VentilationDiffusionVentilation-perfusion RelationshipsGas Transport by the Blood

Respiratory Physiology

Lung Volumes

LUNG VOLUMES AND CAPACITIES Remember: Capacities are always the summation of volumes.TIDAL VOLUME (TV): Volume inspired or expired with each normal breath.INSPIRATORY RESERVE VOLUME (IRV): Maximum volume that can be inspired over the inspiration of a tidal volume/normal breath. Used during exercise/exertion.EXPIRATRY RESERVE VOLUME (ERV): Maximal volume that can be expired after the expiration of a tidal volume/normal breath.RESIDUAL VOLUME (RV): Volume that remains in the lungs after a maximal expiration.  CANNOT be measured by spirometry.INSPIRATORY CAPACITY ( IC): Volume of maximal inspiration:IRV + TVFUNCTIONAL RESIDUAL CAPACITY (FRC): Volume of gas remaining in lung after normal expiration, cannot be measured by spirometry because it includes residual volume:ERV + RVVITAL CAPACITY (VC): Volume of maximal inspiration and expiration:IRV + TV + ERV = IC + ERVTOTAL LUNG CAPACITY (TLC): The volume of the lung after maximal inspiration.  The sum of all four lung volumes, cannot be measured by spirometry because it includes residual volume:IRV+ TV + ERV + RV = IC + FRCDEAD SPACE: Volume of the respiratory apparatus that does not participate in gas exchange, approximately 300 ml in normal lungs.  --ANATOMIC DEAD SPACE: Volume of the conducting airways, approximately 150 ml  --PHYSIOLOGIC DEAD SPACE: The volume of the lung that does not participate in gas exchange.  In normal lungs, is equal to the anatomic dead space (150 ml).  May be greater in lung disease.FORCED EXPIRATORY VOLUME in 1 SECOND (FEV1): The volume of air that can be expired in 1 second after a maximal inspiration.  Is normally 80% of the forced vital capacity, expressed as FEV1/FVC.  In restrictive lung disease both FEV1 and FVC decrease , thus the ratio remains greater than or equal to 0.8.  In obstructive lung disease, FEV1 is reduced more than the FVC, thus the FEV1/FVC ratio is less than 0.8.

Ventilation

Ventilation

PaCO2 – ventilatory status

Ventilation

PaCO2

Ventilation

PaCO2

Ventilation

Minute ventilation (VE) = Tidal volume (VT) x RR

= (VD + VA) RR

VT (RR) = VD(RR) + VA(RR)

VA(RR) = VT (RR) - VD(RR)

Alveolar ventilation = (tidal volume – VD ) RR

VA = RR ( VT – VD )

PaCO2

= =

PaCO2

= =

Ventilation

Anatomic dead space - volume of the conducting airways (150 ml)

Physiologic dead space - volume of gas that does not eliminate CO2

Diffusion

Fick’s Law

The rate of diffusion of a gas through a tissue slice: proportional to the area, partial pressure difference,

solubility of the gas in the tissue inversely proportional to the thickness and the square

root of the molecular weight

Vgas = A . D . (P1 – P2 ) T

Measurement of Diffusing Capacity

Carbon Monoxide Gas of choice Diffusion-limited

Fick’s Law Vgas = A . D . (P1 – P2 )

T

Ventilation-Perfusion Relationships PO2 of air: 20.93% Barometric pressure at sea level:

760mmHg Water vapor pressure of moist inspired

air: 47mmHg

PO2 of inspired air = (.2093) X (760 - 47) =149 mm Hg

Ventilation-Perfusion Relationships

Scheme of the O2 partial pressures from air to tissues

5 Causes of Hypoxemia

1. Hypoventilation2. Diffusion abnormality3. Shunt4. Ventilation-perfusion inequality5. Decreased inspired oxygen

Hypoventilation

Hypoventilation

Causes: Drugs (morphine and barbiturates) Damage to chest wall or paralysis of

respiratory muscles High resistance to breathing (underwater)

Hypoventilation

Increases the PCO2

Decreases the PO2 unless additional O2 is inspired

Hypoxemia is easy to reverse by adding O2

5 Causes of Hypoxemia

1. Hypoventilation

2. Diffusion abnormality3. Shunt4. Ventilation-perfusion inequality5. Decreased inspired oxygen

Shunt

Refers to blood that enters the arterial system without going through ventilated areas of lung Bronchial artery collected by pulmonary veins Coronary venous blood draining through thebesian

veins AV fistula

Hypoxemia responds poorly to added inspired O2 When 100% O2 is inspired, the arterial PO2 does not

rise to the expected level- a useful diagnostic test

VQ Mismatch/Inequality

O2 = 150 mmHgCO2 = 0

Regional Gas Exchange in the Lung

Gas Transport by the Blood

Oxygen is carried in the blood in 2 forms: Dissolved O2

Amount dissolved is proportional to the partial pressure (Henry’s Law)

0.003 ml O2 in 100ml blood/mmHg of PO2 N Arterial blood w/ PO2 of 100mmHg has 0.3ml

O2/IL blood

Combined with hemoglobin

O2 + Hb ↔ HbO2 (oxyhemoglobin)

O2 capacity - maximum amount of 02 that can be combined with Hb

O2 saturation - percentage of the available binding sites that have O2 attached,

O2 combined w/ Hb x 100 O2 capacityO2 saturation of arterial blood with

PO2 100mmHg is 97.5%

O2 Dissociation Curve

Carbon Dioxide

Carried in the blood in 3 forms: Dissolved As bicarbonate As carbamino compounds

Scheme of the uptake of CO2 and liberation of O2 in systemic capillaries

Chloride shift

Some of the H+ ions liberated are bound to reduced hemoglobin (better proton acceptor)

H+ + HbO2 ↔ H+. Hb + 02

Reduced Hb in the peripheral blood helps unload CO2

Haldane effect: deoxygenation of blood increases its ability to carry CO2, mop up H+ ions, and form carbamino-Hb

CO2 Dissociation Curve

Acid-base Status

HCO3 (Metabolic)pH= (6.1) + log --------

PaCO2 (Respiratory)

HCO3--------- = pHPaCO2

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iHCO3

------ = iipHPaCO2

hHCO3

------ = hhpHPaCO2

HCO3

------ = hhpH

iPaCO2

HCO3

------ = iipH

hPaCO2

Metabolic Acidosis

Metabolic Alkalosis

Respiratory Acidosis

Respiratory Alkalosis

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hHCO3

------ = hhpH

iPaCO2

iHCO3

------ = iipH

hPaCO2

CombinedRespiratory & Metabolic

Alkalosis

CombinedRespiratory & Metabolic

Acidosis

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iHCO3------ = iipH PaCO2

Uncompensated

iHCO3------ = ipH iPaCO2

iHCO3------ = N pH iPaCO2

Partly Compensated

Compensated

Metabolic Acidosis

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