Human Respiratory System: MDCAT Biology notes
Human Respiratory System for MDCAT: air passage, alveoli, breathing mechanism, lung volumes, O2 and CO2 transport and lung disorders.
Respiratory surfaces
A good respiratory surface is thin, moist, permeable, has a large surface area and a rich blood supply. Gases cross it by simple diffusion. Ventilation (breathing) is the mechanical process of bringing air into contact with this surface; cellular respiration is the chemical process that uses the oxygen.
The air passage
Nostrils and nasal cavity (filter, warm, moisten air) → pharynx (common passage for food and air) → larynx (voice box, with vocal cords) → trachea (windpipe) → two bronchi → bronchioles → alveolar ducts → alveoli.
| Part | Cartilage | Smooth muscle |
|---|---|---|
| Trachea, bronchi | C-shaped rings / plates | Present |
| Bronchioles | None | Present |
| Alveoli | None | None |
Alveoli are the structural and functional units and the site of gas exchange. Millions of them make the lungs spongy. Their walls are one cell thick (squamous epithelium) supported by collagen and elastic fibres. Alveolar cells secrete surfactant, which lowers surface tension and stops alveoli collapsing; its deficiency in premature babies causes respiratory distress syndrome. The lungs lie in the pleural cavities; the space between them, holding the heart, is the mediastinum.
Mechanism of breathing
Breathing has two phases.
- Inspiration (active): the diaphragm contracts and flattens; external intercostal muscles contract, moving ribs and sternum upward and outward. Chest volume rises, pressure falls, air rushes in.
- Expiration (normally passive): diaphragm and external intercostals relax; elastic lungs recoil and air is pushed out.
Lung volumes
- Tidal volume: air moved in a normal breath, about 500 mL.
- Inspiratory reserve volume: extra air forcibly inhaled after a normal inspiration. Expiratory reserve volume: extra air forcibly exhaled after a normal expiration.
- Residual volume: air that always remains in the lungs.
- Total lung capacity: about 5 to 6 litres in adults.
Exhaled air contains about 4% $CO_2$, compared with about 0.04% in inhaled air.
Transport of gases
Oxygen is carried mainly as oxyhaemoglobin in red cells. Myoglobin in muscle has a higher affinity for oxygen than haemoglobin, so it takes oxygen from blood and stores it.
| Form of $CO_2$ transport | Share |
|---|---|
| Bicarbonate ions ($HCO_3^-$) in plasma | About 70% |
| Carbaminohaemoglobin (bound to amino groups of globin) | About 23% |
| Dissolved in plasma | About 7% |
In red cells, carbonic anhydrase catalyses $CO_2+H_2O\rightleftharpoons H_2CO_3\rightleftharpoons H^++HCO_3^-$. $HCO_3^-$ diffuses into plasma and $Cl^-$ moves into the cell to balance charge (chloride shift). $H^+$ is taken up by haemoglobin (forming HHb) and other blood buffers, so blood does not become acidic. At the tissues oxyhaemoglobin releases oxygen; it is formed in the lungs.
Disorders
- Emphysema: gradual breakdown of alveolar walls, reducing surface for exchange; common in smokers.
- Asthma: narrowing of bronchioles by muscle spasm, often allergic.
- Bronchitis: inflammation of bronchi.
- Pneumonia: infection and inflammation of the lungs.
- Pulmonary tuberculosis: bacterial and contagious. Lung cancer is linked to smoking.
Common MDCAT traps
- Bronchioles lack cartilage; alveoli lack both cartilage and smooth muscle.
- Inspiration needs the diaphragm to contract, not relax.
- Most $CO_2$ travels as bicarbonate, not as carbaminohaemoglobin or dissolved gas.
- $CO_2$ binds haemoglobin's amino groups, not the iron of haem. (Some papers loosely call this "carboxyhaemoglobin", which properly means haemoglobin bound to carbon monoxide.)
- Pharynx is common to food and air; the larynx is for voice.
Quick revision
- Alveoli: site of gas exchange.
- Surfactant deficiency: respiratory distress syndrome.
- Carbonic anhydrase joins $CO_2$ and water.
- Total lung capacity 5 to 6 L.
- Emphysema: alveolar wall breakdown.