Physiology Objectives 44
1. Central organization of respiratory control: feedback from chemoreceptors, mechanoreceptors, and cortex to brainstem control. Brainstem then controls the ventilatory pump. This is done through three major brainstem centers and spinal integration:
a. Medullary respiratory center: dorsal respiratory group is important in mediating inspiration (receives input from CNIX and CNX and outputs through the phrenic nerve to the diaphragm). Ventral respiratory group is important in mediating active expiration and is normally not functioning (passive respiration requires no brainstem input)
b. Apneustic center (lower pons): sends signals to the dorsal respiratory group to increase inspiration length
c. Pneumotaxic center (upper pons): sends signals to end inspiration
d. Spinal integration: control in the rostral (T1-T8) region inhibits respiratory activity while the caudal (T9-T12) region stimulates respiratory activity
Differences between respiratory and cardiac rhythm generation: since the respiratory system maintains homeostasis of many compounds as well as voluntary input, reliance on a single rhythm is insufficient (there are many inputs because ventilation is important in many different pathways)
Multiple input requirement for respiratory control: the respiratory system responds to chemoreceptor, mechanoreceptor, baroreceptor, temperature receptors, cortical input, and brainstem input
2. Input difference between chemical homeostasis and mechanical function: chemical homeostasis is controlled by chemoreceptors while mechanical function is controlled by mechanoreceptors
3. Chemical control of ventilation:
a. Central chemoresceptors (70-80% of PCO2 response): responds to increased H+ in CSF. Increased H+ in CSF is made solely by circulatory CO2 crossing the blood-brain barrier, making H2CO3, and dissociation into H+ and HCO3-. Thus, it does not respond to low PO2 or plasma pH changes.
b. Peripheral chemoreceptors (10-20% of PCO2 response): located in the carotid bodies and aortic arch; respond to changes in PCO2, pH,and PO2 because of direct interaction with bloodstream. The carotid body receptors are more sensitive to chemical homeostasis while the aortic arch receptors are receptive to blood flow.
· Note: carotid body receptors are sensitive to hypoxia, but do not increase their signal sigmoidally in concert with hemoglobin dissociation. Thus, until PO2 reaches below 100 mmHg, carotid sinus input is minimal. Between 100 mmHg and 60 mmHg there is a mild increase, and below 60 mmHg there is a drastic increase.
Anatomic difference between oxygen and carbon dioxide controllers: carbon dioxide controllers are located both peripherally and centrally, whereas oxygen controllers are located peripherally; also, signals from carbon dioxide controllers react linearly while oxygen controllers react sigmoidally.
4. Chemical control response to:
a. Blood pH: at acidotic levels, ventilation increases at low PaCO2, and to minute increases in PaCO2. Conversely, at alkalotic levels, ventilation increases at high PaCO2, and to large increases in PaCO2. This is due to the need for oxygen delivery (associate low pH with high blood [CO2] and low blood [O2]. Ventilation in this case needs to increase to restore homeostasis.)
b. Oxygen: low PaO2 increases the need for oxygen in the tissues. Thus, at low PaO2, there is increased ventilation at low PaCO2 and increased ventilation in response to minute changes in PaCO2.
c. Carbon dioxide: high PaCO2 increases the need for oxygen delivery to tissues, and therefore, ventilation increases sharply in response to low PaO2.