Oxygen Concentration and Partial Pressure in the Alveoli
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Oxygen Concentration and PartialPressure in the Alveoli
The oxygen concentration in the alveoli, and its partial pressure is controlled by:
1. The rate of absorption of oxygen into the blood
2. The rate of entry of new oxygen into the lungs by the ventilatory process. Rate of alveolar ventilation.
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CO2 Concentration and PartialPressure in the Alveoli
Determined by two factors:First, the alveolar
PCO2 increases directly in proportion to the rate of carbon dioxide excretion
Second, the alveolar PCO2 decreases in inverse proportion to alveolar ventilation.
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REGULATION OF RESPIRATIONBy
Dr. Mudassar Ali Roomi (MBBS, M.Phil.)Assist. Prof. Physiology
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Control of respirationTwo types: 1. Nervous control of respiration2. Chemical control of respiration
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Control of repirationComponents:
Sensors gather information
Central controller integrate signals
Effectors muscles
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Respiratory centre
Located bilaterally in medulla oblongata and pons.
Composed of1. Dorsal Respiratory Group (DRG)
2. Ventral Respiratory Group (VRG)
3. Pneumotaxic center
4. Apneustic center
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Respiratory centre
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Pre-Botzinger complex (pre-BOTC)A collection of pace-maker
cells at the upper end of Dorsal Respiratory Group (DRG)
Synaptic connection with DRG
Function: Discharges rhythmic respiratory signals
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Dorsal Respiratory Group (DRG)Extends most of the length of M.
oblongata LOCATION: Neurons located in nucleus
of tractus solitarius and additional neurons in reticular substance of medulla
vagus and glossopharyngeal nerve terminates at Nucleus of tractus solitarius
Both nerves – afferent nerves for resp. signals to center
Pace maker neurons send ramp signals to inspiratory muscles in a Rhythmic fashion
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Ramp signals controlled by
(a) Pneumotaxic center (b) Stretch receptors in the
lungs Significance of ramp signals No gasping Smooth inflation of lungs Full cycle of respiration 5 seconds2sec inspiration 3 sec expiration
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Fibers from respiratory center (DRG) reach the motor neurons in spinal cord between C3 & C5 to form phrenic nerve
Complete lesion of spinal cord above C3 will stop the breathing
Lesion after C5 will not affect the respiration
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The Hering-Breuer Inflation ReflexMuscular portions of the walls of the bronchi and bronchioles throughout the lungs
have stretch receptors
Transmit signals through the vagi into the dorsal respiratory group of neurons when the lungs become overstretched.
Switches Off the inspiratory ramp and thus stops further inspiration
These signals affect inspiration in much the same way as signals from the pneumotaxic center
It also increases rate of respiration
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The Hering-Breuer Inflation ReflexThis reflex is activated when tidal volume increases to
more than three times normal Therefore, this reflex appears to be mainly a protective
mechanism for preventing excess lung inflation
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Lung “J Receptors.”Location: In the alveolar walls in
juxtaposition to the pulmonary capillaries
Stimalation: Stimulated especially when the pulmonary capillaries become engorged with blood or
Example: When pulmonary edema occurs in such conditions as congestive heart failure.
Their excitation may give the person a feeling of dyspnea.
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Ventral Respiratory Group (VRG)LOCATION: Ventral part of medulla
Two nuclei (1) Nucleus Ambiguus rostrally (2) Nucleus Retroambiguus caudally
Both types of neurons – INSPIRATORY & EXPIRATORY
Center remain inactive during quite breathing
Active only in increased pulmonary ventilation, during which signal from DRG spill over to VRG
Stimulation of accessory inspiratory muscles & expiratory muscles
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Pneumotaxic Center
Location: Upper part of Pons
Function: Switches off Ramp Signal
Controls rate and duration of Inspiratory ramp signals
Strong stimulation may reduce Inspiratory phase to 0.5 sec respiratory rate ↑ to 30 – 40/min
Weak stimulation may ↑ Inspiratory phase to 5sec or more respiratory rate ↓ to 3-5/ min
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Apneustic CenterLocated in lower part of pons
Function: Prevent inspiratory neurons from being switched off → prolonged inspiration
Shortens expiration
Such Respiration called – apneusis
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CHEMICAL CONTROL OF RESPIRATIONFollowing chemical stimuli stimulate the respiration:
1. Excess CO2
2. Excess Hydrogen ion
3. Decreased Oxygen
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Central chemosensitive areaStimulated by CO2 & H+ .Oxygen have no effect
Peripheral chemoreceptorsStimulated by O2. CO2 & H+ has little effect
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Location of Chemosenstive areaLocated bilaterally
beneath the ventral surface of medulla
Hydrogen ions are only the main direct stimulus for these group of neurons
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Peripheral ChemoreceptorCarotid bodies
through Hering N to
Glossopharyngeal NAortic Bodies
through Vagus N to DRG
Both bodies are supplied by special minute arteries direct from the arterial trunk
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Stimulation of the Chemoreceptors by Decreased Arterial Oxygen
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Effect of Carbon Dioxide and Hydrogen Ion Concentration onChemoreceptor Activity
They have a weak effect but stimulation by way of the peripheral chemoreceptors occurs as much as five times as rapidly as central stimulation
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