Manuscript: Explicit Constructivism: a missing link in ineffective lectures? Author: E.S.Prakash Supplement 2: Constructivist Lecture Note - Copyrighted images have been removed and replaced with a URL link to those images. This is an open access article distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by-nc-sa/3.0/

Chemical and neural regulation of respiration (a block of 3 lectures) E.S.Prakash, School of Medicine, AIMST University, Malaysia E-mail: dresprakash@gmail.com

How much do we already know How much do we already know? Just write the answers with the question number on a sheet of paper At sea level, barometric pressure is: mmHg Barometric pressure is the same as atmospheric pressure. T/F In the upright position, ventilation-perfusion ratio is highest in the upper lung zones. T/F The maximum volume of air that you can expel after a maximal inspiration is called: The amount of air that remains in the lungs after a tidal expiration is called:

Objective: We want to know how respiration is regulated Learning strategy: Go from known to unknown We breathe in and out throughout our life … Why do we have to breathe? We need oxygen We need to eliminate CO2 Why do we need oxygen? Why do we need to eliminate CO2?

Why do we need oxygen all the time? To power oxidative metabolism; we know, the energy yield of oxidative metabolism is much higher compared to anerobic metabolism Why do we need to eliminate CO2 all the time? CO2 is the end product of oxidative metabolism; it is an acid; if it accumulates in blood and body fluids, the pH of body fluids will drop; and we know the importance of maintaining pH of body fluids

So, logically, the system that regulates breathing should do the following: It MUST sense oxygen concentration in blood; It COULD sense oxygen concentration in cells; It MUST sense pH of blood and body fluids; It MUST be able to initiate appropriate responses so that the above parameters are maintained within acceptable limits.

The receptors that sense blood chemistry: We will call the receptors that sense blood chemistry – “chemoreceptors” Where do you want to have them? Arterial system Or venous system Why do you want them there? On the arterial side of the circulation so that the system could verify the oxygen concentration and pH of blood that will be supplied to all tissues.

Indeed, chemoreceptors that sense the oxygen content of blood are located in the aortic bodies and carotid bodies, on the arterial side of the circulation. http://www.medicine.mcgill.ca/physio/resp-web/Figures/Figtt20.jpg

Now we want to know how these chemoreceptors work: Presumably, these cells have oxygen sensors much like oxygen electrodes that we use to measure PO2 of a blood sample. Presumably, they have pH sensors, much like those that we use in the lab to measure pH of a blood sample. Second, they should transmit this information to regulatory centers in the brain (which could initiate appropriate responses)

Arterial chemoreceptors SENSE… From lessons in school, we recall that respiration is regulated by centers in the medulla. Arterial chemoreceptors SENSE… They transmit this information to medulla Respiratory center in medulla initiates appropriate responses

Sensory innervation of arterial chemoreceptors: http://www.medicine.mcgill.ca/physio/resp-web/Figures/Figtt20.jpg Aortic bodies Sensory branches of X cranial n. Carotid bodies Sensory branch of IX cranial n.

When do you think the chemoreceptors will be called into play? When arterial PO2 is low? When arterial PO2 is high? When arterial PCO2 increases? When arterial PCO2 decreases? When arterial pH falls? When arterial pH rises? All of the above?

So what are the normal values of each? Parameter Normal range Arterial pH 7.35–7.45 Arterial PO2 (PaO2) 81–100 mm Hg Arterial PCO2 (or PaCO2) 35–45 mm Hg

What “should” the response to low arterial PO2 be? Increase in breathing rate? Decrease in breathing rate? Increase in depth of respiration? Decrease in depth of respiration? How would an increase in rate and depth of respiration be a useful response to low PaO2? Increase in minute ventilation is likely to increase PaO2 toward normal – Yes / No

And what should the response to a rise in PaCO2 be? Increase in breathing rate Decrease in breathing rate Increase in depth of respiration Decrease in depth of respiration How would an increase in rate and depth of respiration be a homeostatic response to high PaCO2? Increase in minute ventilation is likely to decrease PaCO2 toward normal – Yes / No

Now, some facts about arterial chemoreceptors: Glomus cells in carotid bodies contain oxygen sensitive K+ channels; They have a very high blood flow 2000 ml/100 g tissue/min; In contrast, cerebral blood flow is 50 ml/100 g/min; So it is 40 times greater than cerebral blood flow

What do high blood flows to the carotid bodies mean? Do you think this might allow carotid body cells to meet their oxygen demands by consuming only oxygen dissolved in blood? Is it possible that they sense the amount of oxygen dissolved in blood rather than the amount of oxygen bound to hemoglobin? We know they are stimulated when PaO2 is low. If our reasoning is correct, carotid bodies would not be stimulated in anemia where [Hb] and total oxygen content in blood is low, but PaO2 is normal. Yes / No

But how important are the carotid bodies for sensing arterial PO2? Can you suggest an animal experiment to test this question? Experiment: Remove both carotid bodies Have the animal breathe O2 poor gas Does it hyperventilate as we would expect? Observation: The ventilatory response to hypoxia is virtually abolished by removal of carotid bodies. Conclusion: The ventilation stimulating effect of hypoxia depends largely on the carotid bodies

How important are the carotid bodies for sensing arterial pH? Can you suggest an animal experiment to test this question? Experiment: Remove both carotid bodies. Inject acid (say lactic acid) intravenously. Now the pH of arterial blood will drop. Does the animal hyperventilate as we would expect? Observation: The ventilatory response to a drop in arterial pH is virtually abolished by removal of carotid bodies. Conclusion: The ventilation stimulating effect of acidosis depends largely on the carotid bodies.

Can you suggest an animal experiment to test this question? We said that a rise in PaCO2 would increase minute ventilation. How is this response mediated? Is this mediated by the carotid bodies? Can you suggest an animal experiment to test this question? Remove both carotid bodies. Have the animal breathe a CO2 rich gas mixture. Hold the oxygen percentage of gas mixture normal (i.e. 21%) so that the animal is not hypoxic. The PaCO2 will rise Does the animal hyperventilate in response to the rise in PaCO2?

Continued.. Hyperventilation in response to hypercapnia (rise in PaCO2) occurs despite removal of carotid bodies. Conclusion: The increase in minute ventilation in response to hypercapnia must be mediated by receptors located elsewhere!

Now where are these receptors located? They were localized to ventral aspect of the medulla; Hence called medullary chemoreceptors Link to Figure – Medullary Chemoreceptors Sometimes called “central chemoreceptors” in contrast to the carotid & aortic bodies (“peripheral chemoreceptors”)

How do medullary chemoreceptors sense PaCO2? Tissue metabolism CO2 (end product of oxidative metabolism) PCO2 in cells and arterial blood rises How are chemoreceptors activated?

CO2 crosses the blood brain barrier (BBB) A rise in PaCO2 lowers CSF pH which is sensed by medullary chemoreceptors CO2 crosses the blood brain barrier (BBB) CO2 blood brain ISF H+ + HCO3 CO2 + H2O H2CO3 Carbonic anhydrase Drop in CSF pH

Problem: The same mechanism can work in the carotid bodies also; but we must explain why central chemoreceptors are more effectively stimulated by a rise in PaCO2 compared to peripheral chemoreceptors Think about it! Clue: Does a rise in PaCO2 lower pH of blood? Yes / No Does a rise in PaCO2 lower pH in brain ISF? Yes Why this difference? CO2 is buffered in arterial blood; What buffers CO2 in blood?

Continued >> CO2 is buffered mainly by: hemoglobin plasma proteins CSF has no hemoglobin and hardly any protein! What is the protein concentration of plasma? 6000–8000 mg/dL What is the protein concentration in CSF? 20 mg/dL CSF has 300-400 times less protein (buffering capacity) compared to plasma

So, is the presence of low concentration of proteins in CSF ‘advantageous’? Yes / No Yes. Otherwise, a rise in PaCO2 will not lead to a drop in CSF pH and ventilation will not be stimulated.

Is there any evidence to support the hypothesis that a low concentration of protein in CSF is advantageous? Take bacterial meningitis for example; The blood brain barrier is inflamed and leaky Proteins in blood leak into brain ISF CSF protein levels rise Eventually respiratory drive is blunted. Why… Eventually, death may occur due to accumulation of CO2 (respiratory acidosis)

Mediate the ventilatory response to a rise in PaCO2 Summary: Chemoreceptor Major Function Systemic arterial chemoreceptors (peripheral chemoreceptors; carotid and aortic bodies) Essential for the ventilatory response to hypoxia and a drop in blood pH (acidosis) Medullary chemoreceptors (central chemoreceptors) Mediate the ventilatory response to a rise in PaCO2

Continuing from where we left off.. Part Two.. Continuing from where we left off..

What is the stimulus that normally drives spontaneous breathing? Is it a rise in PaCO2? Or is it a fall in PaO2? How would you test this question? Experiment: hold your breath for one full minute. What happens to PaCO2 during this time? What happens to PaO2 during this time? How long were you able to hold your breath? 50 seconds? 1 minute? 70 seconds?

Why are you able to hold your breath only for a limited time? Breaking point: the point at which voluntary control of breathing is overridden. Why is breath holding broken? Is it because PaCO2 rises enough to stimulate breathing? Or is it because PaO2 falls enough to stimulate breathing? Or is it due to a combination of both factors? And how would you test it?

Continued.. IF the breaking point is due to a combination of hypoxia and hypercapnia, THEN, breath holding time will be greater if you hold your breath in full inspiration (compared to full expiration). Yes / No? ALSO, breath holding time should be greater when you hold your breath after breathing in 100% oxygen for some time. Yes / No?

You can do it and see: Breath holding time is longer if you hold your breath in full inspiration (compared to full expiration) [True / false] Breath holding time is longer after breathing in a O2 rich gas mixture. [True / false]

We still have this question: what is the stimulus that normally drives breathing? Suggest a different simple experiment.. Hyperventilate for a minute What happens to breathing thereafter? There is a period of apnea… Why this apnea? CO2 (the stimulus for breathing) has been washed out Conclusion: The rise in PaCO2 is probably the most important stimulus for breathing under normal conditions.

Other conclusions from these experiments regarding control of breathing: We can control our breathing (but only to a certain extent) – Voluntary control. What remains to be learnt: what is the neural basis for this? Otherwise, for the most part, breathing is a spontaneous (automatic) rhythm

Neural mechanism of voluntary control of breathing Note: it bypasses the respiratory center in medulla Cerebral cortex (the “will” originates here) (upper motor neurons) Medulla NB: There are both excitatory as well as inhibitory controls from cerebral cortex Spinal cord +/- Phrenic n. Intercostal n. Muscles of expiration: Internal intercostals Diaphragm Intercostal n. External intercostals

How do you expect spontaneous breathing to be achieved? Spontaneous breathing must be paced by some mechanism; there must be a pacemaker for initiating breathing like the SA node initiates the impulse that excites the heart. The pacemaker must receive information from central and peripheral chemoreceptors. In turn, the pacemaker should drive neurons which drive muscles of inspiration and expiration.

Afferents from carotid bodies terminate here How do inputs from central and peripheral chemoreceptors reach respiratory motor neurons? Afferents from carotid bodies terminate here Pons Medullary chemoreceptors Pre-Bottzinger complex; pre-BOTC (Pacemaker) Medulla I neurons Spinal cord Phrenic motor neurons Diaphragm

Afferents from carotid bodies terminate here Neural mechanism of spontaneous breathing: (A hypothetical working draft..) Afferents from carotid bodies terminate here Pons Medullary chemoreceptors Pre-Bottzinger complex; pre-BOTC (Pacemaker) Medulla I neurons Spinal cord Phrenic motor neurons Diaphragm

Component Details Peripheral & central chemoreceptor neurons Project to Pre-Botzinger complex of neurons (the putative pacemaker) Neurons in pre-BOTC Discharge spontaneously (pacemakers) Entrained by input from chemoreceptors (i.e. frequency of discharge is modulated by input from chemoreceptors) I neurons Fire during inspiration Project to lower motor neurons in spinal cord (e.g. phrenic motor neurons)

Some more questions to be answered about spontaneous breathing: Our model explains the mechanism of spontaneous inspiration. But what is the mechanism of spontaneous expiration?

During spontaneous breathing, expiration is due to passive elastic recoil of lung Breathe in Thorax expands Lungs expand i.e., lung parenchyma is stretched Elastic lungs recoil spontaneously

But there is a problem.. If I neurons continue to fire during expiration, then, contraction of muscles of inspiration will oppose expiration and increase the work of breathing. So what? Hypothesis: There could be a mechanism inhibiting discharge of I neurons during expiration.

Interpret this experimental observation: Normal breathing pattern apneusis Breathing after transection of neuraxis between pons and medulla

Diaphragm & intercostal muscles A center in the pons may serve to switch from inspiration to expiration (Hypothetical working draft model..) Pneumotaxic center - Pons Pre-Bottzinger complex; pre-BOTC (Pacemaker) Medulla - I neurons Spinal cord Diaphragm & intercostal muscles Phrenic neurons

But is there anything that drives the Pneumotaxic center to cause the switch from Inspiration to Expiration? How could we test it? Suggest an experiment Breathe in deep Try breathing in further Can you do it? Do you find it difficult? Why? Is a mechanism triggering expiration?? Note the duration of expiration is also longer following a deep breath

This reflex is called Hering-Breuer inflation reflex (stimulus: excessive lung inflation) Experiment: Deep inspiration Observation (response): further inspiration is inhibited expiration is triggered the duration of this expiration is longer this has been shown to be abolished by vagotomy (cutting afferent input from stretch receptors in lungs to pons & medulla) Conclusion: vagal afferent input from lungs inhibits excessive lung stretch (negative feedback)

So what is the pattern of breathing you expect after vagotomy So what is the pattern of breathing you expect after vagotomy? Observations below.. Normal After vagotomy Note the depth of breathing is increased after vagotomy

Hering-Breuer deflation reflex (Stimulus: excessive lung deflation) Experiment: breathe out fully Try breathing out even more Observation: Can you do it? Why? Note the depth of the next inspiration? Response: Further deflation is inhibited (negative feedback) The next inspiration is prolonged. The deep inspiration following a deep expiration is abolished by vagotomy

take a look at this phenomenon.. Breathe in This stimulates further lung inflation + / - feedback?? Positive feedback A paradoxical reflex (Head’s paradoxical reflex) Can you think of one situation in which it might be “useful”?

The first cry – the most crucial moment in life The cry of a newborn Generates very negative intrathoracic pressure Facilitates expansion of collapsed fluid filled lungs Do Hering Breuer reflexes work in this situation?

Summary: Stimulus Response Name of reflex Receptor Excessive lung inflation Inhibition of inflation; lung deflation Hering Breuer inflation reflex Vagal afferents from airways Excessive lung deflation Inhibition of deflation; lung inflation Hering Breuer deflation reflex Lung inflation Further inflation Head’s paradoxical reflex ?

Part 3

Take this problem: If you gradually ascend to an altitude of 2500 m and live there for a month, in which direction would you expect the following variables to change? PaO2 – Increase / decrease. Why? PaCO2 – Increase / decrease. Why? pH of arterial plasma – Increase / decrease. Why? Minute ventilation – Increase / decrease. Why?

Answer: Low PaO2 due to low barometric pressure This drives peripheral chemoreceptors Minute ventilation is increased PaCO2 will fall because of increased minute ventilation Arterial pH raised because of increased elimination of CO2 Hypoxemia with respiratory alkalosis Further question: what is the magnitude of the increase in minute ventilation in response to hypoxemia?

The ventilatory response to hypoxia: 200 Minute ventilation (l/min) 100 0 25 50 75 100 PO2 (mm Hg)

Consider this situation: If we perform moderate exercise (let us say bicycle) at high altitude (2500 m) without being truly accustomed to high altitude, how much would minute ventilation increase during exercise? Not at all / Increase / Increase greatly Explain why?? And what will happen to our work capacity? Decrease / not change / increase

Max. ventilation during exercise Ventilatory response to hypoxia, hypercapnia, severe exercise and maximal voluntary ventilation (MVV) compared 200 Minute ventilation (l/min) MVV: 125-175 l/min Max. ventilation during exercise 100 Response to hypercapnia Response to hypoxia 0 50 100 Alveolar PO2 or PCO2 (mm Hg)

Answer: Regarding the graph in the previous slide: Minute ventilation increases much more; Stimuli for breathing: hypoxia + metabolic (lactic) acidosis; We will be breathless and get exhausted quicker. Regarding the graph in the previous slide: At rest, minute ventilation is about 5 l/min But MVV = 125-175 l/min (higher in males compared to females) Thus, there is a great ventilatory reserve (25 – 35 times); Hypoxia and hypercapnia alone are not as potent as severe exercise in stimulating ventilation. So, other factors also drive ventilation during exercise.

Why do we get breathless during exercise? Besides activation of breathing by carotid bodies, suggest some other potential mechanisms that may contribute to the sensation of breathlessness during intense exercise… What would be the effect of an increase in pulmonary interstitial fluid pressure (PIFP) on breathing pattern? When does this occur? Clue: What happens to PIFP in heart failure? What is the pattern of breathing in this condition?

Some working definitions now: Normocapnia: PaCO2 between 35 and 45 mm Hg Hypocapnia: PaCO2 < 35 mm Hg Hypercapnia: PaCO2 > 45 mm Hg Hypoxemia: PaO2 < 80 mm Hg Note: significant activation of carotid bodies occurs only when PaO2 < 60 mm Hg

This pattern of breathing (shown in brown) was observed in a patient with brain stem disease. Can you suggest a mechanism that can result in such a breathing pattern? Normal

Cheyne-Stokes respiration: Periods of apnea punctuated by periods of hyperpnea It occurs in: congestive heart failure brain stem disease affecting respiratory centers Mechanisms postulated to explain this: Prolonged lung-to-brain circulation time Alterations (increase or decrease) in sensitivity of medullary chemoreceptor or respiratory neurons

What is the effect of voluntary hyperventilation to exhaustion on breathing? Hyperventilate to exhaustion Then, note your pattern of breathing Explain your observations.

Following hyperventilation Activity: Periodic breathing hyperventilation normal Following hyperventilation

Outline of the explanation: Hyperventilation eliminates CO2; Apnea is due to lack of CO2 During apnea, PaO2 falls & stimulates breathing Few breaths eliminate hypoxia Now there is no stimulus for breathing So there is apnea again Normal breathing resumes only when PaCO2 is 40 mm Hg Conclusion: normal breathing pattern is entrained by PaCO2 not PaO2

Could one stay alive after respiratory centers in the medulla are destroyed? Yes / No How?

Ondine’s curse: Spontaneous control of breathing is disrupted; Voluntary control is intact; One could stay alive only by remembering to breathe; Clinical analog: bulbar poliomyelitis affecting respiratory neurons in the brain stem; disease processes compressing the medulla

Some items for self-study: How is breathing regulated during exercise? What is the mechanism of hiccup? What is the mechanism of yawning? What is the mechanism of sneezing? What happens when you sigh?

Now take the Post-Test

In addition, you should also be able to answer these questions: Describe with the help of schematic diagram, the neural mechanism of spontaneous breathing. Describe with the help of schematic diagram, the neural mechanism of voluntary control of respiration. Describe with the help of schematic diagram, the role of systemic arterial chemoreceptors in regulation of alveolar ventilation.

Describe with the help of schematic diagrams the functional organization and functions of medullary chemoreceptors. How does CO2 stimulate breathing? What is the relationship between PaCO2 and minute ventilation? Describe the mechanism responsible for periodic breathing following voluntary hyperventilation.

Explain the factors that affect breath holding time. Briefly explain the effect of damage to the pneumotaxic center on the pattern of breathing Briefly explain the effect of vagotomy on the pattern of breathing in experimental animals.

What is the difference between the effect of acute hypercapnia and chronic hypercapnia on minute ventilation? What is Kussmaul’s respiration? When does it occur? What is the mechanism involved? What is periodic breathing? When does it occur? What is Cheyne-Stokes respiration? What are the Hering Breuer reflexes? What is Head’s paradoxical reflex?

Required reading: Chapter 36. Regulation of respiration. Ganong WF. Review of Medical Physiology, Mc Graw Hill Co, 2005