15.1 Blood
15.1.1 Plasma
Definition: Special connective tissue consisting of a fluid matrix (plasma) and formed elements.
Physical Characteristics: Straw-coloured, viscous fluid constituting nearly 55% of total blood volume.
Composition: 90–92% water; proteins contribute 6–8%.
Major Plasma Proteins
- Fibrinogen: Needed for clotting/coagulation of blood NEET 2011, 2018
- Globulins: Primarily involved in the body's defense mechanisms NEET 2009, 2018
- Albumins: Help maintain osmotic balance NEET 2018
Minerals & Nutrients: Contains small amounts of Na⁺, Ca⁺⁺, Mg⁺⁺, HCO₃⁻, Cl⁻, etc. Na⁺ is the principal plasma cation NEET 1999. Glucose, amino acids, and lipids are also present.
15.1.2 Formed Elements
Abundance: Collectively constitute nearly 45% of blood. Decreasing order: Erythrocytes > Platelets > Leucocytes NEET 2022 Re.
Erythrocytes (Red Blood Cells - RBCs)
- Count & Origin: Most abundant blood cells. 5 to 5.5 million RBCs per mm³. Formed in red bone marrow in adults NEET 2015
- Structure: Devoid of nucleus in most mammals; biconcave shape. Enucleation maximizes space for oxygen transport; mature RBCs lack DNA NEET 2009, 2017. Do not occur in cockroaches NEET 1988
- Haemoglobin: Red, iron-containing complex protein. Healthy individual: 12–16 g/100 mL blood NEET 2010. Breakdown products: bilirubin and biliverdin NEET 1988
- Life Span & Fate: Average lifespan 120 days. Destroyed in spleen ("graveyard of RBCs") NEET 2012, 2021
- High Altitude Adaptation: Altitudes ≥3500 m induce increased RBC production and breathing rate NEET 2010
Leucocytes (White Blood Cells - WBCs)
General Features: Colourless (no haemoglobin), nucleated, short-lived. Count: 6000–8000/mm³. Can squeeze through capillary walls (diapedesis) NEET 2000. Divided into granulocytes and agranulocytes.
| WBC Type | Category | Relative Abundance | Key Functions & Features |
|---|---|---|---|
| Neutrophils | Granulocyte | 60–65% (Most abundant) | Phagocytic cells; destroy foreign organisms NEET 2008, 2013, 2020 |
| Eosinophils | Granulocyte | 2–3% | Resist infections; associated with allergic reactions, release histaminase NEET 2013, 2020 |
| Basophils | Granulocyte | 0.5–1% (Least abundant) | Secrete histamine, serotonin, heparin; involved in inflammatory reactions NEET 2008, 2013, 2023. 2-3 lobed nucleus |
| Monocytes | Agranulocyte | 6–8% | Phagocytic cells NEET 2008, 2023-Manipur |
| Lymphocytes | Agranulocyte | 20–25% | Two major types: 'B' and 'T' forms. Responsible for immune responses NEET 2023-Manipur |
Platelets (Thrombocytes)
- Origin & Count: Cell fragments from megakaryocytes (bone marrow). Count: 1,50,000–3,50,000/mm³
- Function: Release substances involved in blood coagulation/clotting
- Clinical Exception: Reduced count → clotting disorders → excessive blood loss NEET 2016
15.1.3 Blood Groups
ABO Grouping
Basis: Presence/absence of two surface antigens (A and B) on RBCs and two natural antibodies in plasma.
Transfusion Clumping: Mismatching causes severe clumping/destruction of RBCs.
| Blood Group | Antigens on RBCs | Antibodies in Plasma | Can Receive From |
|---|---|---|---|
| A | A | anti-B | A, O NEET 1989 |
| B | B | anti-A | B, O |
| AB | A, B | nil | AB, A, B, O (Universal Recipient) NEET 2014, 2021 |
| O | nil | anti-A, anti-B | O (Universal Donor) NEET 2009, 2011, 2012 |
Universal Donor Key: O individuals have no antigens A and B on RBCs NEET 2001, 2009
Rh Grouping
Basis: Rh antigen (similar to Rhesus monkey) present on RBC surface of ~80% humans (Rh +ve). Absent individuals are Rh -ve. Exposure of Rh -ve person to Rh +ve blood induces anti-Rh antibody formation.
Erythroblastosis Foetalis (Rh Incompatibility)
- Condition: Occurs when an Rh -ve mother carries an Rh +ve foetus NEET 2000, 2020-Covid
- Mechanism: First pregnancy safe (bloods separated by placenta). During first delivery, maternal blood may contact foetal Rh +ve blood → mother synthesizes anti-Rh antibodies. In subsequent pregnancies, maternal Rh antibodies leak into foetal blood → destroy foetal RBCs
- Consequences: Severe anaemia, jaundice, or foetal death NEET 1988
- Prevention: Administer anti-Rh antibodies to mother immediately after first delivery
- Transfusion Match: An A Rh⁻ person can only receive blood from A Rh⁻ or O Rh⁻ donors NEET 2024 Re
15.1.4 Coagulation of Blood
Definition: Mechanism to prevent excessive blood loss via a dark reddish-brown scum called a clot/coagulum.
Composition: Network of threads called fibrins trapping dead/damaged formed elements.
Cascade Mechanism Steps
- Injury stimulates platelets and tissues to release coagulation-initiating factors
- Thrombokinase (enzyme complex) formed by series of linked enzymic reactions (cascade process)
- Inactive prothrombin → active thrombin by thrombokinase
- Inactive fibrinogen → active fibrins by thrombin NEET 2021
15.2 Lymph (Tissue Fluid)
Formation: Blood passing through tissue capillaries filters out water and small water-soluble substances into intercellular spaces, leaving behind larger proteins and formed elements → interstitial/tissue fluid.
Characteristics: Same mineral distribution as plasma. Colourless fluid containing specialized lymphocytes.
Composition Difference: Lymph has more WBCs and no RBCs NEET 2009, 2015 Cancelled. Less protein content than blood.
Functions
- Acts as middleman for exchange of nutrients and gases between blood and cells
- Collected by lymphatic system and drained back into major veins to return interstitial fluid to blood NEET 1995
- Important carrier for nutrients, hormones, etc.
- Fats absorbed through lymph in lacteals within intestinal villi
15.3 Circulatory Pathways
15.3.1 Types of Systems & Vertebrate Hearts
Open Circulatory System: Present in arthropods and molluscs. Blood pumped by heart passes through large vessels into open spaces/body cavities called sinuses. Observed in Periplaneta NEET 2006.
Closed Circulatory System: Present in annelids and chordates. Blood always circulated through closed network of blood vessels. More advantageous due to precisely regulated flow.
Evolutionary Patterns of Vertebrate Hearts
- 2-Chambered Heart: Fishes (one atrium, one ventricle). Pumps deoxygenated blood → gills → body → heart (single circulation)
- 3-Chambered Heart: Amphibians and reptiles (except crocodiles). Two atria, one ventricle. Oxygenated and deoxygenated blood mix in single ventricle → mixed blood pumped out (incomplete double circulation)
- 4-Chambered Heart: Crocodiles, birds, mammals. Two atria, two ventricles. Oxygenated and deoxygenated blood remain separated. Ventricles pump via two separate pathways (double circulation). Whales possess this separate double pathway NEET 2015
15.3.2 Human Circulatory System
Anatomy of the Human Heart
- Origin & Location: Mesodermally derived. Situated in thoracic cavity between lungs, slightly tilted left. Size of clenched fist
- Protection: Double-walled membranous bag called pericardium, enclosing pericardial fluid
- Internal Chambers: Two small upper atria, two larger lower ventricles
- Septa: Inter-atrial septum (thin, muscular), Inter-ventricular septum (thick-walled), Atrio-ventricular septum (thick fibrous tissue)
Valves (Prevent Backward Flow)
- Tricuspid Valve: Three muscular flaps guarding right atrium → right ventricle NEET 2018
- Bicuspid / Mitral Valve: Guards left atrium → left ventricle NEET 2018
- Semilunar Valves: Guard openings of right ventricle → pulmonary artery and left ventricle → aorta NEET 2018
- Wall Thickness: Ventricular walls much thicker than atrial walls
Nodal Tissue (Conduction System)
Nature: Special autoexcitable cardiac musculature that generates action potentials without external stimuli.
Components & Conduction Pathway Sequence
- Sino-atrial Node (SAN): Right upper corner of right atrium. Generates max action potentials (70–75/min). Maintains rhythmic contractile activity; called the Pacemaker NEET 1999, 2003, 2011
- Atrio-ventricular Node (AVN): Lower left corner of right atrium close to inter-atrial septum
- AV Bundle: Continues from AVN, passes through atrio-ventricular septa, splits at top of inter-ventricular septum into right and left bundles
- Purkinje Fibres: Minute fibres from bundle branches distributed throughout ventricular musculature
Complete High-Yield Sequence:
SAN → AVN → AV Bundle → Bundle Branches → Purkinje Fibres → Heart Muscles NEET 1995, 2024
15.3.3 Cardiac Cycle
Duration: Heart beats ~72 times/minute → one complete cycle takes 0.8 seconds.
Phase-by-Phase Mechanism
- Joint Diastole: All four chambers relaxed. Tricuspid & bicuspid valves open; semilunar valves closed. Blood from vena cava & pulmonary veins flows freely into ventricles through atria NEET 2022 Re
- Atrial Systole: SAN generates action potential → simultaneous contraction of both atria → increases blood flow into ventricles by ~30%
- Ventricular Systole: Action potential travels via AVN & Bundle of His to ventricular musculature → ventricular contraction. Atria undergo diastole simultaneously
- Valvular Action in Ventricular Systole: Rising ventricular pressure → immediate closure of tricuspid & bicuspid valves (prevent backflow). Further pressure increase → semilunar valves force open → blood pumped into pulmonary artery & aorta
- Ventricular Diastole: Ventricles relax → ventricular pressure falls → immediate closure of semilunar valves (prevent backflow from arteries). Falling pressure → tricuspid & bicuspid valves open again → return to joint diastole
Volumes & Heart Sounds
Stroke Volume: Volume pumped by each ventricle per cardiac cycle ≈ 70 mL
Cardiac Output Formula:
Average Cardiac Output: ≈ 5000 mL (5 Litres/min) in healthy individual. Can be altered dynamically (e.g., higher in athletes).
If End-Diastolic Vol = 100 mL, End-Systolic Vol = 50 mL → Stroke Volume = 100 - 50 = 50 mL
Given Cardiac Output = 5 L = 5000 mL → Heart Rate = 5000 / 50 = 100 beats/min
Heart Sounds
- First Heart Sound (Lub): Closure of tricuspid & bicuspid valves at beginning of ventricular systole NEET 2024 Re
- Second Heart Sound (Dub): Closure of semilunar valves at beginning of ventricular diastole NEET 2015
15.3.4 Electrocardiogram (ECG)
Definition: Graphical representation of electrical activity of heart during cardiac cycle. Obtained using electro-cardiograph.
Lead Configuration: Standard ECG: three electrical leads (one to each wrist, one to left ankle). Detailed evaluation: multiple chest leads.
High-Yield Waves & Clinical Significance
- P-wave: Represents electrical excitation (depolarisation) of atria → contraction of both atria NEET 2013, 2019
- QRS complex: Represents depolarisation of ventricles → initiates ventricular contraction NEET 2020, 2023. Ventricular contraction begins shortly after Q, marking beginning of systole
- Pulse Counting: Counting QRS complexes in given time period directly determines heartbeat rate NEET 2011
- T-wave: Represents return of ventricles from excited to normal state (repolarisation of ventricles) NEET 2019, 2023
- End of T-wave: Marks end of systole NEET 2023-Manipur
- T-P Gap: Period when heart muscles are electrically silent NEET 2024
15.4 Double Circulation
Blood Vessel Anatomy
Three-Layered Wall Structure
- Tunica intima: Inner lining of squamous endothelium
- Tunica media: Middle layer of smooth muscle & elastic fibres. Comparatively thin in veins NEET 1990
- Tunica externa: External layer of fibrous connective tissue with collagen fibres
Capillary Wall: Made of endothelium only NEET 1993
Dual Circuits
- Pulmonary Circulation: Deoxygenated blood from Right Ventricle → Pulmonary Artery → Lungs (oxygenation) → Pulmonary Veins → Left Atrium NEET 2013
- Pressure/Wall Detail: Pulmonary artery wall thicker than pulmonary vein wall NEET 2000; blood pressure higher in pulmonary artery than pulmonary vein NEET 2016
- Systemic Circulation: Oxygenated blood from Left Ventricle → Aorta → Arteries/Arterioles/Capillaries → Tissues → Venules/Veins/Vena Cava → Right Atrium. Provides nutrients & O₂ while clearing CO₂
- Aorta Pressure: Blood pressure in mammalian aorta maximum during left ventricular systole NEET 2015 Cancelled
Portal and Coronary Systems
- Hepatic Portal System: Unique vascular connection between digestive tract and liver. Hepatic portal vein carries blood from intestine → liver → systemic circulation NEET 2017, 2022 Re. Hepatic vein carries largest amount of urea NEET 2016
- Coronary System: Dedicated network of blood vessels exclusively supplying blood to/from cardiac musculature
15.5 Regulation of Cardiac Activity
- Myogenic Nature: Normal cardiac activities regulated intrinsically (auto-regulated) by specialized nodal tissue musculature without external neural stimuli
- Neural Regulation Center: Located in medulla oblongata; moderates function via Autonomic Nervous System (ANS)
- Sympathetic Nerves: Increase heart rate, strengthen ventricular contraction, increase cardiac output
- Parasympathetic Nerves: Decrease heart rate, slow down action potential conduction, decrease cardiac output NEET 2014
- Hormonal Regulation: Adrenal medullary hormones (adrenaline/epinephrine & noradrenaline) increase cardiac output
15.6 Disorders of the Circulatory System
High Blood Pressure (Hypertension)
- Blood pressure higher than normal standard of 120/80 mm Hg
- 120 mm Hg = Systolic (pumping) pressure; 80 mm Hg = Diastolic (resting) pressure NEET 1998
- Repeated readings of 140/90 mm Hg or higher denote hypertension. Can harm vital organs like brain and kidney NEET 2011
Coronary Artery Disease (CAD) / Atherosclerosis
Affects vessels supplying blood to heart muscle. Caused by lumen narrowing due to deposits of calcium, fat, cholesterol, and fibrous tissues. Thickening of arterial wall = arteriosclerosis NEET 1999
Angina (Angina Pectoris)
Symptom of acute chest pain appearing when insufficient oxygen reaches heart muscle. Occurs due to compromised blood flow; common in middle-aged and elderly individuals.
Heart Failure
State where heart not pumping blood effectively enough to meet body demands. Also termed congestive heart failure because lung congestion is primary symptom.
• Heart Failure ≠ Cardiac Arrest (heart completely stops beating)
• Heart Failure ≠ Heart Attack (heart muscle suddenly damaged by inadequate blood supply)