6.4 Gas Exchange

Some basic ideas:We have to breathe so that we can exchange the carbon dioxide that our cells produce during cell respiration for the oxygen that our cells need for cell respiration

• ventilation – the exchange of air between our lungs and the atmosphere

• Gas exchange – the back and forth diffusion of gases – Exchange occurs between lungs (inhale O2, release CO2)

and capillary beds (cells use O2 and produce CO2)

Why do we need a Ventilation System?

• To maintain the concentration gradients of oxygen and carbon dioxide in the alveoli. – Oxygen needs to diffuse from the alveoli into the

blood. Carbon dioxide needs to diffuse from the blood into the alveoli.

• The body needs oxygen to make ATP via cell respiration. – The body needs to get rid of carbon dioxide which is a

product of cell respiration. – To do so there must be a high oxygen concentration and

a low carbon dioxide concentration in the alveoli.

Tim & Moby: Respiratory System

Anatomy of the Respiratory System1. Nose/Mouth – brings in/removes

gases2. Trachea – transports air to and

from lungs3. Bronchi – the branching airway

to each lung4. Lungs -

1. Bronchioles – smaller branches in lungs leading to alveoli

2. Alveoli – air sacks where the diffusion of gases takes place

Ventilation Process

The alveoli• Alveoli are found in clusters at the ends of the

smallest bronchioles (think about a bunch of grapes)• Surrounding each cluster of alveoli is a capillary bed• Blood entering the capillary beds is from the

pulmonary arteries (CO2-rich/O2-poor) and here the exchange takes place; now the blood (O2-rich/CO2-poor) can return to the heart via the pulmonary veins

• The gases must each diffuse through two membranes (phospholipid bilayers)– The cell membrane of the alveoli– The cell membrane of the capillary

• As long as you continue breathing then you will maintain the proper concentration gradients of O2 and CO2

• Great numbers increase the surface area for gas exchange.

• Wall made up of single layer of cells and so are the walls of the capillaries so diffusion distance is small allowing rapid gas exchange.

• Covered by a dense network of capillaries which have low oxygen and high carbon dioxide concentrations. This allows oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood.

• Some cells in the walls secrete fluid allowing gases to dissolve. (also act to oppose invading pathogens)

Summary of Features of Alveoli that allow for Efficient Gas Exchange:

This process works like Boyle’s Law :P1 x V1 = P2 x V2

Process of Breathing: Volume/Pressure Relationship

Inspiration (air in)/Expiration (air out): Cycle:

Inspiration: diaphragm contracts (pulling muscle down), intercostal muscles contract elevating chest wall and expanding volume of chest which lowers pressure in lungs therefore pulling in air

Expiration: muscles relax, diaphragm resumes dome shape, intercostal muscles allow chest to lower resulting in increase of pressure in chest and expulsion of air

Process of Breathing: Volume/Pressure Relationship

Video Clip

Review Video