating right and left ventricles.

  • Atrio-ventricular Septum: Thick fibrous wall separating atrium and ventricle on the same side, containing valved passages.

  • Valves:

  • Tricuspid Valve: Composed of three muscular flaps/cusps; guards the right atrio-ventricular opening.

  • Bicuspid / Mitral Valve: Composed of two flaps; guards the left atrio-ventricular opening.

  • Semilunar Valves: Guard the exits from the right ventricle into the pulmonary artery and the left ventricle into the aorta.

  • Function: Ensure strictly unidirectional blood flow (atria $\rightarrow$ ventricles $\rightarrow$ pulmonary artery/aorta) and prevent backflow.

• Nodal Tissue & Electrical Conduction:

  • Entire heart consists of cardiac muscle; ventricular walls are significantly thicker than atrial walls.

  • Sino-atrial Node (SAN): Patch of specialized autoexcitable tissue in the upper right corner of the right atrium.

  • Atrio-ventricular Node (AVN): Mass of nodal tissue in the lower left corner of the right atrium near the atrio-ventricular septum.

  • Atrio-ventricular Bundle (AV Bundle / Bundle of His): Originates from the AVN, pierces the atrio-ventricular septa, passes along the top of the inter-ventricular septum, and splits into right and left branches.

  • Purkinje Fibres: Fine nodal fibers branching throughout the entire right and left ventricular musculature.

  • Pacemaker Activity:

  • Nodal musculature generates action potentials automatically (autoexcitable).

  • SAN generates maximum action potential frequency: $70\text{--}75\,\text{min}^{-1}$.

  • SAN initiates and maintains the rhythmic contractile activity of the heart; therefore designated as the pacemaker.

  • Normal resting human heart rate: $70\text{--}75\,\text{beats min}^{-1}$ (average $72\,\text{beats min}^{-1}$).

Cardiac Cycle and Dynamics

• Phases of the Cardiac Cycle:

  • 1. Joint Diastole:

  • All four chambers are relaxed.

  • Tricuspid and bicuspid valves are open; semilunar valves are closed.

  • Blood flows passively from pulmonary veins and vena cava through the atria into the ventricles.

  • 2. Atrial Systole:

  • SAN fires an action potential, triggering simultaneous contraction of both atria.

  • Increases ventricular filling by approximately $30\%$.

  • 3. Ventricular Systole:

  • Action potential travels from SAN $\rightarrow$ AVN $\rightarrow$ AV bundle $\rightarrow$ Bundle of His $\rightarrow$ Purkinje fibers $\rightarrow$ Ventricular myocardium.

  • Ventricles contract; atria relax simultaneously (atrial diastole).

  • Rising intraventricular pressure closes tricuspid and bicuspid valves, producing the first heart sound (lub).

  • Further pressure increases force open the semilunar valves, ejecting blood into the pulmonary artery and aorta.

  • 4. Ventricular Diastole:

  • Ventricles relax; intraventricular pressure drops.

  • Closure of semilunar valves prevents backflow from major arteries, producing the second heart sound (dub).

  • Continued pressure drop opens tricuspid and bicuspid valves as atrial pressure rises, returning the heart to joint diastole.

• Quantitative Cardiac Parameters:

  • Cycle Duration: At $72\,\text{beats min}^{-1}$, duration of one cardiac cycle is $\frac{60}{72} = 0.8\,\text{seconds}$.

  • Stroke Volume: Volume of blood pumped out by each ventricle per beat $\approx 70\,\text{mL}$.

  • Cardiac Output: Volume of blood pumped by each ventricle per minute.     $\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$     $\text{Cardiac Output} = 70\,\text{mL} \times 72\,\text{min}^{-1} \approx 5000\,\text{mL min}^{-1} \text{ (or } 5\,\text{litres min}^{-1}\text{)}$

  • Stroke volume and heart rate adjust to metabolic demand (e.g., cardiac output is significantly higher in athletes than non-athletes).

• Heart Sounds:

  • First Sound (Lub): Associated with the closure of tricuspid and bicuspid valves during ventricular systole.

  • Second Sound (Dub): Associated with the closure of semilunar valves during ventricular diastole.

Electrocardiogram (ECG)

• Definition: Graphical registration of the electrical activity of the cardiac musculature during a cardiac cycle, obtained using an electrocardiograph machine.

• Lead Configuration:

  • Standard ECG: Patient is connected with three electrical leads (one attached to each wrist and one to the left ankle).

  • Detailed Evaluation: Requires multiple diagnostic leads applied across the chest region.

• Waveform Analysis:

  • P-Wave: Represents electrical excitation (depolarisation) of the atria, causing atrial contraction.

  • QRS Complex: Represents depolarisation of the ventricles, triggering ventricular contraction.

  • Ventricular contraction begins shortly after Q and marks the onset of systole.

  • Counting QRS complexes per time unit gives the individual's heart rate.

  • T-Wave: Represents the return of ventricles from excitation to normal resting state (repolarisation).

  • End of the T-wave marks the end of ventricular systole.

  • Clinical value: Structural deviations in ECG geometry indicate specific cardiac pathophysiologies or abnormalities.

Double Circulation and Vascular Structure

• Histology of Blood Vessels (Arteries and Veins):

  • 1. Tunica Intima: Innermost lining composed of single-layer squamous endothelium.

  • 2. Tunica Media: Middle layer composed of smooth muscle fibers and elastic tissue; thin in veins.

  • 3. Tunica Externa: Outermost layer composed of fibrous connective tissue with collagen fibers.

• Circulatory Circuits:

  • 1. Pulmonary Circulation:

  • Right Ventricle (deoxygenated blood) $\rightarrow$ Pulmonary Artery $\rightarrow$ Lungs (oxygenation) $\rightarrow$ Pulmonary Veins $\rightarrow$ Left Atrium.

  • 2. Systemic Circulation:

  • Left Ventricle (oxygenated blood) $\rightarrow$ Aorta $\rightarrow$ Arteries/Arterioles/Capillaries $\rightarrow$ Tissues (gases/nutrients exchange) $\rightarrow$ Venules/Veins/Vena Cava $\rightarrow$ Right Atrium.

  • 3. Hepatic Portal System:

  • Unique vascular bridge connecting the digestive tract to the liver.

  • The hepatic portal vein transports blood directly from the intestine to the liver before entering general systemic circulation.

  • 4. Coronary Circulation:

  • Dedicated network of blood vessels supplying arterial blood to and draining venous blood from the cardiac myocardium.

Regulation of Cardiac Activity

• Intrinsic Regulation (Myogenic Control):

  • Human heart is myogenic; rhythmic contraction is initiated intrinsically by specialized autoexcitable nodal tissues (SAN).

• Extrinsic Neural Regulation:

  • Moderated by the autonomic nervous system (ANS) via a regulatory center located in the medulla oblongata.

  • Sympathetic Nervous System: Signals increase heart rate, elevate ventricular contractile force, and increase cardiac output.

  • Parasympathetic Nervous System: Signals reduce heart rate, slow action potential conduction velocity, and decrease cardiac output.

• Hormonal Regulation:

  • Adrenal Medullary Hormones (epinephrine and norepinephrine) increase cardiac output during stress.

Disorders of the Circulatory System

• High Blood Pressure (Hypertension):

  • Defined as blood pressure consistently exceeding normal baseline values of $120/80\,\text{mmHg}$.

  • $120\,\text{mmHg}$: Systolic (pumping) pressure.

  • $80\,\text{mmHg}$: Diastolic (resting) pressure.

  • Diagnostic threshold: Repeated readings at or above $140/90\,\text{mmHg}$.

  • Pathological effects: Heart disease; causes damage to vital organs including the brain and kidneys.

• Coronary Artery Disease (CAD / Atherosclerosis):

  • Affects vessel branches supplying blood to the heart muscle.

  • Cause: Deposition of calcium, fat, cholesterol, and fibrous tissue in coronary arteries.

  • Result: Progressive narrowing of arterial lumen, restricting blood flow to the myocardium.

• Angina (Angina Pectoris):

  • Symptom: Severe, acute chest pain caused by inadequate oxygen supply to cardiac tissue.

  • Epidemiology: Occurs in both sexes at any age; higher prevalence in middle-aged and elderly individuals.

  • Pathophysiology: Induced by disorders restricting coronary blood flow.

• Heart Failure:

  • State in which the heart fails to pump blood effectively enough to meet the metabolic demands of the body.

  • Often termed congestive heart failure due to lung congestion being a primary clinical feature.

  • Distinctions:

  • Heart Failure: Ineffective pumping action.

  • Cardiac Arrest: Total cessation of heart beat.

  • Heart Attack (Myocardial Infarction): Sudden damage to heart muscle caused by insufficient blood supply.

Review Questions and Exercises

• Formed Elements: Name the components of the formed elements in blood and state one major function of each component.

• Plasma Proteins: Explain the physiological functions of fibrinogen, globulins, and albumins.

• Matching Column Exercise:

  • Eosinophils $\rightarrow$ Resist Infections

  • RBC $\rightarrow$ Gas transport

  • AB Group $\rightarrow$ Universal Recipient

  • Platelets $\rightarrow$ Coagulation

  • Systole $\rightarrow$ Contraction of Heart

• Tissue Classification: Why is blood classified as a connective tissue?

• Fluid Comparison: Contrast the composition and function of lymph with blood.

• Double Circulation: Define double circulation and explain its functional significance in birds and mammals.

• Distinction Categories:

  • Blood versus Lymph

  • Open versus Closed circulatory systems

  • Systole versus Diastole

  • P-wave versus T-wave

• Evolutionary Zoology: Trace the structural evolution of the heart across vertebrate classes.

• Physiological Definitions:

  • Why is the human heart termed myogenic?

  • Why is the sino-atrial node (SAN) designated as the pacemaker?

  • Explain the conduction role of the AVN and AV bundle.

  • Define cardiac cycle, stroke volume, and cardiac output.

  • Describe the genesis of first and second heart sounds.

  • Sketch a standard ECG trace and interpret the P, QRS, and T segments.