PRESENTER: Keagan Kirugo

DISCUSSANTS: The Circle

Central controllers

Effectors

Sensors

Integrated responses

The main function of the lung is to exchange O2 and CO2 between blood and gas and thus maintain normal levels of PO2 and PCO2.

Despite the widely differing demands for O2 uptake and CO2 output made by the body, the arterial PO2 and PCO2 are normally kept within close limits through the mechanisms that will be elaborated further.

The respiratory control system comprises the following:

Sensors that gather information and feed into the

Central controller in the brain which co-ordinates information and then sends impulses to the

Effectors which are the respiratory muscles. They cause the ventilation to occur.

Site Ventral surface of medulla

Evidence Local application of H+ ions or dissolved CO2 to this area stimulates breathing within a few seconds.

The central chemoreceptors are surrounded by brain ECF-Extracellular fluid.

ECF composition is governed by CSF, local blood flow and local metabolism.

CSF is the most important among the three. It is separated from blood by the blood-brain barrier (BBB).

BBB is relatively impermeable to H+ and HCO3-ions but not CO2.

blood PCO2 diffusion of CO2 from the cerebral blood vessels to CSF through ECF

CO2 + H2O H+ + HCO3- and reduced CSF pH

H+ ions stimulate the chemoreceptors leading to hyperventilation and finally decreased PCO2 in blood and hence CSF.

The cerebral vasodilation that accompanies an increased arterial PCO2 enhances diffusion of CO2 into the brain ECF and CSF.

Normal CSF pH is 7.32

Sites Carotid bodies at the bifurcation of the common carotid arteries and the aortic bodies above and below the aortic arch.

Stimulants Decrease in arterial PO2 and PH and Increase in arterial PCO2.

Both receptors are responsible for increase in ventilation that occurs due to hypoxemia.

Response is rapid with PO2 <100 mmHg

PCO2 response is less important compared to the central chemoreceptors. However, the response is rapid.

PH response is by the carotid bodies and not the aortic bodies.

It is the most important factor in the control of ventilation.

Factors leading to a decresa in ventilatoryresponse to CO2

Sleep

Increasing age

Genetics

Training: athletes and divers

Increase in work of breathing

There is an interaction to increase in PCO2 with a fall in PO2 such that an increase in PCO2 leads to increased response to a fall in PO2.

Response to O2 is important in hypoxic environments such as high altitude areas and in patients with severe lung disease.

Patients with severe lung disease have chronic CO2 retention and the pH of the brain ECF has returned to normal in spite of a raised PCO2 . Therefore they have lost most of their increase in stimulus to ventilation from CO2. In addition, the initial depression of blood pH has been nearly abolished by renal compensation hence there is little pH stimulation of the peripheral chemoreceptors. With these conditions; arterial hypoxia becomes the chief stimulant to respiration.

Patients with a partly compensated metabolic acidosis for example; uncontrolled DM who have reduced pH and PCO2, show an increase in ventilation which is responsible for the fall in PCO2.