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.

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.

REGULATION OF RESPIRATIONBy

Dr. Mudassar Ali Roomi (MBBS, M.Phil.)Assist. Prof. Physiology

Control of respirationTwo types: 1. Nervous control of respiration2. Chemical control of respiration

Control of repirationComponents:

Sensors gather information

Central controller integrate signals

Effectors muscles

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

Respiratory centre

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

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

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

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

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

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

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.

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

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

Apneustic CenterLocated in lower part of pons

Function: Prevent inspiratory neurons from being switched off → prolonged inspiration

Shortens expiration

Such Respiration called – apneusis

CHEMICAL CONTROL OF RESPIRATIONFollowing chemical stimuli stimulate the respiration:

1. Excess CO2

2. Excess Hydrogen ion

3. Decreased Oxygen

Central chemosensitive areaStimulated by CO2 & H+ .Oxygen have no effect

Peripheral chemoreceptorsStimulated by O2. CO2 & H+ has little effect

Location of Chemosenstive areaLocated bilaterally

beneath the ventral surface of medulla

Hydrogen ions are only the main direct stimulus for these group of neurons

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

Stimulation of the Chemoreceptors by Decreased Arterial Oxygen

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