Humans rely on the exchange of gases with the external environment for their survival. Carbon dioxide and other waste products of metabolism are released through the lungs during expiration. Oxygen is taken in during inspiration. Once the gases are taken in, they are transported via the bloodstream to the various parts of the body where they are needed. Oxygen is transported throughout the body to cells to be used for cellular respiration. The bodily process that exchanges gas with the environment takes place through the alveoli of the lungs and can be affected by several different factors, including gas concentration, tissue thickness, and pressure inside and outside the lungs. Respiration, or breathing, is controlled by the brain and can be affected by exercise and other physical activities.
At A Glance
- The alveoli and capillaries both have thin walls to maximize gas exchange; alveoli can be found at the ends of the respiratory bronchioles.
- Oxygen and carbon dioxide move down a pressure gradient from an area of higher concentration to an area of lower concentration; the rate of movement is affected by the thickness of the membrane through which the gases diffuse, the surface area available for gas exchange, and the partial pressure of the gases.
- Most oxygen is transported on hemoglobin molecules in the erythrocytes (red blood cells); carbon dioxide is carried through the blood in three ways: dissolved in plasma, converted to carbonic acid, or bound to protein, particularly hemoglobin.
- The oxyhemoglobin-dissociation curve describes hemoglobin's affinity for oxygen at different partial pressures of oxygen; changes in pH, temperature, and bisphosphoglycerate levels will shift the oxyhemoglobin-dissociation curve.
Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin; as a result, the fetus has increased access to oxygen from the maternal bloodstream.
Chemoreceptors in the pons and medulla oblongata detect changes in carbon dioxide levels in blood and cerebrospinal fluid; a decrease (or increase) in pH causes the medulla to send nerve signals to the intercostal muscles and diaphragm to respond with increased (or decreased) ventilation.
Increased respiratory rate during exercise occurs because of a feed-forward mechanism resulting in physiological changes in the respiratory rate.