As mentioned earlier , carbon dioxide is a by-product of cellular respiration . The level of carbon dioxide in the body is kept within a narrow range , as part of the acid – base balance mechanism . This important homeostatic mechanism ensures the body is not too acidic or too alkaline , with the optimum pH being 7.4 . When monitoring end-tidal carbon dioxide ( ETCO 2
) with capnography during anaesthesia , or when looking at a patient ' s blood gases , it is important to be aware of the optimum range and the signs of abnormal levels , to help maintain the correct pH for the body to function .
Control of respiration
Figure 4 . The blood supply of the alveoli . Adapted from Colville [ 1 ] .
The lungs are covered in a thin membrane called the pleural lining ; this acts to lubricate the lungs to reduce friction during breathing . It also helps to adhere the lungs to the thoracic wall , which aids ventilation . Each alveolus is also lined with a surfactant , which helps to prevent the alveoli from collapsing as air moves in and out of them [ 1 ] .
How do animals breathe ?
When compared with atmospheric pressure , the pressure in the thorax is negative , which means it acts like a vacuum . This holds the lungs open against the animal ' s chest wall . When animals breathe in , air is drawn into the lungs by the movement of the diaphragm and intercostal muscles . The diaphragm is dome-shaped when it is relaxed , so when it contracts and flattens , the space in the thoracic cavity increases . The intercostal muscles ( the muscles between the ribs ) contract , which pulls the ribs up and forwards and draws air down into the lungs . When animals breathe out , the opposite happens : the diaphragm and intercostal muscles relax , compressing the lungs and pushing the air out again . The Herring – Breuer reflex signals when the lungs are full and prevents them from over-inflating .
Cellular respiration
In addition to internal and external respiration , there is a third type of respiration : cellular respiration , in which the mitochondria in cells use glucose as a fuel , combined with oxygen and adenosine diphosphate ( ADP ), to produce adenosine triphosphate ( ATP ), carbon dioxide and water . All aerobic organisms require oxygen for cellular respiration . ATP is used for a number of essential cellular processes that cannot occur without oxygen ; without oxygen , cells begin to die within minutes and death of the animal occurs quickly .
Respiratory centres in the medulla oblongata of the brain control respiration . Within these respiratory centres are individual controls for inspiration , expiration and the constant monitoring of the pH of the blood . The body automatically alters the rate and depth of breathing depending on information provided by these centres . You can consciously control your breathing , but not for long , as the automatic system will override this , preventing you from consciously suffocating yourself [ 1 ] .
There are also mechanical stretch receptors , which set the volume of inflation and deflation in the lungs . Preset points during inspiration and expiration send nerve impulses between the lungs and brain , triggering the next breath in or out .
Chemical receptors in the carotid arteries and aorta constantly monitor the pH of the blood and the levels of oxygen and carbon dioxide . If changes are detected outside the normal range , a signal is sent to the medulla , which alters the respiratory rate and depth as required .
In cellular respiration , if an animal is unable to obtain oxygen effectively , ADP is still converted to ATP , but lactic acid is produced as a by-product of this process . As a result , lactate levels increase , because of poor perfusion or shock , for example .
Metabolic acidosis is one of the most common acid – base disorders . The body tries to correct this by increasing the respiratory rate to expel carbon dioxide . The medulla oblongata will detect the rise in the pH of the blood and signal to the respiratory system to speed up the respiratory rate , to get rid of the excess carbon dioxide . Conversely , if carbon dioxide levels decrease , the medulla will slow down respiration to retain carbon dioxide .
When weaning a patient off ventilation , ETCO 2 is allowed to rise slightly , which will encourage the patient to start breathing for themselves . Patients with a low
ETCO 2 will be slow to breathe for themselves , until the level of carbon dioxide rises . To take over a patient ' s breathing when putting it on to a ventilator , a slightly higher respiratory rate is set , so ETCO 2 levels lower and the patient ' s drive to breathe is reduced , allowing the ventilator to take over .
56 Veterinary Nursing Journal