Medical Physiology Block 5 Week 3 Flashcards
Define total ventilation, dead-space ventilation, and arterial ventilation
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
How can dead-space be measured?
anatomical dead-space: measure concentration of expired nitrogen; physiological dead-space: measure concentration of expired carbon dioxide (some dead-space may be alveolar)
What is the equation to calculate alveolar carbon dioxide partial pressure?
0.863 x (rate of carbon dioxide production (200 mL/min)/alveolar ventilation rate normally 4.2 L/min)
What happens to alveolar carbon dioxide partial pressure as alveolar ventilation increases? What happens to pH?
decreases; 0.863 x (200/8.4) = about 20 mm Hg (same as arterial carbon dioxide partial pressure; respiratory alkalosis
What is the equation to calculate alveolar oxygen partial pressure?
inspiratory oxygen partial pressure - (alveolar carbon dioxide partial pressure multiplied by mole fraction of oxygen (.21) x ((1-.21)/respiratory quotient)
What happens to alveolar oxygen partial pressure as alveolar ventilation increases?
partial pressure of oxygen also increases (as carbon dioxide partial pressure falls)
Is ventilation uniform throughout the lung?
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
What is ventilation proportional to?
the change in volume of the lung (a more compliant portion of the lung is “more ventilated”
What is the profile of pulmonary circulation for resistance and pressure?
low resistance and low pressure (does not need to be pumped long distances like the systemic circulation)
Is pulmonary perfusion uniform?
No; blood flow is highest near the base (not at the very base) and declines towards the apex
Describe the state of pulmonary vessels during normal cardiac output? What happens with small increases in pulmonary artery pressure?
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)
Why does blood flow tend to decrease near the base of the lung?
intrapleural pressure decreases (becomes less negative) at the base and this constricts extra-alveolar vessels (proximal to the alveolar vessels
Does hypoxia constrict or dilate pulmonary vessels? Is this unique to the pulmonary circulation?
constricts; this is unique; mechanism: (on smooth muscle cells) decreases potassium conductance causing increased intracellular calcium and contraction
Is ventilation/perfusion ratio uniform throughout the lung?
No; greatest ratio at the apex and lowest at the base
What is the composition of alveolar gases (oxygen and carbon dioxide) at the base? at the apex?
lower oxygen and slightly higher carbon dioxide; higher oxygen and slightly lower carbon dioxide
Describe a shunt.
obstruction in ventilation (V/Q = 0); alveolar gas composition will be similar to mixed-venous blood (hypoxemia and hypercapnia); triggers hypoxic vasoconstriction
Describe alveolar dead-space.
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
Describe the dorsal respiratory group.
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
Describe the ventral respiratory group.
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
Describe peripheral chemoreceptors.
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
Describe central chemoreceptors
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
What are the chemosensitive neurons?
medullary raphe, ventrolateral medulla, and locus coeruleus
What is the normal pH of CSF?
7.3
What is the effect of decreasing the pH of CSF on breathing? is this a quick response? is this a sustained response?
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)
What happens to ventilation in diabetic ketoacidosis?
decreased pH stimulates both central and peripheral chemoreceptors to increase ventilation (Kussmal breathing) and decreases carbon dioxide partial pressure
What are the mechanics of increased ventilation?
first increased depth of breathing followed by increased frequency
Describe integrated responses to hypoxia and respiratory acidosis.
respiratory acidosis increases sensitivity to hypoxia; hypoxia accentuates the acute response to respiratory acidosis (increasing sensitivity of the peripheral chemoreceptors)
