Functions of the Respiratory System

Functions of the Respiratory SystemFunctions of the Respiratory System

1. Gas Exchange

2. Regulation of blood pH

3. Voice production

4. Olfaction

5. Protection

Tracheobronchial TreeTracheobronchial Tree

• Conducting Zone

• Respiratory Zone– Alveoli– Type I pneumocytes– Type II pneumocytes

Thoracic Wall & Muscles of RespirationThoracic Wall & Muscles of Respiration

• Thoracic wall– Thoracic vertebrae– Ribs– Sternum– Muscles

• Thoracic cavity– Thoracic wall & Diaphragm

Thoracic Wall & Muscles of RespirationThoracic Wall & Muscles of Respiration

• Muscles of Respiration– Muscles of Inspiration

• Diaphragm• External intercostals• Pectoralis minor• Scalenes

– Muscles of Expiration• Abdominal muscles• Internal intercostals

PleuraPleura

• Each lung is surrounded by a separate pleural cavity.

• Parietal pleura– Covers the inner layer of the throacic wall.

• Visceral pleura– Covers the surface of the lung.

• Pleural Cavity– Filled with pleural fluid.

Pleural FluidPleural Fluid

• Acts as lubricant allowing parietal and visceral

• Helps hold the parietal and visceral pleural membranes together.

Blood SupplyBlood Supply

• Blood that has passed through the lungs and picked up oxygen is called oxygenated blood.

• Blood that has passed through the tissues and released some of its oxygen is called deoxygenated blood.

VentilationVentilation

• Moving air in & out of the lungs.

• Flow of air into the lungs requires a pressure gradient from the outside of the body to the alveoli.

• If air needs the flow out of the lungs, a pressure gradient needs to exist in the other direction.

• F = (P1 – P2) / R

Pressure & VolumePressure & Volume

• General gas law– P = nRT / V– n, R, and T are all constants, therefore pressure is

inversly proportional to volume.– Called Boyle’s Law.

Airflow into & out of AlveoliAirflow into & out of Alveoli

• Inspiration– Initiated by contraction of diaphragm & external

intercostal muscles.– Result: increase in the size of the thorax.– Leads to a drop in alveolar pressure (Boyle’s

Law).

– Palv < PB

– Lung will expand.


• Expiration– At the end of inspiration, the nerves to the

diaphragm and external intercostals stop firing.– These muscles relax.– Lungs & chest wall passively return to their

original dimensions.


• Expiration– As lungs decrease in size, the pressure increases

(Boyle’s Law).

– Palv > PB

– Air will flow from the alveoli to the atmosphere.

Forced ExpirationForced Expiration

• During exercise, the expiration of larger volumes is achieved by the contraction of the internal intercostals and the abdominal muscles.

Lung ComplianceLung Compliance

• Compliance = “stretchability”.

• Two determinants:– Compliance (stretchability of lung tissue)– Surface tension at the air-water interface of the

alveoli.

• Surfactant

Respiratory Distress SyndromeRespiratory Distress Syndrome

• 2nd leading cause of death in premature infants.

• Normal maturation of surfactant-synthesizing cells is facilitated by hormones (particularly cortisol).

• Secretion of cortisol increases late in pregnancy (7th month).

Respiratory Distress SyndromeRespiratory Distress Syndrome

• Premature babies lack mature type II alveolar cells due to low levels of cortisol.

• Treatment:– Administration of cortisol to mother.– Administer high pressure, oxygen-rich air to infant.– Exogenous source of surfactant to infant.

Pleural PressurePleural Pressure

• Normally lower than the alveolar pressure.

• This helps keep the alveoli expanded.

• If this difference is lost the alveoli will collapse.

• e.g. Pneumothorax

Airway ResistanceAirway Resistance

• F = ΔP / R

• Factors that affect R:– Viscosity– Length– Radius


• Factors that affect radius:– Transpulmonary pressure:

• Exerts a distending force.• Increases during inspiration.

– Lateral traction:• Connective tissue fibers pulling outwards.

– Mucus Accumulation


• Factors that affect radius:– Parasympathetic nerves

• Constriction (ACh)

– Epinephrine• Dilates

– Histamine• Constricts

AsthmaAsthma

• Attacks are characterized by smooth muscle contraction.

• Increases resistance impairing ventilation.

• More mucus may also be secreted.

• Main defect is inflammation of the airways.

• Cause of inflammation varies from person to person (virus, allergy, etc.).

AsthmaAsthma

• Smooth muscle can be hyperresponsive to:– Exercise– Emotional stress– Cold air– Cigarette smoke– Irritants– Certain drugs

AsthmaAsthma

• Therapy:– Anti-inflammatory drugs– Bronchodilators

Chronic ObstructiveChronic ObstructivePulmonary DiseasePulmonary Disease

• Examples:– Emphysema & Chronic Bronchitis

• Emphysema– Destruction of alveolar walls.– Impairs gas exchange.– Destruction of elastic tissue also contributes to

airway collapse (obstruction).

Chronic ObstructiveChronic ObstructivePulmonary DiseasePulmonary Disease

• Examples:– Emphysema & Chronic Bronchitis

• Chronic Bronchitis– Characterized by excessive mucus production in

bronchi.– Results in obstruction.

Pulmonary Volumes & CapacitiesPulmonary Volumes & Capacities

• Spirometry is the process of measuring volumes of air moving in & out of the lungs.

• Important volumes– Tidal volume– Inspiratory reserve volume– Expiratory reserve volume– Residual volume


• Spirometry is the process of measuring volumes of air moving in & out of the lungs.

• Important capacities (= sum of two or more lung volumes)

– Inspiratory capacity– Functional residual capacity– Vital capacity– Total lung capacity


• Forced expiratory vital capacity is a simple clinical pulmonary test.

• Forced expiratory volume in one second(FEV1) is also an important diagnostic value.

• Airway obstructions (asthma, emphysema, chronic bronchitis, etc) cause a decreased FEV1.

Alveolar VentilationAlveolar Ventilation

• Pulmonary Ventilation

• VP = f (VT)

• During inspiration, a portion of the inspired air fills the anatomic dead space before reaching the alveoli.

• This air is not available for gas exchange.

• VA = f (VT – VD)

Principles of Gas ExchangePrinciples of Gas Exchange

• Dalton’s Law

• Partial Pressure

• Nitrogen (78.62%)

• Oxygen (20.84%)

• Sea level– Total atmospheric pressure = 760 mmHg– Partial pressure of nitrogen = 597.5 mmHg– Partial pressure of oxygen = 158.4 mmHg

Inspired air vs. Alveolar airInspired air vs. Alveolar air

• Inspired air and alveolar air are different.– Air entering repiratory system is humidified.– Oxygen diffuses from alveoli into the blood.– Carbon dioxide diffuses from pulmonary

capillaries into the alveoli.– Air in the alveoli is only partially replaced with

atmospheric air during each inspiration.

Diffusion of GasesDiffusion of GasesThrough Respiratory MembraneThrough Respiratory Membrane

• Influenced by:– Thickness of the membrane.– Diffusion coefficient of the gas.– Surface area of the membrane (70 m2).– Difference of the partial pressures of the gas on

either side of the membrane.

Oxygen & Carbon DioxideOxygen & Carbon DioxideTransport in the BloodTransport in the Blood

• Oxygen– Po2 in alveoli is approx. 104 mmHg.

– Po2 in blood in pulmonary capillaries is approx.40 mmHg.

– Therefore, O2 diffuses from the alveoli to the pulmonary capillaries.


• Oxygen– Equilibrium is achieved during the first third of the

pulmonary capillary beds.– Blood leaving the lungs in the pulmonary veins

has a Po2 of approx. 95 mmHg due to mixing with bronchial veins.

– At the tissues, the Po2 is close to 40 mmHg in the interstitial space and approx. 20 mmHg in the individual cells.


• Carbon Dioxide– CO2 is continually produced during cellular

respiration.

– The intracellular Pco2 is approx. 46 mmHg

– Interstitial Pco2 is approx. 45 mmHg

– Arterial Pco2 is close to 40 mmHg

– Venous Pco2 is approx. 45 mmHg

– Because the Pco2 is approx. 40 mmHg in the alveoli, CO2 will diffuse into the alveoli.

Hemoglobin & Oxygen TransportHemoglobin & Oxygen Transport

• Approx. 98.5% of oxygen transported in the blood is combined with hemoglobin.

• The other 1.5% is dissolved in the plasma.

• Effect of Po2 on oxygen transport.

– Oxygen-hemoglobin dissociation curve


• Effect of pH, Pco2, & temperature on oxygen transport.

– pH• Bohr Effect: Decreases in blood pH results in an

decreased ability of Hb to bind to oxygen (decreased affinity) and vice versa.

– Pco2

• Increases in Pco2 decreases the ability of Hb to bind to oxygen (decreased affinity) because of the effect of CO2 on pH and vice versa.


• Effect of pH, Pco2, & temperature on oxygen transport.

– Temperature• Increases in temperature decreases the tendency for

oxygen to remain bound to Hb (decreased affinity).

Transport of Carbon DioxideTransport of Carbon Dioxide

• CO2 is transported in the blood in three different ways.

1. Dissolved in the plasma (7%)

2. Combined with hemoglobin (23%)

3. As bicarbonate ions (70%)


• Dissolved in the plasma (7%)– CO2 simply dissolves in the plasma.


• Combined with hemoglobin (23%)– CO2 combines with Hb in a reversible fashion.

– Many CO2 molecules can combine with one Hb molecule.

– Deoxyhemoglobin binds more readily to CO2 than oxyhemoglobin (Haldane effect).


• As bicarbonate ions (70%)– More than half of the CO2 molecules react with

water (H2O) molecules inside the red blood cell.

CO2 + H20 H2CO3 HCO3- + H+

– As HCO3- accumulates it is exchanged for chloride

ions (Cl-) (chloride shift).


• As bicarbonate ions (70%)

CO2 + H20 H2CO3 HCO3- + H+

– Hb binds to H+ ions which decreases the H+ concentration and therefore increases pH.

– Therefore Hb acts as a buffer.


• As bicarbonate ions (70%)

CO2 + H20 H2CO3 HCO3- + H+

– The reverse occurs in the lungs.

Carbon Dioxide & Blood pHCarbon Dioxide & Blood pH

• CO2 can also bind with H2O in the plasma.

• Therefore, as plasma CO2 levels increase, hydrogen ion levels increase, and blood pH decreases.

• The respiratory system can regulate the pH of the blood by changing plasma CO2 levels.

• Hyperventilation decreases CO2 levels and hypoventilation increases CO2 levels.

Rhythmic VentilationRhythmic Ventilation

• The basic rhythm of ventilation is generated and controlled by the medulla oblongata.

• Medullary respiratory center.– Two dorsal groups

• Primarily responsible for stimulating the contraction of the diaphragm.

– Two ventral groups• Primarily responsible for stimulating the external &

internal intercostals, and abdominal muscles.

Chemical Control of VentilationChemical Control of Ventilation

• Respiratory system maintains O2 & CO2 concentrations, and blood pH in their normal range.

• If these get out of their normal range they will have an effect on respiratory movements.

ChemoreceptorsChemoreceptors

• Respond to hydrogen ion concentration (pH) or changes in Po2.

• Central chemoreceptors– Located in medulla oblongata

• Peripheral chemoreceptors– Located in aortic and carotid bodies

Effect of pHEffect of pH

• Blood pH is closely linked to Pco2.

• Pco2 levels are monitored by medullary chemoreceptors.

• If Pco2 levels rise, blood pH decreases.

• Decreased blood pH values stimulates the respiratory center resulting in elimination of CO2.

• And vice versa…

Effect of COEffect of CO22

• Major regulator of respiration during rest and intense exercise.

• An increase in Pco2 of only 5 mmHg can increase ventilation by 100%!

• Hypercapnia = elevated Pco2

• Hypocapnia = lowered Pco2

• Hypercapnia and hypocapnia affect blood pH and thereby ventilation.

Effect of OEffect of O22

• Affects respiration at a relatively minor level.

• Hypoxia = reduced Po2 levels

• Ventilation will increase, but only after a marked decrease (50% or more) in Po2.

• This is due to the oxygen-Hb saturation curve.

Hering-Breuer ReflexHering-Breuer Reflex

• Limits inspiration and prevents overinflation of lungs.

• Initiated by stretch receptors in the walls of bronchi and bronchioles.

• Inhibitory influence on the respiratory center.

Effect of Exercise on VentilationEffect of Exercise on Ventilation

• Two phases– Abrupt increase in ventilation

• As much as 50%.• Primarily due to nervous system.

– Gradual increase in ventilation• Arterial Po2, Pco2, and pH values play a minor role.

• Nervous system also plays a role.• pH changes at anaerobic threshold.

Respiratory Adaptations to ExerciseRespiratory Adaptations to Exercise

• Training:– Lead to more efficient cardiovascular and

respiratory systems.– Slight decrease in residual volume– Slight increase in vital capacity– Slight decrease in respiratory rate– Therefore, little change in minute ventilation.

Respiratory Adaptations to ExerciseRespiratory Adaptations to Exercise

• Training:– However, minute ventilation is greatly increased

during exercise.• Untrained 120 l/min• Trained 150 l/min• Highly trained 180 l/min

Ventilation in Steady-Rate ExerciseVentilation in Steady-Rate Exercise

• Light to moderate exercise, ventilation increases linearly with O2 consumption and CO2 production.

• Averages about 20 - 25 liters of air for every liter of oxygen consumed.

• Also known as ventilatory equivalent (VE/VO2)

Ventilation in Steady-Rate ExerciseVentilation in Steady-Rate Exercise

• Healthy young adults maintain VE/VO2 of 25 during submax exercise up to approx 55% VO2max

• Children’s values average around 32.

Pulmonary Ventilation During Graded Exercise

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Ventilation in Non-Steady-Rate Ventilation in Non-Steady-Rate ExerciseExercise

• At progressively more intense levels of exercise beyond 55% VO2max, minute ventilation increases sharply.

• Ventilation increases more quickly than O2 consumption.

• Therefore, the ventilatory equivalent increases to values as high as 35 or 40 liters of air per liter of O2 consumed!

Ventilatory Threshold (VVentilatory Threshold (VTT))

• VT describes the point at which pulmonary ventilation increases disproportionately with O2 consumption.

• Ventilation is no longer linked with O2 consumption.

• Increased ventilation is a result of increases in CO2 levels.

Ventilatory Threshold (VVentilatory Threshold (VTT))

• Increased CO2 levels come from the buffering of lactate that accumulates from anaerobic glycolysis.

• Sodium bicarbonate is the buffer in the blood that buffers almost all of the lactate generated.

Lactic acid + NaHCO3 Na Lactate + H2CO3 H2O + CO2

Respiratory Exchange Ratio (R)Respiratory Exchange Ratio (R)

• R is the ratio of CO2 produced and O2 consumed (VCO2/VO2).

• In steady-rate exercise the R value remains less than 1.00.

• Excessive CO2 production due to the buffering of lactate increases R to a value greater than 1.00.


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Functions of the Respiratory System

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