Gas Exchange

Diffusion and solubility of gases

The diffusion rate of gasses depends on the surface area, concentration gradient, permeability and thickness of the membrane. Diffusion of gases will happen between two close surfaces. For example, this occurs between the alveoli and capillaries in the lung.

$Diffusionrate\propto \frac{Surfacearea\times Concentrationgradient\times Membranepermeability}{Membranethickness}$

Partial pressure and gas exchange

Partial pressure refers to the pressure exerted by a particular gas in the mixture of gases and is used to predict the movement of gases. The gas will move from a high partial pressure area to a lower partial pressure area because the higher the difference between the two environments’ partial pressures, the faster the movement of gases.

Mammalian lung in the exchange process

The trachea is a flexible airway supported by the cartilage rings, which prevents the trachea from collapsing when the air pressure inside falls while breathing in. The trachea is divided into two divisions called bronchi. Bronchi then split into a series of bronchioles ending in alveoli - minute air-sacs. The alveolar membrane is the gas exchange surface.

Gas exchange in the lung

When you inhale oxygen, it enters the lungs and travels to the alveoli. Cells lining alveoli and capillaries carrying blood are in close contact with each other. The barrier thickness averages to 1 micron - that’s 1/10 000 of a centimetre! Oxygen passes through the barrier into the blood in capillaries. In turn, carbon dioxide passes into the alveoli from the bloodstream and is exhaled.

Oxygen travels from the lungs to the pulmonary veins, which take oxygen to the left side of the heart, where it is pumped to the rest of the body. Deoxygenated blood returns to the right side of the heart and is pumped through the pulmonary artery back to the lungs.

Fig. 1 - The human gas exchange system

Alveoli have adaptations to facilitate efficient gas exchange. You will learn about four central adaptations.

1. Large surface area - larger area allows for more effective gas exchange.
2. Alveoli walls are only one cell thick - this means they are in extremely close contact with the capillaries.
3. The alveolar walls are moist - the layer of moisture allows the gases to dissolve more quickly.
4. Alveoli have a good blood supply - due to close contact with capillaries making the gas exchange quicker and more efficient.

Fig. 2 - Movement of gases between alveoli and capillaries

Gas exchange process in single-celled organisms

Single-celled organisms have a large surface-to-volume ratio allowing efficient diffusion of gasses. Oxygen diffuses into the cell, and carbon dioxide leaves the cell, followed by respiration. The membrane is entirely permeable to facilitate this exchange. The distance between the internal and the external environments is small enough, and the surface area is large enough for the cell’s needs. No specialised structures are required for the gas exchange.

Fig. 3 - Simple gas exchange in a single-celled organism

Gas exchange process in insects

Insects have an internal network of tubes called tracheae which divide into smaller tracheoles (the end tubes). There is only a short diffusion pathway from tracheoles - the oxygen is brought directly into the respiring tissues.

Gasses move in three ways:

1. Along the diffusion gradient - oxygen is used up during cell respiration, making the concentration fall and the gradient increase, which causes oxygen to diffuse from the atmosphere. In turn, carbon dioxide increases and diffuses down the concentration gradient - into the atmosphere.
2. Mass transport - muscle contraction can squeeze the air in and out, causing a mass movement, which speeds up the process of exchange.
3. The ends of tracheoles are filled with water - during major activities, muscles can start respiring anaerobically, as well as aerobically. Anaerobic respiration produces lactate which lowers the water potential of the muscle cells. Water will begin moving down the gradient by osmosis into the muscle cells. The water in the tracheoles will reduce in volume, and the final gas diffusion will be in the gas phase rather than liquid. The diffusion will be more rapid.

Gas exchange process in fish

Fish have gills to facilitate gas exchange due to their small surface area to volume ratio (similar to mammals’ lungs). Gills are made up of gill filaments. Gill filaments are stacked (imagine stacking your notebooks, just like that). They contain gill lamellae which are at a right angle to the filaments. They increase the surface area for gas exchange.

Did you notice that the movement of water over gills and blood flow is in opposite directions in Figure 6? This is the countercurrent exchange system.

Countercurrent exchange system

The countercurrent exchange system allows the blood to be well-loaded with oxygen when it meets water. Diffusion will occur, and oxygen will move into the blood. Blood with little oxygen will meet water which has the most oxygen. Take a look at Figure 7; what if the flow was parallel instead of countercurrent? The blood would absorb a much lower percentage of 50% of available oxygen, which is why fish prefer to stick to the countercurrent flow, wouldn’t you?

Gas exchange process in the leaf of a plant

Gas exchange in the leaf occurs in the gas phase quicker than in water. Living cells are in close contact with the source of oxygen and carbon dioxide (air).

Leaves, as the main gas exchange surfaces, have adaptations for rapid diffusion:

• Stomata - tiny pores on the leaf’s surface, which allows close air contact with the cells.
• Interconnecting air-spaces throughout mesophyll (the internal ground tissue located between the two epidermal cell layers of the leaf) - gases can quickly contact the mesophyll.
• Large surface area

Factors affecting the rate of diffusion of gases

• The membrane thickness - the thinner the membranes, the faster the diffusion will be. For example, the barrier between alveoli and capillaries is only one cell thick in the lungs.
• The membrane surface area - larger the surface area, more gas exchange can occur. For example, fish contain stacks of gill filaments and gill lamellae, increasing the surface area for exchange.
• The pressure difference across the membrane - more pressure, more gases will diffuse. For example, mass transport in insects, muscle contractions can push gases.
• High diffusion gradient - for example, countercurrent exchange system in fish.