🩺 Zoology · Physiology Part 1 🔥 Day 1

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Physiology Part 1 — NEET Zoology Notes

📗 Full Biology chapter: Human Physiology

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Breathing and Exchange of Gases

1. Introduction · 2. Why Learn This? · 3. Big Picture

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1. Introduction & Objectives

Explore the gas transport system of the human body. By the end of this module, you will understand:

The mechanics of lung ventilation (diaphragm and intercostals).
Alveolar gas exchange dynamics and partial pressures.
Bohr and Haldane shifts in chemical gas transport.

Estimated Reading Time: 20 minutes | Difficulty Level: Moderate | Prerequisites: Cell respiration.

2. Why Should You Learn This?

Understanding respiratory mechanics is vital for treating respiratory failure, managing asthma, and designing ventilators.

3. Big Picture

Oxygenation cycle: \[\text{Atmosphere} \rightarrow \text{Alveoli} \rightarrow \text{Blood} \rightarrow \text{Tissues (cellular respiration)}\]

5. Detailed Explanation & 10. Scientific Terminology

Breathing Mechanics: Diaphragm contraction flattens it, increasing thoracic volume. External intercostals pull ribs up. Air flows in due to negative intrapulmonary pressure.
Gas transport: Oxygen binds hemoglobin (oxyhemoglobin). Bohr effect: low pH, high \(pCO_2\), and high temperature in tissues shift the dissociation curve right, releasing oxygen.

Respiratory Organs & Pathway

Air travels through a conducting part (which warms, moistens and filters air but does no gas exchange) and a respiratory part (where exchange occurs):

Nostrils → Nasal chamber → Pharynx → Larynx (voice box) → Trachea → Primary bronchi → Bronchioles → Alveoli (site of exchange)

The trachea and bronchi are held open by C-shaped cartilage rings.
The epiglottis covers the glottis during swallowing to stop food entering the airway.
Each lung is covered by a double-layered pleura with pleural fluid that reduces friction; the right lung has 3 lobes, the left 2.

Transport of Carbon Dioxide

CO₂ is carried from tissues to lungs in three forms:

≈ 70% as bicarbonate (\(\text{HCO}_3^-\)) in plasma — formed in RBCs by carbonic anhydrase and exchanged for Cl⁻ (the chloride shift / Hamburger's phenomenon).
≈ 20–25% as carbamino-haemoglobin (CO₂ bound to the globin part of Hb).
≈ 7% dissolved in plasma.

The Haldane effect: deoxygenated blood carries more CO₂, aiding CO₂ pickup in tissues and release in lungs.

The Oxygen–Haemoglobin Dissociation Curve

Plotting % saturation of Hb against \(pO_2\) gives a sigmoid (S-shaped) curve. Its position shifts with conditions, changing how readily Hb releases O₂:

Shift	Caused by	Effect
Right (lower Hb–O₂ affinity)	↑ \(pCO_2\), ↑ H⁺ / ↓ pH, ↑ temperature, ↑ 2,3-DPG (all found in active tissues)	favours O₂ unloading to tissues — the Bohr effect
Left (higher Hb–O₂ affinity)	↓ \(pCO_2\), ↑ pH, ↓ temperature, ↓ 2,3-DPG (conditions in the lungs)	favours O₂ loading in the alveoli

This shifting is what lets the same haemoglobin pick up oxygen efficiently in the lungs and give it up efficiently where the body needs it.

Regulation of Respiration & Disorders

Breathing rhythm is set by the respiratory rhythm centre in the medulla oblongata; the pneumotaxic centre in the pons can moderate it, and a chemosensitive area responds mainly to a rise in CO₂ / H⁺ (not directly to O₂).

Disorder	Cause / feature
Asthma	Difficulty in breathing from inflammation/spasm of bronchi and bronchioles
Emphysema	Alveolar walls damaged (chiefly by smoking), reducing the exchange surface
Occupational (asbestosis, silicosis)	Long exposure to dust/asbestos causes fibrosis of the upper lung

13. Partial Pressures & 14. Lung Volumes Box

Gas	Alveoli	Deoxygenated Blood	Oxygenated Blood	Tissues
\(O_2\)	104	40	95	40
\(CO_2\)	40	45	40	45

Lung Capacity Equations: - Vital Capacity: \(VC = TV + IRV + ERV\) (standard value: 3500-4500 mL) - Functional Residual Capacity: \(FRC = ERV + RV\) (standard value: 2100-2300 mL)

Respiratory Volumes & Capacities

Term	Definition	Value (mL)
Tidal Volume (TV)	Air breathed in/out in one normal breath	≈ 500
Inspiratory Reserve Volume (IRV)	Extra air taken in by forced inspiration	2500–3000
Expiratory Reserve Volume (ERV)	Extra air expelled by forced expiration	1000–1100
Residual Volume (RV)	Air left after forced expiration	1100–1200
Inspiratory Capacity (IC)	TV + IRV	3000–3500
Vital Capacity (VC)	TV + IRV + ERV	3500–4500
Total Lung Capacity (TLC)	VC + RV	≈ 6000

💡 CO₂ transport: ~70% bicarbonate, ~20–25% carbamino-Hb, ~7% dissolved. Chloride shift keeps ionic balance in RBCs. Regulation is mainly by CO₂/H⁺, not O₂. TLC ≈ 6 L; VC ≈ 4.5 L.

11. Misconceptions & 22. Altitude Acclimatization

❌ Misconception: The body breathes out only carbon dioxide.
✔ Correction: Exhaled air still contains about 16% oxygen; the body only absorbs a fraction of inhaled oxygen.

Real Life Application: At high altitudes, low \(pO_2\) causes hypoxia. The body acclimates by increasing RBC production (erythropoietin release), breathing rate, and decreasing Hb binding affinity.

6. Visual Learning (O2 Curve)

Figure 1.2: Oxygen-Hemoglobin dissociation curve showing right shift factors

18. PYQs

1. (NEET PYQ) The volume of air that remains in lungs after a normal expiration is called:

Correct Answer: C. Functional Residual Capacity

Explanation: FRC is the volume of air remaining in the lungs after a normal quiet expiration, calculated as \(FRC = ERV + RV\).

2. The maximum share of carbon dioxide is transported in blood as:

Correct Answer: C. Bicarbonate ions

About 70% of CO₂ travels as \(\text{HCO}_3^-\) formed via carbonic anhydrase in RBCs; ~20–25% as carbamino-haemoglobin and ~7% dissolved.

3. The respiratory rhythm centre is located in the:

Correct Answer: B. Medulla oblongata

The respiratory rhythm centre lies in the medulla; the pneumotaxic centre in the pons moderates it. Regulation responds chiefly to CO₂/H⁺ levels.

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Body Fluids and Circulation

Blood formed elements, conduction pathway, cardiac cycle, and ECG waves

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5. Detailed Explanation & 10. Terminology

Formed Elements: RBCs (spleen graveyard), WBCs (neutrophils and monocytes are phagocytic; basophils release histamine/serotonin; lymphocytes produce antibodies), Platelets (blood clotting).
Conduction Pathway: Pacemaker SAN generates action potentials, propagating via AVN, Bundle of His, and Purkinje fibers to ventricular walls.

Blood Composition

Blood is a fluid connective tissue: plasma ≈ 55% (water 90–92%, plasma proteins albumin, globulin and fibrinogen) and formed elements ≈ 45%. Albumin maintains osmotic balance, globulins fight infection, and fibrinogen is essential for clotting.

Blood Groups (ABO & Rh)

Group	Antigen (on RBC)	Antibody (in plasma)	Can donate to
A	A	anti-B	A, AB
B	B	anti-A	B, AB
AB	A, B	none	AB (universal recipient)
O	none	anti-A, anti-B	all (universal donor)

An Rh⁻ mother carrying an Rh⁺ foetus can be sensitised, causing erythroblastosis foetalis in a later Rh⁺ pregnancy — prevented by anti-Rh (anti-D) antibodies.

Coagulation of Blood

Injury exposes tissue factors; platelets release factors that (with Ca²⁺ and vitamin-K-dependent factors) convert prothrombin → thrombin, which converts soluble fibrinogen → insoluble fibrin. The fibrin mesh traps blood cells to form the clot.

Heart, Cardiac Cycle & Double Circulation

Double circulation: blood passes through the heart twice per cycle — a pulmonary circuit (heart→lungs→heart) and a systemic circuit (heart→body→heart); the two are kept separate by the four-chambered heart.
Cardiac cycle: atrial systole → ventricular systole → joint diastole (≈ 0.8 s total). "Lub" is the closure of AV valves (start of ventricular systole); "Dup" is the closure of semilunar valves (start of diastole).
Cardiac output = stroke volume × heart rate ≈ 70 mL × 72 ≈ 5 L/min.

The Cardiac Cycle — 0.8-Second Timeline

At a resting heart rate of 72 beats/min, one cycle lasts 0.8 s, in three phases:

Atrial systole (0.1 s): atria contract (SA-node driven), topping up the ventricles by ~30%. AV valves open, semilunar valves closed.
Ventricular systole (0.3 s): ventricles contract; AV valves slam shut → first sound "Lub"; when pressure exceeds arterial pressure the semilunar valves open and blood is ejected into the aorta and pulmonary artery.
Joint diastole (0.4 s): all chambers relax; back-pressure shuts the semilunar valves → second sound "Dup"; AV valves reopen and the ventricles fill passively.

Blood Pressure, Lymph & Disorders

Normal blood pressure ≈ 120/80 mmHg (systolic/diastolic); hypertension is a persistent reading of 140/90 or higher. Lymph is the tissue fluid that drains into lymph vessels — it lacks RBCs and most large proteins and returns fluid, fats (via lacteals) and lymphocytes to blood.

Disorder	Feature
Coronary Artery Disease (CAD)	Narrowing of coronary arteries by atherosclerosis (plaque deposits)
Angina pectoris	Chest pain when the heart muscle receives too little oxygen
Heart failure	Heart cannot pump enough blood to meet the body's needs

14. Cardiac Cycle Formulas

- Cardiac Output Calculation: \[\text{Cardiac Output} = \text{Stroke Volume (70 mL)} \times \text{Heart Rate (72 beats/min)} \approx 5000 \text{ mL (5.0 L)}\]

Cardiovascular Equations

Cardiac output: \(\text{CO} = \text{HR} \times \text{SV}\)
Stroke volume: \(\text{SV} = \text{EDV} - \text{ESV}\) (end-diastolic − end-systolic volume)
Pulse pressure: \(\text{PP} = \text{Systolic} - \text{Diastolic BP}\)
Mean arterial pressure: \(\text{MAP} = \text{Diastolic} + \tfrac{1}{3}\,\text{PP}\)

6. Visual Learning (ECG)

Figure 2.2: Electrocardiogram waves correlation (P: atrial depolarization, QRS: ventricular depolarization, T: repolarization)

18. PYQs

1. (NEET PYQ) In a standard ECG, the QRS complex represents:

Correct Answer: B. Depolarization of ventricles

Explanation: The QRS complex represents the electrical depolarization of the ventricles, which triggers ventricular contraction.

2. A person with blood group O is called a "universal donor" because their RBCs carry:

Correct Answer: D. Neither A nor B antigen

Group O RBCs have no A or B surface antigens, so they are not agglutinated by anti-A or anti-B antibodies in a recipient — hence "universal donor".

3. The heart sound "Lub" is produced by the closure of the:

Correct Answer: A. Atrioventricular valves

"Lub" (first sound) is the closure of the AV valves at the start of ventricular systole; "Dup" is the closure of the semilunar valves at the start of diastole.