RESPIRATORY PHYSIOLOGY

PART Ib

Overview

Mechanics of breathing refers to the factors that participate

in moving of the lungs and of the chest wall along the respiratory cycle

Breathing

Breathing, or pulmonary ventilation, consists of two phases Inspiration – air flows into the lungs Expiration – gases exit the lungs

Pulmonary Ventilation

A mechanical process that depends on volume changes in the thoracic cavity

Volume changes → pressure changes

→ flow of gases to equalize pressure

Pressure Relationships in the Thoracic Cavity

Respiratory pressure is always described in relation to atmospheric pressure Respiratory pressure of 0 = atmospheric P (-) “ “ of -4 = 756 mmHg (+) “ “ of +4 = 764 mmHg

Pulmonary Pressures

Atmospheric pressure = Patm

Intrapulmonary pressure = Palv

Intrapleural pressure = Pip

Transpulmonary pressure

Atmospheric Pressure

760 mm Hg at sea level decreases as altitude increases increases under water Other lung pressures given

relative to atmospheric (set Patm = 0 mm Hg)

Intra-alveolar Pressure(intrapulmonary pressure)

pressure within the alveoli varies with phase of respiration

During inspiration = negative (less than atmospheric)

During expiration = positive (more than atmospheric)

but it always equalizes with atmospheric P Difference between Palv and Patm

drives ventilation

Alveolar pressure

During inspiration: –1cm of H2O (this slight negative pressure is enough to move about 0.5 liter of air into the lungs in the first 2 second of inspiration)

During expiration: it rises to about +1cm of H2O (this forces 0.5 liter of inspired air out of the lungs during the 2 to 3 seconds of expiration

Intrapleural Pressure

Pressure inside pleural cavityAlways negative under normal conditionsAlways less than Palv

Varies with phase of respirationvaries between -5 and -7.5 cmH2O

(inspiration to expiration)At rest, -4 mm Hg

Intrapleural Pressure Negative pressure due to :

opposing forces of the lungs trying to collapse

chest wall trying to expand

Intrapleural pressure

Always ~ 4 mmHg less than pressure in alveoli

Any condition that equalizes the intrapleural pressure to the intrapulmonary (atm P) Pressure causes immediate lung collapse

Collapse of the lungs If the pleural space communicates with the

atmosphere, i.e. pleural P = atmospheric P the lung will collapse

Causes: Puncture of the parietal pleura

Sucking chest wound Erosion of visceral pleura Also if a major airway is blocked, the air trapped

distal to the block will be absorbed by the blood and that segment of the lung will collapse

Pleura and negative pressure

Pneumothorax: lung collapse due to air entering in the pleural cavity

(not to be confused with atelectasis alveoli collapse)

Transpulmonary Pressure

Transpulmonary pressure = Palv – Pip

Distending pressure across the lung wall Keeps the lungs from collapsing peaks at the end of inspiration Increase in transpulmonary pressure:

Increase distending pressure across lungsLungs (alveoli) expand, increasing volume

Pressure Relationships

Intrapulmonary pressure and intrapleural pressure fluctuate with the phases of breathing

Intrapulmonary pressure always eventually equalizes itself with atmospheric pressure

Intrapleural pressure is always less than intrapulmonary pressure and atmospheric pressure

Pressure Relationships

Figure 22.12

Diagram of Diaphragm

Boyle’s Law

Boyle’s law – the relationship between the pressure and volume of gases As volume of a chamber increases,

pressure of the gas within it decreases As volume of a chamber decreases,

pressure of the gas within it increases

Boyle’s Law

Volume of gas is inversely proportional to pressure (if temperature constant)

Volume = Constant

Pressure

P1V1 = P2V2

Boyle’s Law P1V1 = P2V2 Volume of gas is inversely proportional to pressure (if temperature constant)

Volume = Constant Pressure

Boyle’s Law P1V1 = P2V2 Volume of gas is inversely proportional to pressure (if temperature constant)

Volume = Constant Pressure

CHARLES’ LAW

If pressure is constant, the volume of a gas and its temperature vary proportionately, or V≈T

GENERAL GAS LAW

At constant temperature, an increase in thoracic volume leads to a decrease in pleural pressure

Driving Force for Air Flow

Airflow driven by the pressure difference between atmosphere (barometric pressure) and inside the lungs (alveolar pressure).

760 mmHg

Pressure gradient drives flowAir moves from high to low pressure

Air moves in and out of lungs by bulk flow

Inspiration: pressure in lungs less than atmosphere

Expiration: pressure in lungs greater than atmosphere

Muscles of Inspiration

Diaphragm forces abdominal contents down & forward

External intercostals lift ribs up and outwards during quiet breathing diaphragm works alone during exercise, accessory muscles called into

play

Muscles of Expiration

Expiration during quiet breathing is passive due to elastic recoil of chest cavity

Decrease in lung size force air out of lungs During exercise and voluntary

hyperventilation, rectus abdominus, transverse abdominus: push

diaphragm up internal intercostals: pull ribs downwards

PRESSURE CHANGES AND AIRFLOW Flow of air into and out of the lungs is

governed by two physical principles:

1. Changes in volume result in changes in pressure

*** Boyle’s law

> muscles of respiration change thoracic volume and therefore pressure within the thoracic cavity

2. Air flows from areas of higher to lower pressure

BREATHING CYCLE

AT REST ( before inspiration)

A. alveolar pressure equals atmospheric pressure

B. intrapleural pressure is NEGATIVE

C. lung volume is at FRC

INSPIRATION

EXPIRATION

Normal Breathing Cycle

Normal Breathing Cycle

INSPIRATION Inspiratory muscles contract (diaphragm descends;

rib cage rises)↓

Thoracic cavity volume increases↓

Lungs stretched; intrapulmonary volume increases↓

Intrapulmonary pressure drops ( to -1mmHg)↓

Air flows into the lungs down its pressure gradient until intrapulmonary pressure is 0 (equal to atmospheric

pressure)

EXPIRATION Inspiratory muscles relax (diaphragm rises; rib cage

descends due to recoil of costal cartilages)↓

Thoracic cavity volume decreases↓

Elastic lungs recoil passively ; intrapulmonary volume decreases

↓Intrapulmonary pressure rises ( to +1mmHg)

↓Air flows out of lungs down its pressure gradient until

intrapulmonary pressure is 0

Physiology of respiration

In the intact lung as the thoracic cage expands the pleural and the

alveolar pressures decrease below atmospheric pressure forcing

airflow to the lung, increasing the lung volumes

QUESTIONS?

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