Module XIII

Chapter 23

The Respiratory

System:

Physiology

Respiratory System Anatomy

Functionally , the respiratory system is divided into the conducting zone and the respiratory zone.

The conducting zone n ose, pharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles.

The respiratory zone is the main site of gas exchange and consists of the r espiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

Copyright © John Wiley & Sons, Inc. All rights reserved.

Functions of Respiratory System

The respiratory system functions to supply the body with oxygen and dispose off carbon dioxide

Four processes accomplish this:

Pulmonary ventilation – moving air into and out of the lungs

External respiration – gas exchange between the lungs and the blood

Internal respiration – gas exchange between blood and tissues

Transport of oxygen and carbon dioxide between the lungs and tissues- by blood


Pulmonary ventilation

Pulmonary ventilation is the movement of air between the atmosphere and the alveoli

Inspiration – air flows into the lungs

Expiration – air flows out of the lungs


Pressure Relationships in the Thoracic

Cavity

Respiratory pressures are described relative to atmospheric pressure

Atmospheric pressure

Pressure exerted by the air surrounding the body

At sea level the atmospheric pressure is 760mmHg=

1atm


Pressure Relationships in the Thoracic Cavity

Intrapulmonary pressure – pressure within the alveoli

Intrapulmonary rises & falls with the phases of breathing, but always equalizes itself with atmospheric pressure- 760mmHg


Pressure Relationships in the Thoracic Cavity

Intrapleural pressure – pressure within the pleural cavity

Intrapleural pressure is less than intrapulmonary pressure= 756mmHg


Pulmonary Ventilation

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

Volume changes lead to pressure changes, which lead to the flow of gases to equalize pressure

Boyle’s law – the pressure of a gas varies inversely with its volume

The larger the volume the lesser the pressureV ∝ 1/P

Volume = 1 liter

Pressure = 1 atm

Volume = 1/2 liter

Pressure = 2 atm


Pulmonary Ventilation

Muscles of inspiration ( inhalation):

Diaphragm ( primary muscle of inspiration)

External intercostals

Normal expiration is a passive process

Muscles of forced expiration (exhalation):

Internal intercostals

Abdominal muscles


The recruitment of accessory muscles depends on whether the respiratory movements are

quiet (normal), or forced


Inspiration

Inspiratory muscles contract: diaphragm descends, rib cage rises

Thoracic cavity volume increases

Lungs stretched- intrapulmonary volume increases

Intrapulmonary pressure drops by

2mmHg

Air flows into lungs down the pressure gradient, till intrapulmonary pressure equalizes atmospheric pressure


Expiration

Inspiratory muscles relax; diaphragm rises, rib cage descends

Thoracic cavity volume decreases

Elastic lungs recoil passively

Intrapulmonary volume decreases

Intrapulmonary pressure rises by

2mmHg

Air flows out of the lungs, down the pressure gradient, till intrapulmonary pressure equalizes atmospheric pressure


Factors affecting Pulmonary Ventilation

3 factors affect the ease with which we ventilate:

Surface tension of alveolar fluid

Lung compliance

Airway resistance



1.

The surface tension of alveolar fluid causes the alveoli to assume the smallest possible diameter

The alveoli would collapse each expiration o

Surfactant reduces tension- prevents the collapse of alveoli o Clinical connection: Infant respiratory distress syndrome ( IRDS) o

.



2.

Lung compliance means the ease with which lungs and chest wall expand.

Related to two main factors

Elasticity of the lung tissue

Surface tension of the alveoli

Lungs of healthy people have a high compliance

Compliance is decreased in:

Lung fibrosis, IRDS, intercostal muscle paralysis, emphysema



3. Airway resistance

Gas flow is inversely proportional to resistance (friction)mainly determined by diameter of airways

The smaller the diameter the more the resistance

Sympathetic stimulation dilates bronchi & decreases resistance

Airway resistance increases in:

Asthma attacks, chronic bronchitis-when bronchioles are constricted -decreases ventilation


Measuring Ventilation-

Ventilation can be measured using spirometry .

Lung volumes and Capacities can be measured

Old and new spirometers used to measure ventilation

.


Lung Volumes

Tidal Volume (V

T

) is the volume of air inspired (or expired) during normal quiet breathing (500 ml).

Inspiratory Reserve Volume (IRV) is the volume inspired during a very forced inhalation (3100 ml – height and gender dependent).


Lung Volumes

Expiratory Reserve Volume (ERV) is the volume expired during a forced exhalation (1200 ml).

Residual Volume (RV) is the air still present in the lungs after a force exhalation (1200 ml).

o

The RV is a reserve for mixing of gases but is not available to move in or out of the lungs.


Lung Capacities

Inspiratory capacity: Is the total volume of air that can be inspired after a tidal expiration

IC=TV+IRV

Functional residual capacity: Is the volume of air that remains in the lungs at the end of normal tidal expiration

FRC= RV+ ERV

Vital Capacity (VC) : the total amount of exchangeable air

Is all the air that can be exhaled after maximum inspiration.

It is the sum of the inspiratory reserve + tidal volume + expiratory reserve (4800 ml)

Total lung capacityIs the sum of all lung volumes-6000ml


A graph of spirometer volumes and capacities


Forced vital capacity (FVC)– the volume of air forcibly & rapidly expelled after taking a deep breath

Forced expiratory volume (FEV1) – the volume of air expelled during 1sec (healthy person can expel 80% of

FVC in 1sec) in the FVC test

COPD decreases FEV1, because it increases resistance to flow of air


Only about 350 ml of the tidal volume reaches the respiratory zone – the 150ml remains in the conducting zone (called the anatomic dead space ).

If a single V

T breath = 500 ml, only 350 ml will exchange gases at the alveoli. o With a respiratory rate of 12/min, the minute ventilation rate = 12 x 500 = 6000 ml/min.

o

The alveolar ventilation rate (volume of air/min that actually reaches the alveoli) = 12 x 350 = 4200ml/min.


Respiration

Respiration is the exchange of gases .

External respiration (pulmonary) is gas exchange between the alveoli and the blood.

Internal respiration (tissue) is gas exchange between the systemic capillaries and the tissues of the body.


Exchange of O

2

and CO

2

The respiratory system depends on the medium of the earth’s atmosphere to extract the oxygen necessary for life.

The atmosphere is composed of these gases:

Nitrogen (N

2

)

Oxygen (O

2

)

Carbon Dioxide (CO

2

)

Water Vapor

79%

21%

0.04% variable, but on average around 1%


Exchange of O

2

and CO

2

Using gas laws we can understand the principals of respiration

Dalton’s Law states that each gas in a mixture of gases exerts its own pressure- its partial pressure P p.

Total pressure is the sum of all the partial pressures.

The partial pressure of each gas is directly proportional to its percentage in the mixture


Exchange of O

2

and CO

The partial pressures determine the direction of

2 movement of gases

Each gas diffuses across a permeable membrane from high to low partial pressure

There is a higher P

O2 in the alveoli than in the pulmonary capillaries O

2 moves from the alveoli into the blood .

Since there is a higher P

CO2 in the pulmonary capillaries CO

2 moves into the alveoli


Exchange of O

2

and CO

2

Henry’s law deals with gases and solutions:

The quantity of a gas that will dissolve in a liquid is proportional to the partial pressures of the gas and its solubility.

Increasing the partial pressure of a gas in contact with a solution will result in more gas dissolving into the solution

How much it dissolves also depends on solubility

CO

2 is 24 times more soluble in blood (and soda !) than O

2


Clinical connections

Hyperbaric oxygenhigh pressures of O

2 are used to treat anaerobic bacterial infections such as tetanus, gangrene

Decompression sickness (“the bends”)

Air is mostly N

2

, but very little dissolves in blood due to its low solubility

Insoluble N

2 is forced to dissolve into the blood and tissues because of breathing compressed air in scuba diving o

By ascending too rapidly, the N

2 bubbles out of the tissues and blood


Alveolar air is different in composition from

Atmospheric air

The atmosphere is mostly oxygen and nitrogen, while alveoli contain in comparison more carbon dioxide and less oxygen

These differences result from:

Gas exchanges in the lungs

Mixing of alveolar air that remains, with newly inspired air

Atmospheric air: Alveolar air:

P

O2

= 159 mmHg P

O2

= 105 mmHg

P

CO2

= 0.3 mmHg P

CO2

= 40 mmHg


External Respiration (Pulmonary gas exchange)

O

2 diffuses down its steep PO

2 gradient in the alveoli

(105mmHg) to pulmonary capillary blood (40mmHg)

CO

2 diffuses down its gentler PCO

2 gradient from pulmonary capillary blood ( 45mmHg) to alveoli (40mmHg)- exhaled

Blood in the pulmonary veins entering the left atrium has:

PCO

2

40mmHg

PO

2

100mmHg (due to mixing of blood from bronchial veins)


Internal Respiration

As in gas exchange between blood & alveoli, the gas exchange between blood & tissue cells occurs by simple diffusion, driven by partial pressure gradients

Tissue cells constantly use O2 & produce CO2

P

O2 in tissue is 40mmHg- O2 moves into tissues from blood capillaries

P

CO2 is 45 mm Hg in tissues- CO2 moves into blood

P

O2 of venous blood draining tissues is 40 mm Hg and P

CO2 is 45 mm Hg


Atmospheric air:

P

O2

= 159 mmHg

P

CO2

= 0.3 mmHg

CO

2 exhaled

O

2 inhaled

Alveoli

CO

2

O

2

Alveolar air:

P

O2

= 105 mmHg

P

CO2

= 40 mmHg

To lungs

Pulmonary capillaries

(a) External respiration: pulmonary gas exchange

To left atrium

Deoxygenated blood:

P

O2

= 40 mmHg

P

CO2

= 45 mmHg

To right atrium

(b) Internal respiration: systemic gas exchange

Systemic capillaries

To tissue cells

CO

2

O

2

Systemic tissue cells:

P

O2

= 40 mmHg

P

CO2

= 45 mmHg

Oxygenated blood:

P

O2

= 100 mmHg

P

CO2

= 40 mmHg

Factors affecting gas exchange

Factors influencing the movement of oxygen and carbon dioxide across the respiratory membrane

Partial pressure gradients and gas solubilities

Surface area for gas exchange & thickness of the respiratory membrane

Matching of alveolar ventilation (airflow) to alveoli and pulmonary perfusion (blood flow)


Partial pressure gradients and gas solubility

The more the partial pressure differences, the more is the rate of gas diffusion

During exercise greater differences in P

CO2 and P

O2 between alveolar air and pulmonary blood- greater rate of gas diffusion

Decreased alveolar P

O2 at high altitudes – decreases oxygen diffusion

Solubility:

CO2 diffuses out faster compared to O2 diffusing in


Surface area & respiratory membrane

Respiratory membranes are only 0.5 to 1

 m thickallows efficient gas exchange

Thicken in pulmonary edema - gas exchange is inadequate

The greater is the surface area , the more gases can be exchanged- normally huge

Decrease in surface area: o emphysema, when walls of adjacent alveoli break o mucus, tumors block gas flow into alveoli


Ventilation-Perfusion Matching

Ventilation and perfusion must be matched for efficient gas exchange

In the lungs, pulmonary vasoconstriction occurring in response to hypoxia diverts pulmonary blood from poorly ventilated areas of the lungs to well-ventilated regions pulmonary vasodilation in response to increased ventilation


Transport of O

2

In the blood, some O

2 is dissolved in the plasma as a gas

(only about 1.5% )

Most O

2

(about 98.5% ) is carried attached to Hb .

Oxygenated Hb is called oxyhemoglobin (Hb-O2)


Transport of O

2

The amount of Hb saturated with O

2 is called percent saturation of hemoglobin

Each Hb molecule can carry 1 to 4 molecules of O

2

. Blood leaving the lungs has Hb that is almost fully saturatedthe percent saturation is close to 98%

Partially saturated hemoglobin – when 1-3 heme groups are bound to oxygen


Factors affecting saturation of Hb

Most important factor is PO

2

The relationship between the amount of PO

2 in plasma and the saturation of Hb is called the oxygen-hemoglobin dissociation curve .

The higher the P

O2 dissolved in the plasma, the higher the Hb. saturation

• With P

O2

100mmHg in arterial blood saturation is 98%


P

O2 and percent saturation

contd.

In the venous blood at P

O2

40mmHg

percent saturation is 75%

- only 25% has O2 been unloaded to tissues

With P

O2 between 60-100mmHg, Hb is

90% or more saturated with oxygen

So even with P

O2 as low as 65mmHg

Hb saturation is not so low-

(important for those with lung diseases or living at high altitudes


P

O2 and percent saturation contd.

Between 40 and 20mmHg a small decrease in P

O2 causes a large drop in Hb saturation -with release of oxygen

In actively contracting muscles

P

O2 may drop to 20mmHg – saturation 35%- with oxygen release to muscles


Transport of O

2

Measuring hemoglobin saturation is common in clinical practice- done by Pulse oximeters

3660 Group,

Inc/NewsCom


Factors influencing the affinity of Hb binding with

O

2

-Affect percent saturation of Hb


Bohr Effect

Metabolically active tissues produce H +

H + bind to Hb- change its shape- decreasing affinity of Hb for oxygen- enhancing unloading of O

2 to tissues

The pH decrease shifts the O

2

–Hb saturation curve “to the right”

This is called the Bohr effect


Transport of CO

2

CO

2 is transported in the blood in three different forms:

1.

7% is dissolved in the plasma, as a gas .

2.

70% is transported as bicarbonate ions (HCO

3

– ) through the action of an enzyme called carbonic

3.

anhydrase.

o

CO

2

+ H

2

O H

2

CO

3

H

+

+ HCO

3

-

23% is attached to Hb (to the amino acids) as c arbaminohemoglobin( HbCO2)


Transport of CO

2

At the level of tissues: Carbon dioxide diffuses into RBCs, combines with water to form H

2

CO

3

, (catalyzed by carbonic anhydrase ), which quickly dissociates into hydrogen ions and bicarbonate ions

Cl–)

Bicarbonate diffuses from RBCs into the plasma

The chloride shift – to balance the outrush of negative bicarbonate ions from the RBCs, chloride ions (Cl–) move into the erythrocytes


Transport of CO

2

At the lungs , these processes are reversed

Cl–)


Control of Respiration- Respiratory Center

The medullary rhythmicity area, has centers that control basic respiratory rythm

The inspiratory center stimulates the diaphragm via the phrenic nerve, and the external intercostal muscles via intercostal nerves.

Inspiration normally lasts about 2s .


Control of Respiration-Respiratory Center

Expiration is a passive process- nerve impulses cease for about

3 sec, causing relaxation of inspiratory myscles

The expiratory center is inactive during quiet breathing

During forced exhalation, however, impulses from this center stimulate the internal intercostal and abdominal muscles


Control of Respiration

Other sites in the pons help the medullary centers

The pneumotaxic center limits inspiration to prevent hyperexpansion of lungs

The apneustic center prolongs inhalation


Chemoreceptor Regulation of Respiration:

Central chemoreceptors in medulla only sensitive to P

CO2

Peripheral chemoreceptors sensitive to P

CO2

, P

O2

, arterial pH

P

CO2 levels rise (hypercapnia ) stimulate both the central & peripheral chemoreceptors

Respiratory center stimulated

Hyperventilation – increased rate and depth of breathing occurs in response to hypercapnia- CO2 flushed out


Chemoreceptors

Internal carotid artery

Carotid body

Peripheral Chemoreceptors

Aortic bodies

Heart

Medulla oblongata

Central chemoreceptors glossopharyngeal nerve

(cranial nerve IX)

Carotid sinus vagus nerve

(cranial nerve X)

Arch of aorta

Chemoreceptor Regulation of Respiration

Fall in pH:

Acidosis may occur due to:

Carbon dioxide retention, other metabolic conditions e.g. accumulation of lactic acid

Increased ventilation in response to falling pH is mediated by peripheral chemoreceptors



Arterial P

O2 levels are monitored by the aortic and carotid body peripheral chemoreceptors

Substantial drops in arterial P

O2

(to 60 mm Hg) are needed before oxygen levels become a major stimulus to increase ventilation ( hypoxic drive)




Control of Respiration

Other brain areas also play a role in respiration:

The cerebral cortex has influence over breathing.

Stretch receptors in lungs sense overinflationinhibitory signals are sent to the medullary inspiration center to end inhalation and allow expiration (Herring Breuer reflex)

Emotions (limbic system) affect respiration.


Diseases

Asthma is a disease of hyper-reactive airways (the major abnormality is constriction of smooth muscle in the bronchioles

It presents as attacks of wheezing, coughing, and excess mucus production.

It typically occurs in response to allergens

Bronchodilators and antiinflammatory corticosteroids are mainstays of treatment.

Pulse Picture Library/CMP mages /Phototake


Diseases

Chronic Obstructive Pulmonary Diseases

They are diseases caused by cigarette smoking

Chronic bronchitis is caused by chronic irritation and inflammation

Patients have cough with sputum

Emphysema : destruction of elastic tissue with enlargement of air spaces


Module XIII

Chapter 23

The Respiratory

System:

Physiology

Respiratory System Anatomy

Functions of Respiratory System

Pulmonary ventilation

Pressure Relationships in the Thoracic

Cavity

Pulmonary Ventilation

Pulmonary Ventilation

Inspiration

Expiration

Factors affecting Pulmonary Ventilation

Factors affecting Pulmonary Ventilation

Factors affecting Pulmonary Ventilation

Factors affecting Pulmonary Ventilation

Lung Volumes

Lung Volumes

Lung Capacities

Respiration

Exchange of O

and CO

Exchange of O

and CO

Exchange of O

and CO

Exchange of O

and CO

Clinical connections

Internal Respiration

Factors affecting gas exchange

Surface area & respiratory membrane

Ventilation-Perfusion Matching

Transport of O

Transport of O

Factors affecting saturation of Hb

contd.

Transport of O

Bohr Effect

Transport of CO

Transport of CO

Transport of CO

Control of Respiration- Respiratory Center

Control of Respiration-Respiratory Center

Control of Respiration

Chemoreceptor Regulation of Respiration:

Chemoreceptor Regulation of Respiration

Chemoreceptor Regulation of Respiration

Control of Respiration

Diseases

Diseases

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