Chapter 4: Blood Flow and Metabolism Flashcards
Describe both the relative flow and relative pressure in the pulmonary and systemic circulations
The flows are equal (5-6 liters per minute), but the pressures are very different. Systemic»_space; Pulmonary
What is the mean pressure difference across the pulmonic circulation?
Arterial = 15 mm Hg Venous = 5 mm Hg
delta P = 10 mm Hg
What is the mean pressure difference across the systemic circulation?
Arterial = 100 mm Hg Venous = 2 mm Hg
delta P = ~100 mm Hg
What effect does increasing tension in the surrounding alveolar walls have on the extra-alveolar vessels of the lung?
The extra-alveolar vessels are pulled open
What tissues will you find in the extra-alveolar vessels of the lung?
Smooth muscle and elastic tissue
Where does vascular constriction in response to alveolar hypoxia occur?
Arterioles and small arteries (not veins)
What effect does lung inflation have on the caliber of the extra-alveolar vessels?
The caliber of the extra-alveolar vessels increases
What formula describes the relationship between vascular resistance, blood flow, and pressure?
Analogous to Ohm’s Law (V = IR), we can describe vascular parameters with the relation:
delta P = Q x R
where delta P = arterial - venous pressure
where Q = rate of flow of blood
where R = total resistance in the systemic or pulmonary circuit
What happens to pulmonary vascular resistance on exercise?
Decreases
Why does pulmonary vascular resistance decrease on exercise?
Increased pressure (more flow) results in two compensating mechanisms in the pulmonary vasculature. At low pressure increases, previously closed capillary beds open up, creating more pathways for blood flow. At higher pressure increases, vessel distension accommodates increased flow and pressure, reducing resistance.
Describe the Fick principle as it relates to relating blood flow, arterial and venous constituent concentration, and delivery of blood constituents.
Constituents could be oxygen, carbon dioxide, nutrients, etc. Basically, the difference in concentration of a constituent across some distance, multiplied by the rate of flow of blood across that same distance, describes the amount of that constituent consumed:
d[C]/dt = Q x ([C upstream] - [C downstream])
In which zone(s) of the lung does alveolar pressure exceed arterial pressure
Zone 1 - not seen very often, means the transmural pressure is such that the capillary beds will close down
In which zone(s) of the lung does venous pressure exceed alveolar pressure?
Zone 3 - flow determined by arterial/venous gradient
What is the pressure gradient that determines blood flow in Zone 2?
Arterial - alveolar
What effect would acutely increasing pulmonary venous pressure have on pulmonary vascular resistance?
Pulmonary vascular resistance would decrease. Looking at the relationship R = (delta P)/Q, it is clear that decreasing delta P reduces R. This can be accomplished by bringing up the venous pressure, all other things staying the same.
What happens to the pulmonary vasculature in an individual breathing a 10% oxygen mixture?
Hypoxic pulmonary vasoconstriction
What can happen to pulmonary capillaries under conditions of mechanical ventilation?
Too much positive pressure will compress the capillaries as the alveolar pressure rises
Describe the mechanism by which pulmonary resistance is decreased at birth
At birth, baby takes first breath. Oxygenation reverses the effect of hypoxic vasoconstriction. This probably occurs when the vascular endothelium releases nitric oxide (NO, synthesized from L-arginine via NO synthase (eNOS)), which activates soluble guanylate cyclase to increase synthesis of cGMP. Cyclic GMP relaxes smooth muscle, opening the pulmonary vasulature
Hypoxic vasoconstriction occurs in reaction to what condition?
Low alveolar oxygen, not arterial or venous O2 saturation
Describe the pressure driving blood into or out of capillaries into the interstitium
The net pressure driving fluid flow across the capillary endothelium is a balance between hydrostatic pressure pushing out and osmotic pressure pulling in. Strictly speaking, the relation is:
delta P = K{(Pc - Pi) - sigma(PIc - PIi)}
where Pc and Pi are the capillary and interstitial hydrostatic pressures
where PIc and PIi are the capillary and interstitial osmotic pressures
where K and sigma are coefficients
We assume the K and sigma are 1 for our purposes, which makes calculations easier
What peptides are processed in the lungs? How are they modified?
Angiotensin I –> converted to Angiotensin II by ACE
Angiotensin II –> Unaffected
Vasopressin –> Unaffected
Bradykinin –> Up to 80% inactivated
What amines are processed in the lungs? How are they modified?
Serotonin –> almost completely removed
Norepinephrine –> Up to 30% removed
Histamine –> Unaffected
Dopamine –> Unaffected
What arachidonic acid metabolites are processed in the lungs? How are they modified?
Prostaglandins E2 and F2alpha –> Almost completely removed
Prostaglandin A2 –> Unaffected
Prostacyclin (PGI2) –> Unaffected
Leukotrienes –> Almost completely removed
Side effect of ACE inhibitor on the lungs?
ACE degrades bradykinin. Blocking ACE results in bradykinin build up, leads to coughing fits.
