Overview of Respiration andOverview of Respiration andRespiratory MechanicsRespiratory Mechanics

Dr Shihab Khogali

Ninewells Hospital & Medical School, University of Dundee

This lecture is the first of four-linked lectures …in this lecture:

Understand what is meant by the terms “internal respiration” and “external respiration”

Know the four steps of external respiration

Understand Ventilation - the first step of external respiration

What is This LectureAbout?

See blackboard for detailed learning objectives

Know that gases move from higher to lower pressure, with the Boyle’s Law.

Understand the respiratory mechanics and the relationship between atmospheric, intra-alveolar, and intrapleural pressures.

understand the significance of transmural pressure gradient. Know that peumothorax abolishes the transmural pressure gradient.

Understand that inspiration is an active process and that normal resting expiration is a passive process.

Know the inspiratory muscles and the accessory muscles of respiration (link with anatomy).

Describe the role and importance of pulmonary surfactant, with the Law of Laplace and alveolar stability.

Know the lung volumes and capacities. Understand the changes in dynamic lung volumes in obstructive and restrictive lung disease.

Know the factors which influence airway resistance.

Define the compliance of lungs and thorax.

Understand what is meant by the term work of breathing.

Understand ventilation (Step 1 of external respiration).

Our body systems are made of cells

These cells need a constant supply of oxygen (O2) to produce energy and function

The carbon dioxide (CO2) produced by the cellular reactions must continuously be removed from our bodies

The internal respiration refers to the intracellular mechanisms which consumes O2 and produces CO2

‘food’ + O2

‘energy’ + CO2

Internal Respiration

The term external respiration refers to the sequence of events that lead to the exchange of O2 and CO2 between the external environment and the cells of the body

External respiration is the topic for our four-linked physiology lectures

External respiration involves four steps

Atmosphere

Tissue cell

Alveoli of lungs

Pulmonarycirculation

Systemiccirculation

CO2O2

Food + O2 CO2 + HO2 + HTP

O2

CO2

CO2

O2

External Respiration

Atmosphere

Tissue cell

Alveoli of lungs

Pulmonarycirculation

Systemiccirculation

CO2O2

Food + O2 CO2 + HO2 + ATP

O2

CO2

CO2

O2

1

Steps of external respiration

Ventilation or gas exchange betweenthe atmosphere and air sacs (alveoli)in the lungs

Exchange of O2 and CO2 between air

in the alveoli and the blood

Transport of O2 and CO2 between the

lungs and the tissues

Exchange of O2 and CO2 between the

blood and the tissuesInternal respiration

2

3

4

Fig. 13-1, p. 452

The Four Steps of External Respiration

VentilationThe mechanical process of moving gas in and out of the lungs

Gas exchange between alveoli and bloodThe exchange of O2 and CO2 between the air in the alveoli and

the blood in the pulmonary capillaries

Gas transport in the bloodThe binding and transport of of O2 and CO2 in the circulating

blood

Gas exchange at the tissue levelThe exchange of O2 and CO2 between the blood in the systemic

capillaries and the body cells

The Respiratory System

The Cardiovascular System

The Haematology System

Atmosphere

Tissue cell

Alveoli of lungs

Pulmonarycirculation

Systemiccirculation

CO2O2

Food + O2 CO2 + HO2 + HTP

O2

CO2

CO2

O2

Three body systems are involved in external respiration

The mechanical process of moving air between the atmosphere and alveolar sacs

Ventilation

Air flow down pressure gradient from a region of high pressure to a region of low pressure

The intra-alveolar pressure must become less than atmospheric pressure for air to flow into the lungs during inspiration. How is this achieved?

Before inspiration the intra-alveolar pressure is equivalent to atmospheric pressure

During inspiration the thorax and lungs expand as a result of contraction of inspiratory muscles

But: How the movement of the chest wall expand the lungs as there is no physical connection between the lungs and chest wall?

as the volume of a gas increases the pressure

exerted by the gas decreases

Boyle’s Law

At any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas

Ventilation

Linkage of Lungs to Thorax

Two forces hold the thoracic wall and the lungs in close opposition:

(1) The intrapleural fluid cohesiveness: The water molecules in the intrapleural fluid are attracted to each other and resist being pulled apart. Hence the pleural membranes tend to stick together.

(2) The negative intrapleural pressure: the sub-atmospheric intrapleural pressure create a transmural pressure gradient across the lung wall and across the chest wall. So the lungs are forced to expand outwards while the chest is forced to squeeze inwards.

Three Pressures are Important in Ventilation

Inspiration is an active process depending on muscle contraction

The volume of the thorax is increased vertically by contraction of the diaphragm (major inspiratory muscle), flattening out its dome shape.

Phrenic nerve from cervical 3,4 and 5

The external intercostal muscle contraction lifts the ribs and moves out the sternum.

The “bucket handle” mechanism.

Inspiration is an active process brought about by contraction of inspiratory muscles

The chest wall and lungs stretched

The Increase in the size of the lungs make the intra-alveolar pressure to fall

This is because air molecules become contained in a larger volume (Boyle’s Law)

The air then enters the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressure

Inspiration

759

Size of thorax oncontraction ofinspiratory muscles

Size of lungs as theyare stretched to fillthe expanded thorax

754

760

Normal expiration is a passive process brought about by relaxation of inspiratory muscles

The chest wall and stretched lungs recoil to their preinspiratory size because of their elastic properties

The recoil of the lungs make the intra-alveolar pressure to rise

This is because air molecules become contained in a smaller volume (Boyle’s Law)

The air then leaves the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressure

761

Size of thorax onrelaxation ofinspiratory muscles

Size of lungs asthey recoil

756

760

Expiration

Inspiration Expiration

Atmosphericpressure

Intra-alveolarpressure

Intrapleuralpressure

Transmural pressuregradient across thelung wall

Changes in intra-alveolar and intra-pleural pressures during the respiratory cycle

Pneumothorax (air in the pleural space) abolishes the transmural pressure gradient

What causes the lungs to recoil during expiration?(i.e. what gives the lungs their elastic behaviour)

Elastic connective tissue in the lungsThe whole structure bounces back into shape

But even more important is the alveolar surface tension

What is alveolar surface tension? Attraction between water molecules at liquid air interface

In the alveoli this produces a force which resists the stretching of the lungs

If the alveoli were lined with water alone the surface tension would be too strong so the alveoli would collapse

According to the law of LaPlace: the smaller alveoli (with smaller radius - r) have a higher tendency to collapse

Pulmonary surfactant is a complex mixture of lipids and proteins secreted by type II alveoli

It lowers alveolar surface tension by interspersing between the water molecules lining the alveoli

Surfactant lowers the surface tension of smaller alveoli more than that of large alveoli

This prevent the smaller alveoli from collapsing and emptying their air contents into the larger alveoli

Surfactant Reduces the Alveolar Surface Tension

If we regard the alveoli as spherical bubles, then:

r

TP

2=P = inward directed collapsing pressureT = Surface Tensionr = radius of the buble

(LaPlace’s Law)

Surfactant prevent thiS happening

Respiratory Distress Syndrome of the New Born

Developing fetal lungs are unable to synthesize surfactant until late in pregnancy

Premature babies may not have enough pulmonary surfactant

This causes respiratory distress syndrome of the new born

The baby makes very strenuous inspiratory efforts in an attempt to overcome the high surface tension and inflate the lungs.

Another factor which helps keep the alveoli open is: The Alveolar Interdependence

If an alveolus start to collapse the surrounding alveoli are stretched and then recoil exerting expanding forces in the collapsing alveolus to open it

Fig. 13-11, p. 459

See Practical Class and Online Tutorial

Lung Volumes and Capacities

Predicted normal values vary with age, height, gender,..

Lung Volumes and CapacitiesDescription Average

Value

Tidal volume (TV)

Volume of air entering or leaving lungs during a single breath

500 ml

Inspiratory reserve volume (IRV)

Extra volume of air that can be maximally inspired over and above the typical resting tidal volume

3000 ml

Inspiratory capacity (IC)

Maximum volume of air that can be inspired at the end of a normal quiet expiration (IC =IRV + TV)

3500 ml

Expiratory reserve volume (ERV)

Extra volume of air that can be actively expired by maximal contraction beyond the normal volume of air after a resting tidal volume

1000 ml

Residual volume (RV)

Minimum volume of air remaining in the lungs even after a maximal expiration

1200 ml

Lung Volumes and CapacitiesDescription Average

Value

Functional residual capacity (FRC)

Volume of air in lungs at end of normal passive expiration (FRC = ERV + RV)

2200 ml

Vital capacity (VC) Maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC = IRV + TV + ERV)

4500 ml

Total lung capacity (TLC)

Maximum volume of air that the lungs can hold (TLC = VC + RV)

5700 ml

Forced expiratory volume in one second (FEV1): Dynamic volume

Volume of air that can be expired during the first second of expiration in an FVC (Forced Vital Capacity) determination

FEV1% = FEV1/FVC ratio

Normal >75%

Volume time curve - allow you to determine:

FVC = Forced Vital Capacity (maximum volume that can be forcibly Expelled from the lungs following a maximum inspiration)

FEV1 = Forced Expiratory volume in one second

FEV1% = FEV1/FVC ratio

Spirometry for Dynamic Lung Volumes

Normal

Obstructive Lung Disease

Airway Resistance

Resistance to flow in the airway normally is very low and therefore air moves with a small pressure gradient

Primary determinant of airway resistance is the radius of the conducting airway

Parasympathetic stimulation causes bronchoconstriction Sympathetic stimulation causes bronchodilatation Disease states (e.g. COPD or asthma) can cause

significant resistance to airflow Expiration is more difficult than inspiration

RPF ∆=

F: Flow P: Pressure R: Resistance

Dynamic Airway Compression

If there is an obstruction (e.g. COPD), the driving pressure between the alveolus and airway is lost over the obstructed segment. This causes a fall in airway pressure along the airways resulting in airway compression by the transairway pressure during active expiration.

During inspiration the airways are pulled open by the expanding thorax. Therefore in cases of increased airway resistance expiration tends to be more difficult.

The transairway pressure tends to compress airways during active expiration -pleural pressure rises during expiration (increases airway resistance)

If no obstruction: the increased airway resistance causes an increase in airway pressure upstream. This helps open the airways (i.e. reduce thecompressive transairway pressure)

Transairway Pressure = Airway Pressure – Pleural pressure

Gives an estimate of peak flow rate

The peak flow rate assess airway function

The test is useful in patients with obstructive lung disease (e.g. asthma and COPD)

It is measured by the patient giving a short sharp below into the peak flow meter

The average of three attempts is usually taken

The peak flow rate in normal adults vary with age and height

You will practice taking the peak flow rate in the Clinical Skills Centre

Peak Flow Meter

Compliance

During inspiration the lungs are stretched– Compliance is measure of effort that has to go

into stretching or distending the lungs

– Volume change per unit of pressure change across the lungs

– The less compliant the lungs are, the more work is required to produce a given degree of inflation

– Decreased by factors such as pulmonary fibrosis

Work of Breathing

Normally requires 3% of total energy expenditure for quiet breathing

Lungs normally operate at about “half full”

Work of breathing is increased in the following situations– When pulmonary compliance is decreased– When airway resistance is increased– When elastic recoil is decreased– When there is a need for increased ventilation