hello welcome to bite size med this video is on the regulation of respiration and in part two we're going to look at chemoreceptors just a quick recap of part one respiratory control is both voluntary and involuntary voluntary controls by the cortex involuntary control is by the brain stem that's the medulla and the pons there are four groups of neurons two each in the medulla and the pons the medulla has the dorsal and ventral group of neurons the dorsal group controls inspiration and the ventral group has both inspiratory and expiratory neurons but mostly controls forced exploration the pawns has a pneumotaxic and amnestic center the apneustic center stimulates the inspiratory center and the pneumotaxic center inhibits the apnositic center and switches off the center for inspiration so it controls the rate and depth of breathing these centers they have to get their information from somewhere and that's from receptors and receptors pick up stimuli there are chemical and non-chemical stimuli non-chemical stimuli are like when the lung stretches it gets picked up by stretch receptors proprioception picked up by the proprioceptors of the muscles in the joints but what we're looking at right now is the chemical stimuli and what picks that up is a chemoreceptor there are two types of chemoreceptors central and peripheral central are in the central nervous system in the brainstem they form a chemosensitive area in the ventral medulla the peripheral chemoreceptors are in the periphery the carotid and the aortic bodies first we're going to look at the central chemoreceptors that chemosensitive area on the ventral medulla right near the inspiratory center the ultimate goal of this respiratory regulation is to maintain a constant blood oxygen carbon dioxide and hydrogen ion concentration so the receptors have to detect these changes now carbon dioxide and hydrogen ions they go together when carbon dioxide levels increase hydrogen ions increase as well and so the ph comes down so we'll put those two together changes in these two are detected by the central chemoreceptors not oxygen oxygen cannot directly influence the central chemoreceptors they're more important for the peripheral chemoreceptors this chemosensitive area it's very sensitive to hydrogen ions but there's an issue there hydrogen ions cannot cross the blood-brain barrier easily but carbon dioxide can and remember that they both go together so when the partial pressure of arterial carbon dioxide increases the carbon dioxide crosses the blood-brain barrier combines with water and by carbonic anhydrase it forms carbonic acid which then dissociates into hydrogen ions and bicarbonate ions now these hydrogen ions can stimulate the chemosensitive area which in turn communicates with the respiratory center so there's an increased rate and depth of breathing clearing off the excess carbon dioxide bringing down the arterial carbon dioxide to normal levels this effect would last for a few days and then the kidneys would take over throwing out the excess hydrogen ions by increasing their secretion and also increasing bicarb reabsorption ultimately maintaining the ph of blood so this is how central chemoreceptors work now on to peripheral chemoreceptors there are two the carotid and the aortic bodies the carotid bodies are at the bifurcation of the carotid artery while the aortic bodies are on the arch of the aorta the afferent neurons from these bodies are the glossopharyngeal and the vagus nerves so the ninth and the tenth nerve the carotid sinus nerve or the herrings branch of the glossopharyngeal nerve that carries information from the carotid body and the aortic branch of the vagus nerve carries information from the aortic body both of them reach the dorsal respiratory group of the medulla that's the center of inspiration each of these bodies is called the glomus and agglomus has two types of cells the type 1 and type 2 cells the type 1 cells are the glomus cells they're important for functioning the type 2 cells are supportive to the type 1 cells so there's one type 2 cell for a few gloma cells the gloma cells are the ones that are going to respond to changes in the oxygen levels in blood so they are associated with afferent neurons now let's see how the glomus cell can detect low oxygen and tell the centers to increase respiration the carotid body weighs around two milligrams they have a high blood supply like two liters per hundred gram of tissue per minute now the hundred grams is for comparison so compared to other organs that's a lot of blood in blood oxygen can be dissolved or bound now since they're getting that much blood they can get what they need just from the dissolved oxygen so bound oxygen cannot stimulate the glomus cell like in anemia when low hemoglobin reduces bound oxygen it's dissolved oxygen that does stimulate it if low remember that dissolved oxygen is what exerts a partial pressure not the bound form so reduction in the partial pressure of arterial oxygen will stimulate the gloma cells now how does that happen the cells they have potassium and calcium channels the potassium channels are oxygen sensitive and when the arterial oxygen reduces these potassium channels close reducing efflux of potassium which opens the voltage-gated calcium channels so calcium enters the cell that stimulates the release of a neurotransmitter which excites the afferent neuron the impulses then travel to the dorsal respiratory group stimulating inspiration so low arterial oxygen stimulates inspiration but how low is low normal arterial oxygen is around 100 millimeters of mercury it's only when it goes below 60 that the gloma cells get stimulated what about 60 to 100 why doesn't that work there are two reasons if there's low oxygen that means there's lesser oxygen bound to hemoglobin reduced hemoglobin is a weaker acid it binds to hydrogen ions easier lowering the hydrogen ions in blood slightly remember what we went over earlier with central chemoreceptors high hydrogen ions stimulate respiration so low hydrogen ions would inhibit respiration even if the ventilation were to increase there's more carbon dioxide being breathed out that lowers the alveolar carbon dioxide again inhibiting respiration so there are two blockers inhibiting respiration until the oxygen goes below 60. that's when this stimulus becomes stronger than these two inhibitory stimuli increasing respiration now all of this was oxygen the peripheral chemoreceptors can also directly detect changes in arterial carbon dioxide and in the ph so that's the hydrogen ions not as much as the central but yes they can unlike the central the detection of ph changes is independent of the carbon dioxide so even if the ph change is coming from metabolic acidosis or alkalosis the peripheral receptors can detect it but only the carotid bodies and that's important for how the lungs compensate in metabolic acidosis and alkalosis and so in maintaining acid-base balance so by maintaining the blood oxygen carbon dioxide and hydrogen ions the central and the peripheral chemoreceptors they work together if this video helped you give it a thumbs up and subscribe to my channel thanks for watching and i'll see you in the next one