VPB 121: Veterinary Physiology - II (2+1)

HUMORAL CONTROL

• Humoral control refers to those factors in the body fluids that influence ventilation like CO₂, O₂ and H⁺ .
• Their concentrations in the blood affect alveolar ventilation in several ways:
  • Carbon dioxide increase causes alveolar ventilation to increase; its decrease causes alveolar ventilation to decrease.
  • H⁺ increase causes alveolar ventilation to increase; its decrease causes alveolar ventilation to decrease.
  • Oxygen decrease causes alveolar ventila­ tion to increase; its increase causes alveolar ventilation to decrease.

• The respiratory system functions to bring in O₂ and eliminate CO₂ from the body. This function is assisted by specialized receptors called chemoreceptors that monitor the levels of CO₂, O₂ and H⁺, and then send such information to the respiratory center.
• These chemoreceptors are located in several loca­tions. There are central chemoreceptors found in the medulla oblongata that respond to changes in cerebro­spinal fluid H⁺ and PCO₂. Because of the much greater diffusibility of carbon dioxide, as com­pared with H⁺, it is distributed more quickly from the blood to the interstitial fluid of the medulla and to the cerebrospinal fluid than hydrogen ions.
• The H⁺ concentration of the interstitial fluid of the brain stem is the deciding stimulus for respiratory drive. The influence of CO₂ is exerted by its conversion to H⁺ through the hydration reaction.
• Peripheral chemoreceptors include the aortic bodies and carotid bodies whose removal eliminates a respiratory response to hypoxia.
• The aortic bodies are a cluster of chemoreceptors in the aortic arch; the carotid bodies are oval nodules in the wall of the left and right common carotid arteries, where they bifurcate into the internal and external carotid arteries.
• Axons from the chemoreceptors in the aortic bodies are part of the vagus nerve (cranial nerve X), whereas those of the carotid bodies project in the glossopharyngeal nerves (cranial nerve IX).
• The levels of CO₂ and H⁺ are highly correlated.
• Throughout the body, CO₂ is quickly converted to car­bonic acid catalyzed by the enzyme carbonic anhy­drase. Carbonic acid dissociates into HC0₃^- and H⁺. Therefore, increase in PCO₂ lead to increases in H+ while decreases in CO₂ lead to decreases in H⁺. As a result, has a large affect on respiration, whereas P O₂ affects respiration only if its levels change substantially.
• Increases in arterial blood CO₂, called hypercapnia, cause an increase in H⁺. This has a particularly large effect on central chemoreceptors since there is little protein within the cerebrospinal fluid to buffer the H⁺. Activation of the central chemoreceptors causes increased respiration rate, possibly causing hyperven­tilation. Conversely, low arterial blood CO₂, called hypocapnia, inhibits respiration. Large drops in arte­rial PO₂ increase ventilation by stimulating peripheral chemoreceptors.