Respiratory: Gasseous exchange: O2

Question 1

Normal partial pressures of oxygen are:

Atmospheric air

Alveolar gas

Arterial blood

Mixed venous blood

Answer 1

Atmospheric air: PiO2 = 21.2 kPa (159.2 mmHg)

Alveolar gas: PAO2 = 14 kPa (10.5 mmHg)

Arterial blood: PaO2 = 13.3 kPa (100 mmHg)

Mixed venous blood: PvO2 = 5.3 kPa (40 mmHg)

Question 2

Airway Anatomy:

Describe anatomy and function of conducting zone

Describe anatomy and function of respiratory zone

Answer 2

Conducting zone:

• No alveoli
• Includes generations 1 – 16, trachea to terminal bronchioles
• Volume 150 ml
• Function
  
  • Bulkflow during inspiration and expiration
  • Warming humidification and inspired air

Respiratory zone

• Has alveoli
• Includes generations 17 to 23, respiratory bronchioles to alveolar sacs
• Volume 3000 ml
• No bulk flow, gases move by diffusion down concentration gradient
• Function
  
  • Gas exchange

Question 3

The factors that affect gas exchange are:

Answer 3

Dead space

• The proportion of tidal volume not involved in gas exchange

Diffusing capacity

• The volume of a gas that can be transferred across a membrane per unit time

Shunt

• The proportion of the blood entering the left side of the heart that has bypassed the oxygenation process of the lungs

Question 4

• Dead Space is?
• Anatomical deadspace is?
• Alveolar minute ventilation (AMV)?
• Alveolar deadspace?
• Physiological dead space (DS)?

Answer 4

• Dead space is the proportion of tidal volume not involved in gas exchange.
• Anatomical deadspace
  
  • Upper airway – nose and pharynx
  • Conducting zone
  • 150mls
• AMV = (TV – DSV) x RR
• Alveolar dead space is proportion of AMV not taking part in gas exchange due to entering unperfused alveoli or underperfused alveoli.
• Physiological dead space (DS) = anatomical DS + alveolar DS (i.e. the total proportion of tidal volume not taking part in gas exchange)

Question 5

• Diffusion Capacity is?
• relevant variables for transfer of oxygen between the alveoli and the erythrocytes within the pulmonary capillaries are?

Answer 5

• The volume of a gas that can be transferred across a membrane per unit time.
• Variables:
  • The surface area of lungs
  • The diffusion constant for 02
  • The thickness of the alveolar/capillary membrane
  • The difference between the partial pressure of oxygen in the alveoli and the blood

Question 6

Ficks Law of Diffusion

Answer 6

Combines variables that affect diffusion

A = surface area of lungs

D = diffusion constant for O2

T = thickness of the alveolar/capillary membrane

P1 = Partial pressure of O2 in the alveolus

P2 = Partial pressure of O2 in the capillary

Question 7

Oxygen Diffusion

1. Oxygen diffusion from the alveoli to the circulation at rest
2. Equilibration across the membrane
3. Red blood cells transit time

Answer 7

1. 250 ml per minute
2. 0.25 seconds
3. 0.75 seconds

Allows for full oxygenation

Question 8

Shunt

1. Define shunt
2. Define venous admixture
3. Causes of:
   
   1. Normal extrapulmonary shunt
   2. Normal pulmonary shunt
   3. Pathological extrapulmonary shunt
   4. Pathological pulmonary shunt

Answer 8

1. The total proportion of the circulation entering the LEFT side of the heart which has bypassed the oxygenation process of the lungs
2. The calculated amount of mixed venous blood required to be mixed with pulmonary end capillary blood to produce the observed difference between arterial and alveolar PO2
3. Causes
   1. Part of the bronchial circulation

Thebesian drainage, from heart muscle directly into the left ventricle

2. Areas of lung with V/Q \>0 and \<1
3. Congenital heart disease, heart disease
4. Pneumonia, atelectasis

Question 9

V/Q Ratio

Normal V/Q

Answer 9

Describes the relationship between:

V: the amount of ventilation of the lung

Q: the amount of perfusion of the lung

Gives a global figure for the entire lung

Ideal should be 1 (V=Q)

Normal AMV = 4000 ml

Normal CO = 5000 ml

Therefore Normal / = 0.8

Question 10

Alveloar gas equation

Answer 10

Question 11

1. Alveolar-arterial (A-a) Gradient is?
2. Under normal conditions A-a gradiant is?
3. Increases in shunt or venous admixture causes..?
4. Examples of increased A-a gradient?

Answer 11

1. PAO2 – PaO2 = A-a gradient
2. Under normal conditions (i.e. V/ Qratio of 0.8) in healthy lungs the A-a gradient is less than 2 kPa.
3. change in V/Q which then leads to an increased A-a gradient.
4. Abnormal anatomical shunt e.g. congenital heart disease with a right to left shunt

Pulmonary pathology such as a large tumour or pneumonia

Lung collapse secondary to obstructing tumour or pneumothorax

Question 12

O2 content in blood

1. Bound to Hb
   
   1. % of O2 carrying capacity?
   2. Affect PO2?
   3. Hb O2 carrying capacity
2. Dissolved in solution
   
   1. Affect PO2?

Answer 12

1. 98% of total O2 carrying capacity
2. O2 bound to Hb does not affect PO2
3. Hb O2 carrying capacity
   = Hb concentration (g/100 ml) x 1.39 x % SaO2/100
4. 39 = ml O2 carried per gram of fully saturated Hb
5. O2 in solution is reflected in the PO2
6. O2 in solution is proportional to PO2
   = 0.003 ml/100 ml/mmHg 37°
7. At normal PaO2 of 100 mmHg (13.3 kPa)
   = 0.3 ml/100 ml

Question 13

Total Oxygen Content

Answer 13

Total O2 carrying capacity =

(Hb X 1.39 x SaO2) + dissolved O2

Question 14

Describe Adult Haemoglobin

Answer 14

Adult haemoglobin consists of 4 sub-units:

• 2 α globin chains
• 2 β globin chains

Each subunit:

• Has a haem group
• Can bind 1 O2 molecule

Therefore, each Hb molecule can reversibly bind 4 O2 molecules.

Question 15

Draw the 02 Dissociation Curve

1. what gives the curve its shape?
2. what is the physiological benefit of this?
3. what is the P50

Answer 15

1. As each subunit is oxygenated, the binding of oxygen to the remaining units is enhanced.
2. The physiological benefit is that the last free subunits bind oxygen rapidly despite the limited number of binding sites available.
3. P50 = PO2 at which 50% of the binding sites are occupied.

Question 16

The Bohr Effect

Causes of left shift

Answer 16

Various factors cause a change in the shape of the Hb molecule.

This changes the access and the affinity of O2 to haem, causing a shift in the curve:

When affinity increases, P50 decreases, i.e. the curve shifts to the left

When affinity idecreases, P50 increases, i.e. the curve shifts to the right

Causes of right shift:

1. decrease pH
2. increase temp
3. increase 2,3-DPG

Question 17

Describe the bohr effect in vivo

Answer 17

In the lungs:

• CO2 is eliminated and pH rises
• The dissociation curve shifts to the left
• O2 binding to haem is enhanced
• For any given PO2 the oxygen saturation is increased, facilitating O2 uptake

In the tissues:

• Metabolism leads to CO2 and H+ production
• pH falls and the curve shifts to the right
• O2 release from heam to the tissues is facilitated

Question 18

2,3-diphosphoglycerate

Answer 18

2,3-diphosphoglycerate:

• Is produced by a side reaction from glycolysis
• Is present in erythrocytes
• Binds to β chains of Hb to reduce affinity for oxygen
• Is increased in anaemic states and at low oxygen tension, helping to off load oxygen at the tissues
• Is reduced in stored blood

Question 19

Oxygen Delivery

Oxygen Consumption

Answer 19

Oxygen Delivery

DO2 = blood O2 content x cardiac output

= 20 (ml/dl) x 10 x 5 (L/min)

= 1000 ml/min

Oxygen Consumption

VO2 = cardiac output x (arterial O2 content - mixed venous O2 content)

Normal mixed venous PO2 is approximately 5.3 kPa. At this PO2 the Hb is 75% saturated with oxygen.

100% saturated blood contains 20 ml/100 ml (approx) of oxygen.

Therefore:

O2

= 5000 x (20-15)/100

= 250 ml/min