Chapter 18 Gas Exchange and transport

transportation of oxygen in the body

• diffuses from alveoli to blood

• primarily bound to hemoglobin

transportation of carbon dioxide

• produced through metabolism

• diffuses out of cells and into alveoli

partial pressure drives

movement of gases across cell membranes

hypoxic hypoxia

low arterial PO2

histotoxic hypoxia

failure to use O2 and metabolic poisons

anemic hypoxia

decreased Hb-O2 and blood loss

Ischemic hypoxia

reduced blood flow, heart failure, shock, and blood clot

factors influencing alveolar gas exchange

alveolar PO2 levels, Alveolar ventilation, Altered O2 levels, and diffusion of oxygen

Alveolar PO2 levels

adequate O2 has to reach alveoli and composition of inspired air

Alveolar ventilation

decreased compliance, increased resistance, and CNS depression

Diffusion of oxygen between alveoli and blood

respiratory membrane, surface area, partial gradients, permeability, and distance

respiratory membrane

Type I pneumocytes and capillary endothelium

gas solubility

how easily gas moves from air to liquid phase

O2 solubility

low

CO2 solubility

high

Emphysema

destruction of alveoli leading to less surface area for gas exchange, and PO2 normal or low

Pulmonary Edema

fluid in interstitial space increases diffusion distance causing higher solubility of CO2 in water and normal arterial PO2

fibrotic lung disease

thickened alveolar membrane slows gas exchange and loss of lung compliance causes decrease in alveolar pressure

asthma

increased airway resistance decreases alveolar ventilation

oxyhemoglobin

hemoglobin binds to 4 O2 molecules

Hb saturation

amount of O2 bound to Hb

Normal arterial PO2 saturation

98%

resting cell PO2 saturation

75%

exercising muscle PO2 saturation

35%

how pH affects hemoglobin-oxygen binding

decreased pH facilitates unloading of oxygen

transport of CO2 in blood

1. CO2 diffuses out of cells into systemic capillaries

2. 7% of CO2 remains dissolved in plasma

3. ¼ of CO2 binds to hemoglobin

4. 70% of CO2 load is converted to bicarbonate and H+

5. HCO-3 enters plasma in exchange for Cl-

6. at lungs dissolved CO2 diffuses out of plasma

7. CO2 unbinds from hemoglobin and diffuses out of RBC

8. carbonic acid reverses pulling bicarbonate back into RBC and converting back to CO2

peripheral chemoreceptors

carotid and aortic bodies and glomus cells activated by low PO2 levels

Glomus cells activation

• close O2 sensitive K+ channels

• cell depolarizes

• open v-g Ca2+ channels

• causes exocytosis

• NT diffuses to sensory neuron

• action potential

Central chemoreceptors

in medulla and control centers monitoring CO2 levels

a decrease in CO2 levels

slows ventilation

and increase in CO2

speeds up ventilation