1.5 Gas exchange and the transport of oxygen in living organisms

Describe how surface area to volume ratio changes with organism size.

As organism size increases, the surface area to volume (SA:V) ratio decreases. Smaller organisms have a larger SA:V ratio, allowing sufficient exchange of substances by diffusion across their surface. In larger organisms, SA:V ratio becomes too small to meet metabolic demands, so diffusion alone is insufficient.

Why do multicellular organisms require specialised exchange and transport systems?

In multicellular organisms, the diffusion pathway from the surface to the centre of the body is too long, and the SA:V ratio is smaller. Diffusion alone would be too slow to supply oxygen and nutrients or remove waste, so specialised exchange surfaces and transport systems are required.

Explain how body shape affects heat exchange.

Flattened or elongated shapes increase surface area relative to volume, increasing the rate of heat exchange. Compact, spherical shapes decrease SA:V ratio, reducing heat exchange and conserving heat.

State adaptations that increase efficiency of gas exchange in animals and plants.

• Mammals: thin alveolar walls, large surface area, rich blood supply.
• Fish: large surface area from gill filaments, counter-current flow.
• Plants: large surface area of leaves, thin cell walls, stomata for gas exchange.
• Animals: circulatory systems deliver oxygen/nutrients and remove waste.
• Plants: xylem and phloem transport water, minerals, and sugars.

Explain the relationship between metabolic rate and S

Small animals with a larger SA:V ratio lose heat faster per unit body mass, so they require a higher metabolic rate to maintain body temperature. Metabolic rate is the amount of energy expended in a given time, often measured by oxygen uptake. Higher metabolic rates require faster aerobic respiration and greater oxygen intake.

Describe how fish gills are adapted for efficient gas exchange.

• Protected by the operculum.
• Large surface area: many gill filaments covered in thin lamellae.
• Short diffusion pathway: lamellae are thin.
• Good blood supply in lamellae.
• Counter-current exchange maintains concentration gradient: water and blood flow in opposite directions, ensuring oxygen concentration in water remains higher than in blood along the whole lamella.

Explain the importance of counter-current exchange in fish.

Counter-current flow maintains a steep concentration gradient across the entire lamella, so oxygen diffuses into blood along the whole length. This ensures maximum oxygen uptake, which is vital as water has a lower oxygen concentration than air.

Describe the insect gas exchange system.

Insects exchange gases through spiracles (pores on exoskeleton) leading to tracheae (air tubes supported by chitin) and tracheoles (thin, highly branched tubes with large surface area). Tracheoles provide a short diffusion pathway directly to cells. Oxygen diffuses into cells, and rhythmic abdominal movements ventilate the system.

How do insects adapt to minimise water loss while maintaining gas exchange?

• Spiracles can close to reduce evaporation.
• Waxy cuticle on body surface reduces evaporation.
• Hairs around spiracles trap moisture.
• Small SA:V ratio reduces evaporative loss.

This system balances oxygen uptake with minimal water loss.

Explain how gases are exchanged in dicotyledonous plants.

Gases diffuse in and out through stomata controlled by guard cells.
• When guard cells lose water by osmosis they become flaccid, causing stomata to close and reducing water loss.
• When guard cells gain water by osmosis they become turgid, opening stomata.
Gas exchange occurs at the surface of mesophyll cells in leaves. Many mesophyll cells and air spaces provide a large surface area. The short diffusion pathway across cell walls and membranes allows rapid diffusion of gases. Chloroplasts use CO₂ in photosynthesis and produce O₂, maintaining concentration gradients.

Describe xerophytic adaptations to conserve water.

Xerophytes live in dry, windy, warm conditions and have adaptations to reduce transpiration, including:
• Hairs around stomata trapping water vapour.
• Stomata sunk into pits to trap vapour.
• Curled leaves reducing air movement.
• Thick waxy cuticle increasing diffusion distance and reducing transpiration.
• Reduced number of stomata, or spines instead of leaves (cacti), lowering SA:V ratio.

These adaptations reduce the water potential gradient between leaf air spaces and the outside air, minimising evaporative water loss.

Describe the gross structure of the human gas exchange system.

• Trachea: flexible airway with C-shaped cartilage rings, lined with ciliated epithelium and goblet cells.
• Bronchi: two divisions of the trachea leading to each lung, supported by cartilage, lined with cilia and mucus.
• Bronchioles: smaller branching airways with muscular walls controlling airflow.
• Alveoli: tiny air sacs at ends of bronchioles, site of gas exchange, surrounded by capillary network.
• Lungs: paired lobed structures, spongy and elastic, protected by ribcage.

How are alveoli adapted for efficient gas exchange?

• Many alveoli provide a large surface area.
• Alveolar epithelium is one cell thick, giving a short diffusion pathway.
• Capillary endothelium is also one cell thick.
• Oxygen and CO₂ diffuse across alveolar epithelium into blood and out of blood.
• Ventilation and blood flow maintain steep concentration gradients.

Explain the process of ventilation in humans.

• Inspiration (active process, requires ATP): diaphragm contracts and flattens, external intercostal muscles contract, ribcage moves up/out, volume of thoracic cavity increases, pressure decreases below atmospheric pressure, air moves in down pressure gradient.
• Expiration (passive): diaphragm relaxes (dome shape), external intercostals relax, ribcage moves down/in, volume of thoracic cavity decreases, pressure rises above atmospheric, air moves out down pressure gradient.
• Forced expiration is active, requiring contraction of internal intercostals.

Define the following measurements of lung function.

• Tidal volume: volume of air inhaled/exhaled at rest.
• Forced expiratory volume (FEV₁): maximum air volume exhaled in one second.
• Ventilation rate: number of breaths per minute.
• Forced vital capacity (FVC): maximum volume exhaled after deepest breath.
• Pulmonary ventilation rate = ventilation rate × tidal volume.

Explain how fibrosis affects gas exchange.

Fibrosis forms scar tissue in lungs (e.g., after infection such as TB). Scar tissue is thicker and less elastic, so lungs cannot expand properly. Tidal volume and FVC are reduced, ventilation rate increases to compensate, but diffusion rate is slower due to thickened alveolar epithelium, reducing oxygen uptake. Symptoms include shortness of breath, dry cough, chest pain, fatigue.

Explain how emphysema affects gas exchange.

Caused by long-term smoking or air pollution. Phagocytes release elastase that breaks down elastin in alveoli, reducing elasticity. Alveoli cannot recoil properly, surface area decreases, FEV₁ is reduced, ventilation rate increases, and diffusion rate decreases. Symptoms: shortness of breath, wheezing.

Describe the structure and function of haemoglobin.

Haemoglobin is a protein of four polypeptide chains, each with a haem group containing iron (Fe²⁺). Each haem binds one O₂ molecule, so one haemoglobin can carry four O₂. Binding is reversible, allowing oxygen loading at high pO₂ (lungs) and unloading at low pO₂ (respiring tissues). Found in red blood cells.

Explain the oxygen–haemoglobin dissociation curve.

S-shaped curve:
• At high pO₂ (e.g., alveoli), haemoglobin has high affinity and loads oxygen.
• At low pO₂ (e.g., respiring tissues), affinity decreases, oxygen is unloaded.
• Binding of first O₂ changes haemoglobin’s quaternary structure, making binding of subsequent O₂ molecules easier (positive cooperativity).

What is the Bohr effect?

At higher partial pressures of CO₂, dissociation curve shifts right. Haemoglobin affinity decreases due to lower blood pH (more acidic). Oxygen unloads more readily at respiring tissues where CO₂ is high, increasing oxygen supply for aerobic respiration.

How do different organisms’ haemoglobins adapt to environment?

• High metabolic demand (e.g., small mammals): haemoglobin has lower affinity, dissociation curve shifts right, oxygen unloaded more readily.
• Low oxygen habitats (e.g., high altitude, water environments): haemoglobin has higher affinity, dissociation curve shifts left, oxygen loads readily at low pO₂.

Describe the human circulatory system.

Consists of the heart and blood vessels. Double circulation: pulmonary circulation (heart → lungs → heart) and systemic circulation (heart → rest of body → heart). Needed due to low SA:V ratio in larger organisms. Blood transports oxygen, carbon dioxide, glucose, amino acids, urea, hormones, etc.

Explain how arteries are adapted to their function.

Arteries carry blood away from the heart under high pressure. They have thick muscle walls, thick elastic tissue for stretch and recoil, folded endothelium for expansion, smooth endothelium to reduce friction, and outer collagen for strength.

Explain the role of arterioles.

Arterioles branch from arteries to capillaries, walls contain muscle that contracts (vasoconstriction) or relaxes (vasodilation) to control blood flow to tissues.

Describe the adaptations of capillaries for exchange.

Endothelium one cell thick (short diffusion pathway), narrow lumen increases surface area for exchange, networks form capillary beds, close contact with cells.

Explain how veins are adapted to their function.

Veins return blood at low pressure to the heart. They have thinner walls, less muscle and elastic tissue, wide lumen, valves to prevent backflow, and outer collagen for support.

Describe the structure of the human heart.

• Four chambers with muscular walls: two atria and two ventricles.
• Right side pumps deoxygenated blood to lungs; left side pumps oxygenated blood to body.
• Ventricles have thicker walls than atria for stronger contraction.
• Left ventricle wall is thicker than right to generate higher pressure for systemic circulation.
• Atrioventricular (AV) and semi-lunar valves prevent backflow.
• Cords (tendinous cords) prevent AV valves from being forced into atria.
• Coronary arteries supply oxygenated blood to heart muscle.

Outline the three main stages of the cardiac cycle.

1. Atrial systole: Ventricles relax, atria contract, decreasing volume and increasing pressure. AV valves open, blood forced into ventricles.

2. Ventricular systole: Atria relax, ventricles contract, decreasing volume and increasing pressure. AV valves close, semi-lunar valves open, blood forced into aorta and pulmonary artery.

3. Diastole: Atria and ventricles relax. Semi-lunar valves close, atrial volume increases, AV valves open, blood flows passively into ventricles.

Explain how pressure changes control valve opening and closing.

Valves open when pressure is higher behind them and close when pressure is higher in front. E.g., when atrial pressure > ventricular pressure, AV valves open; when ventricular pressure > atrial pressure, AV valves close. Semi-lunar valves open when ventricular pressure > artery pressure, and close when artery pressure > ventricular pressure.

Define stroke volume and cardiac output.

• Stroke volume = volume of blood pumped by the left ventricle in one heartbeat.
• Cardiac output (cm³ min⁻¹) = stroke volume (cm³) × heart rate (bpm).

State the vessels entering and leaving the heart.

• Vena cava: brings deoxygenated blood from body into right atrium.
• Pulmonary artery: carries deoxygenated blood from right ventricle to lungs.
• Pulmonary vein: carries oxygenated blood from lungs to left atrium.
• Aorta: carries oxygenated blood from left ventricle to body.

State the vessels entering and leaving the liver.

• Hepatic artery: carries oxygenated blood from aorta to liver.
• Hepatic vein: carries blood from liver to vena cava.
• Hepatic portal vein: carries blood rich in absorbed nutrients from small intestine to live