6.4 Gas Exchange

Topic 6 Human Health and Physiology

6.4.1 Distinguish between ventilation, gas exchange and cell respiration.

6.4.2 Explain the need for a ventilation system.

A ventilation system is needed to maintain high concentration gradients in the alveoli.

6.4.3 Describe the features of alveoli that adapt them to gas exchange.

This should include a large total surface area, a wall consisting of a single layer of flattened cells, a film of moisture and a dense network of capillaries.

6.4.4 Draw and label a diagram of the ventilation system, including trachea, lungs, bronchi, bronchioles and alveoli.

Students should draw the alveoli in an inset diagram at a higher magnification.

6.4.5 Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by theinternal and external intercostal muscles, the diaphragm and abdominal muscles.

Aim 7: Data logging involving spirometers or ventilation rate monitors is possible here.

The Need for Gas Exchange

All living cells need energy all of the time for its metabolic processes The required energy is provide by the process of Cell Respiration.

This process occurs in the mitochondria. Cell respiration uses oxygen to break down large organic molecules

and release energy which is stored in the molecule ATP. Cell respiration can be aerobic (with oxygen) or anaerobic (without

oxygen). Aerobic respiration releases more energy per molecule of glucose than anaerobic respiration. Anaerobic respiration release about 5-7% of the energy that aerobic respiration does.

Hence the need for oxygen. Carbon Dioxide is the waste product of cell respiration.

The Need for Gas Exchange

Humans therefore must take in oxygen from their surroundings and release Carbon Dioxide. This process is called Gas Exchange.

Gas exchange happens in the Alveoli of the lungs. Oxygen from the air in the lungs passes through the thin walls of the

alveoli, through the walls of the capillaries and into the blood. Carbon dioxide moves in the opposite direction, from blood into the lungs.

The exchange of oxygen and carbon dioxide is driven by the concentration gradients between the air in the lungs and the blood.

To maintain these concentration gradients, the air in the alveoli must be refreshed frequently.

The process of bringing fresh air into the alveoli and removing stale air is called Ventilation.

Parts of the Ventilation System.

Ref: Rowland p205

The Ventilation System

Some main parts you should understand the function of include: The Lungs The Trachea The Alveoli The Diaphragm Intercostal Muscles

Air enters the nasal and buccal cavities and passes down the trachea to the bronchi. There are two bronchi, one for each lung. The large bronchi branch into smaller Bronchioles and then into tiny Alveoli.

The Alveoli

There are a number of features of Alveoli that make them highly efficient in their job of gas exchange.

They have a large surface area: There are hundreds of millions of alveoli giving a huge overall surface area.

Remember SA/V ratio. They have very thin walls:

The walls of the alveoli are only a single layer of flattened cells, meaning short diffusion distances. The capillary walls are also very thin.

The walls are kept moist: Cells in the alveolus walls secret a fluid which keeps the inner surface moist,

allowing gases to dissolve. They have a very good blood supply

There is a dense network of blood capillaries with low oxygen and high carbon dioxide concentrations surrounding the alveoli.

The Alveoli

Ref: Core Biology.

Breathing

Breathing is the process of ventilating the lungs. This process is involuntary and is controlled by the

medulla oblongata (a part of the brain). Taking air in is called Inhaling (inspiration) and removing

air from the lungs is called Exhaling (expiration). Inhaling:

The intercostal muscles contract and moves the rib cage up and out. The diaphragm contracts, moving down. This increases the volume of the thorax. The pressure inside the thorax drops below atmospheric pressure and so air flows into the lungs from outside the body.

Breathing

Exhaling The intercostal muscle relax and the ribcage moves down and in to its original

position. The diaphragm relaxes and moves up. This decrease the volume of the thorax. The pressure inside the thorax rises above atmospheric pressure and so air flows out from the lungs until the pressure inside the lungs equals atmospheric pressure.

A normal breath will move about 500cm3 of air into the lungs. This is called the tidal volume.

After a normal breath, you can inhale an extra 3000cm3, the inspiratory reserve volume.

If you breathe out as much air as you can you can expire about 4500cm3 of air, your vital capacity.

You can never exhale all of the air out of your lungs. The air you can exhale after a normal breath is about 1100cm3, the

expiratory reserve volume.

Breathing

Ref: Core Biology

6.4.1 Distinguish between ventilation, gas exchange and cell respiration.

6.4.2 Explain the need for a ventilation system.

A ventilation system is needed to maintain high concentration gradients in the alveoli.

6.4.3 Describe the features of alveoli that adapt them to gas exchange.

This should include a large total surface area, a wall consisting of a single layer of flattened cells, a film of moisture and a dense network of capillaries.

6.4.4 Draw and label a diagram of the ventilation system, including trachea, lungs, bronchi, bronchioles and alveoli.

Students should draw the alveoli in an inset diagram at a higher magnification.

6.4.5 Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by theinternal and external intercostal muscles, the diaphragm and abdominal muscles.

Aim 7: Data logging involving spirometers or ventilation rate monitors is possible here.