Medical Physiology Block 5 Week 3

Question 1

Define total ventilation, dead-space ventilation, and arterial ventilation

Answer 1

Total ventilation = tidal volume multiplied by respiratory frequency (WORK); dead-space ventilation = volume of the dead-space multiplied by respiratory frequency; arterial ventilation = (total-dead space) x respiratory frequency

Question 2

How can dead-space be measured?

Answer 2

anatomical dead-space: measure concentration of expired nitrogen; physiological dead-space: measure concentration of expired carbon dioxide (some dead-space may be alveolar)

Question 3

What is the equation to calculate alveolar carbon dioxide partial pressure?

Answer 3

0.863 x (rate of carbon dioxide production (200 mL/min)/alveolar ventilation rate normally 4.2 L/min)

Question 4

What happens to alveolar carbon dioxide partial pressure as alveolar ventilation increases? What happens to pH?

Answer 4

decreases; 0.863 x (200/8.4) = about 20 mm Hg (same as arterial carbon dioxide partial pressure; respiratory alkalosis

Question 5

What is the equation to calculate alveolar oxygen partial pressure?

Answer 5

inspiratory oxygen partial pressure - (alveolar carbon dioxide partial pressure multiplied by mole fraction of oxygen (.21) x ((1-.21)/respiratory quotient)

Question 6

What happens to alveolar oxygen partial pressure as alveolar ventilation increases?

Answer 6

partial pressure of oxygen also increases (as carbon dioxide partial pressure falls)

Question 7

Is ventilation uniform throughout the lung?

Answer 7

No; because of the lung’s weight, intrapleural pressure is more negative at the apex than at the base when the subject is upright; at functional residual capacity the apex is relatively overinflated and thus has less compliance with inspiration

Question 8

What is ventilation proportional to?

Answer 8

the change in volume of the lung (a more compliant portion of the lung is “more ventilated”

Question 9

What is the profile of pulmonary circulation for resistance and pressure?

Answer 9

low resistance and low pressure (does not need to be pumped long distances like the systemic circulation)

Question 10

Is pulmonary perfusion uniform?

Answer 10

No; blood flow is highest near the base (not at the very base) and declines towards the apex

Question 11

Describe the state of pulmonary vessels during normal cardiac output? What happens with small increases in pulmonary artery pressure?

Answer 11

some vessels are open but the pressure is not great enough to allow flow, some vessels are collapsed and other vessels may conduct (pressure gradient must be higher than alveolar pressure); with small increases in pressure, the vessels conducting distend, the open non-conducting vessels begin to conduct blood and the closed vessels may be open (recruitment)

Question 12

Why does blood flow tend to decrease near the base of the lung?

Answer 12

intrapleural pressure decreases (becomes less negative) at the base and this constricts extra-alveolar vessels (proximal to the alveolar vessels

Question 13

Does hypoxia constrict or dilate pulmonary vessels? Is this unique to the pulmonary circulation?

Answer 13

constricts; this is unique; mechanism: (on smooth muscle cells) decreases potassium conductance causing increased intracellular calcium and contraction

Question 14

Is ventilation/perfusion ratio uniform throughout the lung?

Answer 14

No; greatest ratio at the apex and lowest at the base

Question 15

What is the composition of alveolar gases (oxygen and carbon dioxide) at the base? at the apex?

Answer 15

lower oxygen and slightly higher carbon dioxide; higher oxygen and slightly lower carbon dioxide

Question 16

Describe a shunt.

Answer 16

obstruction in ventilation (V/Q = 0); alveolar gas composition will be similar to mixed-venous blood (hypoxemia and hypercapnia); triggers hypoxic vasoconstriction

Question 17

Describe alveolar dead-space.

Answer 17

obstruction in pulmonary circulation (V/Q=infinity); alveolar gas composition will be similar to inspired air (hyperoxia and hypocapnia); triggers bronchoconstriction and reduction in surfactant production

Question 18

Describe the dorsal respiratory group.

Answer 18

Found around the NTS in the medulla; receive afferent information from vagus nerve (X; aortic bodies) and glossopharyngeal nerve (IX; carotid bodies); primarily inspiratory; may synapse on phrenic motor nuclei (diaphragm) or on VRG

Question 19

Describe the ventral respiratory group.

Answer 19

scattered around the medulla (Botzinger complex = rostral, pre-Botzinger complex, and caudal VRG); motor functions (no sensory) and has both inspiratory and expiratory neurons (caudal); innervates upper airways (pharynx and larynx); pre-Botzinger complex may be CPG

Question 20

Describe peripheral chemoreceptors.

Answer 20

found in aortic and carotid bodies; neuro-ectodermal origin (glomus cell); receive very high perfusion and are highly metabolic; hypoxia can stimulate neurotransmitter release through changes in heme moieties, redox reactions, and changes in cAMP (decrease in potassium conductance; depolarization; calcium influx); innervate medullary DRG; hypercapnia is sensed by decreased intracellular pH; decreased extracellular pH is transmitted into the cell via the balance of acid loaders and extruders

Question 21

Describe central chemoreceptors

Answer 21

protected by blood brain barrier; rather insensitive to hypoxia and very sensitive to pH and carbon dioxide; carbon dioxide can diffuses from lumen of blood in brain extracellular fluid where it combines with water to form bicarbonate and a proton

Question 22

What are the chemosensitive neurons?

Answer 22

medullary raphe, ventrolateral medulla, and locus coeruleus

Question 23

What is the normal pH of CSF?

Answer 23

7.3

Question 24

What is the effect of decreasing the pH of CSF on breathing? is this a quick response? is this a sustained response?

Answer 24

hyperventilation; no (may take up to 10 minutes); no: choroid plexus pumps bicarbonate ions into the CSF to compensate for the increased carbon dioxide partial pressure (make take hours to days)

Question 25

What happens to ventilation in diabetic ketoacidosis?

Answer 25

decreased pH stimulates both central and peripheral chemoreceptors to increase ventilation (Kussmal breathing) and decreases carbon dioxide partial pressure

Question 26

What are the mechanics of increased ventilation?

Answer 26

first increased depth of breathing followed by increased frequency

Question 27

Describe integrated responses to hypoxia and respiratory acidosis.

Answer 27

respiratory acidosis increases sensitivity to hypoxia; hypoxia accentuates the acute response to respiratory acidosis (increasing sensitivity of the peripheral chemoreceptors)