Human respiratory system
Air enters through your nose like a visitor at a gate: it is warmed, cleaned and moistened before reaching the lungs, where the real exchange happens.
The human respiratory system consists of the nose, pharynx, larynx, trachea, bronchi, bronchioles and the lungs with their tiny air sacs, the alveoli, where oxygen enters the blood and carbon dioxide leaves it.
In the figure : Path of air from the nostrils to the alveoli.
Rule 1: Passage of air — Air enters through the nostrils into the nasal chamber, whose lining of hair and mucus filters it. It passes through the pharynx and the larynx (voice box). A cartilaginous flap, the epiglottis, covers the glottis during swallowing so that food does not enter. The trachea is a straight tube supported by incomplete cartilage rings; it divides into right and left primary bronchi, which divide again and again into bronchioles ending in alveoli.
- Easy : The epiglottis covers the windpipe when you swallow.
- Medium : The tracheal rings are incomplete so that the food bolus can pass in the oesophagus behind.
- Exam-type : Q: What prevents food from entering the trachea? A: The epiglottis.
Rule 2: Lungs and alveoli — The two lungs are in the thoracic cavity, which is airtight, covered by a double-layered pleura with fluid between the layers to reduce friction. The right lung has three lobes and the left two. Alveoli are thin-walled, richly supplied with capillaries and give a very large surface for diffusion. The wall of an alveolus, the capillary wall and the tissue between them together form the diffusion membrane, which is extremely thin.
- Easy : Alveoli look like a bunch of grapes with blood vessels wrapped around them.
- Medium : The thin wall and the large number of alveoli make diffusion fast and efficient.
- Exam-type : Q: How many lobes does the left lung have? A: Two.
Order: nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli. Alveoli are the exchange sites.
Saying that gas exchange takes place in the trachea or bronchi. It takes place only in the alveoli.
Memory clue: N-P-L-T-B-B-A — Nice People Like To Breathe Big Air.
Spot the difference
| Part | Function |
|---|---|
| Nasal chamber | Filters, warms and moistens air |
| Larynx | Sound production; epiglottis protects |
| Trachea | Carries air to the lungs |
| Alveoli | Gas exchange |
| Pleura | Protects lungs and reduces friction |
Practice questions
- 1Name the part where the exchange of gases takes place.
- 2What is the voice box called?
- 3What covers the glottis during swallowing?
- 4How many lobes are there in the right lung?
- 5What is the pleura?
- 6Why are the cartilage rings of the trachea incomplete?
- 7List the features of alveoli that help gas exchange.
- 8Trace the path of air from the nose to a capillary of the lung.
Show answers (try them yourself first)
- 1The alveoli.
- 2The larynx.
- 3The epiglottis.
- 4Three.
- 5A double membrane around each lung with fluid between its layers.
- 6To let the food pass easily through the oesophagus behind it.
- 7Thin walls, large surface area and rich blood supply.
- 8Nostril, nasal chamber, pharynx, larynx, trachea, bronchus, bronchioles, alveolus and then capillary.
Mechanism of breathing and respiratory volumes
A bicycle pump draws in air when its handle is pulled and pushes it out when pressed. The chest does the same by changing its volume.
Breathing has two steps: inspiration (taking air in) and expiration (giving air out). Air flows because the pressure inside the lungs is made lower or higher than the atmosphere's.
In the figure : Inspiration: muscle contraction increases thoracic volume, so air flows in.
Rule 1: Inspiration and expiration — In inspiration, the diaphragm contracts and flattens and the external intercostal muscles lift the ribs and sternum. This increases the volume of the thoracic cavity, lowers the pressure inside the lungs below atmospheric pressure and air rushes in. In expiration, the diaphragm and intercostals relax, the volume decreases, lung pressure rises above atmospheric pressure and air flows out. A healthy person breathes 12 to 16 times a minute.
- Easy : Place a hand on your belly: it rises as you breathe in because the diaphragm flattens.
- Medium : Pressure falls during inspiration and rises during expiration, relative to atmosphere.
- Exam-type : Q: What happens to the diaphragm in inspiration? A: It contracts and flattens.
Rule 2: Respiratory volumes and capacities — Tidal volume (TV) is about 500 mL. Inspiratory reserve volume (IRV) is 2500 to 3000 mL and expiratory reserve volume (ERV) is 1000 to 1100 mL. Residual volume (RV) is 1100 to 1200 mL. Capacities are sums: IC = TV + IRV; EC = TV + ERV; vital capacity VC = ERV + TV + IRV; total lung capacity TLC = VC + RV. Functional residual capacity is ERV + RV.
- Easy : TV x breathing rate, 500 mL x 12 per minute, gives 6000 mL per minute.
- Medium : If IRV is 2500 mL, TV 500 mL and ERV 1000 mL, then VC = 2500 + 500 + 1000 = 4000 mL.
- Exam-type : Q: With VC = 4000 mL and RV = 1200 mL, what is TLC? A: 5200 mL.
Inspiration = active, volume up, pressure down. Expiration = passive, volume down, pressure up. VC = IRV + TV + ERV.
Adding the residual volume to vital capacity incorrectly or forgetting it in TLC. TLC = VC + RV.
Memory clue: T-I-E-R — Tidal, Inspiratory reserve, Expiratory reserve, Residual: the four volumes.
Spot the difference
| Term | Meaning | Approximate value |
|---|---|---|
| TV | Normal breath | 500 mL |
| IRV | Extra air breathed in | 2500 to 3000 mL |
| ERV | Extra air breathed out | 1000 to 1100 mL |
| RV | Air left in lungs | 1100 to 1200 mL |
Practice questions
- 1Name the muscles used in normal inspiration.
- 2What is tidal volume?
- 3What is residual volume?
- 4Is normal expiration active or passive?
- 5Define vital capacity.
- 6How does pressure change in the lungs during inspiration?
- 7A person has TV 500 mL, IRV 3000 mL and ERV 1100 mL. Find the VC.
- 8If the person above has RV 1200 mL, what is TLC and what is the volume of air per minute at 12 breaths?
Show answers (try them yourself first)
- 1Diaphragm and external intercostal muscles.
- 2The volume of air inspired or expired in a normal breath.
- 3Air remaining in the lungs after forced expiration.
- 4Passive.
- 5The maximum air that can be breathed out after a deepest inspiration (ERV + TV + IRV).
- 6It falls below atmospheric pressure.
- 73000 + 500 + 1100 = 4600 mL.
- 8TLC = 4600 + 1200 = 5800 mL; air per minute = 500 x 12 = 6000 mL.
Exchange of gases by diffusion
A drop of ink spreads from a crowded region into clear water. Oxygen and carbon dioxide also move from where they are more concentrated to where they are less.
Exchange of gases occurs by simple diffusion in the alveoli and in the tissues, driven by differences in the partial pressure (pO2 and pCO2) of the gases. Diffusion depends on the pressure gradient, the solubility of the gas, and the thickness of the membrane.
In the figure : Oxygen moves from higher to lower partial pressure from alveoli to tissues.
Rule 1: Partial pressures — Atmospheric air: pO2 159 and pCO2 0.3 mm Hg. Alveoli: 104 and 40. Deoxygenated blood: 40 and 45. Oxygenated blood: 95 and 40. Tissues: 40 and 45. So oxygen moves from alveoli to blood and from blood to tissues; carbon dioxide moves the other way, from tissues to blood and from blood to alveoli.
- Easy : O2 moves from 104 (alveoli) to 40 (deoxygenated blood) because 104 is higher.
- Medium : CO2 moves from 45 (blood) to 40 (alveoli) because 45 is higher.
- Exam-type : Q: In which direction does CO2 diffuse at the alveolus? A: From blood to alveolus.
Rule 2: Why CO2 diffuses easily — Although the pressure difference for CO2 (45 against 40) is small, CO2 is about 20 to 25 times more soluble in the membrane than O2. Hence a large amount of CO2 can diffuse even through a small difference. The diffusion membrane is very thin, which also aids diffusion.
- Easy : Pressure difference for O2 is 64; for CO2 only 5.
- Medium : Even so, CO2 diffuses well because of its higher solubility.
- Exam-type : Q: Why does CO2 diffuse rapidly despite a small pressure gradient? A: It is much more soluble than O2.
Gases diffuse from higher to lower partial pressure. O2: alveoli to blood to tissues. CO2: tissues to blood to alveoli.
Thinking oxygen is actively pumped into blood. It simply diffuses.
Memory clue: 104-40-95-40 — Oxygen path: 104 (alveoli), 40 (deoxygenated blood), 95 (oxygenated blood), 40 (tissues).
Spot the difference
| Place | pO2 | pCO2 |
|---|---|---|
| Atmospheric air | 159 | 0.3 |
| Alveoli | 104 | 40 |
| Deoxygenated blood | 40 | 45 |
| Oxygenated blood | 95 | 40 |
| Tissues | 40 | 45 |
Practice questions
- 1By which process do gases cross the alveolar membrane?
- 2What is partial pressure?
- 3What is pO2 in the alveoli?
- 4What is pCO2 in deoxygenated blood?
- 5Name three factors affecting diffusion of gases.
- 6Explain why oxygen moves from blood to tissue.
- 7Why does CO2 diffuse well despite a small gradient?
- 8If pO2 in tissues were raised to 110, what would happen to oxygen delivery?
Show answers (try them yourself first)
- 1Simple diffusion.
- 2The pressure contributed by one gas in a mixture of gases.
- 3104 mm Hg.
- 445 mm Hg.
- 5Pressure gradient, solubility of gas and thickness of the membrane.
- 6pO2 in oxygenated blood (95) is higher than in tissues (40).
- 7It is 20 to 25 times more soluble than O2.
- 8Oxygen would not diffuse from the blood to tissues because the gradient would be reversed.
Transport of oxygen and carbon dioxide
Oxygen travels in blood like passengers in train carriages: the carriers are haemoglobin molecules in red blood cells.
About 97 per cent of oxygen is carried by haemoglobin in red blood cells as oxyhaemoglobin, and 3 per cent is dissolved in plasma. Carbon dioxide is mostly carried as bicarbonate (about 70 per cent), with the rest as carbamino-haemoglobin (20 to 25 per cent) and dissolved in plasma (about 7 per cent).
In the figure : Three ways in which blood carries carbon dioxide.
Rule 1: Oxygen transport — Each haemoglobin molecule can carry four O2 molecules. Binding depends on pO2, pCO2, H+ and temperature. Where pO2 is high and pCO2, H+ and temperature are low (alveoli), oxyhaemoglobin forms. Where pO2 is low and pCO2, H+ and temperature are high (tissues), oxygen is released. A graph of haemoglobin saturation against pO2 is S-shaped (oxygen dissociation curve). 100 mL of oxygenated blood delivers about 5 mL of O2 to tissues.
- Easy : Each Hb carries up to four oxygen molecules.
- Medium : A rise in pCO2 and a fall in pH shifts the curve right so that O2 is released more easily to active muscle.
- Exam-type : Q: How many O2 molecules can one Hb carry? A: Four.
Rule 2: Carbon dioxide transport — In red cells the enzyme carbonic anhydrase speeds the reaction CO2 + H2O to H2CO3, which becomes H+ and HCO3-. At the tissues pCO2 is high, so the reaction goes forward and bicarbonate is carried in plasma. At the alveoli pCO2 is low, so the reaction reverses and CO2 is released. About 100 mL of deoxygenated blood delivers about 4 mL of CO2 to the alveoli.
- Easy : Carbonic anhydrase is found in high concentration in red blood cells.
- Medium : The same enzyme works in either direction depending on pCO2.
- Exam-type : Q: Which enzyme speeds the formation of carbonic acid in RBCs? A: Carbonic anhydrase.
O2: 97% on Hb, 3% in plasma. CO2: 70% bicarbonate, 20 to 25% carbamino-Hb, 7% dissolved.
Saying that CO2 is mainly carried on haemoglobin. Most is carried as bicarbonate.
Memory clue: 70-25-7 for CO2 — Seventy bicarbonate, about twenty-five on Hb, seven dissolved.
Spot the difference
| Gas | Main form | Smaller forms |
|---|---|---|
| O2 | Oxyhaemoglobin (97%) | Dissolved in plasma (3%) |
| CO2 | Bicarbonate (70%) | Carbamino-Hb (20-25%), dissolved (7%) |
Practice questions
- 1What is the pigment that carries oxygen?
- 2What percentage of oxygen is carried by RBCs?
- 3In what form is most CO2 carried?
- 4Which enzyme is important in CO2 transport?
- 5Name two factors that favour the release of O2 from oxyhaemoglobin.
- 6What is the shape of the oxygen dissociation curve?
- 7Why does oxygen bind to haemoglobin in lungs and get released in tissues?
- 8A runner's muscles produce much CO2 and heat. How does this help oxygen supply?
Show answers (try them yourself first)
- 1Haemoglobin.
- 2About 97 per cent.
- 3As bicarbonate.
- 4Carbonic anhydrase.
- 5High pCO2, high H+ (or high temperature), low pO2 (any two).
- 6Sigmoid (S-shaped).
- 7In lungs pO2 is high and pCO2 low; in tissues pO2 is low and pCO2 high.
- 8Higher pCO2, H+ and temperature make haemoglobin release more oxygen.
Regulation of respiration and disorders
You never remember to breathe, yet you can breathe faster when you climb stairs. Your brain adjusts breathing to suit your body's needs.
Breathing is controlled by the respiratory rhythm centre in the medulla oblongata, with the pneumotaxic centre in the pons and a chemosensitive area next to the rhythm centre that responds to CO2 and H+.
In the figure : Rising CO2 and H+ stimulate the rhythm centre to increase breathing.
Rule 1: Control of breathing — The rhythm centre sets the basic pattern of inspiration and expiration. The pneumotaxic centre can switch off inspiration, so it changes the breathing rate. A rise in CO2 and H+ activates the chemosensitive area, which signals the rhythm centre to make changes. Receptors in the aortic arch and carotid artery also detect changes in CO2 and H+. The role of oxygen in regulation is quite insignificant.
- Easy : After a sprint, higher CO2 makes you breathe faster.
- Medium : If CO2 is low (after deep breathing), breathing slows for a moment.
- Exam-type : Q: Which part of the brain contains the respiratory rhythm centre? A: Medulla oblongata.
Rule 2: Disorders — Asthma: difficulty in breathing with wheezing due to inflammation of the bronchi and bronchioles. Emphysema: damaged alveolar walls reduce the surface for gas exchange; cigarette smoking is a major cause. Occupational disorders: long exposure to dust such as silica or asbestos causes inflammation and fibrosis, leading to serious lung damage; workers in grinding and stone-breaking industries need protection, for example masks.
- Easy : A person with asthma has wheezing because the airways are narrowed.
- Medium : A quarry worker wearing a mask reduces the risk of dust-caused lung disease.
- Exam-type : Q: What is the main cause of emphysema? A: Damage to alveolar walls, often due to smoking.
Medulla = rhythm; pons = pneumotaxic. CO2 and H+ drive breathing more than oxygen.
Thinking that low oxygen is the main stimulus for breathing. CO2 and H+ are.
Memory clue: MeRhythm, PonsPneu — Medulla = rhythm centre; Pons = pneumotaxic centre.
Spot the difference
| Disorder | Cause | Effect |
|---|---|---|
| Asthma | Inflammation of bronchi and bronchioles | Wheezing, difficult breathing |
| Emphysema | Alveolar wall damage (smoking) | Less surface area for exchange |
| Occupational | Dust such as silica or asbestos | Lung fibrosis |
Practice questions
- 1Where is the respiratory rhythm centre?
- 2What is the function of the pneumotaxic centre?
- 3Which substances activate the chemosensitive area?
- 4What is asthma?
- 5What is emphysema?
- 6What is the main cause of emphysema?
- 7Why do stone-grinding workers risk lung damage?
- 8Why does breathing quicken during exercise?
Show answers (try them yourself first)
- 1In the medulla oblongata.
- 2It can moderate the functions of the rhythm centre and shorten inspiration.
- 3CO2 and hydrogen ions.
- 4A breathing difficulty with wheezing due to inflammation of the bronchi and bronchioles.
- 5A disorder with damaged alveolar walls, reducing the gas exchange surface.
- 6Smoking.
- 7Inhaled dust causes inflammation and fibrosis of the lungs.
- 8Rising CO2 and H+ in blood stimulate the rhythm centre.