6.4 Gas Exchange

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.

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.

Alveolus

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

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.

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.

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

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)

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.

Great gas exchange video for ib