Chapter 23 The Respiratory System: Physiology. Copyright © John Wiley & Sons, Inc. All rights reserved. Respiratory System Anatomy Functionally, the respiratory

Chapter 23

The Respiratory

System:

Physiology

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

Respiratory System Anatomy

Functionally, the respiratory system is

divided into the conducting zone and the

respiratory zone.

The conducting zone - nose, pharynx,

larynx, trachea, bronchi, bronchioles

and terminal bronchioles.

The respiratory zone is the main site of

gas exchange and consists of the

respiratory bronchioles, alveolar

ducts, alveolar sacs, and alveoli.


Functions of Respiratory System

The respiratory system functions to supply the body with oxygen and dispose off carbon dioxideFour processes accomplish this:

Pulmonary ventilation – moving air into and out of the lungsExternal respiration – gas exchange between the lungs and the bloodInternal respiration – gas exchange between blood and tissuesTransport 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



Intrapulmonary pressure– pressure within the

alveoli

Intrapulmonary rises & falls with the phases of

breathing, but always equalizes itself with

atmospheric pressure- 760mmHg



Intrapleural pressure– pressure within the pleural cavityIntrapleural pressure is less than intrapulmonary pressure= 756mmHg


Pulmonary VentilationA 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 pressure- V ∝ 1/P

Volume = 1 literPressure = 1 atm

Volume = 1/2 literPressure = 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


InspirationInspiratory 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


ExpirationInspiratory 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 alveolio 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 airwaysThe smaller the diameter the more the resistanceSympathetic stimulation dilates bronchi & decreases resistanceAirway 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 (VT) 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 capacity- Is 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 VT 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 O2 and CO2

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 (N2) 79%

Oxygen (O2) 21%

Carbon Dioxide (CO2) 0.04%

Water Vapor variable, but on

average

around 1%



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 Pp.

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



The partial pressures determine

the direction of movement of

gases

Each gas diffuses across a

permeable membrane from high to

low partial pressure

There is a higher PO2 in the alveoli

than in the pulmonary capillaries O2

moves from the alveoli into the

blood.

Since there is a higher PCO2 in the

pulmonary capillaries CO2 moves

into the alveoli



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

CO2 is 24 times more soluble in

blood (and soda !) than O2


Clinical connections

Hyperbaric oxygen- high pressures of O2 are used

to treat anaerobic bacterial infections such as

tetanus, gangrene

Decompression sickness (“the bends”)

Air is mostly N2, but very little dissolves in blood

due to its low solubility

Insoluble N2 is forced to dissolve into the blood

and tissues because of breathing compressed air in

scuba diving

o By ascending too rapidly, the N2 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:

PO2 = 159 mmHg PO2 = 105 mmHg

PCO2 = 0.3 mmHg PCO2 = 40 mmHg


External Respiration (Pulmonary gas exchange)

O2 diffuses down its steep PO2 gradient in the

alveoli (105mmHg) to pulmonary capillary blood

(40mmHg)

CO2 diffuses down its gentler PCO2 gradient from

pulmonary capillary blood ( 45mmHg) to alveoli

(40mmHg)- exhaled

Blood in the pulmonary veins entering the left

atrium has:

PCO2 40mmHg

PO2 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

PO2 in tissue is 40mmHg- O2 moves into tissues

from blood capillaries

PCO2 is 45 mm Hg in tissues- CO2 moves into

blood

PO2 of venous blood draining tissues is 40 mm

Hg and PCO2 is 45 mm Hg

CO2 exhaledO2 inhaled

Atmospheric air:PO2 = 159 mmHgPCO2 = 0.3 mmHg

Alveolar air:PO2 = 105 mmHgPCO2 = 40 mmHg

Oxygenated blood:PO2 = 100 mmHgPCO2 = 40 mmHg

Deoxygenated blood:PO2 = 40 mmHgPCO2 = 45 mmHg

Systemic tissue cells:

PO2 = 40 mmHgPCO2 = 45 mmHg

Pulmonary capillariesPulmonary capillaries

(a) External respiration:

pulmonary gasexchange

(b) Internal respiration:

systemic gasexchangeSystemic capillariesSystemic capillaries

To lungs

To right atrium

To left atrium

To tissue cells

AlveoliCO2CO2 O2

O2

CO2 O2


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 PCO2

and PO2 between alveolar air and pulmonary

blood- greater rate of gas diffusion

Decreased alveolar PO2 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

thick- allows 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 O2

In the blood, some O2 is dissolved in the

plasma as a gas (only about 1.5%)

Most O2 (about 98.5%) is carried attached to

Hb.

Oxygenated Hb is called oxyhemoglobin

(Hb-O2)


Transport of O2

The amount of Hb saturated with O2 is called

percent saturation of hemoglobin

Each Hb molecule can carry 1 to 4 molecules of

O2. Blood leaving the lungs has Hb that is almost

fully saturated-

the 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 PO2

The relationship between the amount of PO2 in

plasma and the saturation of Hb is called the

oxygen-hemoglobin dissociation curve.

The higher the PO2 dissolved in

the plasma, the higher the Hb.

saturation

• With PO2 100mmHg in

arterial blood saturation is 98%


PO2 and percent saturation contd.

In the venous blood at PO2 40mmHg

-percent saturation is 75%

- only 25% has O2

been unloaded to tissues

With PO2 between 60-100mmHg, Hb is

90% or more saturated with oxygen

So even with PO2 as low as 65mmHg

Hb saturation is not so low-

(important for those with lung diseases

or living at high altitudes


PO2 and percent

saturation contd.

Between 40 and

20mmHg a small

decrease in PO2

causes a large drop in

Hb saturation -

with release of oxygen

In actively contracting

muscles PO2 may drop to

20mmHg – saturation 35%-

with oxygen release to

muscles


Transport of O2

Measuring

hemoglobin

saturation is

common in clinical

practice- done by

Pulse oximeters

3660 Group, Inc/NewsCom


Factors influencing the affinity of Hb binding

with O2 -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

O2 to tissues

The pH decrease shifts the O2–Hb saturation curve

“to the right”

This is called the Bohr effect


Transport of CO2

CO2 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 (HCO3–) through the action of an

enzyme called carbonic anhydrase.

o CO2 + H2O H2CO3 H+ + HCO3

-

3. 23% is attached to Hb (to the amino acids)

as carbaminohemoglobin( HbCO2)


Transport of CO2At the level of tissues: Carbon dioxide diffuses into RBCs, combines with water to form H2CO3, (catalyzed by carbonic anhydrase), which quickly dissociates into hydrogen ions and bicarbonate ions

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

Cl–)


Transport of CO2

At the lungs, these processes are reversed

Cl–)


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


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 PCO2

Peripheral chemoreceptors sensitive to PCO2, PO2,

arterial pH

PCO2 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

Medulla oblongata Central chemoreceptorsglossopharyngeal nerve(cranial nerve IX)

Carotid bodyCarotid sinusvagus nerve(cranial nerve X)

Arch of aorta

Aortic bodies

Internal carotidartery

Heart

Chemoreceptors

Peripheral Chemoreceptors


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 PO2 levels are monitored by the aortic

and carotid body peripheral chemoreceptors

Substantial drops in arterial PO2 (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

overinflation-

inhibitory 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 anti-

inflammatory 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

Documents

Chapter 23 The Respiratory System: Physiology. Copyright © John Wiley & Sons, Inc. All rights reserved. Respiratory System Anatomy Functionally, the respiratory