Physiology 1, Fall 2008, LPC

Chapter 17 - Mechanics of Breathing

I – Respiratory System

A. Introduction

4 primary function of respiratory system

Exchange of gases between atmosphere & blood
Homeostatic regulation of body pH
Protection from inhaled pathogens & irritating substances
Vocalization

Bulk flow of air

Flow from regions of higher pressure to lower pressure
Muscular pump creates pressure gradients
Resistance to air flow influenced primarily by diameter of air flow tubes

B. Respiration

4 processes

exchange of O2 and CO2 between lungs & blood
Transport of O2 and CO2 by blood
Exchange of gases between blood and cells = cellular respiration

Anatomy

Airways

Upper = mouth, nasal cavity, pharynx, larynx
Lower = trachea, two bronchi, bronchioles, lungs

Alveoli – site of gas exchanges
Bones & muscles of thorax and abdomen (abdominal muscles & diaphragm)

Pleural Sacs

Double-walled sac surrounds outer surface of lung and inside of thorax
Pleural fluid between two surfaces of membrane allows lungs to slide easily & holds lungs tight against the thoracic wall

Airways connect lungs to external environment
Alveoli

Type I - gas exchange
Type II – secrete surfactant -> decreases surface tension
Many elastic fibers -> alveoli constrict automatically
Many capillaries -> large volume of blood for gas exchange

Circulation

Large amount of blood in lungs (entire right ventriclar output) -> low pressure in lungs -> low hydrostatic pressure & filtering rate

II – Gas Laws
Table 17-1

1. Total pressure of a mixture of gases is sum of pressures of individual gases

a. pressure exerted by an individual gas is determined by its relative abundance in the mixture (partial pressure of O2 = atmospheric pressure x 21%)

1. Gases move from area of higher pressure to lower
2. If volume of a container of gas changes, pressure changes
   Boyle’s Law: P1V1 = P2V2 (if volume changes, pressure changes inversely)

III – Ventilation

A. Lung Volumes

tidal volume – air that moves during quiet inspiration or expiration
inspiratory reserve volume – air that can be inhaled in addition to tidal volume
expiratory reserve volume – air that can be exhaled in addition to tidal volume
residual volume – air in the respiratory system after maximal exhalation

B. Total lung capacity = vital capacity (maximal air that can be voluntarily moved) + residual volume

C. Airways

Warm, Humidify, Filter inspired air
Body heat warms air
Moisture in mucus humidifies air
Mucus traps dirt, bacteria, pollen, etc
Cilia move mucus towards throat to be swallowed
Watery saline secretion allows cilia to move mucus

D. Diaphragm

creates pressure gradient by increasing or decreasing thoracic cavity volume

E. Inspiration

Diaphragm down -> increase in thoracic volume
Ribs out -> increase in thoracic volume
alveoli expand in response to decreased intrathoracic volume & pressure

F. Expiration

Elastic recoil of lungs and thoracic cage -> passive expiration
Active expiration – voluntary exhalation and/or quick rate

G. Intrapleural Pressure

Elastic recoil of chest wall tries to pull chest wall outward
Elastic recoil of lungs creates inward pull
These two opposing forces -> vacuum in intrapleural pressure, -> lungs “stuck” to thoracic cage.  If thoracic cage moves, lungs move with it.
Pneumothorax = break in pleural cavity seal, -> lung collapse

H. Surfactant

Surfactant decreases surface tension of alveolar fluid, -> decreases resistance of lung to stretch
Surfactant more concentrated in smaller alveoli, making their surface tension less than in larger alveoli. Helps equalize pressure among various sizes of alveoli

I. Airway Diameter Primary Determinant of Airway Resistance

R ∞ (length x viscosity)/radius 4
Length of airways and viscosity of air remain essentially constant
Radius can change via physical obstruction or constriction or dilation of bronchioles

J. Ventilation = rate X volume moved

alveolar ventilation more important than pulmonary ventilation
deep breathing -> more fresh air into alveoli because dead space volume remains constant
rate affects ventilation (more or less fresh air inhaled)

K. Matching ventilation & alveolar blood flow

alveolar blood flow depends on local factors
air flow regulated by exhaled CO2 levels in bronchioles
decreased O2 (or increased CO2) -> pulmonary arterioles constrict  -> less blood flow -> route blood to well ventilated alveoli
increased O2 -> pulmonary arterioles dilate  -> more blood flow

Note: opposite of effect in systemic circulation

If blood flow or air flow blocked, no part of lung has normal ventilation or perfusion, local control mechanisms not effective regulator