Body Fluids and Circulation

• Introduction to Body Fluids and Circulation

• Living cells require a continuous supply of nutrients, $\text{O}_2$, and other essential substances to maintain cellular metabolism and tissue health.

• Waste or harmful substances generated during cellular processes must be continuously removed to prevent toxicity and maintain homeostasis.

• Transport Mechanisms across animal groups:

  • Simple organisms (such as sponges and coelenterates) circulate water from their surroundings through body cavities to facilitate exchange of substances directly with cells.

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

  • Blood is the primary body fluid used by most higher organisms, including humans.

  • Lymph (tissue fluid) serves as an additional body fluid that aids in transporting specific substances.

Blood Composition and Components

• Blood is classified as a special connective tissue composed of a fluid matrix, plasma, and formed elements.

Plasma

• Physical characteristics: A straw-coloured, viscous fluid constituting approximately $55\%$ of the total blood volume.

• Chemical composition:

  • Water makes up $90\text{--}92\%$ of plasma.

  • Proteins contribute $6\text{--}8\%$ of plasma.

• Major Plasma Proteins and Functions:

  • Fibrinogen: Essential for blood clotting (coagulation).

  • Globulins: Primarily involved in defense mechanisms of the body.

  • Albumins: Crucial for maintaining osmotic balance.

• Inorganic Minerals Present: Small amounts of ions including $\text{Na}^+$, $\text{Ca}^{++}$, $\text{Mg}^{++}$, $\text{HCO}_3^-$, and $\text{Cl}^-$.

• Organic Nutrients in Transit: Glucose, amino acids, lipids, and other substrates continuously present in plasma as they are in transit in the body.

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

• Serum: Plasma from which clotting factors have been removed.

Formed Elements

• Constitute nearly $45\%$ of the blood.

• Comprises erythrocytes, leucocytes, and platelets.

• Erythrocytes (Red Blood Cells / RBCs):

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

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

  • Structure: Devoid of nucleus in most mammals; biconcave in shape.

  • Haemoglobin Content: Contains a red-coloured, iron-containing complex protein called haemoglobin.

  • Normal Concentration: A healthy individual has $12\text{--}16\,\text{gms}$ of haemoglobin in every $100\,\text{ml}$ of blood.

  • Function: Plays a significant role in the transport of respiratory gases.

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

• Leucocytes (White Blood Cells / WBCs):

  • General Characteristics: Colourless due to the lack of haemoglobin; nucleated; generally short-lived.

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

  • Classification: Divided into Granulocytes and Agranulocytes.

  • Granulocytes:

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

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

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

  • Agranulocytes:

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

  • Lymphocytes: Constitute $20\text{--}25\%$ of total WBCs; major types are 'B' and 'T' forms, both responsible for immune responses of the body.

• Platelets (Thrombocytes):

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

  • Concentration: Normally contains $1,500,00\text{--}3,500,00\,\text{platelets mm}^{-3}$.

  • Function: Release a variety of substances, most of which are involved in the coagulation or clotting of blood.

  • Clinical Impact: A reduction in platelet number leads to clotting disorders resulting in excessive loss of blood from the body.

Human Blood Groups and Compatibility

ABO Grouping

• Antigen Basis: Based on the presence or absence of two surface antigens (chemicals that can induce immune response) on RBCs, namely A and B.

• Antibody Basis: Plasma contains two natural antibodies (proteins produced in response to antigens), designated as anti-A and anti-B.

• Transfusion Matching: Blood of a donor must be carefully matched with the blood of a recipient before transfusion to avoid severe problems of clumping (destruction of RBC).

• Distribution Breakdown:

  • Group A: Antigens on RBCs = A; Antibodies in Plasma = anti-B; Donor's Group = A, O.

  • Group B: Antigens on RBCs = B; Antibodies in Plasma = anti-A; Donor's Group = B, O.

  • Group AB: Antigens on RBCs = A, B; Antibodies in Plasma = nil; Donor's Group = AB, A, B, O.

  • Group O: Antigens on RBCs = nil; Antibodies in Plasma = anti-A, B; Donor's Group = O.

• Universal Classifications:

  • Universal Donors: Group 'O' blood can be donated to persons with any other blood group; hence, group 'O' individuals are called universal donors.

  • Universal Recipients: Group 'AB' persons can accept blood from persons with AB as well as other groups; hence, they are called universal recipients.

Rh Grouping

• Antigen Overview: The Rh antigen, similar to one present in Rhesus monkeys, is observed on the surface of RBCs of nearly $80\%$ of humans.

  • Rh Positive ($\text{Rh}^{+\text{ve}}$): Individuals possessing the Rh antigen.

  • Rh Negative ($\text{Rh}^{-\text{ve}}$): Individuals in whom the Rh antigen is absent.

• Sensitization: An $\text{Rh}^{-\text{ve}}$ person exposed to $\text{Rh}^{+\text{ve}}$ blood forms specific antibodies against Rh antigens. Therefore, Rh group matching is required before transfusions.

• Erythroblastosis Foetalis (Rh Incompatibility in Pregnancy):

  • Occurs between an $\text{Rh}^{-\text{ve}}$ blood of a pregnant mother and $\text{Rh}^{+\text{ve}}$ blood of the foetus.

  • First Pregnancy: Rh antigens of the foetus do not get exposed to the $\text{Rh}^{-\text{ve}}$ blood of the mother as the two bloods are well separated by the placenta.

  • Delivery Exposure: During delivery of the first child, maternal blood may be exposed to small amounts of $\text{Rh}^{+\text{ve}}$ blood from the foetus.

  • Maternal Response: Mother starts preparing antibodies against Rh antigen in her blood.

  • Subsequent Pregnancies: Rh antibodies from the $Rh^{-ve}$ mother leak into the blood of the $Rh^{+ve}$ foetus and destroy foetal RBCs.

  • Consequences: Can be fatal to the foetus or cause severe anaemia and jaundice to the baby.

  • Prevention: Avoided by administering anti-Rh antibodies to the mother immediately after the delivery of the first child.

Blood Coagulation Mechanism

• Function: Blood exhibits coagulation or clotting in response to an injury or trauma as a mechanism to prevent excessive loss of blood from the body.

• Clot Structure: A dark reddish brown scum formed at the site of injury over time; it is a clot or coagulum formed mainly of a network of threads called fibrins in which dead and damaged formed elements of blood are trapped.

• Enzymatic Cascade Steps:

  1. Inactive prothrombin in plasma is converted to thrombin.

  2. The enzyme complex thrombokinase is required for converting prothrombin to thrombin.

  3. Thrombokinase is formed by a series of linked enzymic reactions (cascade process) involving a number of factors present in plasma in an inactive state.

  4. Thrombin converts inactive fibrinogen in plasma into fibrin.

• Activation Triggers:

  • Injury or trauma stimulates platelets in blood to release factors that activate clotting.

  • Factors released by tissues at the site of injury also initiate coagulation.

• Role of Ions: Calcium ions ($\text{Ca}^{++}$) play a very important role in clotting.

Lymph and Tissue Fluid

• Interstitial Fluid Formation:

  • As blood passes through capillaries in tissues, water along with many small water-soluble substances move out into spaces between tissue cells.

  • Larger proteins and most formed elements remain inside blood vessels.

  • The released fluid is called interstitial fluid or tissue fluid.

• Chemical Composition: Has the same mineral distribution as plasma.

• Function: Exchange of nutrients, gases, etc., between blood and cells always occurs through tissue fluid.

• Lymphatic System & Lymph:

  • An elaborate network of vessels called the lymphatic system collects tissue fluid and drains it back to major veins.

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

• Properties and Functions of Lymph:

  • Colourless fluid containing specialized lymphocytes responsible for immune responses of the body.

  • Important carrier for nutrients, hormones, etc.

  • Dietary Fat Absorption: Fats are absorbed through lymph in lacteals present in the intestinal villi.

Comparative Circulatory Pathways

Open vs. Closed Circulatory Systems

• Open Circulatory System:

  • Blood pumped by the heart passes through large vessels into open spaces or body cavities called sinuses.

  • Present in arthropods and molluscs.

• Closed Circulatory System:

  • Blood pumped by the heart is always circulated through a closed network of blood vessels.

  • Present in annelids and chordates.

  • Considered more advantageous as fluid flow can be more precisely regulated.

Evolutionary Patterns of Vertebrate Hearts

• Fishes:

  • 2-chambered heart ($1$ atrium and $1$ ventricle).

  • Single Circulation: Heart pumps out deoxygenated blood, which is oxygenated by gills and supplied to body parts; deoxygenated blood returns to the heart.

• Amphibians and Reptiles (except Crocodiles):

  • 3-chambered heart ($2$ atria and $1$ single ventricle).

  • Incomplete Double Circulation: Left atrium receives oxygenated blood from gills/lungs/skin and right atrium receives deoxygenated blood from other body parts. They get mixed up in the single ventricle, which pumps out mixed blood.

• Crocodiles, Birds, and Mammals:

  • 4-chambered heart ($2$ atria and $2$ ventricles).

  • Double Circulation: Oxygenated and deoxygenated blood received by left and right atria pass to ventricles of the same side. Ventricles pump it out without mixing, creating two separate circulatory pathways.

Anatomy of the Human Heart

• Structure & Location:

  • Muscular organ derived from the mesoderm.

  • Situated in the thoracic cavity, in between the two lungs, slightly tilted to the left.

  • Size of a clenched fist.

• Protection: Protected by a double-walled membranous bag called the pericardium, enclosing pericardial fluid.

Chambers and Internal Partitioning

• Possesses 4 chambers: two relatively small upper chambers (atria) and two larger lower chambers (ventricles).

• Partition Walls:

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

  • Inter-ventricular Septum: A thick-walled structure separating the left and right ventricles.

  • Atrio-ventricular Septum: A thick fibrous tissue separating the atrium and ventricle of the same side, provided with an opening connecting the two chambers.

Cardiac Valves

• Tricuspid Valve: Formed of three muscular flaps or cusps; guards the opening between right atrium and right ventricle.

• Bicuspid Valve (Mitral Valve): Guards the opening between left atrium and left ventricle.

• Semilunar Valves: Guard the openings of the right and left ventricles into the pulmonary artery and aorta, respectively.

• Function: Allows blood flow only in one direction (atria to ventricles, and ventricles to pulmonary artery or aorta), preventing backward flow.

Musculature & Nodal Tissue

• Heart is made entirely of cardiac muscles; walls of ventricles are much thicker than atria.

• Nodal Tissue (Specialized Auto-excitable Cardiac Musculature):

  • Sino-atrial Node (SAN): A patch of tissue in the right upper corner of the right atrium. Generates maximum action potentials ($70\text{--}75\,\text{min}^{-1}$); acts as the pacemaker initiating and maintaining rhythmic contractile activity.

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

  • AV Bundle (Atrio-ventricular Bundle): Bundle of nodal fibres continuing from AVN through atrio-ventricular septa to the top of inter-ventricular septum, dividing into right and left bundles.

  • Purkinje Fibres: Minute fibres arising from bundle branches throughout the ventricular musculature of respective sides.

  • Auto-excitability: Nodal musculature can generate action potentials without external stimuli.

Mechanics of the Cardiac Cycle

Sequence of Events

• Normal Heart Rate: Heart normally beats $70\text{--}75\,\text{times in a minute}$ (average $72\,\text{beats min}^{-1}$).

• Duration of Cycle: Calculated as $0.8\,\text{seconds}$ per cardiac cycle.

• Phases:

  1. Joint Diastole: All four chambers are in a relaxed state. Tricuspid and bicuspid valves are open; semilunar valves are closed. Blood from pulmonary veins and vena cava flows into left and right ventricles through left and right atria.

  2. Atrial Systole: SAN generates action potential stimulating simultaneous contraction of both atria. Increases blood flow into ventricles by about $30\%$.

  3. Ventricular Systole: Action potential conducted to ventricular side by AVN and AV bundle, then Bundle of His transmits it through ventricular musculature. Ventricles contract; atria undergo relaxation (diastole) simultaneously.

  • Ventricular pressure increases $\rightarrow$ closure of tricuspid and bicuspid valves due to attempted backflow of blood.

  • Pressure increases further $\rightarrow$ semilunar valves guarding pulmonary artery and aorta are forced open, blood flows into circulatory pathways.

  1. Ventricular Diastole: Ventricles relax and pressure falls $\rightarrow$ semilunar valves close preventing backflow of blood into ventricles.

  • Declining pressure allows tricuspid and bicuspid valves to be pushed open by pressure in atria exerted by emptying veins.

  • Blood moves freely to ventricles again; returns to Joint Diastole.

Output Parameters

• Stroke Volume: Volume of blood pumped out by each ventricle during a cardiac cycle, approximately $70\,\text{mL}$.

• Cardiac Output: Volume of blood pumped out by each ventricle per minute.

  • Formula: $\text{Cardiac Output} = \text{Stroke Volume} \times \text{Heart Rate}$

  • Average Value: $70\,\text{mL} \times 72 = 5000\,\text{mL}$ or $5\,\text{litres}$ per minute in a healthy individual.

  • Body can alter stroke volume and heart rate (e.g., cardiac output of an athlete is higher than an ordinary man).

Heart Sounds

• First Heart Sound (lub): Associated with the closure of tricuspid and bicuspid valves.

• Second Heart Sound (dub): Associated with the closure of semilunar valves.

• Clinical Diagnostic Significance: Easily heard through a stethoscope to assess cardiac health.

Electrocardiogram (ECG)

• Definition: Graphical representation of the electrical activity of the heart during a cardiac cycle, obtained using an electro-cardiograph machine.

• Patient Setup: Standard ECG uses 3 electrical leads attached to one to each wrist and to the left ankle; multiple leads attached to chest region for detailed evaluation.

• Waveform Breakdown:

  • P-wave: Represents electrical excitation (or depolarisation) of the atria, leading to contraction of both atria.

  • QRS Complex: Represents depolarisation of the ventricles, initiating ventricular contraction. Contraction starts shortly after Q and marks the beginning of systole.

  • T-wave: Represents the return of ventricles from excited to normal state (repolarisation). End of T-wave marks the end of systole.

• Clinical Utility: Counting QRS complexes over a time period determines heart rate; shape deviations indicate potential abnormality or disease.

Double Circulation and Vascular Anatomy

Blood Vessel Histology

• Arteries and veins consist of three distinct wall layers:

  • Tunica Intima: Inner lining of squamous endothelium.

  • Tunica Media: Middle layer of smooth muscle and elastic fibres; comparatively thin in veins.

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

Pathways of Circulation

• Pulmonary Circulation:

  • Deoxygenated blood pumped by right ventricle enters pulmonary artery.

  • Passed to lungs for oxygenation; oxygenated blood carried by pulmonary veins into left atrium.

• Systemic Circulation:

  • Oxygenated blood pumped by left ventricle enters aorta.

  • Carried by network of arteries, arterioles, and capillaries to tissues.

  • Deoxygenated blood collected by venules, veins, and vena cava is emptied into right atrium.

  • Provides nutrients, $\text{O}_2$, and essential substances to tissues and removes $\text{CO}_2$ and harmful substances.

• Specialized Networks:

  • Hepatic Portal System: Unique vascular connection between digestive tract and liver; hepatic portal vein carries blood from intestine to liver before systemic circulation.

  • Coronary System: Specialized blood vessels present exclusively for blood circulation to and from cardiac musculature.

Regulation of Cardiac Activity

• Myogenic Regulation: Heart is intrinsically regulated / auto-regulated by specialized muscle tissues (nodal tissue).

• Neural Regulation: Specialized neural centre in medulla oblongata moderates cardiac function through Autonomic Nervous System (ANS):

  • Sympathetic Nerves: Increase heart rate, strength of ventricular contraction, and cardiac output.

  • Parasympathetic Neural Signals: Decrease heart rate, speed of action potential conduction, and cardiac output.

• Hormonal Control: Adrenal medullary hormones increase cardiac output.

Disorders of the Circulatory System

• High Blood Pressure (Hypertension):

  • Normal BP: $120/80\,\text{mm Hg}$ ($120\,\text{mm Hg}$ = systolic/pumping pressure, $80\,\text{mm Hg}$ = diastolic/resting pressure).

  • Hypertension Criterion: Repeated checks showing $140/90\,\text{mm Hg}$ ($140\text{ over } 90$) or higher.

  • Effects: Leads to heart diseases and affects vital organs like brain and kidney.

• Coronary Artery Disease (CAD) / Atherosclerosis:

  • Affects vessels supplying blood to heart muscle.

  • Caused by deposits of calcium, fat, cholesterol, and fibrous tissues, narrowing the arterial lumen.

• Angina (Angina Pectoris):

  • Symptom of acute chest pain appearing when not enough oxygen reaches heart muscle.

  • More common among middle-aged and elderly; occurs due to conditions affecting blood flow.

• Heart Failure:

  • State of heart when it is not pumping blood effectively enough to meet body needs.

  • Called congestive heart failure because lung congestion is a main symptom.

  • Distinctions:

  • Heart Failure: Ineffective pumping to meet body needs.

  • Cardiac Arrest: Heart stops beating.

  • Heart Attack: Heart muscle is suddenly damaged by an inadequate blood supply.

SHORT NOTES SECTION:

Introduction to Body Fluids and Circulation

• Function: Supply nutrients/$\text{O}_2$ and remove cellular wastes to maintain homeostasis.

• Transport Mechanisms: Simple organisms use surrounding water (sponges); complex organisms use blood and lymph.

Blood Composition and Components

• Blood: Connective tissue consisting of plasma ($55\%$) and formed elements ($45\%$).

• Plasma: $90\text{--}92\%$ water, $6\text{--}8\%$ proteins.

  • Proteins: Fibrinogen (clotting), Globulins (defense), Albumins (osmotic balance).

  • Serum: Plasma minus clotting factors.

• Formed Elements:

  • Erythrocytes (RBCs): $5\text{--}5.5\,\text{million mm}^{-3}$, biconcave, non-nucleated, contain haemoglobin ($12\text{--}16\,\text{g/100 mL}$), lifespan $120\,\text{days}$, destroyed in spleen.

  • Leucocytes (WBCs): $6000\text{--}8000\,\text{mm}^{-3}$, nucleated.

    • Granulocytes: Neutrophils ($60\text{--}65\%$, phagocytic), Eosinophils ($2\text{--}3\%$, allergy), Basophils ($0.5\text{--}1\%$, inflammation).

    • Agranulocytes: Monocytes ($6\text{--}8\%$, phagocytic), Lymphocytes ($20\text{--}25\%$, immune responses).

  • Platelets: $1.5\text{--}3.5\,\text{lakh mm}^{-3}$, megakaryocyte fragments required for blood clotting.

Blood Groups and Coagulation

• ABO Grouping: Based on A and B antigens on RBCs.

  • Group O = Universal Donor; Group AB = Universal Recipient.

• Rh Grouping: Present in $80\%$ of humans. Mismatch in pregnancy causes Erythroblastosis Foetalis.

• Coagulation: Inactive prothrombin $\rightarrow$ active thrombin (via thrombokinase) $ ightarrow$ converts fibrinogen to fibrin clot. Requires $\text{Ca}^{++}$ ions.

Lymph and Tissue Fluid

• Interstitial Fluid: Water and solutes filtered out of tissue capillaries.

• Lymph: Colourless fluid containing lymphocytes; transports nutrients, hormones, and absorbs dietary fats via lacteals.

Comparative Circulatory Pathways

• Systems: Open (arthropods/molluscs in sinuses) vs Closed (annelids/chordates in vessels).

• Heart Structure:

  • Fishes: 2-chambered (single circulation).

  • Amphibians/Reptiles: 3-chambered (incomplete double circulation).

  • Crocodiles, Birds, Mammals: 4-chambered (double circulation).

Human Heart Anatomy & Cardiac Cycle

• Structure: Mesodermal organ, protected by double-walled pericardium. 4 chambers (2 atria, 2 ventricles).

• Valves: Tricuspid (right AV), Bicuspid/Mitral (left AV), Semilunar (aortic and pulmonary outlets).

• Nodal System: Auto-excitable; SAN is the pacemaker ($70\text{--}75\,\text{bpm}$).

• Cardiac Cycle: Duration $0.8\,\text{s}$.

  • Stroke Volume: $70\,\text{mL}$.

  • Cardiac Output: $\text{Stroke Volume} \times \text{Heart Rate} \approx 5\,\text{L/min}$.

• Heart Sounds: Lub (closure of AV valves), Dub (closure of semilunar valves).

ECG, Vessels, and Regulation

• ECG Waves: P-wave (atrial depolarisation), QRS complex (ventricular depolarisation), T-wave (ventricular repolarisation).

• Blood Vessels: 3 layers — Tunica intima, Tunica media (thinner in veins), Tunica externa.

• Circuits: Pulmonary (heart-lungs) and Systemic (heart-body).

• Regulation: Myogenic (nodal tissue), neural (ANS in medulla oblongata), and hormonal (adrenal medulla).

Disorders of Circulatory System

• Hypertension: BP $\ge 140/90\,\text{mm Hg}$ (Normal: $120/80\,\text{mm Hg}$).

• Coronary Artery Disease (CAD): Atherosclerosis narrowing lumen of coronary arteries.

• Angina Pectoris: Acute chest pain from inadequate oxygen supply to cardiac muscles.

• Heart Failure: Ineffective pumping of blood by the heart to meet body needs.