Respiratory System is part of Biology. Below are notes on 10 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Anatomy of the Human Respiratory System

  • Human Respiratory System consists of the Nose, Pharynx, Larynx, Trachea, Bronchi, Bronchioles, Lungs (containing Alveoli), Diaphragm, and intercostal muscles.
  • Larynx (voice box) is located between the pharynx and trachea and contains the vocal cords.
  • Trachea (windpipe) is a tube extending from the larynx, bifurcating into two primary bronchi.
  • Bronchi further divide into smaller bronchioles within the lungs, leading to alveoli.
  • Diaphragm is the primary muscle of respiration, separating the thoracic and abdominal cavities.
  • Bowman's capsule is a part of the nephron in the kidney and is not involved in the respiratory system or breathing.

Common Respiratory Disorders and Diseases (e.g., asthma, emphysema, pneumonia)

  • Pneumonia: Acute inflammatory infection primarily affecting the lungs' air sacs (alveoli).
  • Lung healing after quitting smoking: Occurs partially due to the regeneration/synthesis of new mucosa.

Comparative Anatomy of Respiratory Organs (e.g., gills, tracheae, lungs)

  • Aquatic animals respire faster than terrestrial animals due to the lower concentration of dissolved oxygen in water.
  • Aquatic/marine animals extract oxygen directly from water.
  • Frog tadpoles respire through gills; adult frogs respire through lungs and moist skin.
  • Whales and Dolphins are mammals; their main respiratory organs are lungs, and they breathe through blowholes.
  • Earthworms respire through their moist skin (cutaneous respiration).
  • Fish respire through gills.
  • Snakes and Cows (terrestrial vertebrates) respire through lungs.

Gaseous Exchange at Alveolar and Tissue Levels

  • Alveoli are the primary sites for gaseous exchange in the lungs.
  • Gaseous exchange (O2 and CO2) occurs by simple diffusion down partial pressure gradients.
  • The amount/partial pressure of Nitrogen (N2) in air does not change significantly during human respiration.
  • Brain cells are obligate aerobes; they cannot respire anaerobically and are highly sensitive to oxygen deprivation.
  • Respiratory Quotient (RQ) for carbohydrates is 1.

Mechanism of Breathing: Inspiration and Expiration

  • Breathing (Pulmonary Ventilation): Process of air moving into (inspiration) and out of (expiration) the lungs.
  • Mechanism Principle: Air movement occurs due to pressure gradients between the lungs and the atmosphere. Air flows from higher pressure to lower pressure.
  • Inspiration (Inhalation): Active process.
    • Diaphragm: Contracts, moves downwards, flattening.
    • External Intercostal Muscles: Contract, pulling ribs and sternum upwards and outwards.
    • Thoracic Cavity Volume: Increases (both antero-posterior and dorsoventral axes).
    • Intrapulmonary Pressure: Decreases (becomes less than atmospheric pressure).
    • Air Flow: Atmospheric air rushes into the lungs.
  • Expiration (Exhalation): Passive process during normal breathing.
    • Diaphragm: Relaxes, moves upwards to its original domed position.
    • External Intercostal Muscles: Relax, allowing ribs and sternum to move downwards and inwards.
    • Thoracic Cavity Volume: Decreases.
    • Intrapulmonary Pressure: Increases (becomes greater than atmospheric pressure).
    • Air Flow: Air is expelled from the lungs.
  • Forced Expiration: Active process involving contraction of internal intercostal muscles and abdominal muscles, further decreasing thoracic volume and expelling more air.

Pleural Membranes and Intrapleural Pressure

  • Pleural Membranes: Two serous layers enclosing each lung.
  • Visceral Pleura: Inner layer, directly adheres to lung surface.
  • Parietal Pleura: Outer layer, lines thoracic cavity, diaphragm, and mediastinum.
  • Pleural Cavity/Space: Thin fluid-filled space between visceral and parietal pleura.
  • Pleural Fluid: Present in pleural cavity, acts as a lubricant (reduces friction) and creates surface tension, allowing the lungs to adhere to the thoracic wall.
  • Intrapulmonary/Alveolar Pressure (Palv): Pressure inside the alveoli (and thus the lungs).
  • At rest (between breaths), Palv equals atmospheric pressure (Patm).
  • Intrapleural Pressure (Pip): Pressure within the pleural cavity.
  • Pip is always negative (subatmospheric) relative to both atmospheric and intrapulmonary pressure.
  • Typical Pip value at rest: Approximately -4 to -5 mmHg relative to Patm.
  • Reason for negative Pip: Opposing elastic recoils of the lung (inward collapse) and the thoracic wall (outward expansion) pull the pleurae apart, creating suction.
  • Transpulmonary Pressure (Ptp): The difference between intrapulmonary and intrapleural pressure (Palv - Pip).
  • Ptp is always positive (since Pip is negative), keeping the lungs inflated and preventing collapse.
  • Pneumothorax: Air entering the pleural cavity, which equalizes Pip with Patm, eliminating Ptp and leading to lung collapse.

Regulation of Respiration (Neural and Chemical)

  • Respiration regulation: Neural & Chemical mechanisms.
  • Neural Regulation:
  • Respiratory Rhythm Centre: Located in medulla oblongata; primary center for generating basic breathing rhythm.
  • Pneumotaxic Centre: Located in pons; moderates respiratory rhythm, limits inspiration duration, resulting in faster and shallower breathing.
  • Apneustic Centre: Located in pons; prolongs inspiration (if not inhibited by pneumotaxic centre or vagus nerve).
  • Chemical Regulation:
  • Chemosensitive Area: Located adjacent to the rhythm centre in medulla; highly sensitive to CO2 and H+ concentration in blood.
  • Peripheral Chemoreceptors: Located in aortic arch and carotid artery; sensitive to CO2, H+, and significant drops in O2 concentration.
  • Increased CO2 and H+ concentration (acidosis) are the primary and most potent stimuli for increasing respiratory rate and depth, acting via chemoreceptors.
  • Decreased O2 concentration (hypoxia) primarily stimulates peripheral chemoreceptors, increasing respiration only when O2 levels are significantly low.

Respiratory

Volumes

and

Capacities

(Spirometry)

  • An average human inhales approximately 16 kg of air per day.

Role of Diaphragm and Intercostal Muscles

  • Diaphragm: dome-shaped at rest (relaxed state).
  • Inspiration (active process):
  • Diaphragm contracts, flattens, and moves downwards (lowered).
  • External intercostal muscles contract, pulling ribs and sternum upwards and outwards.
  • Result: Thoracic cavity volume increases; intra-pulmonary pressure decreases; air enters lungs.
  • Expiration (normal, passive process):
  • Diaphragm relaxes, returns to dome shape (arched), and moves upwards.
  • External intercostal muscles relax, allowing ribs and sternum to return to original position.
  • Result: Thoracic cavity volume decreases; intra-pulmonary pressure increases; air exits lungs.
  • Forced Expiration (active process):
  • Internal intercostal muscles contract, pulling ribs inwards and downwards.
  • Abdominal muscles contract, pushing diaphragm further up.

Transport of Oxygen by Blood

  • Oxygen (O₂) transport in blood occurs mainly via hemoglobin (97%) in Red Blood Cells (RBCs) and dissolved in plasma (3%).
  • Hemoglobin (Hb) is an iron-containing (Fe²⁺) protein; each molecule can bind up to four O₂ molecules reversibly.
  • Oxyhemoglobin forms in high partial pressure of O₂ (pO₂) environments (e.g., lungs) and dissociates in low pO₂ (e.g., tissues).
  • Factors promoting O₂ dissociation from Hb (Bohr effect): high pCO₂, high H⁺ concentration (low pH), increased temperature, and high 2,3-bisphosphoglycerate (2,3-BPG).
  • Respiratory Pigments & Associated Organisms:
    • Haemoglobin: Vertebrates, some Annelida, some Molluscs.
    • Chlorocruorin: Some marine Annelida (e.g., Sabellidae, Serpulidae).
    • Hemerythrin: Brachiopods (e.g., Lingula), Priapulids, some Annelida.
    • Haemocyanin: Most Molluscs, most Arthropods.
  • Cellular Respiration: A metabolic process that converts biochemical energy from nutrients into ATP. Major steps include Glycolysis, Krebs Cycle (Citric Acid Cycle), and Electron Transport Chain.
  • Calvin cycle: A part of photosynthesis (carbon fixation), not involved in respiration.
  • Key intermediate steps in Glycolysis (Respiration):
    • Glucose → Glucose-6-phosphate.
    • Fructose-6-phosphate → Fructose 1,6bisphosphate.
    • 1,3-bisphosphoglycerate (1,3-di PGA) → 3-phosphoglycerate (3PGA).
  • Krebs Cycle (Citric Acid Cycle): Fumaric acid is converted into malic acid by hydration (addition of water), catalyzed by the enzyme fumarase.

12. Circulatory System

Practice MCQs: Respiratory System

Anatomy of the Human Respiratory System

  1. Which vitamin complex includes thiamine, niacin and riboflavin?

    Asked in: Section Officer(LDCE) 2021 22 2021 · UPSC NDA 1 2018

    1. Vitamin A
    2. Vitamin C
    3. Vitamin K
    4. Vitamin B
    Show answer

    Answer: D. Vitamin B

    Thiamine, niacin, and riboflavin are B vitamins. D. Trachea Ans: B The process of breathing involves the respiratory system, which includes organs like the trachea, bronchi, lungs, and muscles like the diaphragm. A) Bronchi: Tubes that carry air from the trachea into the lungs; they are part of the respiratory system. B) Bowman's capsule: A cup-like structure that surrounds the glomerulus in the nephron of the kidney. It is the site of filtration in the process of urine formation, which is part of the excretory system, not the respiratory system. C) Diaphragm: A large, dome-shaped muscle located at the base of the chest cavity that plays a crucial role in inhalation and exhalation, making it an essential part of the breathing mechanism. D) Trachea: Also known as the windpipe, it is a tube that connects the larynx (voice box) to the bronchi, allowing the passage of air into the lungs. It is part of the respiratory system. Therefore, Bowman's capsule is the only option not involved in breathing.

  2. Vocal Cords occur in

    Asked in: WBCS Main Paper 4 2023

    1. pharynx
    2. larynx
    3. glottis
    4. bronchial tube
    Show answer

    Answer: B. larynx

    The larynx, commonly known as the voice box, is an organ in the neck responsible for breathing, producing sound, and protecting the trachea against food aspiration. The vocal folds (true vocal cords) are located within the larynx.

Common Respiratory Disorders and Diseases (e.g., asthma, emphysema, pneumonia)

  1. Pneumonia is an infection of

    Asked in: BPSC Prelims Exam 67th Cancelled

    1. nerve
    2. blood
    3. skin
    4. lungs
    Show answer

    Answer: D. lungs

    Pneumonia is an infection that inflames the air sacs in one or both lungs. The air sacs (alveoli) may fill with fluid or pus, causing cough with phlegm or pus, fever, chills, and difficulty breathing.

  2. After a person quits smoking, the lungs can heal to a certain extent, because of ______.

    Asked in: KPSC KAS Prelims 2017

    1. Decrease in carbon monoxide
    2. Increase in oxygen uptake
    3. Synthesis of new mucosa
    4. Long-term inflammatory changes
    Show answer

    Answer: C. Synthesis of new mucosa

    - Smoking damages the delicate tissues lining the airways and lungs, including the cilia (tiny hair-like structures that clear mucus and debris) and the mucosal cells. - When smoking stops, the body's natural repair mechanisms can begin. The damaged cilia can regenerate, and new, healthy mucosal cells are synthesized, helping to restore the lung's ability to clear irritants and fight infections.

Comparative Anatomy of Respiratory Organs (e.g., gills, tracheae, lungs)

  1. Aquatic animals breathe much faster than terrestrial animals because of :

    Asked in: UPSC CAPF 2009

    1. low amount of dissolved oxygen in water
    2. low amount of hemoglobin in their blood
    3. high metabolic rate of aquatic animals
    4. large amount of dissolved nitrogen in water
    Show answer

    Answer: A. low amount of dissolved oxygen in water

    - Dissolved oxygen levels in water are much lower than oxygen levels in the air. - Aquatic animals breathe faster to compensate for the lower oxygen concentration in their environment.

  2. How do marine animals survive in water without air contact?

    Asked in: UPSC CAPF 2012

    1. They do not require any oxygen
    2. They take oxygen from water
    3. They only produce oxygen in their body
    4. They get oxygen from water plants
    Show answer

    Answer: B. They take oxygen from water

    Marine animals, like most living organisms, require oxygen for respiration. They do not obtain it directly from the air like terrestrial animals do. Instead, they extract dissolved oxygen from the water using specialized respiratory organs, such as gills.

  3. Which one among the following animals can perform respiration either through gills or through lungs ?

    Asked in: UPSC CISF AC EXE 2021

    1. Fish
    2. Frog
    3. Snake
    4. Cow
    Show answer

    Answer: B. Frog

    Respiration is the process of gas exchange (oxygen intake and carbon dioxide release). Different animals have evolved various respiratory organs depending on their environment. Gills are specialized structures for extracting oxygen from water, while lungs are adapted for gas exchange in air. Amphibians exhibit a dual mode of respiration, reflecting their transition from aquatic to terrestrial life.

  4. Which one of the following organs is the main respiratory organ of whale?

    Asked in: WBPSC CS Prelims 2018

    1. Integuments
    2. Gills
    3. Lungs
    4. Trachea
    Show answer

    Answer: C. Lungs

    Whales are marine mammals, and like all mammals, they breathe air using lungs. They surface to breathe air through blowholes located on top of their heads.

  5. Which of the following statements is/are correct? 1. Dolphins and Whales breathe through blowholes. 2. Earthworm breathe through their skin. Select the correct answer from the code given below : Code : (a) Only 2 (b) Both 1 and 2 (c) Neither 1 nor 2 (d) Only 1

    Asked in: uppsc 2025 2025

    1. Only 2
    2. Both 1 and 2
    3. Neither 1 nor 2
    4. Only 1
    Show answer

    Answer: B. Both 1 and 2

    Statement 1 is correct: Dolphins and whales are marine mammals and breathe air through a blowhole located on the top of their heads. Statement 2 is correct: Earthworms are invertebrates that respire through their moist skin, absorbing oxygen from the soil.

Gaseous Exchange at Alveolar and Tissue Levels

  1. Site of gaseous exchange in lungs is

    Asked in: UPPCS Prelims 2021

    1. Tracheoles
    2. Bronchioles
    3. Pulmonary vein
    4. Alveoli
    Show answer

    Answer: D. Alveoli

    Alveoli are tiny air sacs in the lungs where oxygen from inhaled air diffuses into the blood in surrounding capillaries, and carbon dioxide from the blood diffuses into the alveoli to be exhaled.

  2. During respiration in human beings, the actual place of gaseous exchange is

    Asked in: UPSC CAPF 2020

    1. Bronchi
    2. Nasal passage
    3. Alveoli
    4. Trachea
    Show answer

    Answer: C. Alveoli

    Alveoli are tiny air sacs in the lungs where the thin walls of the alveoli and surrounding capillaries allow for efficient diffusion of gases between the air and the blood.

  3. Amount of which of the following components in air does not change in the process of respiration

    Asked in: RPSC RAS Prelims 2003

    1. Carbon dioxide (CO2)
    2. Oxygen
    3. Water vapors
    4. Nitrogen
    Show answer

    Answer: D. Nitrogen

    During the process of respiration, humans inhale air which is primarily composed of Nitrogen (about 78%), Oxygen (about 21%), Argon (about 0.9%), Carbon Dioxide (about 0.04%), and trace gases, including water vapor. The body uses oxygen for cellular respiration and produces carbon dioxide and water vapor as byproducts, which are then exhaled. Nitrogen, being largely inert for biological processes in the lungs, is inhaled and exhaled without being significantly absorbed or used by the body. Therefore, the amount of nitrogen in the air does not change during respiration.

  4. Which cells of our body can not respire anaerobically?

    Asked in: District Statistical Officer Assistant Director 2025

    1. Mature RBC
    2. Brain cells
    3. Muscle cells
    4. Newly formed RBCs
    Show answer

    Answer: A. Mature RBC

    Aerobic respiration requires oxygen and occurs in mitochondria to produce a large amount of ATP. Anaerobic respiration occurs without oxygen and produces less ATP.

  5. What is the Respiratory Quotient (RQ) value of carbohydrates ?

    Asked in: Co operative Supervisor & Environment Supervisor (Group C) Exam 2023

    1. Less than one
    2. One
    3. More than one
    4. None of these
    Show answer

    Answer: B. One

    The RQ value varies depending on the substrate being respired. It is a crucial concept in understanding cellular respiration and metabolic rates.

Mechanism of Breathing: Inspiration and Expiration

  1. During expiration the air will leave the lungs when the pressure inside the lungs must become

    Asked in: HAS Prelims 2021

    1. equal to the atmospheric pressure
    2. lower than atmospheric pressure
    3. higher than atmospheric pressure
    4. diaphragm must contract
    Show answer

    Answer: C. higher than atmospheric pressure

    - Respiration (breathing) is driven by pressure differences between the lungs and the atmosphere. - Inspiration occurs when intrapulmonary pressure is lower than atmospheric pressure (diaphragm contracts, thoracic cavity expands). - Expiration occurs when intrapulmonary pressure is higher than atmospheric pressure (diaphragm relaxes, thoracic cavity volume decreases). - The diaphragm contracts during inspiration, not expiration (except forced expiration).

Pleural Membranes and Intrapleural Pressure

  1. Pressure within the lungs is called

    Asked in: WBCS Main Paper 4 2023

    1. Inter-pulmonary pressure.
    2. Intra-pulmonary pressure.
    3. Interstitial pressure.
    4. Alveolar pressure.
    Show answer

    Answer: B. Intra-pulmonary pressure.

    Intra-pulmonary pressure (also known as intra- alveolar pressure) is the pressure inside the air passages and alveoli of the lungs. It is compared to atmospheric pressure to determine the direction of airflow during breathing. "Intra-" means within, while "Inter-" (as in option A) means between. Interstitial pressure refers to pressure in the tissue spaces, not the air spaces within the lungs. Alveolar pressure specifically refers to the pressure within the alveoli, which is essentially the same as intrapulmonary pressure, but "Intra-pulmonary pressure" is often used as a general term for pressure within the lungs' air spaces.

Regulation of Respiration (Neural and Chemical)

  1. Which part of the brain controls breathing?

    Asked in: lower devision clerk 2025

    1. Cerebrum
    2. Medulla oblongata
    3. Cerebellum
    4. Pons
    Show answer

    Answer: B. Medulla oblongata

    The medulla oblongata is the most crucial part of the brain controlling the automatic process of breathing.

Respiratory Volumes and Capacities (Spirometry)

  1. How much air does a man normally inhale in a day?

    Asked in: WBCS Main Paper 4 2022

    1. 5 kg
    2. 16 kg
    3. 20 kg
    4. 25 kg
    Show answer

    Answer: B. 16 kg

    The amount of air inhaled varies depending on activity level, age, and health, but a typical adult at rest breathes about 7-8 liters of air per minute. Over 24 hours (1440 minutes), this amounts to roughly 10,000-12,000 liters. The density of air is approximately 1.225 grams per liter at sea level. Therefore, 10,000-12,000 L 1.225 g/L ≈ 12,250 - 14,700

Role of Diaphragm and Intercostal Muscles

  1. During normal respiration in a mammal, the diaphragm is :

    Asked in: Paper I 4

    1. Lowered
    2. Fully contracted
    3. Arched
    4. Flattened
    Show answer

    Answer: A. Lowered

    The diaphragm moves downwards and flattens during inspiration in normal breathing.

Transport of Oxygen by Blood

  1. Match the following : Respiratory Pigment Animal/Animal group a. Haemoglobin I. Annelida b. Chlorocruorin II. Vertebrates c. Hemerythrin III. Lingula d. Haemocyanin IV. Molluscs

    Asked in: MPSC CS Prelims 2019

    1. a-II, b-III, c-I, d-IV
    2. a-II, b-I, c-III, d-IV
    3. a-III, b-II, c-IV, d-I
    4. a-IV, b-III, c-I, d-II
    Show answer

    Answer: B. a-II, b-I, c-III, d-IV

    Different animal groups utilize various respiratory pigments for oxygen transport. Haemoglobin is found in vertebrates and some invertebrates (like annelids). Chlorocruorin is characteristic of certain marine annelids. Hemerythrin is found in some annelids, sipunculans, and brachiopods like Lingula. Haemocyanin is found in molluscs and arthropods.

  2. Which among the following is not involved in Respiratory Process ?

    Asked in: MPSC CS Prelims 2021

    1. Calvin cycle
    2. Citric acid cycle
    3. Glycolysis
    4. Electron transport chain
    Show answer

    Answer: A. Calvin cycle

    Cellular respiration and photosynthesis are distinct processes. Respiration breaks down glucose to release energy, while photosynthesis builds glucose using light energy.

  3. Which one of the following is not an intermediate step in Respiration ?

    1. Glucose → glucose-6-phosphate
    2. Fructose 1-6 diphosphate → Fructose-6-phos‐ phate
    3. Fructose-6-phosphate → Fructose 1,6-phos‐ phate
    4. 1-3 di PGA → 3PGA
    Show answer

    Answer: B. Fructose 1-6 diphosphate → Fructose-6-phos‐ phate