RESPIRATION

Dr. Faraz A. Bokhari

If you cant breathe, nothing else matters!

(American Lung Association)

Introduction• Why respire?

• Gas exchange• Host defense (barrier b/w outside and inside)

• Metabolism (produces/metabolizes compounds)

• Gas exchange• Pulmonary ventilation• Diffusion of O2 and CO2 b/w alveoli and blood• Transport of O2 and CO2 in blood and body fluids to &

from body's tissue cells• Regulation of ventilation

Physiological Anatomy

• Upper airway• All structures from nose to

the vocal cords, including sinuses and the larynx

• Conditioning of inspired air

• Lower airway• Trachea, airways & alveoli

The Airway Tree:Conducting zone

• Consists of trachea and first 16 generations of airway branches

– First four generations are subjected to changes in -ve and +ve pressures, & contain a considerable amount of cartilage (to prevent airway collapse)

– Cartilage present up to lobar and segmental bronchi– Disappears in bronchioles

» Bronchioles are suspended by elastic tissue of lung parenchyma

» This elasticity keeps them patent

• Blood supply: Bronchial vessels• No gas exchange

The Airway Tree:Respiratory zone

• Composed of last seven generations

• Consists of respiratory bronchioles, alveolar ducts and alveoli

• Blood supply: pulmonary circulation• Pulmonary circulation receives all of CO

The Airway Tree:Respiratory zone

• Adult lungs contain 300 to 500 million alveoli• Combined internal surface area: 75 m2

• Represents one of the largest biological membranes in the body

• With age number and size increases till adolescence!– after adolescence, alveoli only increase in size – Smoking induced damage can be reversed in a limited way

only!

Physiological Anatomy

• Lung are covered by the visceral pleura and are encased by the parietal pleura– Potential space b/w these 2 layers– Layer of fluid allows for smooth gliding of lung as

it expands in the chest– Pressure within this space is normally kept

negative• Pneumothorax• Hemothorax

Lung-Chest Wall Interaction

Mechanics of Pulmonary Ventilation

• Lungs can be expanded and contracted in 2 ways:

• By downward and upward movement of the diaphragm

– To lengthen or shorten the chest cavity

• By elevation and depression of the ribs – To increase and decrease the anteroposterior

diameter of the chest cavity

• Physical events• Pump Handle movement

• Bucket-handle movement

Pump Handle – AP Diameter

Bucket Handle – Lateral Expansion

Muscles of Respiration– INSPIRATION

• Principal– Diaphram (domes descend – increase longitudinal

aspect)– External intercostals (elevate ribs – increase AP

aspect)• Accessory

– Sternocleidomastoid muscles (lift upward on the sternum)

– Scaleni (lift the first two ribs)

– EXPIRATION• Quiet breathing

» Passive recoil of lungs• Active breathing

» Internal intercostals (depress ribs)» Abdominal recti (depress lower ribs, compress

abdominal contents)» External/internal oblique

Inspiratory Sequence – General Concept

Quiet Inspiratory Sequence - General Concept

Quiet Expiratory Sequence - General Concept

Pressures Involved in Breathing

• Barometric pressure (Pb)

• Intrapleural pressure (Pip)

• Alveolar pressure (Palv)

• Transpulmonary pressure (Ptp)

Pressures Involved in Breathing• Barometeric P

– P exerted by the air we breathe– @ sea level: 760 mmHg– Dalton’s law:

• Pb is equal to sum of partial pressures of individual gases

• Pb = PN+PO2+PH2O+PCO2

– Changes in respiratory pressures during breathing are often expressed as pressure relative to atmospheric P

• When relative pressures are used, Pb = zero

Partial pressures and percentages of Respiratory Gases at Sea Level (PB = 760 mm Hg)

Gas

AmbientDry Air

(mm Hg) (%)

MoistTracheal Air

(mm Hg) (%)

Alveolar Air

(mm Hg) (%)

Systemic Arterial Blood

(mm Hg)

MixedVenous Blood

(mm Hg)

02 160 21 150 20 102 14 95 40

C02 0 0 0 0 40 5 40 46

Water vapor 0 0 47 6 47 6 47 47

N2 600 79 563 74 571 751 571 571

Total 760 100 760 100 760 100 760 7042

Pressures Involved in Breathing

• Intrapleural Pressure• Less than Pb

» Since the 2 elastic recoils are opposite

• Pip in fact is intrathoracic pressure• In upright subject:

– Greatest vacuum (least Pip) – lung apex

– Lowest vacuum (highest Pip) – lung base

• Average (Resting) value = -5 cm water

Pressures Involved in Breathing

• Alveolar pressure (Palv)– Pressure inside the

alveoli– Decreases during

inspiration– Atmospheric air fills in

• Transpulmonary pressure (Ptp)– Distending pressure– Ptp = Palv – Pip

Interaction of Pressures Involved in Breathing

Pneumothorax

Static Vs Dynamic Lung• Re-expanding:

• Cadaver lung• Collapsed lung

– Pneumothorax!

– Initially pressure is required to ‘regain’ original lung & chest wall volume (static component)

– The lung is now expanded– In vivo, over and above ‘static P’, more pressure is

required to overcome inertia and resistance of tissues (airways) & air molecules (dynamic component)

Static Vs Dynamic Lung• Ptp = Palv – Pip

• Pip = (-Ptp) + Palv

• Thus, Pip has 2 aspects*:– Transpulmonary pressure (Ptp) – Static component

– Alveolar pressure (Palv) – Dynamic component

• Compliance (dV/dP) varies• Static compliance

» Change in volume for a given change in Ptp with zero gas flow

• Dynamic compliance» Measurements made by monitoring TD used» While intra thoracic pressure (Pip) measured during the

instance of zero air flow occurring at the end inspiritory and expiratory levels with each breath

Compliance*• Extent to which lungs

will expand for each unit increase in Ptp

• C=dV/dP

• Stages of compliance:– Stage1 [Stable VL):

– Less volume change for pressure change

– Surface tension makes it difficult to open an airway

– Stage2 (airway start opening):

– Stepwise decreases in PIP beyond -8 - produce dV

– dV first small, then larger

Compliance– Stage3

• Linear expansion of open airways

– Stage4• Limit of airway inflation

• Hysteresis• Mostly due to surface

tension• Less due to elastic forces

Compliance Vs Elastance

• Compliance is a measure of distensibility• Elastance is a measure of elastic recoil• These both oppose each other!

– Compliance decreases as Elastance increases:» Pulmonary fibrosis (restrictive lung disease)» Pulmonary hypertension/congestion» Decreased surfactant – increased surface tension

(prematurity, artificial ventilation)– Compliance increases as Elastance decreases

» Normal ageing (alteration in elastic tissue)» Asthma (unknown reason)» Emphysema* (obstructive lung disease)

Compliance - Emphysema

Compliance & Surface Tension• ELASTIC FORCES of

the lungs:– (1) Lung tissue elastic

forces (elastin & collagen

fibers)– (2) Elastic forces caused

by surface tension (Tension created

by fluid-air interface)

• Lung tissue elastic forces (air-filled lung)

I/3of total• Surface tension

forces - 2/3

Surfactant

• Surface active agent• Greatly reduces surface tension of water

• Secreted by Type II alveolar epithelial cells

• Most important components:• Dipalmitoylphosphatidylcholine• Surfactant apoproteins • Calcium ions

• Premature babies lack surfactant

Lung-Thoracic CageCompliance

• Thoracic cage has its own compliance!– Compliance (lung-cage): 110 ml/cm

water– Compliance (lung): 200 ml/cm water

• When the lungs are expanded to high volumes/ compressed to low volumes - checked by chest compliance limitations