Body Fluids and Circulation - Comprehensive Study Guide

Body Fluids and Circulatory Mechanisms

• Living cells require continuous supply of nutrients, oxygen ($O_2$), and other essential substances to maintain cellular metabolism and tissue health.

• Harmful waste products generated during cellular activities must be eliminated continuously from the body to prevent toxicity.

• Lower animals utilize environmental medium for exchange:

  • Sponges and Coelenterates: Circulate ambient water through their body cavities, allowing direct exchange of nutrients and wastes between cells and water.

• Higher animals possess specialized circulatory fluids for systemic distribution and waste collection:

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

  • Lymph (Tissue Fluid): A secondary body fluid involved in transporting specific substances and maintaining fluid balance.

Blood Composition and Properties

• Blood is a specialized connective tissue comprising a fluid matrix called plasma, along with formed elements.

Plasma

• Physical Properties: Straw-coloured, viscous fluid constituting approximately $55\%$ of total blood volume.

• Water Content: Makes up $90\text{--}92\%$ of plasma volume.

• Protein Content: Contributes $6\text{--}8\%$ of plasma volume.

• Major Plasma Proteins:

  • Fibrinogen: Essential for blood coagulation and clotting processes.

  • Globulins: Primarily involved in defense mechanisms and immune responses.

  • Albumins: Crucial for maintaining osmotic balance and oncotic pressure.

• Inorganic and Organic Solutes:

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

  • Nutrients and Metabolites: Glucose, amino acids, and lipids are continuously present in plasma while in transit throughout the body.

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

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

Formed Elements

• Formed elements constitute approximately $45\%$ of blood volume and comprise erythrocytes, leucocytes, and platelets.

Erythrocytes (Red Blood Cells - RBCs)

• Abundance: Most abundant cells in blood, averaging $5\text{ million to }5.5\text{ million}\,\text{RBCs mm}^{-3}$ of blood in a healthy adult male.

• Site of Formation: Formed in the red bone marrow of adults.

• Structural Features: Devoid of a nucleus in most mammals; possess a biconcave shape to maximize surface area for gas diffusion.

• Haemoglobin: Red-coloured, iron-containing complex protein.

  • Concentration: A healthy individual possesses $12\text{--}16\,\text{g}$ of haemoglobin per $100\,\text{mL}$ of blood.

  • Function: Essential for transport of respiratory gases ($O_2$ and $CO_2$).

• Lifespan and Destruction: Average lifespan is $120\,\text{days}$, after which they are destroyed in the spleen (known as the "graveyard of RBCs").

Leucocytes (White Blood Cells - WBCs)

• Physical Properties: Colourless due to the absence of haemoglobin; possess nuclei.

• Cell Count and Lifespan: Average count is $6000\text{--}8000\,\text{mm}^{-3}$ of blood; generally short-lived.

• Classification: Divided into two major categories: Granulocytes and Agranulocytes.

• Granulocytes:

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

  • Eosinophils: Constitute $2\text{--}3\%$ of WBCs; involved in resisting infections and mediating allergic responses.

  • Basophils: Least abundant WBC type ($0.5\text{--}1\%$ of WBCs); secrete histamine, serotonin, heparin, and participate in inflammatory reactions.

• Agranulocytes:

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

  • Lymphocytes: Constitute $20\text{--}25\%$ of WBCs; consist of two functional types: B lymphocytes and T lymphocytes, both responsible for adaptive immune responses.

Platelets (Thrombocytes)

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

• Cell Count: Range from $150,000\text{ to }350,000\,\text{mm}^{-3}$ of blood.

• Function: Release substances critical for blood coagulation.

• Clinical Relevance: A reduction in platelet count causes clotting disorders, leading to severe and uncontrolled blood loss.

Human Blood Group Systems

ABO Blood Grouping System

• Based on the presence or absence of two surface antigens (immunogenic surface molecules) on RBCs: Antigen A and Antigen B.

• Plasma contains natural antibodies (proteins synthesized in response to non-self antigens): anti-A and anti-B.

• Clumping (agglutination and destruction of RBCs) occurs if incompatible blood is transfused.

• Universal Donors: Individuals with group O blood, as their RBCs lack A and B surface antigens and can be transfused into recipients of any blood group.

• Universal Recipients: Individuals with group AB blood, as their plasma lacks anti-A and anti-B antibodies, allowing them to accept blood from any group.

Rh Blood Grouping System

• Based on the presence or absence of the Rh antigen (first discovered in Rhesus monkeys) on the RBC surface.

• Approximately $80\%$ of humans are Rh-positive ($\text{Rh}^{+\text{ve}}$); those lacking this antigen are Rh-negative ($\text{Rh}^{-\text{ve}}$).

• An $\text{Rh}^{-\text{ve}}$ individual exposed to $\text{Rh}^{+\text{ve}}$ blood develops specific anti-Rh antibodies.

Erythroblastosis Foetalis

• A condition resulting from Rh incompatibility between an $\text{Rh}^{-\text{ve}}$ pregnant mother and her $\text{Rh}^{+\text{ve}}$ fetus.

• Mechanism:

  • During the first pregnancy, fetal $\text{Rh}^{+\text{ve}}$ antigens are isolated from maternal $\text{Rh}^{-\text{ve}}$ blood by the placenta.

  • During delivery of the first child, maternal blood may be exposed to small amounts of fetal $\text{Rh}^{+\text{ve}}$ blood, inducing maternal anti-Rh antibody production.

  • In subsequent pregnancies, maternal anti-Rh IgG antibodies cross the placenta into fetal circulation and destroy fetal RBCs.

• Consequences: Severe anemia, jaundice, brain damage, 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

• A protective dynamic response to physical injury or vascular trauma preventing excessive blood loss.

• Clot/Coagulum Structure: Dark reddish-brown mesh formed by an insoluble network of fibrin threads trapping damaged blood cells.

Enzymatic Cascade Process

• Coagulation relies on a sequential cascade involving inactive plasma factors:

  1. Injury stimulates blood platelets and damaged tissue cells to release thromboplastic factors.

  2. These factors trigger a cascade reaction producing the enzyme complex thrombokinase.

  3. Thrombokinase converts inactive prothrombin in plasma into active thrombin.

  4. Thrombin acts as an enzyme to convert soluble, inactive fibrinogen into insoluble fibrin polymers.

  5. Fibrin threads entangle formed elements to complete the clot formation.

• Role of Calcium: Calcium ions ($\text{Ca}^{++}$) are required as crucial cofactors at multiple stages of the clotting cascade.

Lymph and the Lymphatic System

• Formation of Interstitial/Tissue Fluid:

  • As blood flows through tissue capillaries, water and small water-soluble substances diffuse out into extracellular spaces.

  • Large plasma proteins and formed elements remain inside the capillaries.

  • The resulting fluid in the intercellular spaces is called interstitial fluid or tissue fluid.

  • Tissue fluid has an inorganic mineral concentration identical to plasma.

  • Serves as the principal medium for exchange of gases, nutrients, and metabolic wastes between blood capillaries and body cells.

• Lymphatic System Structure and Function:

  • An extensive vascular network of lymphatic vessels collects interstitial fluid and returns it to major systemic veins.

  • The fluid contained in the lymphatic system is called lymph.

  • Properties: Colourless fluid containing specialized lymphocytes responsible for immune responses.

  • Transport Roles: Serves as a carrier for nutrients and hormones.

  • Fat Absorption: Dietary fats are absorbed into lymph within specialized lymphatic capillaries called lacteals located in intestinal villi.

Evolutionary Patterns of Circulatory Systems

Types of Circulatory Systems

• Open Circulatory System: Blood pumped by the heart enters large vessels that empty into open body cavities or spaces called sinuses. Present in Arthropods and Molluscs.

• Closed Circulatory System: Blood pumped by the heart circulates continuously within a closed network of blood vessels. Allows precise pressure regulation and targeted organ perfusion. Present in Annelids and Chordates.

Diversity in Vertebrate Cardiac Architecture

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

  • Single Circulation: Deoxygenated blood from the heart is pumped to gills for oxygenation and distributed directly to body tissues; deoxygenated blood then returns to the heart.

• Amphibians and Reptiles (except Crocodiles; 3-Chambered Heart: 2 Atria, 1 Ventricle):

  • Incomplete Double Circulation: Left atrium receives oxygenated blood from gills/lungs/skin; right atrium receives deoxygenated blood from the body. Both atria empty into a single ventricle, causing mixing of oxygenated and deoxygenated blood before systemic ejection.

• Crocodiles, Birds, and Mammals (4-Chambered Heart: 2 Atria, 2 Ventricles):

  • Double Circulation: Left and right sides are completely separated by septa. Left atrium receives oxygenated blood and passes it to the left ventricle; right atrium receives deoxygenated blood and passes it to the right ventricle. Oxygenated and deoxygenated blood streams remain completely separate.

Anatomy of the Human Heart

• Origin and Location: Derived from mesoderm; located in the thoracic cavity between the lungs, slightly tilted toward the left side; size of a closed fist.

• Pericardium: Double-walled membranous sac enclosing the pericardial fluid, which reduces friction during heart contractions.

Internal Chambers and Septa

• Atria: Two smaller upper chambers receiving blood.

• Ventricles: Two larger lower pumping chambers with thick muscular walls.

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

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

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

Cardiac Valves and Directional Flow

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

• Bicuspid (Mitral) Valve: Formed of two flaps; guards the left atrio-ventricular opening.

• Semilunar Valves: Located at the exits of the right ventricle into the pulmonary artery and left ventricle into the aorta.

• Function: Ensure unidirectional blood flow (Atria $\rightarrow$ Ventricles $\rightarrow$ Arteries) and prevent backflow.

Myocardium and Conducting (Nodal) System

• Entire heart composed of specialized cardiac muscle tissue.

• Ventricular walls are substantially thicker than atrial walls to withstand high pumping pressures.

• Nodal Tissue: Specialized, autoexcitable cardiac musculature capable of generating intrinsic action potentials without neural stimulation.

  • Sino-atrial Node (SAN): Located in the upper right corner of the right atrium. Generates the highest frequency of action potentials ($70\text{--}75\,\text{min}^{-1}$); establishes the rhythm of heartbeats, serving as the pacemaker.

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

  • Atrio-ventricular Bundle (AV Bundle / Bundle of His): Nodal fiber bundle extending from the AVN, traversing the atrio-ventricular septum, emerging at the top of the inter-ventricular septum, and dividing into right and left branches.

  • Purkinje Fibres: Fine conduction fibers extending throughout the ventricular myocardium from the bundle branches.

• Heart Rate: Average resting heart rate is $70\text{--}75\,\text{beats min}^{-1}$ (mean resting rate: $72\,\text{beats min}^{-1}$).

The Cardiac Cycle

• The sequence of electrical and mechanical events repeated during each heartbeat, consisting of atrial and ventricular systole (contraction) and diastole (relaxation).

Phases of the Cardiac Cycle

1. Joint Diastole:

   • All four chambers are relaxed.

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

   • Blood from pulmonary veins and venae cavae flows passively into the left and right ventricles through the atria.

2. Atrial Systole:

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

   • Atrial contraction increases blood flow into the ventricles by approximately $30\%$.

3. Ventricular Systole and Atrial Diastole:

   • Impulse propagates from SAN $\rightarrow$ AVN $\rightarrow$ AV Bundle $\rightarrow$ Bundle of His $\rightarrow$ Purkinje fibers $\rightarrow$ Ventricular myocardium.

   • Ventricular contraction begins, increasing intraventricular pressure.

   • Rising pressure causes closure of tricuspid and bicuspid valves, preventing blood backflow into atria.

   • Atria undergo diastole (relaxation) concurrently.

   • Further pressure rise forces open the semilunar valves of the pulmonary artery and aorta, propelling blood into systemic and pulmonary circulations.

4. Ventricular Diastole and Joint Diastole:

   • Ventricles relax, dropping intraventricular pressure.

   • Closure of semilunar valves occurs as blood attempts to flow back from major arteries.

   • Continued pressure drop causes intraventricular pressure to fall below atrial pressure, reopening tricuspid and bicuspid valves.

   • Blood resumes passive flow into ventricles, restoring joint diastole.

Quantitative Cardiac Metrics

• Duration: At a baseline rate of $72\,\text{beats min}^{-1}$, one cardiac cycle lasts $0.8\,\text{seconds}$.

• Stroke Volume (SV): Volume of blood ejected by each ventricle during a single cycle; averages approximately $70\,\text{mL}$.

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

• Dynamic Range: Cardiac output changes dynamically; for example, trained athletes possess significantly higher stroke volumes and cardiac outputs than untrained individuals.

Heart Sounds

• Auditory signals generated during cardiac valve movements, detectable via stethoscope:

  • First Heart Sound ("Lub"): Lower pitched sound associated with the closure of tricuspid and bicuspid valves at the onset of ventricular systole.

  • Second Heart Sound ("Dub"): Higher pitched sound associated with the closure of semilunar valves at the onset of ventricular diastole.

  • Serve as clinical diagnostic markers for cardiovascular function.

Electrocardiogram (ECG)

• An electrocardiogram (ECG) is a graphical trace of electrical potentials generated by cardiac muscle cells during each cardiac cycle, recorded using an electrocardiograph.

Standard ECG Electrode Configuration

• Standard Lead Placement: Three leads attached to the patient (one lead to each wrist and one lead to the left ankle) for basic rhythm analysis.

• Multi-lead Evaluation: Multiple chest leads attached across the precordium for detailed anatomical diagnostic evaluation.

Wave Components and Clinical Significance

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

• QRS Complex: Represents ventricular depolarisation, triggering ventricular contraction. Contraction begins shortly after the Q peak and marks the onset of ventricular systole.

• T-Wave: Represents ventricular repolarisation (recovery from excited to resting state). The conclusion of the T-wave marks the end of ventricular systole.

• Heart Rate Determination: Counting the number of QRS complexes over a defined interval yields the patient's heart rate.

• Diagnostic Value: Deviations from standard ECG wave geometry indicate specific pathologies, arrhythmias, or myocardial damage.

Double Circulation

• Human blood flow occurs entirely within blood vessels through two separate circulatory circuits.

Histology of Blood Vessels

• Both arteries and veins consist of three distinct tunic layers:

  • Tunica Intima: Innermost layer of simple squamous endothelium resting on a basement membrane.

  • Tunica Media: Middle layer composed of smooth muscle fibers and elastic fibers (substantially thinner in veins than in arteries).

  • Tunica Externa: Outer connective tissue layer containing collagen fibers.

Dual Circulatory Circuits

• Pulmonary Circulation:

  • Deoxygenated blood ejected from Right Ventricle $\rightarrow$ Pulmonary Artery $\rightarrow$ Pulmonary Capillaries of Lungs (gas exchange) $\rightarrow$ Oxygenated blood via Pulmonary Veins $\rightarrow$ Left Atrium.

• Systemic Circulation:

  • Oxygenated blood ejected from Left Ventricle $\rightarrow$ Aorta $\rightarrow$ Systemic Arteries, Arterioles, and Capillaries $\rightarrow$ Tissue exchange (delivers $O_2$/nutrients, collects $CO_2$/wastes) $\rightarrow$ Deoxygenated blood collected by Venules, Veins, and Venae Cavae $\rightarrow$ Right Atrium.

Specialized Vascular Systems

• Hepatic Portal System: A unique vascular connection in which the hepatic portal vein carries blood from the digestive tract (intestines) to the liver before returning to systemic circulation.

• Coronary System: A dedicated network of coronary arteries and cardiac veins providing blood flow exclusively to and from the myocardium.

Regulation of Cardiac Activity

• Myogenic Regulation: The heart is myogenic; baseline rhythmicity is regulated intrinsically by specialized autoexcitable nodal tissues (SAN and AVN).

Nervous and Endocrine Modulation

• Central Nervous Regulation: A cardiac control center located in the medulla oblongata modulates heart function through the Autonomic Nervous System (ANS):

  • Sympathetic Nervous System: Releases neural signals that increase heart rate, enhance the force of ventricular contraction, and increase cardiac output.

  • Parasympathetic Nervous System: Releases neural signals (via vagal impulse) that decrease heart rate, slow action potential conduction velocity, and lower cardiac output.

• Hormonal Regulation: Adrenal medullary hormones (adrenaline and noradrenaline) increase heart rate and cardiac output during stress responses.

Disorders of the Circulatory System

• High Blood Pressure (Hypertension):

  • Standard baseline resting blood pressure: $120/80\,\text{mmHg}$ ($120\,\text{mmHg}$ systolic pumping pressure; $80\,\text{mmHg}$ diastolic resting pressure).

  • Clinical Hypertension Criteria: Repeated blood pressure readings of $140/90\,\text{mmHg}$ or higher.

  • Health Effects: Increases cardiac workload; leads to heart disease; causes tissue damage in vital organs such as the brain and kidneys.

• Coronary Artery Disease (CAD) / Atherosclerosis:

  • Etiology: Deposition of calcium, fats, cholesterol, and fibrous connective tissues within the tunica intima and media of coronary arteries.

  • Pathology: Progressive narrowing of arterial lumen, reducing blood supply to the myocardium.

• Angina (Angina Pectoris):

  • Symptom: Severe chest pain resulting from transient myocardial hypoxia (insufficient oxygen supply reaching heart muscle).

  • Demographics: Can affect individuals of any age, but displays higher incidence in middle-aged and elderly populations.

• Heart Failure:

  • Pathophysiology: A clinical state in which the heart cannot pump blood effectively enough to meet systemic metabolic demands.

  • Terminology: Often called congestive heart failure due to pulmonary congestion resulting in fluid accumulation in lungs.

  • Clinical Distinctions:

  • Heart Failure: Ineffective ventricular pumping capacity.

  • Cardiac Arrest: Total cessation of cardiac electrical and mechanical activity (heart stops beating).

  • Heart Attack (Myocardial Infarction): Sudden ischemic necrosis of myocardial tissue caused by acute blockage of blood flow.