Respiratory PhysiologyPart II

Bio 219

Napa Valley CollegeDr. Adam Ross

Gas exchange

• Gas exchange in the lungs (to capillaries) occurs by diffusion across respiratory membrane due to differences in partial pressure

• Partial Pressure: is the driving force for the diffusion of gasses (PO2

PCO2)• Gas will diffuse from high to low partial pressure

• Dalton’s Law: total pressure = sum of all partial pressures in a mixture• Ptotal = PN2 + PO2 + PCO2 + PH2O

• PO2 = Ptotal x (% O2/100) = 760 x 0.21 = 160 mm Hg

Pulmonary gasses and diffusion

• O2 diffuses from air in alveoli to blood in pulmonary capillaries

• CO2 diffuses from pulmonary capillaries into alveoli • - high diffusion efficiency due to:

• (1) high surface area of alveoli

• (2) thin respiratory membrane

Air and Blood Gases PO2 PCO2 O2 saturation

inspired air 160 0.3alveolar air 100 40

pulmonary veins& systemic arteries

100 40 98% arterial blood

vena cava& pulmonary arteries

40 46 75% mixed venous blood

Hemoglobin (Hb)

• Iron containing protein found in erythrocytes• Capable of binding both oxygen and CO2

• Carries oxygen to tissues

• Helps carry (10%) CO2 away from tissues (85% is dissolved in blood as bicarbonate, 5% = free CO2 in solution)

Bohr Shift

• When Hb is bound by CO2 its affinity for O2 is reduced, causing oxygen to be released at tissues that are releasing CO2

• Low pH has the same effect (remember CO2 = H+)

Control of Ventilatory Effort

• Respiratory centers:• Primary respiratory control located in brainstem

• Contains Inspiratory (I) and Expiratory (E) neurons

• Medulla Oblongata: central pattern generator, generates breathing rhythm• Dorsal Respiratory Group: Mostly I neurons

• Ventral Respiratory Group: E and I neurons

• Pons: pontine respiratory group, smoothes out breathing rhythm

Central Chemoreceptors

• Medulla:• Sensitive to PCO2+ via [H+] of cerebrospinal fluid

• ↑ arterial PCO2 → ↑ PCO2 of CSF

• CO2 + H2O → H2CO3 → H+ + HCO3- ↑ [H+] in CSF → stimulates ↑ ventilation

• Central chemoreceptor has the dominant role in regulating breathing at rest

Peripheral Chemoreceptors

• Carotid bodies • - sensitive to low PO2, also PCO2 and pH of arterial blood

• - stimulate ventilation directly at very low PO2 (< 60 mm Hg)

• - increase sensitivity of central response to CO2

• - contribute to increase in ventilation during exercise

Ventilation

• Ventilation is normally regulated to maintain constant arterial PCO2 (normal = 40 mm Hg) • hypoventilation - ↑ PCO2 (> 45 mmHg)

• hyperventilation - ↓ PCO2 (< 35 mmHg)

Hering-Breuer Reflex

• Prevents over inflation of the lungs

• Lungs have stretch receptors that can sense fullness of lung• Pulmonary stretch receptors in smooth muscle of the airways

• When lung inflates the send APs to the respiratory centers in the brain• Inhibits the INSPIRATORY centers of the medulla and ends inspiration

Hypoventilation

• Decrease in ventilation leading to an increase in arterial PCO2(hypercapnia)

• Carbon dioxide will start to build up throughout the body

• The increase in PCO2will cause a decrease in pH (respiratory acidosis)

• This will activate chemoreceptors to increase respiratory rate

Hyperventilation• Increase in ventilation by an increase in respiratory rate

and/or increasing tidal volume leading to a decrease in PCO2

(hypocapnia)• Rate of ventilation is higher than what is needed to

remove carbon dioxide from blood• A decrease in PCO2

will decrease the inspiratory drive (Are able to hold breath for a longer period of time)

• Prolonged hyperventilation will lead to respiratory alkalosis (increase in pH) which can cause arterioles in the brain to constrict -> decrease in blood flow to the brain -> dizziness

Exercise• Hyperpnea : increase in ventilation matching an

increase in metabolic activity (ex. Exercise)• Ventilation rate matches demand for carbon dioxide

removal so there is no decrease in arterial PCO2 that was

seen in hyperventilation

• Exercise increases demand for oxygen and produces more carbon dioxide

• There is an increase in perfusion of the upper lungs (that are normally closed at rest) to increase gas exchange, because the increase in CO during exercise increases pulmonary vascular pressure

• The mechanisms that control the respiratory response to exercise are not understood well