IB Biology SL 2024-2025 Topic B3.1 Gas Exchange in Humans

Breathing and Respiration

  • Breathing is NOT respiration.

Ventilation

  • Mechanical process of moving air in (inspiration) and out (expiration) of the lungs.

  • Controlled by the movement of the diaphragm and rib cage.

Gas Exchange

  • Diffusion of O2andandCO2 between the blood and the alveoli and respiring tissues.

Diffusion and Concentration Gradients

  • Diffusion tends to reduce concentration gradients.

  • This could decrease the rate and eventually stop gas exchange if the concentration gradient becomes equal.

Gas Exchange and Partial Pressures

Partial pressure drives gas exchange.

Ventilation System

Efficient Gas Exchange

  • Relies on high concentration gradients maintained in the alveoli.

Breathing In

  • Increases the concentration gradient of O2O_2 between the alveoli and the blood, so it diffuses into the blood.

Breathing Out

  • Removes CO2 (and unused O2), increasing the concentration gradient of CO2 between blood and the alveolus, so CO2 will diffuse out.

Equilibrium

  • If the alveoli were not ventilated, equilibrium would be reached, and no gas could be exchanged because there would be no concentration gradient.

Alveoli

  • Millions of alveoli increase the surface area for gas exchange.

  • Each has a network of capillaries.

  • A rich blood supply maintains a high concentration gradient.

  • Surfaces are wet so gasses are dissolved making diffusion easier.

Membranes

  • Membranes are very thin (both capillaries and alveoli) so the diffusion path is short.

  • Surfaces are wet so gasses are dissolved making diffusion easier.

Alveoli Adaptations for Gas Exchange

Capillaries

  • A dense network of capillaries means a rich blood supply maintains a high concentration gradient of O<em>2O<em>2 & CO</em>2CO</em>2 necessary for gas exchange.

Pulmonary Surfactant

  • A fluid (pulmonary surfactant) is secreted by the cells in the alveolus wall that keeps the lining of the alveolus moist, allowing oxygen to dissolve.

  • A film of moisture allows gasses to dissolve into solution, making diffusion easier.

  • The surfactant reduces the surface tension & prevents the water from causing the sides of the alveoli to stick together when air is exhaled.

  • This helps to prevent collapse of the lung.

Large Surface Area

  • Provided by having about 300 million alveoli in a pair of adult lungs.

  • Each alveolus is tiny, but because there are so many of them, the total area is very large.

  • About 400 times greater than the outer surface of the body.

Membrane Transport

Which process(es) of membrane transport are being used in gas exchange at the membranes of the alveoli?

Airways

  • Ventilate each lung.

  • Consist of branching bronchioles, ending in alveolar ducts, each of which leads to a group of five or six alveoli air sacs).

Lung Structure & Ventilation

Location

  • The lungs are in the thorax.

Airways

  • Air can only get into or out of the thorax through airways.

  • The airway used to ventilate the lungs consist of the nose, mouth, trachea, bronchi and bronchioles.

Pressure

  • If gas is free to move, it will always flow from regions of higher pressure to regions of lower pressure.

Muscle Contractions

  • During ventilation, muscle contractions cause pressure changes inside the thorax that pull extra air into the alveoli and then push it out again.

  • The muscles causing this are:

    • The diaphragm that divides the thorax and abdomen.

    • Muscles in the front wall of the abdomen.

    • Intercostal muscle between the ribs, in two layers (internal and external) that are antagonistic.

Mechanics of Ventilation

  • The diaphragm muscle expands the size of the thoracic cavity.

  • Intercostal muscles control movement of the rig cage.

Two Stages in Ventilation

Inspiration

  • External intercostals muscles contract, Internal intercostal muscle relax & diaphragm contracts (drops). Abdominal muscles relax.

  • Chest volume increases, pressure in lungs decrease, air enters.

Expiration

  • Internal intercostal muscle contract, External intercostals muscles relax & diaphragm relaxes(rises). Abdominal muscles contract.

  • Chest volume decreases, pressure in lungs increases, air leaves.

Muscle Work

  • Different muscles are required for inspiration and expiration because muscles only do work when they contract. Need antagonistic pairs of muscles.

Process of Controlling Ventilation Rate During Exercise

Exercise

  • Exercise increases aerobic respiration.

CO2 Concentration

  • CO2CO_2 concentration in blood increases.

pH Levels

  • Drop in pH of blood detected.

Breathing Centers

  • Breathing centers send impulses to diaphragm & intercostal muscles.

Contraction Rate

  • Increase rate of contraction.

Ventilation Rate

  • Increase in ventilation rate increases oxygen uptake.

Pleural Sac

  • A doubled-walled closed sac called the pleural sac separates each lung from the thoracic wall and other surrounding structures. It is lubricated with intra-pleural fluid.