Ventilation – Control

THE VRG

Controlling your breathing are two respiratory centersBrainstem regions that regulate breathing. in the medulla oblongataThe lowest part of the brainstem controlling vital functions like breathing and heart rate..  The ventralRelating to the front or belly side of the body. respiratory center or VRG sets the basicA solution with a pH above 7, having a lower concentration of H⁺ ions. rhythm of respirationThe process of gas exchange, including ventilation, external and internal respiration..  Within the VRG are two collections of neuronsThe functional cells of the nervous system that transmit signals.. One group causes inspirationThe process of inhaling, driven by diaphragm and external intercostal contraction. and is known as I neurons. The other group causes expirationThe process of exhaling, which can be passive (relaxation of respiratory muscles) or active (involvi and is known as E neurons.  Both sets of neurons contain pacemaker cellsThe basic structural and functional units of life., similar to those in the AV and SA nodes of the heart.  These neurons spontaneously depolarize and when they do, they inhibit the other collection of neurons.  The I neurons firing, inhibiting the E neurons for about 2 seconds, or the time it takes to inhale.  Then, the E neurons fire, inhibiting the I neurons for about 3 seconds.  This cycle produces the respiratory rhythm which is basically just your respirations per minute.

The medulla oblongata in your brainstemThe lower part of the brain that connects to the spinal cord and controls vital functions. has pacemaker cells that spontaneously depolarize. These cells of the medulla oblongata control your diaphragm and intercostal muscles by way of the phrenic nerveA nerve from the cervical plexus that controls the diaphragm.. These cells set the rate of breathing so that there is a cycle of inhalation and exhalation.  The phrenic nerve is the sole innervation for your diaphragm, and it comes out of your cervical plexusA network of nerves that supplies the neck, shoulders, and diaphragm.. 

The DRG and PRG

The dorsalRelating to the back side of the body. respiratory group or the DRG does not set the rhythm. However, it can modify the rhythm by influencing the rate and depth of respiration. The DRG interprets all the sensory inputs. It organizes these inputs and decides how to adjust the depth and rate of breathing. 

The DRG can receive information from ascending spinal tractsBundles of nerve fibers in the CNS that carry signals between brain regions. carrying info from various chemoreceptors and baroreceptors.  It also receives input from the cerebrumThe largest part of the brain, responsible for thought, memory, and voluntary movements., hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions., and other structures of the limbic systemA group of brain structures responsible for emotions and memory. (your emotional brain). The VRG is setting the rhythm. The DRG is receiving input from the ascending tractsBundles of axons that carry sensory information from the spinal cord to the brain.. Meanwhile, the pontine respiratory group is located in the ponsA part of the brainstem that connects the cerebrum to the cerebellum and helps regulate breathing.. It is receiving input from the cerebrum. It also receives signals from the limbic system and other higher brain centers.  The PRG is important in adapting your breathing for things like exercise, sleep, laughing, crying, etc..

Sensory Receptors

Recall that carbon dioxide is acidicA solution with a pH below 7, having a higher concentration of H⁺ ions..  Therefore, many of our chemoreceptors that influence breathing are pHA measure of hydrogen ion concentration in a solution. sensors.  This is especially important in the CSF filling the central canalA hollow canal in an osteon containing blood vessels and nerves. and ventricles of the brain.  Central chemoreceptors are involved in monitoring the CNSComposed of the brain and spinal cord; integrates and processes information. . 

Peripheral chemoreceptors are strategically placed to monitor the large elastic arteriesBlood vessels that carry oxygenated blood away from the heart (except pulmonary arteries, which carr leaving the heart.  The carotid bodies are such chemoreceptors, monitoring the oxygen and carbon dioxide content of the blood going to the brain.  Aortic bodies are placed right on the aorta. They monitor the blood leaving the left ventricle. They are concerned with the blood going to the body.  Both of the chemoreceptors feed back to the DRG via the vagus and glossopharyngeal cranial nervesNerves that arise from the brain and control head and neck functions..

ChemoreceptorsSensory receptors that detect chemical stimuli, such as odors or blood pH. are located at the opening of the nose and mouthThe opening of the digestive tract where food enters and mastication begins.. They work to detect any irritants in the air.  They feed back to the DRG, which might initiate coughing. 

Baroreceptors are also keep in feeding info the DRG.  The inflation reflex, also called the Hering-Breuer reflex, prevents you from overinflating the lungs.  It controlsVariables that remain constant to ensure a fair test. that special point at which your brain is like, “STOP! Enough inhalation, switch to exhalation!”

All of these inputs reach the DRG or the PRG. They play a role in adjusting the rate of ventilation. They also influence the depth of ventilation.

Ventilation Perfusion Coupling

At first look this concept of ventilation perfusion coupling seems very confusing. However it is quite simple. Every time you take a breath, a certain percentage of your alveoliMicroscopic air sacs in the lungs where gas exchange occurs between air and blood. will not expand. If that alveolus cannot exchange gases in alveolar respiration, then I do not want to send blood into the capillary beds. These surround those alveoli that do not inflate. Let’s say this another way. Every time you take a breath there is a certain percentage of your alveoli that will expand with that breath. Those alveoli will be able to aid in alveolar respiration by exchanging gases with the blood and the air. I want to send blood to those areas.

Ventilation is referring to the expansion of alveoli, and perfusion is referring to the blood flow to said alveoli. The expansion of the alveolus and the blood flow in the capillary bed to it need to be coupled. If the alveolus doesn’t inflate I do not want to vasodilate capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues. surrounding that alveolus. If the alveo list does inflate I do want to dilate capillaries surrounding that alveolus.

We can describe this process with 4 very critical terms. I probably don’t use them enough in my mini lectures: vasoconstrictionThe narrowing of blood vessels due to contraction of smooth muscle, increasing blood pressure and re, vasodilationThe widening of blood vessels due to relaxation of smooth muscle, decreasing blood pressure and incr, bronchoconstriction, and bronchodilation. If I bronchodilate the airway leading to a bronchopulmonary segment of the lung, the capillaries in that lung should vasodilate. This would couple the ventilation and the perfusion in that bronchopulmonary segment.

We can think of ventilation perfusing coupling on a very small scale, such as one alveolus. We could consider it on the scale of one cluster of alveoli. This is because we know that with the alveolar poresSmall openings between adjacent alveoli that allow air to circulate between them, helping to equaliz, one cluster inflates altogether. However, I could go even further out and discuss ventilation perfusion coupling with bronchopulmonary segmentsFunctional units of the lungs, each served by its own tertiary bronchus and blood supply.. It could also be considered with entire lobes of the lung. If you have a patient with an issue confined to one lobe of the lung, such as pneumoniaA lung infection causing inflammation and fluid buildup in the alveoli., there is something you can assume. The patient’s ventilation perfusion coupling is likely not sending blood to the affected lobe. This is because that lobe is not capable of doing any alveolar gas exchange.