Gas Exchange in humans

Name all structures in the human gas exchange system.

• Air first enters the trachea.

• Air travels into the two bronchi, with one bronchus going to each lung.

• Air travels into smaller airways called bronchioles.

• Air travels into clusters of air sacs called alveoli at the end of the bronchioles.

What is the trachea?

Airway leading from mouth & nose to bronchi, held up by cartilage

What are the bronchi?

At the bottom of the trachea, there are 2, narrower than trachea but still similar, bronchi lead to bronchioles

What are the bronchioles?

Narrow tubes that carry air from bronchi to alveoli, narrow and have no cartilage and so can collapse

What are the alveoli?

Main site of gas exchange in lungs, tiny sacs with many structural adaptations to enable efficient gas exchange, EG their thin walls & large SA:V ratio

What are some adaptations of alveoli?

• alveoli have a lining of thin and squamous epithelium, that allows for gas exchange- squamous epithelium forms structure of alveolar wall & so very thin & permeable for easy diffusion of gases

• Each alveolus is surrounded by an extensive network of capillaries- CO2 diffuses out of capillaries into alveoli to be exhaled, while O2 diffuses other way from alveoli & into capillaries to be carried around body

What are the intercostal muscles?

Muscles found between the ribs

What are the 2 types of intercostal muscles?

Internal and external

What does ventilation consist of?

Consists of inspiration (breathing in) and expiration (breathing out), controlled by movements of the diaphragm, internal and external intercostal muscles and ribcage

What is inspiration?

During inspiration:

• external intercostal and diaphragm muscles contact

• causes ribcage to move up and out, diaphragm flattens

• increases volume of thoracic cavity

• decreases pressure of thoracic cavity (lower than atmospheric pressure)

• air always flows from high pressure to lower pressure (down a pressure gradient)

• inspiration is active process- requires energy

What is expiration?

During expiration:

• external intercostal muscles and diaphragm muscles relax

• ribcage moves down and in, diaphragm curves up again- it becomes dome-shaped

• volume of thoracic cavity decreases

• pressure in thoracic cavity increases above atmospheric pressure

• air is forced down pressure gradient & out of lungs

Explain forced expiration.

• external intercostal muscles relax & internal intercostal muscles contract

• pulls ribcage further down & in

• during this, movement of the 2 sets of intercostal muscles are said to be antagonistic (opposite)

Describe the movement of oxygen from the alveolus to the red blood cells.

Oxygen has to diffuse across the alveolar epithelium and then the capillary endothelium before it can reach the red blood cells (to bind w haemoglobin)- down concentration gradient

What do the epithelial cells line?

Line your organs

what do endothelial cells line?

Line your blood vessels

How are the lungs adapted for efficient gas exchange (fast rate of diffusion)?

Fick’s Law = concentration gradient (increase) x surface area (increase) ———————————————————————————————— diffusion pathway (decrease)

surface area:

• many alveoli - surface area (increases)

• provide large SA:V ratio

• many capillaries (increases)

diffusion pathway:

• alveolus epithelium is squamous

• capillary endothelium is squamous

• capillaries are narrow so RBCs move in single file

• RBCs are close to alveoli- allows for more time for diffusion to take place, all RBCs are oxygenated

concentration gradient:

• ventilation- brings in oxygen rich air and remove oxygen poor air so there’s always a higher concentration of oxygen in the alveoli compared to the blood in the capillaries

Describe and explain how the lungs are adapted to allow rapid exchange of O2 between air in alveoli & blood in capillaries around them. (8)

• The capillary wall is narrow (1 cell thick) so allow time for diffusion to occur, only 1 RBC can pass at a time

• Alveoli have short diffusion pathway