Mechanisms for regulating ventilation and pulse rates

To breath is a completely mechanical process. A rule to remember about the mechanics: Volume changes lead to pressure changes, which lead to the flow of gases to equalise the pressure. The mechanical process depends on the volume changes in the thoracic cavity. - Essentials of Human Anatomy and Physiology (Elaine N Marieb).


The rate at which we inhale and exhale is controlled by the respiratory centre, within the Medulla Oblongata in the brain. Inspiration occurs due to increased firing of inspiratory nerves and so the increased recruitment of motor units within the intercostals and diaphragm. Exhalation occurs due to a sudden stop in impulses along the inspiratory nerves.

The diaphragm flattens and moves downwards and the intercostal muscles move the rib cage upwards and out. This increase in size decreases the internal air pressure and so air from the outside (at a now higher pressure that inside the thorax) rushes into the lungs to equalise the pressures.


When we exhale the diaphragm and intercostal muscles relax and return to their resting positions. This reduces the size of the thoracic cavity, thereby increasing the pressure and forcing air out of the lungs.

The breathing rate is all controlled by chemoreceptors within the main arteries which monitor the levels of Oxygen and Carbon Dioxide within the blood. If oxygen saturation falls, ventilation accelerates to increase the volume of Oxygen inspired.


Arteries have the ability to constict or dilate rapidly during excercise to ensure blood flow demands are met. During this time the arteries dilate in the muscles so that blood flow can increase to the capillaries. This increase in blood flow allows the muscles to increase in the exchange of oxygen, the release of heat and the removal of waste products such as lactic acid and carbon dioxide. If levels of Carbon Dioxide do increase a substance known as carbonic acid is released into the blood which causes Hydrogen ions (H+) to be formed. An increased concentration of H+ in the blood stimulates increased ventilation rates.