Home / Notes / Biology / Respiration
Biology · Unit 13

Respiration: MDCAT Notes and MCQs

Parts of the respiratory system

  • Nose / nasal cavity: hairs and mucus filter, warm and moisten air. Pharynx: common passage for food and air.
  • Larynx (voice box): vocal cords; epiglottis closes glottis during swallowing.
  • Trachea: supported by C-shaped cartilage rings (prevent collapse); lined by ciliated mucous epithelium.
  • Bronchi → bronchioles (no cartilage, smooth muscle) → alveoli (≈ 700 million in both lungs, total surface ≈ 70 m2).
  • Lungs: right lung has 3 lobes, left lung 2 lobes; enclosed in pleural membranes.
  • Breathing: inspiration — diaphragm contracts (flattens) & external intercostals contract → chest volume ↑, pressure ↓ → air in. Expiration — mostly passive (relaxation).
  • Breathing rate ≈ 16–20/min; tidal volume ≈ 500 ml; vital capacity ≈ 4.5–4.8 L. Controlled by the medulla (sensitive to CO2/H+).

Gas exchange in alveoli

  • Exchange by diffusion across the thin alveolar + capillary walls (≈ 0.2–0.5 µm), moist surface, rich blood supply, huge area. Surfactant prevents alveolar collapse.
  • pO2: alveoli ≈ 100 mmHg → blood ≈ 40 mmHg, so O2 enters blood. pCO2: blood ≈ 46 mmHg → alveoli ≈ 40 mmHg, so CO2 leaves.
  • O2 transport: ≈ 97% as oxyhaemoglobin (each Hb carries 4 O2), ≈ 3% dissolved in plasma.
  • CO2 transport: ≈ 70% as bicarbonate ions (carbonic anhydrase in RBCs; chloride shift), ≈ 23% as carbaminohaemoglobin, ≈ 7% dissolved.

Effects of smoking

  • Tar, nicotine, CO and other carcinogens. Carbon monoxide binds Hb ≈ 200–250 times more strongly than O2 (carboxyhaemoglobin) → less O2 transport.
  • Paralyses cilia → mucus builds up → "smoker's cough", chronic bronchitis.
  • Emphysema: alveolar walls break down → less surface area, breathlessness.
  • Lung cancer (tar carcinogens), also heart disease, atherosclerosis, hypertension (nicotine).
PMDC learning outcomes for this unit
  • Human Respiratory System: 13.1 Discuss the functions of main parts of respiratory system.
  • Human Respiratory System: 13.2 Discuss the process of gas exchange in human lungs.
  • Human Respiratory System: 13.3 Discuss the effect of smoking on respiratory system.

Respiration MCQs with answers

Practise and track your score →
Q1
The trachea is prevented from collapsing by:
ABones
BC-shaped cartilage rings
CPleura
DSmooth muscles
Show answer
Answer: B (C-shaped cartilage rings). Incomplete C-shaped cartilage rings keep the trachea open.
Q2
Which structure prevents food from entering the trachea?
AEpiglottis
BUvula
CGlottis
DLarynx
Show answer
Answer: A (Epiglottis). The epiglottis covers the glottis during swallowing.
Q3
Gas exchange between alveoli and blood occurs by:
AActive transport
BDiffusion
CPinocytosis
DOsmosis
Show answer
Answer: B (Diffusion). Gases move down their partial-pressure gradients by simple diffusion.
Q4
Most CO2 is transported in blood as:
ACarbaminohaemoglobin
BDissolved CO2
CBicarbonate ions
DCarbonic acid
Show answer
Answer: C (Bicarbonate ions). About 70% of CO2 is carried as HCO3− in plasma.
Q5
One haemoglobin molecule can carry:
A2 O2 molecules
B4 O2 molecules
C8 O2 molecules
D1 O2 molecule
Show answer
Answer: B (4 O2 molecules). Each of the 4 haem groups binds one O2 molecule.
Q6
During inspiration the diaphragm:
AMoves upward
BHas no role
CContracts and flattens
DRelaxes and becomes dome-shaped
Show answer
Answer: C (Contracts and flattens). Contraction flattens the diaphragm, increasing thoracic volume.
Q7
Breakdown of alveolar walls due to smoking causes:
ATuberculosis
BAsthma
CPneumonia
DEmphysema
Show answer
Answer: D (Emphysema). Emphysema reduces alveolar surface area, causing breathlessness.
Q8
Carbon monoxide in cigarette smoke is harmful because it:
ACauses cancer directly
BParalyses cilia
CBinds strongly with haemoglobin
DDissolves surfactant
Show answer
Answer: C (Binds strongly with haemoglobin). CO forms stable carboxyhaemoglobin, reducing O2-carrying capacity.
Q9
The right lung of humans has:
A4 lobes
B1 lobe
C2 lobes
D3 lobes
Show answer
Answer: D (3 lobes). Right lung = 3 lobes; left lung = 2 lobes (space for the heart).
← ImmunityDigestion →