The Human Need for Oxygen
To live, the human being must produce heat and energy from food eaten. Eaten food is converted into simple food products and transferred to the tissues by the blood. It is then oxidized to provide this heat and energy. To oxidize the food, oxygen has to be supplied to the living cells in the body.
The waste product, carbon dioxide, is then carried away from the tissues and expelled from the body. This process is respiration. The definition of respiration is given below:
"The exchange of the respiratory gases, O2 and CO2, between the organism and its environment."
The volume of air breathed in and out in a single breath. When resting this is approximately 500 cm³. The maximum volume that can be breathed in and out is approximately:
- Men: 2500 cm³
- Women: 1500 cm³
The breathing process consists of two phases:
- Breathing In: Inspiration
- Breathing Out: Expiration
The respiratory system is made up of the following:
- Mouth and nose.
- Bronchiole tree.
When a human breathes, air is drawn in through the mouth or nose to the Pharynx. The Pharynx, which is found at the back of the throat, warms, humidifies and filters the air before it passes down the trachea into the two bronchi. The bronchi split into the bronchiole tree as the air passes into the lungs.
The lungs are set inside the chest cavity, or thoracic cavity, wrapped in an airtight sac called the pleura. At the ends of each branch of the bronchiole tree are air sacs, alveoli. These air sacs are very small and are surrounded by capillaries which are small blood vessels.
The thin walls of the alveoli and capillaries allow oxygen to diffuse into the blood and CO2 into the alveoli. The lungs in the average man can hold approximately 6 litres of air, a woman, 4 litres.
Inspiration and Expiration
The chest cavity is surrounded by the ribs on the sides and separated from the abdominal cavity by the diaphragm, a large flat sheet of muscle. The chest cavity has only one opening. Any change in volume to the chest cavity will ventilate the airspace in the lungs. The chest size is altered by a muscular action that raises and lowers the diaphragm and by contraction and relaxation of the muscles between the ribs.
Inspiration and expiration circulate air in and out of the lungs efficiently.
The constant turnover of air provides the mechanism for both O2 to diffuse into the blood and CO2 to diffuse into the lungs.
This gaseous exchange can be explained by looking at the partial pressure each gas exerts. In air outside the lungs the partial pressure of O2 is 160 mmHg.
Carbon Dioxide has a low partial pressure in outside air of approximately 0.3 mmHg. The difference in pressure of these gases between the alveoli and the blood is how the gaseous exchange between the lungs and the bloodstream occurs.
- Blood entering the lungs has a lower ppO2 than the alveolar air, so oxygen diffuses into the blood.
- The ppCO2 is higher in the blood entering the lungs than in the alveoli, so CO2 diffuses out of the bloodstream and into the lungs.
Most of the oxygen is taken into the blood, and carried, by the protein haemoglobin. Haemoglobin is found within the red blood cells and is an Iron rich compound. The Haemoglobin bond ensures that the body can receive enough Oxygen for the body's needs.
If blood diffused directly into the blood solution only, then the body would be starved of sufficient Oxygen necessary for the human to survive. Oxygen remains bound to the haemoglobin until it reaches the tissues of the body, an area of low oxygen tension. This oxygen is then released into the tissues to oxidize food.
About 95% of the oxygen is transported by haemoglobin, as an oxy-haemoglobin bond, and the remainder is diffused directly into the blood solution. Some Carbon Dioxide binds to the haemoglobin but the majority diffuses into the blood and is carried in solution as carbonic acid.
Both Oxygen and Carbon Dioxide bind weakly to the Haemoglobin as a strong bond would result in difficulties in releasing the gases to either the tissues or the lungs.
Control of Breathing
Control of breathing is centred in the respiratory centre of the brain. The human requires no conscious effort to breathe; although the rate of breathing can be altered voluntarily. Inspiration is the active phase of breathing; expiration the passive phase. The rate and depth of breathing can be adjusted to meet any change in the consumption of oxygen and expiration of carbon dioxide.
Under normal conditions the body is slightly alkaline (pH7.4).
- The partial pressure of carbon dioxide elevates
- The acidity level increases
- The pH value lowers to less than 7.4
Any increase in the CO2 concentration in the blood stimulates an increase in the ventilation rate. As blood flows through muscle capillaries the dissociation of oxy-haemoglobin to release oxygen is increased by:
- Low O2 concentration in muscle tissue
- High CO2 concentration
- High temperature
Too little CO2 causes the blood to become more alkaline and the pH value to rise. The human body maintains the equilibrium within narrow limits, any shift in the blood pH and ppCO2 levels are sensed by the respiratory centres of the brain. When unusual levels occur, chemical receptors trigger the respiratory process to help return the ppCO2 and pH levels to normal limits. For the uptake of O2 by the blood and the release of that O2 to tissues the extreme limits of the pH of the body are regarded to be 7.2 to 7.6.
The brain monitors the levels of both carbon dioxide and oxygen in order to make any changes in the respiration rate.
Note: A healthy body is more sensitive to changes in the carbon dioxide balance of the body than to oxygen.