Chapter 2: Breathing and Exchange of Gases

Why organisms need gas exchange

• Oxygen use: Cells use oxygen to oxidise food molecules such as glucose, fatty acids and amino acids, releasing energy for life activities.
• Carbon dioxide: Catabolic reactions produce carbon dioxide, which must be removed from the body.
• Reductionist approach: Studying life through its physico-chemical components is called the reductionist approach; neither this approach alone nor an organism-level approach alone fully explains living phenomena.

• Variation: Respiratory mechanisms differ among animal groups, mainly according to habitat and level of organisation.

Airway structures and alveoli

• Pharynx: A shared passage for food and air.
• Epiglottis: A thin, elastic, cartilaginous flap that covers the glottis during swallowing, preventing food from entering the larynx.
• Larynx: A cartilaginous box that produces sound; also called the sound box.
• Cartilage support: Incomplete cartilaginous rings support the trachea, bronchi up to the tertiary level, and the initial bronchioles.
• Alveoli: Very thin-walled, irregular, vascularised air sacs arising from terminal bronchioles; close contact with capillaries enables gas exchange.
• Number: Human lungs contain approximately 300 million alveoli.

Pleural membranes

• Pleura: Each lung is enclosed by a double-layered membrane; the outer layer contacts the thoracic lining and the inner layer contacts the lung surface.
• Pleural cavity: The narrow space between the two pleural layers contains pleural fluid.
• Pleural fluid: Lubricates the layers so they slide over one another with little friction as the lungs expand and contract.

• Surfactant: A lipid that lowers surface tension and prevents alveoli from collapsing during respiration.
• Development: Surfactant secretion develops only by the eighth or ninth month of pregnancy.
• Clinical significance: In some newborns, an absent lecithin film can cause lung collapse and may be fatal.

• Thoracic chamber: An airtight chamber containing the lungs; bounded dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and below by the dome-shaped diaphragm.
• Volume relationship: The lungs are arranged so that a change in thoracic-cavity volume is reflected in the pulmonary cavity and lungs.

Basic Principle

• Breathing has two stages: inspiration draws atmospheric air in; expiration releases alveolar air out.
• Pressure gradient: Air moves into or out of the lungs because pressure differs between the lungs and atmosphere.
• Main muscles: The diaphragm and the external and internal intercostal muscles help create this pressure gradient.

Air Pathway During Inspiration

• External nares → nasal cavities → internal nares → pharynx → glottis → larynx → trachea → bronchi → bronchioles → alveolar ducts → alveoli.

Air Pathway During Expiration

• Alveoli → alveolar ducts → bronchioles → bronchi → trachea → larynx → glottis → pharynx → internal nares → nasal cavities → external nares → outside.
• Forceful breathing: Additional abdominal muscles can increase the strength of inspiration and expiration.

Breathing Rate and Timing

• Average rate: A healthy human breathes about 12–16 times per minute.
• Duration: Inspiration takes about 2 seconds; expiration takes about 3 seconds.

Measurement and Variation

• Sex differences: Respiratory volumes and capacities are about 20%–25% lower in women than in men.
• Physical build: Values are generally greater in athletic people than in asthenic (thin, non-exercising) people.
• Spirometer: All volumes and capacities can be measured with a simple spirometer except residual volume (RV) and functional residual capacity (FRC).

• Main site: Alveoli are the primary sites of gas exchange.
• Other site: Gases are also exchanged between blood and body tissues.
• Oxygen pathway: Oxygen diffuses from alveoli into blood, then from blood to tissues.
• Carbon dioxide pathway: Carbon dioxide diffuses from tissues into blood, then from blood to alveoli.
• Mechanism: Both gases move by simple diffusion, mainly down their partial-pressure or concentration gradients.

• Oxygen gradient: Higher \(pO_2\) in alveoli drives oxygen into deoxygenated blood; blood then carries it to tissues with lower \(pO_2\).
• Carbon dioxide gradient: Higher \(pCO_2\) in tissues drives carbon dioxide into blood and onward to alveoli.
• Diffusion rate: Gas solubility and the thickness of the diffusion membrane affect how rapidly a gas diffuses.
• Carbon dioxide: Its solubility is about 20–25 times that of oxygen, so more \(CO_2\) diffuses per unit difference in partial pressure.

• Layers: The diffusion membrane consists of thin squamous alveolar epithelium, alveolar-capillary endothelium, and basement substance between them.
• Basement substance: It includes the thin basement membranes supporting the alveolar epithelium and surrounding the capillary endothelial cells.
• Thickness: The complete membrane is much less than \(1\) mm thick, providing a short diffusion path.
• Blood leaving alveoli: Arterial blood has almost the same gas partial pressures as alveolar air: \(pO_2 \approx 104\) mm Hg and \(pCO_2 \approx 40\) mm Hg.

• Large surface area: Provides extensive area for gas exchange.
• Short diffusion path: Keeps the distance between alveolar air and blood very small.
• Partial-pressure gradients: Drive oxygen from alveoli to blood and carbon dioxide from blood to alveoli.
• Respiratory pigment: Haemoglobin increases the blood's carrying capacity for oxygen and carbon dioxide.
• Respiratory membrane: Must be thin, moist and large.

Haemoglobin and oxygen loading

• Haemoglobin: RBCs contain haemoglobin, made of four polypeptide chains and four haem groups, each with iron in the ferrous state (\(\mathrm{Fe^{2+}}\)).
• Binding capacity: Each haemoglobin molecule can bind four oxygen molecules reversibly.
• In the lungs: High alveolar \(p_{\mathrm{O_2}}\) favours oxygen diffusion into RBCs and oxyhaemoglobin formation.
• Effect of carbon dioxide: As \(\mathrm{CO_2}\) diffuses from blood into the alveoli, blood \(p_{\mathrm{CO_2}}\) falls, favouring further oxygen uptake.

Bohr effect

• Bohr effect: Increased blood \(p_{\mathrm{CO_2}}\) shifts the oxygen–haemoglobin curve to the right, promoting oxygen release at tissue capillaries.

Carbon dioxide carriage and release

• Carbonic anhydrase: This zinc-containing enzyme makes bicarbonate formation rapid inside RBCs; the reaction is very slow in plasma.
• At tissues: High \(p_{\mathrm{CO_2}}\) from cellular catabolism drives carbon dioxide into blood and favours formation of \(\mathrm{HCO_3^-}\) and \(\mathrm{H^+}\).
• At alveoli: Low \(p_{\mathrm{CO_2}}\) favours the reverse reactions, producing carbon dioxide and water; carbon dioxide then diffuses into the alveoli.
• Delivery: Each \(100\,\mathrm{mL}\) of deoxygenated blood delivers about \(4\,\mathrm{mL}\) of carbon dioxide to the alveoli.

Haldane effect

• Haldane effect: Proposed by J. S. Haldane; oxygen binding makes haemoglobin a stronger acid and tends to displace carbon dioxide from blood.
• Importance: It promotes carbon dioxide exchange in both tissues and lungs and is more important for carbon dioxide transport than the Bohr effect is for oxygen transport.

• Symptoms: Difficulty in breathing, wheezing and coughing.

• Major cause: Cigarette smoking.

• Symptoms: Persistent cough and greenish-yellow mucoid sputum.
• Causes: Infection, cigarette smoking and air pollutants such as carbon monoxide.