Understanding Respiratory Control, Gas Exchange, and Lung Volumes
Breathing Control Center
The breathing control center is located in the pons and medulla of the brainstem. It consists of multiple cores that generate and modify the basic ventilation rate and is the central ventilatory pattern generator, which works automatically. There are two different medullary groups:
- Dorsal respiratory group: Inspiratory neurons that control the diaphragm, mainly.
- Ventral respiratory group: Controls muscles acting in active expiration and some inspiratory muscles during an intense effort.
Chemoreceptors
Central chemoreceptors: Located on the ventrolateral surface of the medulla in the CNS. They respond to pH and PCO2 of the extracellular fluid that surrounds them, modulating ventilation. These receptors send signals to the central pattern generator to increase the speed and depth of ventilation, decreasing PCO2 and increasing the pH to normal.
Peripheral chemoreceptors: Specialized cells found in the arch of the aorta (aortic bodies) and at the bifurcation of the right and left carotid arteries (carotid bodies) around the neck. They detect PO2 (these are the only chemoreceptors that respond to this stimulus), PCO2, or pH. Again, these chemoreceptors send afferent information to the central respiratory control center.
Gas Exchange and Diffusion
Fick’s Law of Diffusion
Fick’s law of diffusion states that the rate of diffusion of a gas in a liquid is mainly due to its partial pressure difference on both sides of the alveolar-capillary barrier.
Large differences in partial pressures exist between CO2 and O2. In arterial blood, alveolar and arterial PO2 = 100 mmHg, and arterial PCO2 = 40 mmHg. In venous blood, PO2 = 40 mmHg, and PCO2 = 46 mmHg.
The VA/Q ratio = 0.8 in a healthy, rested individual. A mismatch generates a disturbance in the transfer between CO2 and O2. At rest, VA = 5 L/min, and Q = 6 L/min.
Gas Laws
Boyle’s Law: Describes the pressure-volume relationship. This law states that if the volume is halved, the pressure is doubled, and vice versa. Pressure and volume are inversely proportional: P1 x V1 = P2 x V2.
Dalton’s Law: The partial pressure of a gas is directly proportional to its concentration, or the total pressure of a gas equals the sum of partial pressures of gases that make up the mix: Pp gas = [Concentration of a gas in %] x total pressure of the mixture. Therefore, P total = PP1 + PP2 + PP3 + … PPn
Henry’s Law: The volume of a gas dissolved in a liquid, which does not combine chemically, is directly proportional to its partial pressure: Pp dissolved gas = V gas x K (solubility coefficient for that gas)
Lung Volumes
VC (Tidal Volume): Air mobilized in normal breathing. Average value at rest: 500 ml.
IRV (Inspiratory Reserve Volume): Air volume inspired above tidal volume.
ERV (Expiratory Reserve Volume): Air forcibly exhaled after a normal exhalation.
RV (Residual Volume): Air retained in the lung after a maximal expiration. Average value: 150 ml.
VC (Vital Capacity): Maximum amount of air that can move voluntarily in a full breath: VC = TV + IRV + ERV.
TLC (Total Lung Capacity): Total volume of air that the lung can contain: TLC = VC + RV.
IC (Inspiratory Capacity): Total volume of air after a maximal inspiration: IC = TV + IRV.
FRC (Functional Residual Capacity): Volume of air that the lung contains at the end of normal expiration: FRC = ERV + RV.
Factors Affecting Hemoglobin Affinity
pH (Bohr effect), temperature, CO2 (Haldane effect), and metabolites such as 2,3-DPG change the affinity of Hb for O2, shifting the saturation curve to the right.
Gas Diffusion and Solubility
The amount of gas that diffuses into a liquid is proportional to the gradient of partial pressure of gas and its solubility (k) in the liquid (CO2 is 20 times more soluble than O2, so it is involved in Hb).