Gaseous exchange and lung volumes Aims: To understand the concept of partial pressures of gases. To know has gaseous exchange takes place. Define the terms minute ventilation, tidal volume and respiratory frequency.

Gaseous exchange. Where does gaseous exchange take place in our body? The exchange of O2 and CO2 occurs in two places known as external respiration (lungs-blood) and internal respiration (blood-tissue). Gaseous exchange relies on a process called diffusion. “The movement of gases from an area of high concentration to an area of low concentration”.

Gaseous Exchange Continued… The difference in between high and low pressure is called a diffusion gradient. The bigger the gradient, the greater the gaseous exchange.

Partial Pressure. Partial pressure is the pressure a gas exerts within a mixture of gases. The greater the concentration of gas the greater the partial pressure will be. This is measured in mmHg (millimetres of mercury) In the alveoli, PO2 is 100mmHg and in the blood from the heart (in the pulmonary capillaries) it is 40mmHg. A diffusion gradient therefore exists and O2 flows through the semi-permeable membrane walls into the capillary.

External (Alveoli) respiration. Inspired air = high PO2 (100mmHg) and low PCO2 (40 mmHg) Pulmonary (Alveoli) capillaries = high PCO2 (45 mmHg) and low PO2 (deoxygenated blood) (40 mmHg). Because of this what will happen between the two gases? DIFFUSION!

Internal Respiration. Oxygenated blood (Oxyhaemoglobin) is pumped from the heart around the body. As it flows slowly past muscle cells O2 diffuses into the cell (due to the high PO2 in the blood and the low PO2 in the muscle cells). O2 combines with MYOGLOBIN to form OXYMYOGLOBIN. It then moves into the MITOCHONDRIA. CO2 diffuses the other way into the blood.

Factors which make the diffusion of O2 from alveoli so efficient: Permeability of alveoli and capillary cell walls (only one cell thick), Short distance between alveoli and capillary, Readiness of haemoglobin to combine with O2 and form oxyhaemoglobin, Diffusion gradient caused by different partial pressures, Large surface area of alveoli.

What Happens During Exercise? The breakdown of glucose in the muscle to produce energy for the ATP production increase and uses more O2. Therefore the PO2 in the muscle decreases (can go as low as 0 mmHg). As the diffusion gradient is larger the speed and amount of O2 transferred increases. The breakdown of glucose also results in more CO2 being produced and PCO2 can increase to 90 mmHg. The returning blood now has a high PCO2. Therefore the CO2 diffuses back into the alveoli and the process begins again……

The Control of Breathing Rate and depth of breathing is controlled in the respiratory centre in the medulla oblongata (in the brain). This uses both neural and chemical control. During quiet breathing (at rest) nerve impulses cause the intercostal muscles and diaphragm to contract, and then stop sending the impulses to cause relaxation.

Chemical control of breathing during exercise: Increase in CO2 makes blood more acidic. Increase in lactic acid also lowers pH. The change in blood acidity stimulates chemoreceptors which tell the respiratory centre to increase ventilation until blood acidity returns to normal. It is changes in levels of CO2, not O2, which stimulate changes in breathing.

Neural control of breathing during exercise: Activity from brain caused by anticipation of exercise. Increased stimulation from proprioceptors in muscles and joints. Increases in body temperature.

Past exam questions. Explain how gas is exchanged between the blood and the muscle tissues during exercise. Why is this beneficial to performance? [5] Answer: Diffusion gradient. Partial pressures. Differences in O2/CO2 concentrations. Single cell walls. Haemoglobin/Myoglobin. Faster uptake of O2. Etc.