Chapter 8: Respiratory Systems in Humans and Animals

8.1 Types of Respiratory System

• Larger organisms have a small total surface area to volume ratio (TSA/V) and cannot rely on body-surface diffusion. (p.128)
• Adaptations of respiratory structures: large TSA/V, thin (one cell thick), moist, network of blood capillaries (except insects). (p.128)
• Insects: tracheal system with spiracles, trachea and tracheoles; tracheoles are thin, moist and numerous; some have air sacs. (p.129)
• Fish: gills with filaments and lamellae; many filaments give large surface; lamellae are thin with many capillaries. (p.129)
• Frogs: lungs with folded surface; inactive frogs use thin, moist, permeable skin with capillaries beneath. (p.130)
• Humans: alveoli are numerous, moist, surrounded by capillaries and one cell thick. (p.131)

Respiratory structures

(p.131)

Characteristic	Insects	Fish	Frogs	Humans
Structure	Tracheole	Filament and lamella	Skin and lungs	Alveolus
Large TSA/V by	Many tracheoles	Many filaments and lamellae	Folded lung surface, whole skin	Many alveoli

8.2 Mechanisms of Breathing

• Insects: relaxed abdominal muscles reduce tracheal pressure and air enters through spiracles; contraction increases pressure and forces air out. (p.132)
• Frogs (active): lowering the buccopharyngeal floor draws air in through nostrils; raising it with the glottis open pushes air into the lungs; lungs contract to expel air. (p.132)
• Fish inhalation: mouth opens, buccal floor lowered, operculum opening closed, pressure falls and water enters. (p.133)
• Fish exhalation: mouth closes, buccal floor raised, pressure rises and water flows out through the operculum, passing over the gills. (p.133)
• Human inhalation: external intercostal muscles contract and diaphragm contracts and flattens; rib cage moves up and outwards; thoracic volume increases and pressure decreases. (p.134)
• Human exhalation: external intercostals relax, diaphragm relaxes and domes upwards; rib cage moves down and inwards; thoracic volume decreases and pressure increases. (p.134)
• All involve changes in volume and pressure of the respiratory cavity using muscular structures. (p.135)

8.3 Gaseous Exchange in Humans

• Gases diffuse down a partial pressure gradient. (p.136)
• In the lungs, CO₂ diffuses from capillaries into alveoli; O₂ diffuses from alveoli into capillaries and combines with haemoglobin to form oxyhaemoglobin. (p.136)
• In tissues, oxyhaemoglobin releases O₂ to body cells; CO₂ from cells diffuses into tissue capillaries. (p.136)
• Partial pressure of oxygen in the atmosphere at sea level is 160 mm Hg (0.21 × 760). (p.137)
• CO₂ transport: 70% as bicarbonate ion, 23% as carbaminohaemoglobin, 7% dissolved as carbonic acid. (p.137)
• In tissues, carbonic anhydrase in erythrocytes catalyses CO₂ + H₂O to carbonic acid, which splits into HCO₃^- and H⁺. (p.137)
• In the lungs, bicarbonate re-enters erythrocytes and reforms CO₂ and water, which diffuses into the alveolus. (p.137)

8.4 Health Issues Related to the Human Respiratory System

• COPD comprises asthma, chronic bronchitis and emphysema. (p.138)
• Emphysema: alveoli lose elasticity and are damaged, so surface area and gas exchange efficiency fall. (p.138)
• Chronic bronchitis: bronchioles are inflamed and blocked with mucus, causing continuous coughing; cilia are damaged. (p.138)
• Asthma: bronchiole walls swell and thicken, narrowing the air passage and causing breathlessness. (p.138)