Human Physiology (Comprehensive Review)

Core Concept

Human Physiology focuses strictly on Homeostasis—the active maintenance of a steady internal state. Every single body system operates via mechanical gradients (like pressure/osmolarity) or precise molecular signals (like neurotransmitters/hormones). For the IAT, intensely focus on the direction of flow and the regulatory feedback loops.

Ultra Quick Revision

SA node: Natural pacemaker of the heart.
CO₂: Main regulator of respiration (not O₂).
Loop of Henle: Acts as the counter-current multiplier.
Insulin & Glucagon: Insulin ↓ glucose | Glucagon ↑ glucose.
Sliding Filament Theory: Myofilaments slide; they do not shorten.

1. Breathing and Exchange of Gases

Mechanism

Diaphragm & External Intercostal Muscles (EICM) contract → Thoracic volume ↑ → Intra-pulmonary pressure ↓ (drops below atmospheric) → Inspiration.

Volumes & Capacities

TV: 500 mL

IRV: 2500–3000 mL

ERV: 1000–1100 mL

Residual Volume (RV): 1100–1200 mL.
Trap: RV cannot be measured by simple spirometry.

Vital Capacity (VC): TV + IRV + ERV.

Gas Transport

Oxygen: 97% transported as Oxyhemoglobin. The Dissociation Curve (Sigmoid) Shifts Right (meaning it releases O₂) with ↑pCO₂, ↑H⁺, and ↑Temp.
CO₂: 70% as Bicarbonate (HCO₃⁻), 23% as Carbamino-Hb, 7% simply dissolved in plasma.
Chloride Shift (Hamburger’s): Cl⁻ enters the RBC to strictly balance the electrical charge when HCO₃⁻ exits into the plasma.

Regulation of Respiration

Medulla oblongata: Contains the Respiratory rhythm center.
Pons: Contains the Pneumotaxic center (can moderate rhythm center functions).
Regulation: Highly sensitive to CO₂ and H⁺ levels (O₂ has a negligible role).

2. Body Fluids and Circulation

Blood Groups

ABO System: Based on antigens (A and B) on the RBC surface.
- Universal Donor: Group O.
- Universal Recipient: Group AB.
Rh Factor: Presence (Rh⁺) or absence (Rh⁻) of Rh antigen. Important in pregnancy (Erythroblastosis fetalis).

Cardiac Cycle (0.8s): Atrial Systole (0.1s) → Ventricular Systole (0.3s) → Joint Diastole (0.4s).
Heart Sounds: LUB (Start of Ventricular Systole; AV valves loudly close); DUB (Start of Ventricular Diastole; Semilunar valves close).
ECG Waves: P (Atrial Depolarization), QRS (Ventricular Depolarization/Contraction), T (Ventricular Repolarization).
Double Circulation: Systemic + Pulmonary. The Hepatic Portal System critically connects the digestive tract to the liver (for nutrient processing) before returning blood to the heart.

3. Excretory Products and Elimination

Urine Formation

Ultrafiltration: Glomerulus; entirely non-selective; driven purely by Hydrostatic Pressure.
PCT: The "Workhorse." Vigorously reabsorbs 70-80% of electrolytes, and exactly 100% of Glucose and Amino acids.

Micturition

Urine release is controlled by a neural reflex.
Process: Stretch receptors on bladder wall → Signals to CNS → Bladder contraction + Sphincter relaxation.

Counter-Current Mechanism

Involves the Loop of Henle (concentrates) and Vasa Recta (maintains the gradient).

Descending limb: Highly permeable to water; impermeable to salts.
Ascending limb: Permeable to NaCl; completely impermeable to water.

Regulation (RAAS)

Low BP → JGA actively releases Renin → Angiotensin II (Strong Vasoconstrictor) → Aldosterone (reabsorbs Na⁺ and H₂O).

ADH (Vasopressin): Makes the distal collecting duct highly permeable to water (prevents diuresis/water loss).

4. Locomotion and Movement

Muscle Structure

Sarcomere (functional unit between two Z-lines).

A-band: Dark (Contains thick Myosin).
I-band: Light (Contains thin Actin only).

Sliding Filament Theory

Action potential → Ca²⁺ massive release from Sarcoplasmic Reticulum → Ca²⁺ tightly binds to Troponin → Myosin-Actin cross-bridge forms → Power stroke.

Key Change: During contraction, the I-band noticeably shortens, the H-zone disappears entirely, but the A-band length stays completely constant.

Joints

Fibrous (immovable), Cartilaginous (slightly movable), Synovial (Pivot, Hinge, Ball & Socket - highly movable).

5. Neural Control and Coordination

Neuron Impulse

Resting: -70 mV; High Na⁺ outside, High K⁺ inside.
Depolarization: Na⁺ channels snap open; rapid Na⁺ influx; interior abruptly becomes +30 mV.
Repolarization: K⁺ channels open; K⁺ efflux; efficiently restores the negative resting potential.

Synapse & Brain

Chemical synapse utilizes specific neurotransmitters (e.g., Acetylcholine) to cross the synaptic cleft.

Brain: Hypothalamus (Thermoregulation, hunger); Cerebellum (Balance/Precision); Medulla (Respiration/Heart rate).

6. Chemical Coordination and Integration

Hormone Mechanisms

Water-soluble (Proteins): Bind exclusively to surface receptors; actively generate Second Messengers (cAMP, IP₃, Ca²⁺) inside the cell.
Lipid-soluble (Steroids): Enter the cell directly; bind to Intracellular receptors; directly affect gene expression in the nucleus.

Key Glands

Pancreas: Insulin (rapidly lowers blood glucose); Glucagon (raises it).
Adrenal: Epinephrine (Fight/Flight response); Cortisol (Stress management/Metabolism).
Thyroid: T₃, T₄ (Controls Basal Metabolic Rate - BMR).

Hormones of Other Organs

Heart: ANF (Atrial Natriuretic Factor) - reduces Blood Pressure.
Kidney: Erythropoietin (stimulates RBC production).
GIT: Gastrin, Secretin, CCK (Cholecystokinin), GIP.

7. Important Disorders

Respiratory: Emphysema (Alveolar damage); Asthma (Bronchial inflammation).
Circulatory: Hypertension (High Blood Pressure); Angina pectoris.
Excretory: Uremia (Urea in blood); Renal Calculi (Kidney stones).
Muscular/Skeletal: Myasthenia gravis (Autoimmune); Osteoporosis.
Hormonal: Goitre (Iodine deficiency/Thyroid); Diabetes Mellitus (Insulin deficiency).

8. Common Mistakes

ECG Trap: The T-wave precisely marks the beginning of ventricular relaxation, not the end. The QRS complex is where heart rate is actually measured.
Descending Loop: It is fundamentally Impermeable to electrolytes. Salt only leaves in the ascending limb.
Muscular Contraction: Myosin and Actin filaments do not shrink or shorten; they simply slide past each other.

9. IAT Exam Focus Points

Bohr Effect Factors: Extremely high yield (precisely how pH, Temp, and CO₂ affect O₂ affinity to hemoglobin).
Counter-Current Multiplier: Understanding exactly where the medullary interstitium becomes most heavily concentrated (at the very bottom of Henle's Loop).
Hormone Mechanisms: Distinguishing firmly which hormones use cAMP (Peptides) vs. which actually enter the nucleus (Steroids/Thyroxine).
Neural Reflex Arc: The strict unidirectional pathway from Sensory → Interneuron (in Spinal Cord) → Motor.
Joint Examples: Rapidly matching the joint to the physiological location (Pivot = Atlas/Axis; Saddle = Thumb).

10. Practice Mock Test

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