Respiratory Physiology

Main sections:

Pulmonary ventilation

Diffusion of gases

Transport of gases

Control of respiration

The aim is to understand the function and control of the respiratory system and the application of this knowledge. The changes in abnormal physiological and common pathological conditions affecting respiration.Textbooks:1- Guyton and Hall Textbook of Medical Physiology2- Ganong Review of Medical Physiology

Sections One - VentilationLecture 1:

General definitions

Functional anatomy

Muscles of respiration

Objectives:

i. To describe the anatomy of the respiratory system and its relation to function.

ii. To explain the structure of the chest wall and diaphragm and to relate these to respiratory mechanics.

iii. To describe the inspiratory and expiratory process involving the chest wall, diaphragm, pleura and lung parenchyma.

Respiration

Ventilation: Movement of air into and out of lungs

External respiration: Gas exchange between air in lungs and blood

Transport of oxygen and carbon dioxide in the blood

Internal respiration: Gas exchange between the blood and tissues

Respiratory System Functions

Gas exchange: Oxygen enters blood and carbon dioxide leaves

Regulation of blood pH: Altered by changing blood carbon dioxide levels

Voice production: Movement of air past vocal folds makes sound and speech

Olfaction: Smell occurs when airborne molecules drawn into nasal cavity

Protection: Against microorganisms by preventing entry and removing them

Respiratory System Divisions

Upper tract

Nose, pharynx and associated structures

Lower tract

Larynx, trachea, bronchi, lungs

Nasal Cavity and Pharynx

Nose and Pharynx

Nose

External nose

Nasal cavity

Functions

Passageway for air

Cleans the air

Humidifies, warms air

Smell

Along with paranasalsinuses are resonating chambers for speech

Pharynx

Common opening for digestive and respiratory systems

Three regions Nasopharynx

Oropharynx

Laryngopharynx

Larynx

Functions Maintain an open passageway for air movement

Epiglottis and vestibular folds prevent swallowed material from moving into larynx

Vocal folds are primary source of sound production

Vocal Folds

Trachea

Windpipe

Divides to form

Primary bronchi

Carina: Cough reflex

Tracheobronchial Tree

Conducting zone

Trachea to terminal bronchioles which is ciliated for removal of debris

Passageway for air movement

Cartilage holds tube system open and smooth muscle controls tube diameter

Respiratory zone

Respiratory bronchioles to alveoli

Site for gas exchange

Tracheobronchial Tree

The gas exchange airway is the functional unit of the lung.

It consists of the respiratory bronchioles, alveolar ducts,

and alveolar sacs, which collectively comprise the terminal

respiratory unit (TRU).

It is the site of gas exchange with the pulmonary

capillary blood.

Alveoli line the walls of both the respiratory bronchioles and

alveolar ducts, both of which are perfused with pulmonary

capillary blood. Some smooth muscle also is present in

respiratory bronchioles and alveolar ducts. This muscle can be

stimulated by vasoactive substances in the pulmonary blood.

Bronchioles and Alveoli

Alveolus and Respiratory Membrane

The normal adult human lung weighs about 1000g and consists of about 50% blood and 50% tissue by weight. About 10% of the total lung volume is composed of various types of conducting airways and some connective tissue. The remaining 90% is the respiratory or gas exchange portion of the lung, composed of alveoli and supporting capillaries.

Thoracic WallsMuscles of Respiration

Mechanics of VentilationAir is delivered to alveoli as a consequence of respiratory

muscle contraction. These muscles include the diaphragm and

the external intercostal muscles of the rib cage and

accessory inspiratory muscles (scalenes and

sternocleidomastoids which are not active in

eupnea). Contraction of these muscles enlarges the thoracic

cavity, creating a subatmospheric pressure in the alveoli.

Contraction of the diaphragm leads to downwards displacement

of the thoracic cavity and contraction of external intercostals

muscles leads to lifting of the thoracic cage leading to increase

in the antero-posterior diameter.

As alveolar pressure declines, atmospheric air moves

into the alveoli by bulkflow until the pressure is

equalized. The process of inflating the lung is called

inspiration. Expiration is usually passive, resulting from

relaxation of the inspiratory muscles and powered by

elastic recoil of lung tissue that is stretched during

inspiration. With relaxation of the inspiratory muscles and

lung deflation, alveolar pressure exceeds atmospheric

pressure, so gases flow from the alveoli to the

atmosphere by bulk flow. Active expiration is due to

internal intercostals muscles and the abdominal

recti.

Thoracic Volume

Lecture 2: Pleura and its relation to thoracic pressure. Intrapleural, alveolar and transpulmonary pressure. Opposing forces and compliance.

Objectives:i. To describe the elastic properties of the chest wall

and to plot pressure-volume relationships of the lung, chest wall and the total respiratory system.

ii. To define compliance and relate this to the elastic properties of the lung.

iii. To describe the properties of surfactant and relate these to its role in influencing respiratory mechanics.

Pleura

Pleural fluid produced by pleural membranes Acts as lubricant

Helps hold parietal and visceral pleural membranes together

Ventilation

Movement of air into and out of lungs

Air moves from area of higher pressure to area of lower pressure

Pressure is inversely related to volume

Alveolar Pressure Changes

Changing Alveolar Volume

Lung recoil

Causes alveoli to collapse resulting from

Elastic recoil and surface tension Surfactant: Reduces tendency of lungs to collapse

Pleural pressure

Negative pressure can cause alveoli to expand

Pneumothorax is an opening between pleural cavity and air that causes a loss of pleural pressure

Normal Breathing Cycle

Pressures:

*Atmospheric pressure: Is the pressure of the air surrounding the

body or at the nose and mouth, it is 760mmHg at sea level {consider

as zero (0mmHg)}.

*Alveolar pressure: Is the pressure inside the lung alveoli. It is

(0mmHg) which mean it is the same as atmospheric pressure

between breaths (i.e. at the end of normal expiration). During

ventilation, for the air to enter inside the lungs (inspiration) it must

fall to a value (-1cmH2O) below atmospheric pressure and must rise

to (+1cmH2O) for the air to move out of the lung (expiration).

*Intrapleural pressure: Is the hydrostatic pressure of the

intrapleural fluid. It is normally a slight suction which means a

slightly negative pressure. Normally it is -5 cmH2O at the beginning

of inspiration. It is needed to hold the lungs open to its resting level.

Increased negatively from -5 to -7.5 cmH2O during inspiration.

Transpulmonarypressure [pressure difference between the alveolar pressure and the pleural pressure]. It is the measure of the elastic forces that leads to collapse of the lung and it is called the recoil pressure.

The Opposing Force of Pulmonary Elastance or Compliance

The lung is an elastic structure with an anatomical organization that promotes its collapse to essentially zero volume, much like an inflated balloon.

The term elastic means a material deformed by a force tends to return to its initial shape or configuration when the force is removed. It oppose lung inflation. Elastance.

Compliance (distensibility) is the reciprocal of elastance, is a measure of the ease of deformation (inflation).

Compliance

Measure of the ease with which lungs and thorax expand

The greater the compliance, the easier it is for a change in pressure to cause expansion

A lower-than-normal compliance means the lungs and thorax are harder to expand

Conditions that decrease compliance Pulmonary fibrosis

Pulmonary edema

Respiratory distress syndrome

Lung compliance: Which equals to change in volume divided by change in pressure (1 cm = 200 ml). That is, every time the transpulmonary pressure increases 1 centimeter of water, the lung volume, after 10 to 20 seconds, will expand 200 milliliters.

1/3 to overcome pleural pressure

2/3 to overcome surface tension

Effect of thoracic cage: Compliance of both lung + cage = 110 ml (instead of 200ml/cm)

Surfactant: The surface active agent in water and it consists of lipids, protein and ions.

Fetal lung surfactant also is not fully functional until about the seventh month of gestation. Respiratory Distress Syndrome (RDS) is related to non-functional alveolar surfactant.

RDS in infants is manifested by a low lung compliance and

severe hypoxemia. Surfactant deficiency or inactivation can

also occur in adults who breathe 100% O2 for prolonged

periods, or who have prolonged occlusion of the pulmonary

artery, such as associated with heart-lung bypass

procedures.

Lecture 3: Work of breathing. Lung volumes and capacities. Ventilation volume and dead space.

Objectives:i. To describe the work of breathing and its

components.ii. To explain the measurement of lung volumes and

capacities, and to indicate the normal values .iii. To define dead space and describe its effects on

pulmonary ventilation.

Work of breathing:

Compliance work: against elastic forces of lung + cage

Tissue resistance work: against viscosity of both lung and cage

Airway resistance work

During quite breathing, 3-5% of total energy of the body are spent for respiration, while in heavy exercise, it increases up to 50 folds

Lung Volumes and Capacities

Lung volumes measured by spirometry are basically

anatomical measurements of lung gas volumes. A lung

volume refers to a basic volume of the lung, whereas lung

capacities, also a volume measurement, are the sum of two

or more basic lung volumes. The following lung volumes

can be measured directly or indirectly with a spirometer:

Pulmonary Volumes

Tidal volume Volume of air inspired or expired during a normal inspiration or

expiration

Inspiratory reserve volume Amount of air inspired forcefully after inspiration of normal tidal

volume

Expiratory reserve volume Amount of air forcefully expired after expiration of normal tidal

volume

Residual volume Volume of air remaining in respiratory passages and lungs after

the most forceful expiration

Pulmonary Capacities

Inspiratory capacity Tidal volume plus inspiratory reserve volume

Functional residual capacity Expiratory reserve volume plus the residual volume

Vital capacity Sum of inspiratory reserve volume, tidal volume, and expiratory

reserve volume

Total lung capacity Sum of inspiratory and expiratory reserve volumes plus the tidal

volume and residual volume

Volumes Capacities

1- Tidal vol. (500ml) 1- Inspiratory cap.

(3500ml)

2- Inspiratory reserve vol.

(3000ml)

2- Functional residual

cap. (2300ml)

3- Expiratory reserve vol.

(1100ml)

3- Vital cap. (4600ml)

4- Residual vol. (1200ml) 4- Total lung cap.

(5800ml)

Spirometer and Lung Volumes/Capacities

Minute and Alveolar Ventilation

Minute ventilation: Total amount of air moved into and out of respiratory system per minute

Respiratory rate or frequency: Number of breaths taken per minute

Anatomic dead space: Part of respiratory system where gas exchange does not take place

Alveolar ventilation: How much air per minute enters the parts of the respiratory system in which gas exchange takes place

Respiratory dead space Is the space where no gas exchange occurs. It is either

anatomically (150 ml) (anatomical dead space (nose, pharynx, larynx, trachea, bronchi, bronchioles); or physiological dead space whereby some alveoli are not functional because of absent or partial blood supply (normally it should be zero).

So the total dead space is the sum of anatomical and physiological dead spaces and so equals to 150 ml. So the alveolar ventilation per minute equals to pulmonary ventilation per minute minus dead space and equals to 500-150 = 350 ml/min X 12 = 4200 ml/min.

Lecture 4:

Nerve supply.

Blood supply.

Lung zones.

Perfusion across the alveolar wall.

Pulmonary edema.Objectives:

i. To define the cough and sneezing reflexes.

ii. To explain normal ventilation-perfusion matching, including the mechanisms for these as well as the normal values.

iii. To relate the vertical lung distance effect on blood capillary pressure changes.

iv. To explain the diffusion of fluids across the alveolar membrane.

v. To define pulmonary edema , enumerate its causes and safety factor.

Nerve stimulation (sympathetic, i.e., adrenalin dilatation; parasympathetic, i.e., Ach. constriction).

Cough reflex: afferent vagus nerve medulla autonomic inspiration of 2.5 liters closure of epiglottis and vocal cords contraction of abdominal muscles sudden opening expel air at a velocity of 400 miles per hour + narrowing of trachea and bronchi.

Sneeze reflex: Similar except to nasal passages instead of lower airways. Afferent is fifth cranial medulla similar but depression of uvula so that large amounts of air pass through the nose.

Pulmonary circulation:

Blood supply to the lungs goes to bronchi (nutrition) and respiratory units (gaseous exchange).

Has higher blood flow than systemic circulation.

When O2 concentration drops to 70% (73mmHg),

pulmonary blood vessels constricts (opposite to

other capillaries) and this is important to shift the

blood to more aerated areas.

Right atrial pressure is 25 mmHg systolic and 0 mmHg diastolic.

Pulmonary artery pressure is 25mmHg systolic and 8mmHg diastolic (mean arterial pressure equals 15 mmHg)

Lung Zones

In the normal, upright adult, the lowest point in the lungs is about 30 centimeters below the highest point. This represents a 23 mm Hg pressure difference, about 15 mm Hg of which is above the heart and 8 below.

For the whole lung, an ideal ventilation to perfusion ratio is between 0.8 to 1.0

Zone 1: No blood flow during all portions of the cardiac cycle

Zone 2: Intermittent blood flow only during the pulmonary arterial

pressure peaks because the systolic pressure is then greater than the

alveolar air pressure, but the diastolic pressure is less than the alveolar

air pressure.

Zone 3: Continuous blood flow because the alveolar capillary pressure

remains greater than alveolar air pressure during the entire cardiac

cycle.

-Normally, the lungs have only zones 2 and 3 blood flowzone 2

(intermittent flow) in the apices, and zone 3 (continuous flow) in all the

lower areas, when a person is in the upright position.

- when a person is lying down, blood flow in a normal person is entirely

zone 3 blood flow, including the lung apices.

- Zone 1 Blood Flow Occurs Only Under Abnormal

Conditions:

(1) if an upright person is breathing against a positive air

pressure so that the intra-alveolar air pressure is at least

10 mm Hg greater than normal but the pulmonary systolic

blood pressure is normal.

(2) upright person whose pulmonary systolic arterial

pressure is exceedingly low, as might occur after severe

blood loss.

During Exercise: the pulmonary vascular pressures

rise enough to convert the lung apices from a zone 2

pattern into a zone 3 pattern of flow.

Ventilation-Perfusion Matching

The adult lung has about 300 million alveoli. Each alveolus is

surrounded by a capillary mesh. Gas exchange is optimal in

alveoli where ventilation is closely matched to blood flow or

perfusion. More specifically, gas exchange is optimal in

alveoli where the fraction of alveolar ventilation ( A) is

matched to the fraction of cardiac output ( ) perfusing that

alveolus. For the whole lung, an ideal ventilation to perfusion

ratio (A / ) is between 0.8 to 1.0. However, with 300

million alveoli, it is unlikely that ventilation will be precisely

matched to perfusion in each alveolus.

Perfusion across capillaries Capillary pressure equals 7mmHg

(while it is 17 in general circulation). Plasma colloid equals 28 mmHg. Interstitial colloid 14 mmHg (7 in

general circulation) -ve interstitial pressure equals 8

mmHg Total = 29, so 29-28 = 1mmHg which

removed by lymphatics and evaporation

Pulmonary edema:Pulmonary edema is the accumulation of fluid in interstitial spaces of the lung or

in the air spaces (alveoli).

Any factor that causes the pulmonary interstitial fluid pressure to rise from the

negative range into the positive range will cause sudden filling of the pulmonary interstitial spaces and alveoli with large amounts of free fluid (pulmonary edema).

The most common causes of pulmonary edema are:

1- Left-sided heart failure or mitral valve diseases with consequent increase in

venous pressure and engorgement of pulmonary capillaries. The resultant rises

in the capillary pressure above its normally very low value, lead to increased

filtration of fluid out of the capillaries into interstitial fluid.

2- Damage to pulmonary capillary membrane caused by infections such as

pneumonia or by breathing noxious substance such as chlorine gas or SO2 gas

(sulfur dioxide). These gases cause rapid leakage of the plasma proteins and

fluid out of the capillaries.

Pulmonary edema safety factor:

Three factors must be overcome to induce pulmonary edema:

1- The normal negativity of interstitial fluid pressure.

2- Lymphatic pumping of fluid out of the interstitial space.

3- The increased osmosis of fluid into the pulmonary capillaries

caused by decreased protein in the interstitial fluid when lymph

flow increases.