Breathing and Exchange of Gases

Human respiratory system

Breathing means taking in air with oxygen (O₂) and giving out air with carbon dioxide (CO₂). Respiration is bigger: it also includes gas exchange and the use of O₂ by cells to get energy.

Different animals breathe in different ways. Sponges and cnidarians use their whole body surface. Earthworms use moist skin. Insects use tubes called tracheae. Fish use gills. Frogs, birds, reptiles and mammals use lungs.

Parts in humans

• External nostrils → nasal chamber: hair and mucus clean the air; it is warmed and wetted.
• Pharynx: a common passage for food and air.
• Larynx (voice box): a cartilage box that makes sound. The epiglottis, a flap, closes the glottis when we swallow so food does not enter the windpipe.
• Trachea: a tube held open by C-shaped rings of cartilage. It splits into right and left primary bronchi.
• Bronchi → bronchioles → alveoli: the tubes branch again and again and end in thin, cup-like sacs called alveoli.
• Lungs: two lungs covered by a double layer called the pleura, with fluid between the layers to cut friction.
• Thoracic chamber: an air-tight box made by the backbone, sternum, ribs and, at the floor, the dome-shaped diaphragm.

The part from nostrils to terminal bronchioles is the conducting part (it moves, cleans, warms and wets air). The alveoli and their ducts are the respiratory or exchange part.

Steps of respiration

1. Breathing (pulmonary ventilation).
2. Diffusion of gases across the alveolar wall.
3. Transport of gases by blood.
4. Diffusion of O₂ and CO₂ between blood and tissues.
5. Use of O₂ by cells (cellular respiration).

Mechanism of breathing

Air flows from high pressure to low pressure. So to pull air in, the lungs must make their inside pressure lower than the air outside. They do this by changing the size of the chest.

Inspiration (breathing in)

• The diaphragm contracts and becomes flat, so the chest gets longer from top to bottom.
• The external intercostal muscles (between ribs) contract and lift the ribs and sternum, so the chest gets deeper front to back.
• Chest volume goes up → lung volume goes up → pressure inside falls below air pressure → air comes in.

Expiration (breathing out)

• The diaphragm and intercostal muscles relax. The diaphragm goes back to its dome shape and the ribs drop.
• Chest and lung volume go down → pressure inside rises above air pressure → air goes out.
• Quiet expiration is mostly passive. In forced breathing, abdominal muscles and internal intercostals help push air out.

A healthy adult breathes about 12 to 16 times a minute. A spirometer measures the volumes of air moved.

Respiratory volumes and capacities

Volumes (single amounts)

• Tidal Volume (TV): air in or out in one normal breath, about 500 mL. So a person moves about 6000 to 8000 mL per minute.
• Inspiratory Reserve Volume (IRV): extra air you can pull in with force after a normal breath in, about 2500 to 3000 mL.
• Expiratory Reserve Volume (ERV): extra air you can push out with force after a normal breath out, about 1000 to 1100 mL.
• Residual Volume (RV): air that stays in the lungs even after the hardest breath out, about 1100 to 1200 mL. It keeps alveoli from collapsing.

Capacities (sums of volumes)

• Inspiratory Capacity (IC) = TV + IRV
• Expiratory Capacity (EC) = TV + ERV
• Functional Residual Capacity (FRC) = ERV + RV (air left after a normal breath out)
• Vital Capacity (VC) = ERV + TV + IRV (most air you can breathe out after the deepest breath in)
• Total Lung Capacity (TLC) = VC + RV = RV + ERV + TV + IRV

Tip: RV cannot be breathed out, so any capacity that includes RV (FRC, TLC) cannot be measured by a simple spirometer.

Exchange of gases

Gas exchange happens in two places: between alveoli and blood, and between blood and tissues. It happens by simple diffusion.

Partial pressure is the share of total pressure caused by one gas in a mixture, written pO₂ or pCO₂ (in mm Hg). A gas moves from higher partial pressure to lower.

• At the alveolus: O₂ 104 → 40 goes into blood; CO₂ 45 → 40 comes out.
• At tissues: O₂ 95 → 40 goes into cells; CO₂ 45 → 40 goes into blood.

CO₂ has a small pressure difference, but it dissolves 20 to 25 times better than O₂, so plenty still moves. The barrier is very thin (less than a millimetre; made of alveolar lining, a basement layer and capillary lining), which also helps.

Transport of oxygen and carbon dioxide

Oxygen

• About 97% of O₂ is carried by haemoglobin in red blood cells as oxyhaemoglobin. Each haemoglobin can hold up to 4 O₂ molecules.
• About 3% is dissolved in plasma.
• Every 100 mL of oxygenated blood gives about 5 mL of O₂ to the tissues in normal conditions.

Oxygen dissociation curve

If we plot % saturation of haemoglobin against pO₂, we get an S-shaped (sigmoid) curve. In the lungs (high pO₂, low pCO₂, less H⁺, lower temperature), O₂ binds. In the tissues (low pO₂, high pCO₂, more H⁺, higher temperature), O₂ is let go. These tissue conditions shift the curve to the right.

Carbon dioxide

• About 20 to 25% joins haemoglobin as carbamino-haemoglobin.
• About 70% travels as bicarbonate (HCO₃⁻). The enzyme carbonic anhydrase in RBCs speeds this up: CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺. At tissues it goes right; in the lungs it goes left and CO₂ is released.
• About 7% is dissolved in plasma.
• Every 100 mL of deoxygenated blood gives about 4 mL of CO₂ to the alveoli.

Regulation of respiration

• The respiratory rhythm centre in the medulla sets the basic breathing rhythm.
• The pneumotaxic centre in the pons can change the rhythm centre's work, making breaths shorter and faster.
• A chemosensitive area next to the rhythm centre is very sensitive to CO₂ and H⁺. When these rise, it tells the rhythm centre to breathe more to remove them.
• Receptors in the aortic arch and carotid artery also sense CO₂ and H⁺ and send signals.
• Oxygen plays only a small role in controlling normal breathing. CO₂ is the main driver.

Disorders of the respiratory system

• Asthma: the bronchi and bronchioles swell and tighten, so breathing is hard and you hear wheezing. Triggers include dust, pollen and smoke.
• Emphysema: the alveolar walls break down, so there is less surface for gas exchange. Cigarette smoking is the main cause. It is long-lasting.
• Occupational respiratory disorders: people in grinding, stone-breaking, mining or cotton mills breathe fine dust for years. The body's defence cannot handle it all, so lungs become inflamed and scarred (fibrosis). Masks and dust control prevent it.

Key formulas and definitions

• IC = TV + IRV
• EC = TV + ERV
• FRC = ERV + RV
• VC = IRV + TV + ERV
• TLC = VC + RV = IRV + TV + ERV + RV
• Minute ventilation = breathing rate × tidal volume
• CO2 + H2O ⇌ H2CO3 ⇌ HCO3⁻ + H⁺ (carbonic anhydrase)

Worked examples

Common mistakes

• Saying the lungs suck in air by themselves. Lungs have no muscles; the diaphragm and intercostal muscles change the chest size.
• Thinking residual volume can be breathed out. RV always stays in the lungs, so a spirometer cannot measure it.
• Thinking most CO2 is carried by haemoglobin. About 70% travels as bicarbonate in plasma; only 20–25% is carbamino-haemoglobin.
• Believing low oxygen is the main trigger for breathing. The main trigger is rising CO2 and H⁺ sensed by the chemosensitive area.