Respiratory: Gas Exchange with the Atmosphere L16

Question 1

Give typical values for oxygen pressure in various mediums.

Answer 1

PO2 = 159mmHg
PiO2 = 149 mmHg (less than barometric 159 due to water vapour pressure)
PAO2 = 100mmHg
PaO2 = 95mmHg(arterial blood gas)
PvO2 =40mmHg (What’s left after cells use the O2)

Question 2

Give typical values for carbon dioxide pressure in various mediums.

Answer 2

PACO2 = 40 mmHg
PaCO2 = 40mmHg
PvCO2 = 46mmHg (this is higher because the cells produce carbon dioxide)

Question 3

Why is arterial (PaO2) less than oxygen in alveoli (PAO2)?

Answer 3

Oxygen transfer across the alveoli is not complete, therefore some venous blood will not be oxygenated.

Question 4

Why does PACO2 and PaCO2 have the same?

Answer 4

Carbon dioxide diffuses faster than oxygen.

Question 5

How are these arterial values measured?

Answer 5

Using an arterial blood gas (ABG) procedure to sample blood from an artery (usually the radial artery).

Question 6

Give the equation for Fick’s Law of diffusion. Indicate what each variable stands for.

Answer 6

F =[ A x D x (P1-P2) ] / T
F= flow of gases. A high flow indicates a large amount of diffusion, such as would be expected from a big breath in a normal, healthy person.
A = surface area. Alveoli in the lungs create a huge surface area for diffusion.
T = thickness of the membrane where diffusion occurs. The respiratory membrane that lines alveoli is very thin, which allows efficient diffusion.
D = Diffusion constant. This relates to both the solubility and molecular weight of a gas. Molecules of small molecular weight and high solubility will have a high D. Carbon dioxide has a much higher solubility than oxygen, even though both gases have a similar molecular weight. Carbon dioxide diffuses 20 times faster than oxygen
P1-P2 = Pressure gradient driving diffusion. Gases flow from an area of high pressure to an area of low pressure. At the alveoli, P1 is the pressure in the alveoli, and P2 is the pressure in the capillaries.

Question 7

Describe Fick’s law of diffusion.

Answer 7

Governs the process of diffusion across fluid membranes (which enables gas exchange to occur in the body).

Question 8

Use Fick’s law of diffusion to explain diseases affecting flow.

Answer 8

Emphysema = destruction of alveolar walls and dilation of alveolar spaces (typically due to cigarettes). This causes a reduction in surface area (A), and in turn flow (F).

Pulmonary Fibrosis = the alveolar membrane becomes scarred and thickened (many reasons including chronic inflammation, pollutants, smoking), and an increase in thickness causes a decrease in diffusion, and a decrease in flow (F).

Question 9

What influences partial pressures of gases at the alveoli?

Answer 9

Composition of inspired air and partial pressure of gases in the atmosphere: what fraction of the air a gas makes up, and the pressure that gas exerts.
Alveolar ventilation: how much of the inspired air reaches the alveoli. Deep slow breathing increases alveolar ventilation, shallow fast breathing decreases it.
Metabolism of tissues: how much oxygen is being used and carbon dioxide produced.
The matching of the alveolar ventilation to the blood flow in the pulmonary capillaries: this involves respiratory and cardiovascular system working together.

Question 10

Give equation showing how partial pressure of oxygen at the alveoli depends on the balance between alveolar ventilation and oxygen use?

Answer 10

PAO2 = PiO2 - (VO2/VA)

Alveolar pressure of oxygen= Inspired oxygen minus (oxygen consumption/alveolar ventilation).

Question 11

Give equation showing how partial pressure of CO2 at the alveoli depends on the balance between alveolar ventilation and CO2 production?

Answer 11

PACO2 = PiCO2 + (VCO2/VA)

Note: You add rather than subtract like with oxygen.

Question 12

Give equation for measuring carbon dioxide production, and give typical values.

Answer 12

Note: Because there is little carbon dioxide inspired, we assume it 0.
VCO2 = VE x FECO2.
VCO2 is usually 200-250 ml/min, but can increase to 5L/min during exercise due to anaerobic metabolism.

Question 13

Give equation for measuring oxygen use in one minute, and give typical values.

Answer 13

Calculated by measuring proportion of oxygen in the air expired in one minute, and comparing it to the proportion of oxygen in inspired air.
VO2 = VE x (FiO2- FEO2)
VO2 is usually about 250-300ml /,in, but can increase to 3.5L/min.

Question 14

What is the respiratory quotient (R)? What is a typical value?

Answer 14

Represents ratio between carbon dioxide production and oxygen usage.
R = VCO2/VO2
Typical value is 0.8.
200/250 = 0.8.

Question 15

What is the alveolar gas equation?

Answer 15

Involves R, can be used to work out PAO2:

PAO2 = PiO2 - (PACO2/R)

Question 16

What is hyperventilation?

Answer 16

Increased alveolar ventilation, without an increase in metabolism (tissue demand for oxygen). This leads to a build up of oxygen in the alveoli, and a decrease in carbon dioxide. A person may hyperventilate when blowing a balloon, or when anxious (emotional hyperventilation).

Question 17

What is hypoventilation?

Answer 17

Decrease in alveolar ventilation without a change in metabolism. CO2 levels build up, oxygen drops. This occurs slightly when sleeping, and asthma.
A person’s lungs constrict, so even as the person’s respiratory rate increases, alveolar ventilation will fall as an insufficient volume of air is able to travel down blocked airways.

Question 18

What is eupnoea?

Answer 18

Normal quiet breathing

Question 19

What is hypernoea?

Answer 19

Increased alveolar ventilation, in response to increased metabolism, occurs during exercise.

Question 20

What is Dyspnoea?

Answer 20

Difficulty breathing, occurs in hard exercise and various diseases.

Question 21

What is apnoea?

Answer 21

Complete cessation of breathing. This occurs if you voluntarily hold your breath, if there is depression of respiratory centres in the CNS (e.g head injury), in sleep apnoea, or in death.