Exchange Surfaces

Gaseous exchange- The movement of gases across a surface; O2 from alveolar air to capillary blood and CO2 from capillary blood to alveolar air.

Body tissues require a constant supply of O2, however the volume of O2 that can be taken in is dependent on the surface:volume ratio.

Unicellular organisms such as bacteria and yeast consist of a single cell whose surface is in complete contact with the environment.  Therefore they are able to efficiently exchange and remove material.

Organisms have evolved, growing to become multicellular. As the volume of cells increases, the number in contact with the surface decreases making gaseous exchange more difficult.  Multicellular organisms such as tapeworms are elongated and flat, evolving to have a large surface area to maximise gaseous exchange.  

Humans have evolved large specialised organs to increase surface area for efficient gaseous exchange.  The total surface area of adult lung tissue is approximately 60 to 70m2.  Every time you inhale, you expose to air an area roughly equal to that of a tennis court.

Small Organisms:

• Does not need a specialised exchange/transport system.
• Has a LARGE Surface Area to Volume ratio.
• Most of its cells are also very close to the surface.
• Being small also implies a LOW level of activity.
• Therefore able to acquire the oxygen and glucose it needs by simple DIFFUSION over its surface.
• DIFFUSION is fast enough to meet the oxygen demands.

Large Organisms:

• Needs a specialised exchange/transport system
• Has a SMALL Surface Area to Volume ratio.
• Most of its cells are also deep within the body.
• Being large also implies a HIGHER level of activity.
• Therefore not able to acquire the oxygen and glucose it needs by simple DIFFUSION over its surface.
• DIFFUSION is too slow to meet  the oxygen demands.
• Requires a heart, lungs, blood vessels and blood to distribute O₂ and glucose around the body.  

Alveoli are minute air sacs providing the site for gaseous exchange. There are approximately 300 million alveoli in the lungs, providing a vast surface area.

The walls of the alveoli are a single-cell layer of squamous epithelial cells surrounded by pulmonary capillaries. They have the biggest capillary network of anywhere in the body, which provides a short diffusion path to diffuse and exchange oxygen.

Gaseous exchange in the alveoli

Gaseous exchange relies on diffusion.  Where there is a difference in gaseous concentration across membranes there is said to be a ‘concentration gradient’.

General features of an efficient exchange surface are:

• Large surface area
• Thin barrier
• Mechanism to maintain a concentration gradient
• Moist (gases diffuse in solution)

Recap: Define diffusion.

Net movement of molecules from high to low concentration.

Bronchioles- highly branched- Large number of pathways for air to enter and leave the lungs

Alveoli- millions of alveoli, folded surface, elastic fibres, surfactant- Great surface area for gaseous exchange/ expand on inspiration and recoil on expiration/  surfactant lowers alveoli surface tension/energy required for inspiration/ antibacterial effect

Alveolar squamous epithelium cells- 0.1-0.5 µm thin and flattened-  Short diffusion path = rapid diffusion(close contact with capillaries)

Capillary endothelium wall- single-cell, flattened-  Short diffusion path = rapid diffusion (close contact with alveoli)

Capillary Network- dense narrow- Erythrocytes flattened against capillary wall increasing surface area/short diffusion path/ slow blood flow = maximum time + area for gaseous exchange

Capillary blood from the pulmonary artery- high concentration of carbon dioxide- Steep diffusion gradient (increased during exercise)

Alveolar air- high concentration of oxygen- Steep diffusion gradient (increased during exercise)

Connective tissue ( Cartilage ) - plays a structural role in supporting the trachea and bronchi holding the airways open to prevent them from collapsing under low air pressure.

Smooth muscle tissue - can contract to constrict the airway making the lumen narrower. This is a response to harmful substances in the air.

Elastic fibres- used to allow the airways to recoil back to their original size after the smooth muscle has finished contracting.  This helps to expel air.

Squamos Epithelium- found in Alveoli- Lining tissue, Protective function, Short distance between alveolar air and capillary blood, Efficient gaseous exchange.

Ciliated epithelium- Trachea- ‘Waft’ mucus back up the trachea to the throat where is can be swallowed. Dirt, bacteria and dust get trapped in mucus.

Glandular tissue (goblet cells) - Trachea- Secrete mucus (a glycoprotein).Dirt, bacteria and dust get trapped in mucus.

The lungs have no muscles, so they cannot move by themselves. Breathing movements are caused by two sets of muscles –the intercostal muscles between the ribs and the diaphragm muscles.These muscles result in ventilation of the lungs. Ventilation- The process whereby air enters and leaves the lungs. Inspiration- Inhalation or breathing in Expiration- Exhalation or breathing out.

Breathing in:

• External Intercostal muscles contract lifting the rib cage upwards and outwards
• Diaphragm muscle contracts pulling downwards
• Volume of chest cavity increases
• Air pressure in the thorax falls, falling below atmospheric pressure
• Air flows into lungs down a pressure gradient

Breathing out:

• External Intercostal muscles relax, the rib cage falls
• Diaphragm relaxes and recoils upwards (pushed by organs underneath)
• Volume of chest cavity decreases
• Air pressure in the thorax increases rising above atmospheric pressure
• Air flows out of the lungs

1- Nasil cavitivity

2- Mouth

3- Larynx

4-Right lung

5- Bronchus

6- Diaphragm

7- Pharynx

8- Trachea

9- Bronchus

10- Brochioles

11- Alveoli

Tidal Volume- Volume of air taken into the lungs in one breath at rest (measured in dm3)

Vital capacity- Volume of air taken into the lungs in one breath during exercise/deep breathing (measured in dm3)

Residual volume- Volume of air left in lungs after fully breathing out-impossible to push out (measured in dm3)

Reserve volume- The difference in volume between air breathed in at Tidal Volume and at Vital Capacity (measured in dm3)

Breathing rate- Number of breaths taken every minute (measured in Breaths per Minute-BPM)

Ventilation rate- Total volume of air breathed into lungs in one minute-calculated by:  Breath Volume x Breathing Rate (measured in dm3 min -1)

Fish are relatively large animals and so have a small surface area to volume ratio. Diffusion of gasses through their outer surface would not provide sufficient oxygen for their large volume, therefore they have evolved specialised gas exchange surfaces called gills.

Fish are water breathers. Water is a dense, viscous medium that is difficult to move over gas exchange surfaces.

They obtain oxygen in solution from the surrounding water which contains approximately 1% dissolved oxygen, 20% less oxygen than in air. Fish obtain enough oxygen by passing large amounts of water over their gills.

What makes them good for gas exchange?

• 1 Many lamellae so large surface area
• 2 One cell thick gill plates so short diffusion distance
• 3 Maintains a concentration gradient

Insects have a tracheal system -which delivers oxygen directly to the cells without involving the blood.

A network of tubes called tracheae (large) and tracheoles (smaller) lead from holes called spiracles on the external surface of the thorax and abdomen on the side of the body.

Spiracles can open and close like valves.

The ends of the tracheoles are often fluid-filled so oxygen diffuses into cells in solution.

Air flow in the tracheae is tidal (like the lungs), although in some more highly evolved insects, anterior spiracles take in air and posterior spiracles exhale air.

Breathing movements (such as movements of the abdomen) can assist gas exchange by ventilating the trachea.