Q41

Discuss the factors that determine airway resistance.

1. Flow resistance R=8x viscosity x length / pieXr4

Pass: Need to say that radius is the most important determining factor, 2 out of 3 to pass.

2. Directly proportional to viscosity & length. Inversely proportional to radius to the power of 4 (le:  half the radius increases resistance 16 fold).

What factors affect the radius of the airway?

  1. Bronchial smooth muscle tone: sympathetic and parasympathetic activity
  2. Lung volume

Need 2 to pass

 


Q42

Describe the relationship of pressure and wall tension in connected bubbles.

Law of Laplace: P = 4T/r. Two bubbles connected (same surface tension), the smaller with higher pressure will blow up the larger with lower pressure. Smaller bubble will collapse.

What are the effects of surfactant in alveoli?

Surfactant reduces surface tension. Alveolar bubbles are stable because of very low surface tension when small (on expiration). Hysteresis curve demonstrates very low pressures on expiration to small volumes = bubble stability. Increased compliance = ease of expansion. Also keeps alveoli dry = opposes transudation fluid into bubble.

How does surfactant achieve this?

  • Bipolar molecules oppose the normal increasing attracting forces as molecules get closer in a smaller surface
  • The ends of surfactant molecules repel each other and oppose collapse.

Q43

What are the causes of hypoxaemia in a patient breathing room air?

  1. Hypoventilation
  2. Diffusion limitation
  3. Shunt
  4. Ventilation/perfusion (V/Q) inequality

Pass Criteria:

  • 3 of 4 to pass

How does the ventilation/perfusion ratio change in different regions of the lung?

  • V/Q ratio is high at apex (blood flow minimal) and decreases down the lung to the base
  • PO2 highest at apex but blood flow is greatest at the base where PO2 is lowest (can be 40 mmHg difference)
  • Respiratory exchange ratio (CO2 output/O2 uptake) highest at apex where blood flow is lower

Pass Criteria:

  • Bold + general concepts to pass

What is the effect of ventilation-perfusion inequality on arterial PO2 and arterial PCO2?

PROMPT – Why does V/Q inequality cause reduced arterial PO2 while arterial PCO2 remains relatively normal?

  • Much greater influence on PO2 than CO2.
  • O2 dissociation curve nonlinear. Areas with high V/Q ratio add relatively little O2 with increased ventilation. Whereas areas with low V/Q ratio have lower PO2 (close to mixed venous) overall PO2 is reduced
  • CO2 dissociation curve is linear in the working range. Chemoreceptor stimulation increases ventilation and CO2 output especially in lung areas with high V/Q ratios. Normal PCO2 (minimal change).

Pass Criteria:

  • Bold + demonstrates understanding

Q44

How is oxygen carried in the blood?

  • Dissolved: amount dissolved proportional to partial pressure (Henry’s law) – 0.3mL O2/100mL blood at PO2 100mmHg
  • Most combined with Hb: 20.8mL O2/100mL blood (at Hb level of 15g/dL).

Pass Criteria:

  • Bold to pass

Draw and label the oxygen dissociation curve.

Pass Criteria:

  • Draw correct shape and have 2 points of saturations e.g.
    • 27mmHg SaO2 50%
    • 30mmHg SaO2 60%
    • 40mmHg SaO2 75%
    • 56 mmHg SaO2 90%
    • 80mmHg SaO2 95%
    • 90mmHg SaO2 97%

What are the implications of this curved shape?

PROMPT – What happens to the top and bottom?

  • Upper – If PO2 alveolar gas falls (e.g. ARDS in acute pancreatitis) loading of O2 little affected
  • Lower – Steep lower part means large amounts of O2 unloaded at peripheral tissues for only small drop in capillary PO2

Pass Criteria:

  • Explain concept of loading and unloading of oxygen

Q45

What is DEAD SPACE?

  • Portion of the tidal volume that does not participate in gas exchange – VT = VD + VA

Pass Criteria:

  • Demonstrate principle of bold to pass

What types of dead space are there?

PROMPT – Explain difference between the two types.

  • ANATOMICAL
    • Volume of conducting airways – trachea, bronchi, terminal bronchi (16 generations)
    • About 150mLs of 500mL VT
    • Measured by Fowler’s method
    • Determined by
      • Increased diameter of airways during inspiration
      • Size & posture of individual
  • PHYSIOLOGICAL
    • Volume of gas that does not eliminate CO2
    • Same as anatomical dead space in normal individuals
    • Increased in lung disease because of inequality of blood flow and ventilation within the lung

Pass Criteria:

  • Two types dead space and describe

How is dead space measured?

  • Measured by Bohr method

Q46

Describe the different types of tissue hypoxia.

PROMPT  – Hypoxia is a deficiency of O2 at at the tissue level.

  • Hypoxaemia (hypoxic hypoxia) – arterial PO2 reduced
  • Anaemic hypoxia – arterial PO2 normal but Hb reduced
  • Ischaemic/stagnant hypoxia – blood flow & O2 delivery decreased
  • Histotoxic hypoxia – because of toxin cells cannot use it

Pass Criteria:

  • 3 to pass

Describe the respiratory mechanisms leading to hypoxaemia and give examples.

  • Reduced ventilation – Asthma
  • VQ mismatch – Pulmonary Embolism
  • Shunt – Congenital heart disease
  • Diffusion limitation – Acute pulmonary oedema, left ventricular failure, ulmonary fibrosis)

Pass Criteria:

  • 2 mechanisms and correct example

Describe the clinical effects of acute hypoxia.

  • Disorientation
  • Confusion
  • Headache
  • Loss of consciousness
  • Tachycardia
  • Hypertension
  • Hypotension
  • Acute myocardial infarction
  • Arrest
  • Diaphoresis
  • Tachypnoea

Pass Criteria:

  • 2 to pass

Q47

Please draw and label the oxygen dissociation curve.

Pass Criteria:

  • Draw correct shape – have points of 90% (58-60) saturation.

What factors can cause the curve to shift to the right (reduced affinity of Hb for O2)?

  • Increased temperature
  • Increased PCO2
  • Increased 2,3 DPG
  • Drop in pH (increased H+)

Pass Criteria:

  • At least 3

What are the physiological advantages of the curved shape?

  • Upper
    • If pO2 alveolar gas falls, loading of O2 little affected. Also as RBC takes up O2 along pulmonary capillary, diffusion process hastened as large partial pressure different maintained when most of O2 has been transferred.
  • Lower
    • Steep lower part means peripheral tissues can withdraw large amounts of O2 for only small drop in capillary pO2.

Pass Criteria:

  • Concept of loading and unloading of oxygen being facilitated

Q47

Please draw and label the oxygen dissociation curve.

Pass Criteria:

  • Draw correct shape – have points of 90% (58-60) saturation.

What factors can cause the curve to shift to the right (reduced affinity of Hb for O2)?

  • Increased temperature
  • Increased PCO2
  • Increased 2,3 DPG
  • Drop in pH (increased H+)

Pass Criteria:

  • At least 3

What are the physiological advantages of the curved shape?

  • Upper
    • If pO2 alveolar gas falls, loading of O2 little affected. Also as RBC takes up O2 along pulmonary capillary, diffusion process hastened as large partial pressure different maintained when most of O2 has been transferred.
  • Lower
    • Steep lower part means peripheral tissues can withdraw large amounts of O2 for only small drop in capillary pO2.

Pass Criteria:

  • Concept of loading and unloading of oxygen being facilitated

Q48

How is carbon dioxide transported from the tissues to the lungs?

  • In plasma:
    • Dissolved
    • Carbamino compounds with plasma proteins
    • Hydration – H+ buffered – HCO3- in plasma
  • In RBC
    • Dissolved
    • Formation of carbamino-Hb
    • Hydration – H+ buffered – 70% of HCO3- enters plasma
  • Each 49mL CO2/dL arterial blood – 5% dissolved, 5% in carbamino compounds, 90% hydrated as HCO3

Pass Criteria:

  • Bold to pass

Draw and explain the carbon dioxide dissociation curve.

Pass Criteria:

  • Concept to pass

What is meant by the term 'chloride shift'?

  • 70% of HCO3- formed in red cells enters the plasma in exchange for chloride – exchange is the chloride shift

Pass Criteria:

  • Reasonable definition to pass

Q49

What are the major components of the control of ventilation (or respiration)?

  • Voluntary versus automatic
  • Medulla pacemaker cells
  • Pons pneumotactic centre modifies th emedulla activity
  • Higher centres – hypothalamus, limbic system, cerebral cortex
  • Vagal afferents from lung
  • Central chemoreceptors – CSF (medulla, floor 4th ventricle) – increased H+
  • Peripheral chemoreceptors – carotid and aortic bodies – pO2, decreased pH, increased pCO2
  • Integrated response: PaCO2, PO2, pH
  • Lung receptors – stretch, irritant, bronchial C fibres (J receptors)

Pass Criteria:

  • 5 of 7 bold

How does a rise in CO2 affect ventilation?

  • Direct effect on central and peripheral chemoreceptors, due to both high CO2 and lower pH
  • Increase in rate and depth of ventilation

Pass Criteria:

  • 4 of 5 bold 

Q50

How is CO2 carried in the blood?

  • CO2 is carried in the blood in 3 forms:
    • Dissolved – approximately 5-10%
    • As bicarbonate – approximately 90%
    • Combined with proteins as carbamino compunds – approximately 5-10%

Pass Criteria:

  • Bold + 1 other

How is bicarbonate formed in the blood?

PROMPT – Can you write an equation?

  • CO2+ H2O  H2CO3  H+ + HCO3
  • The first reaction is very slow in plasma but fast within the red blood cell because of the presence there of an enzyme carbonic anhydrase (CA). The second reaction, ionic dissociation is fast without an enzyme.

Pass Criteria:

  • Talk through equation + bold 

What is the chloride shift?

  • HCO3– diffuses easily out of the cell. Hdoesn’t because the cell membrane is relatively impermeable to cations. Therefore to maintain cell neutrality, Cl- diffuses from the plasma into the cell.

Pass Criteria:

  • At least 1 bold 

What is the Haldane effect?

  • H+ + HbO2  H+Hb + O2
  • The Haldane effect: Deoxy Hb binds more H+ than oxyHb and forms carbamino compounds more readily. Binding of O2 to Hb reduces its affinity for CO2
    1.  Enhances the removal of CO2 from O2 consuming tissues (e.g. muscles) into the blood. CO2 can bind to amino groups on Hb to form carbaminoHb. CarbaminoHb is the major contributor to the Haldance effect.
    2. Promotes the dissociation of CO2 from Hb in the presence of O2 (e.g. the lungs) which is vital for alveolar gas exchange.

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