Respiratory physiology 4

Question 1

Compare hypoxia and hypoxemia

Answer 1

Hypoxia
reduced availability of O2 at tissues
Under-oxygenation of organs, tissues and cells
Impairs normal metabolism
If severe, can lead to cellular death

Hypoxaemia
- an abnormally low concentration of oxygen in the blood.

Question 2

List the common causes of tissue hypoxia

Answer 2

Four main types

Cytopathic

• cells unable to utilise O2
• Eg mitochondrial dysfunction and septic shock

Anaemic

• Blood not carrying enough O2

Stagnant

• low flow of blood
• Eg low cardiac output and low tissue perfusion

Hypoxemic

• low Pa O2, leading to low delivery of O2 to tissues

Question 3

Explain the steps by which the body corrects hypoxia

Answer 3

learn. more in-depth in later years

Question 4

List the mechanisms by which hypoxaemia can occur (4)

Answer 4

The main 4

• hypoventilation or low PiO2
• Diffusion impairment
• Right to left shunt
• V/Q mismatch

Question 5

Describe how decreases in PIO2 and hypoventilation can lead to hypoxaemia

Answer 5

Decrease in PIO2 means that there is a decrease in the concentration of the O2 which is reaching the lungs.

• causes of this = hyperbaric conditions (altitude)

Hypoventilation =

• fibrosis = increased airway resistance
• Emphysema = Decreased pulmonary compliance

Question 6

Describe how a diffusional impairment can lead to hypoxaemia

Answer 6

nadequate amount of gas exchange at the blood-gas barrier

Depends on

Thickness of membrane

Surface area

Diffusion coefficient

Partial pressure of the gas difference

Question 7

Describe how a right to left shunt leads to hyperaemia

Answer 7

Non-Deoxygenated mixes with the oxygenation of the blood, which leads to mixing of the blood, lowering the concentration of O2 in the blood being pumped out.

Question 8

Describe how a ventilation defect impacts oxygen saturation

Answer 8

Inequality of alveolar ventilation and perfusion

= a mismatch between the amount of air being ventilated and blood flow

Ventilation reduced = Low V/Q

= not enough ventilation

• lung disease
• Air way obstruction
• Stiffening of lung due to inflammation

Question 9

Describe how a perfusion defect impacts oxygen saturation

Answer 9

Perfusion of blood reduced = High V/Q ratio

= not enough perfusion, more ventilation than the blood can carry

• vascular obstruction
• Pulmonary hypotension

Question 10

Describe the mechanisms by which horses experience arterial hypoxaemia and hypercapnia during high intensity exercise and quantify their relative contributions

Answer 10

Post pulmonary shunts between the bronchial an pulmonary circulation around 1%

V/Q mismatch: 25-40% of the increase in the widening of the alveolar-arterial O2 gradient

60-75% related to diffusion limitations and/or alveolar hypoventilation

Question 11

List factors which should contribute to improve diffusion during exercise

Answer 11

PvO2 as low as 16mmHg during intense exercise

= widens the alveolar-arterial O2 gradient

Increased surface area available for gas exchange

= dilation and recruitment of poorly- perfused or non-perfused sections

= 50-60% increase in volume

However the 8 times increase in cardiac output during high intensity exercise

= substantially decreased capillary transit time and decreased time for O2 equilibration

Question 12

Describe hypercapnia and list the mechanisms by which it can occur

Answer 12

Excess carbon dioxide in the blood

mechanisms

• decreased alveolar ventilation
• Severe V/Q mismatch = low V/Q ratio = hypercapnia (the opposite of O2 V/Q ratios)
• CO2 production occurring without appropriate ventilatory compensation