1. 6.4 Gas Exchange

2. U6.4.1 Ventilation maintains concentration gradients of Oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries
Gas Exchange: absorption of one gas from the environment and release of another.
Example: Leaves absorb CO2 and release oxygen.
Humans absorb oxygen and release CO2
Alveoli: the small air sacs in the lungs that exchange gas.
Diffusion between air in alveoli and blood flowing in the adjacent capillaries. Occurs because of concentration gradient: air in alveolus has a higher concentration of oxygen and lower concentration of carbon dioxide than the blood capillary.
Ventilation: the maintenance of the concentration gradients where fresh air is pumped into the alveoli and stale air must be removed.

3. 6.4.2 Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange
Lungs contain huge numbers of alveoli with very large total surface area for diffusion.
Walls of alveolus consist of single layer of epithelium. Most of cells are Type I pneumocytes.
Type I pneumocytes: Flattened cells, thickness about 0.15μm of cytoplasm
Diffusion of oxygen and carbon dioxide very small because the walls of the adjacent capillaries consists of a single layer of thin cells. Air in alveolus and the blood capillaries only about 0.5μm apart.

4. Alveolus

5. Occupy about 5% of alveolar surface area.
6.4.3 Type Ii pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension
Type II pneumocytes:
Rounded cells
Occupy about 5% of alveolar surface area.
Secrete fluid to coat inner surface of alveoli which allows oxygen in alveolus to dissolve and diffuse to blood of alveolar capillaries. Also CO2 can evaporate into air and be exhaled.
Fluid contains pulmonary surfactant
Structure similar to phospholipids
Form a monolayer

6. Fluid contains pulmonary surfactant
6.4.3 Type Ii pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension
Type II pneumocytes:
Fluid contains pulmonary surfactant
Structure similar to phospholipids
Form a monolayer –hydrophilic heads facing the water and hydrophobic tails facing the air.
Reduces the surface tension
Prevents water from causing the alveoli to adhere when exhalation occurs. Preventing the collapse of the lung.

7. Infant respiratory distress syndrome Premature babies
6.4.3 Type Ii pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension
Infant respiratory distress syndrome
Premature babies
Often born with insufficient pulmonary surfactant
Treatment
Give oxygen
Give surfactant extracted from animal lungs.

8. 6.4.4 Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles
Air pathway through ventilation system:
Nose and mouth
Down trachea
Rings of cartilage in wall keep it open with low air pressure inside or high pressure in surrounding tissue
Divides to form two bronchi
Bronchi
One to each lung
Walls strengthened with cartilage
Bronchioles
Bronchi, Inside lung, divide repeatedly for form tree-like structure of narrower airways
Smooth muscle fibres in walls allowing for varying width.
Alveoli
At end of narrowest bronchioles
Allow gas exchange

9. Physics of ventilation of lungs
6.4.5 muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them
Physics of ventilation of lungs
Gas spread out to occupy smaller volume => pressure of gas becomes lower and vice versa.
If gas is free to move will always flow from regions of higher pressure to lower pressure.
During ventilation
Muscles contract causing the pressure inside the thorax to drop below atmospheric (atm) pressure (P)=> inspiration: air drawn into lungs until reaches atm. P.
Muscle contractions than cause P inside thorax to rise above atm. So air forced out of lungs to atm. (expiration)

10. Muscle has 2 states – contracting and relaxing
6.4.6 Different muscles are required for inspiration and expiration because muscles only do work when they contract
ANTAGONISTIC MUSCLE
Muscle has 2 states – contracting and relaxing
Contracting: muscles do work by exerting a pulling force (tension) that causes movement. They become shorter.
Relaxing: muscles lengthen passively but most muscles are pulled into an elongated state by the contraction of another muscle.
DO NO WORK AT THIS TIME
Muscles can only cause movement in one direction by themselves. Takes two muscles to cause opposite movement- one contracts and one relaxes. Known as antagonistic pairs.

11. Great gas exchange video for ib