The Respiratory System
Humans need to get oxygen into the blood and get rid of the carbon dioxide produced by respiring cells. The process by which this happens is VENTILATION.
As you breath in, air enters the trachea.
The trachea then splits into two bronchi.
The bronchi then branch off into bronchioles.
And finally, attatched to the ends of the bronchioles are minute air sacs called alveoli.
Two sets of muscles are also involved in the process of ventilation:
The diaphragm- a sheet of muscle seperating the thorax from the abdomen
(Internal and External) Intercostal muscles- these are situated between the ribs
Inspiration (or breathing in), an active process requiring energy, occurs as follows:
- The EXTERNAL intercostal muscles contract while the INTERNAL intercostal muscles relax
- This causes the ribcage to move upwards and outwards: increasing the volume of thorax
- The diaphragm also contracts causing the muscle to flatten: again increasing the volume of thorax
- The increase in the volume of thorax results in a reduction of the pulmonary pressure within the lungs
- This means the atmospheric pressure is now greater then the pulmonary pressure, causing air to be forced into the lungs
Expiration (or breathing out), a passive process therefore not requiring energy, occurs as follows:
- The INTERNAL intercostal muscles contract while the EXTERNAL intercostal muscles relax
- This causes the ribcage to move inwards and downwards: decreasing the volume of thorax
- The diaphragm muscles relax causing the muscle to return to its domed position: decreasing the volume of thorax further
- The decrease of thorax results in an increase of pulmonary pressure within the lungs
- This means the pulmonary pressure is now greater than the atmospheric pressure, causing air to be forced out of the lungs
Gaseous Exchange Summary
The site of gas exchange (oxygen and carbon dioxide) are the minute air sacs situated on the ends of bronchioles inside the lungs.
There's a huge number of alveoli in the lungs resulting in a large surface area for the exchange of gases.
Alveoli are surrounded by a network of capillaries.
- Oxygen diffuses across the alveolar epithelium and the capillary endothelium into haemoglobin in the blood.
- Carbon dioxide diffuses into the alveoli from the blood and is then breathed out.
Features of Exchange Surfaces
To ensure the efficient transfer of materials by diffusion or active transport, exchange surfaces have the following features:
- Large surface area to volume ratio: speeding up the rate of exchange
- Very thin: decreasing the diffusion pathway to speed up the rate at which materials are transferred
- Constant movement of the environmental medium (eg, air): to maintain a diffusion gradient
- Constant movement of the internal medium (eg, blood): again to maintain a diffusion gradient
Adaptation and the Role of the Alveoli
(Alveoli: lined with epithelial cells. Capillaries: lined with endothelial cells)
Diffusion of gases is very rapid because:
- red blood cells are slowed as they pass through the capillaries: allowing more time for diffusion
- red blood cells are flattened against the capillary walls as the size of the capillaries are so narrow: reducing the diffusion pathway between the alveolar and red blood cells
- the walls of the capillaries and alveolar are one cells thick: reducing the diffusion pathway
- alveoli and capillaries have a large surface area: bigger volume of gases are able to diffuse at one time
- the constant expiration, inspiration and pumping of the heart: maintain steep concentration gradient
The factors that effect the rate of diffusion:
1) Concentration gradient: greater the difference in concentration either side of the exchange surface the faster the rate of diffusion
2) Area over which diffusion takes place: larger the area of the exchange surface the faster the rate of diffusion
3) Thickness of exchange surface: thinner the exchange surface the faster the rate of diffusion
Fick's Law- Diffusion is proportional to:
Surface area x Difference in concentration
length of diffusion pathway