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*USMLE* > Physio > Flashcards

Physio Flashcards

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USMLE® Step 1 flashcards
1
Q

Cardiac Pressures

sys + dias for 4 chambers, pulmonary artery + aorta

A 
RA - <5 mmHg
RV - 25/5
PA - 25/10
LA - <10
LV - 120/10
Aorta - 120/80
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2
Q

Renal Excretion Rate calculation

what is used to estimate GFR and how?

A

Inulin clearance estimates GFR (freely filtered + not secreted or reabs. in tubules)

RER = total filtration rate - total tubular reabsorption rate

total filtration rate = GFR x plasma conc. of substance in question

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3
Q

Fluid Balance Changes in…

Diabetes Insipidus

A

Hyperosmotic volume contraction…

loss of HYPOtonic urine causes overall volume loss (ECF and ICF) with HYPERosmotic blood

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4
Q

Fluid Balance changes in…

GI hemorrhage or diarrhea

A

Isotonic ECF loss

blood is ECF and is obviously isotonic with blood…
no changes in ICF or blood osmolarity

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5
Q

Fluid balance changes in…

adrenal insufficiency

A

Hyposmotic volume contraction (with ICF gain)

no aldo > NaCl and ECF loss > hypoosmotic plasma > fluid shift to IC space

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6
Q

Fluid balance changes in…

hypertonic saline infusion

A

Hypertonic volume expansion (with IFC loss)

hypertonic fluid addition to ECF (plasma) > water drawn out of ICF > further ECF expansion

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7
Q

Fluid balance changes in…

primary polydipsia -or- SIADH

A

Hyposmotic volume expansion (clinical euvolemia)

hyposmotic intake > fluid shifts into cells > both ICF and ECF increase + osmolarity decreases

ECF increases less due to normalization via ANP/aldo balance

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8
Q

High Altitude Sickness

initial ABG?

compensated ABG (when?) ?

A

“Hypobaric Hypoxia” - % of air that is O2 is same, but lower barometric pressure means decreased pO2

Initial - pH is significantly high (7.5+); pCO2 is low (hyperventilation) and pO2 is low

Compensated WITHIN 48 HOURS - bicarb excretion leaves pH NEAR-NORMAL, while pCO2 and pO2 are still low

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9
Q

Vessel with lowest PO2

A

Coronary sinus

Myocardial oxygen extraction is the highest in the body (60-75%)

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10
Q

Formula for expected CO2 level during respiratory compensation of metabolic acid-base disorder

A

Winter formula

PaCO2 = [1.5 x HCO3] + 8 +/- 2

gives a range of 4 mmHg, if the CO2 falls below that range = additional respiratory alkalosis

if above that range = additional respiratory acidosis

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11
Q

Which substance can be used to estimate renal blood or plasma flow? Why?

A

PAH

it is filtered AND actively secreted, so the rate at which the kidneys clear PAH reflects RPF.

Must use both urinary and plasma concentrations to calculate, plus urine flow rate.

(remember PAH for Plasma!)

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12
Q

How can RPF be calculated with PAH?

A

RPF = [urine PAH] x urine flow / [plasma PAH]

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13
Q

What affects filtration fraction?

A

FF is decreased by DECREASED GFR or INCREASED RPF

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14
Q

Functions of DAG and IP3 in the Gq > PLC > Ca release pathway?

A

PLC hydrolyzes PIP2 into DAG and PIP, then…

DAG - direct PKC STIMULATION

IP3 - mediates Ca release from ER > major stimulator of PKC

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15
Q

effect of ADH other than vasoconstriction + water reabsorption

A

increases urea reabsorption in the INNER MEDULLARY COLLECTING DUCT

this accentuates the medullary concentration gradient > maximizes free water reabsorption

large releases of ADH as in hypovolemic shock can thus result in ELEVATED SERUM UREA and a BUN:CREAT RATIO >20:1

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16
Q

what 2 molecules are FREELY FILTERED by the glomerulus and NOT REABSORBED / SECRETED by the tubules?

A

INULIN

MANNITOL

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17
Q

what 3 substances are FREELY FILTERED by the glomerulus and REABSORBED by the tubules?

there are many more like this, but these are some classic examples

A

SODIUM - heavily reabsorbed, FENa normally <1%

UREA - passive resorption in pct and inner medullary collecting duct; passive secretion in thin parts of Henle loop; regulated by ADH (increases inner medullary collecting duct resorption + thus water resorption)

GLUCOSE

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18
Q

What 2 substances are FREELY FILTERED by the glomerulus and actively SECRETED by the tubules?

A

PAH

CREATININE (is secreted somewhat, but no nearly as much as pah)

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19
Q

PTH kidney effects

A

increased Ca resorption

decreased Pi resorption

20
Q

PTH bone effects

A

indirect osteoclast activation by…

osteoblast activation > RANKL secretion

decreased OPG secretion > higher RANKL activity

21
Q

What is the “Haldane effect”? (2 effects, really)

A

In the lungs, at high pO2, hemoglobin will have…

1) DECREASED CO2 AFFINITY - unloads CO2 carried from tissues (off carbamino terminals of alpha/beta subunits); abt 10% CO2 carried to lungs in this form
2) INCREASED ACIDITY OF Hb MOLECULE - stabilization in the “T” tense state > release of H+ from histidine side chains; most CO2 is carried to lungs in HCO3- > H+ ions combine with bicarb to form carbonic acid, then water and CO2 for further co2 release

22
Q

What is the “Bohr effect”?

more specifically, what happens to O2 and CO2 during the Bohr effect?

A

in peripheral tissues, HIGH pCO2 causes an INCREASED AMBIENT ACIDITY (H+ conc.) that shifts Hb dissociation curve RIGHTWARD > unloading of O2

abt 10% of CO2 forms “carbamino adducts” with Hb molecule itself

the rest forms H+ and HCO3- via RBC carbonic anhydrase > H+ binds Hb to stabilize deoxygenated form

HCO3- is exchanged (out to plasma) for chloride (into RBC) via the “chloride shift”

23
Q

Proximal convoluted tubule

handling of water? solutes? relationship of tubular fluid to plasma?

A

water and solutes BOTH reabsorbed

tubular fluid is ISOTONIC (300 mOSm/L) to blood

24
Q

Descending limb of loop of Henle

handling of water? solutes? relationship of tubular fluid to plasma?

A

ONLY WATER is reabsorbed, not solutes

tubular fluid becomes HYPERTONIC (> 300 mOSm/L, and up to 1200 in the medulla)

25
Q

Ascending limb of loop of Henle

handling of water? solutes? relationship of tubular fluid to plasma?

A

ONLY SOLUTES are reabsorbed, not water

tubular fluid becomes HYPOTONIC (< 300 mOSm/L)

(thick + thin ascending limbs are the main region of urine dilution, mainly via NaCl resorption)

26
Q

Distal convoluted tubule

handling of water? solutes? relationship of tubular fluid to plasma?

A

is relatively impermeable to water + SOLUTES ARE STILL REABSORBED, so…

tubular fluid is the MOST HYPOTONIC of all segments in the DCT (about 100 mOsm/L)

27
Q

Collecting Duct

handling of water? solutes? regulation?

A

primary region of urine concentration via ADH-mediated water resorption

ADH mediates aquaporin translocation (V2 > Gs > cAMP > Aq-2) and the high medullary interstitial osmolarity drives diffusion of water through the aquaporins out of the tubules

28
Q

Hepcidin

what stimulates/inhibits its release?

A

it’s an ACUTE PHASE REACTANT

stimulated by - HIGH IRON and INFLAMMATION

inhibited by - HYPOXIA and ERYTHROPOIESIS

29
Q

How does HEPCIDIN influence body iron stores?

A

Hepcidin binds the FERROPORTIN on enterocytes and macrophages > causes INTERNALIZATION of ferroportin and less release of iron from GI into circ or from macrophages

30
Q

Baroreceptor firing rate

how does hypotension affect it?
hypertension?

A

hypotension - firing rate DECREASES and baroreceptor reflex helps restore bp via increased HR

hypertension - firing rate INCREASES and baroreceptor reflex lowers bp via decreased HR

31
Q

what changes are detected by CAROTID baroreceptors? and by AORTIC baroreceptors?

A

carotid detects BOTH increase and decrease in BP

aortic detects ONLY increases

32
Q

What are the signaling mechanisms and functions of the V1 and V2 receptors for vasopressin?

A

V1 - Gq > VASOCONSTRICTION, myocardial hypertrophy, platelet agg, GLYCOGENOLYSIS and uterine contraction

V2 - Gs > insertion of Aquaporin-2 into apical membrane of collecting duct cells; release of vWF and FACTOR VIII; plus VASODILATION

33
Q

What is the LENGTH CONSTANT in nerve conduction physiology?

aka?

how does myelination affect it?

A

aka SPACE CONSTANT

the DISTANCE an AP can propagate along an axon WITHOUT ACTIVE REGENERATION BY ION CHANNELS

myelination INCREASES length constant

34
Q

What TWO RESISTANCES does the LENGTH CONSTANT of a nerve depend on and how?

A

AXOPLASMIC resistance and MEMBRANE resistance

when axoplasmic R is lower than membrane R, the AP will propagate forward along the axon rather than dissipating out through the membrane

MYELINATION increases membrane R > aids axonal propagation

35
Q

what is TIME CONSTANT in nerve conduction physiology?

is based on what?

A

time it takes for membrane potential to respond to a change in permeability

based on membrane RESISTANCE and CAPACITANCE

36
Q

How does myelination affect the time constant?

A

it decreases capacitance and increases resistance of the membrane, resulting in a LOWER TIME CONSTANT

meaning the membrane potential can CHANGE FASTER when nerve is myelinated

37
Q

How is bicarbonate dealt with along the PCT?

A

it is ACTIVELY REABSORBED due to CARBONIC ANHYDRASE activity (inhibition > excretion of bicarb > metabolic acidosis; eg, acetazolamide)

so on a chart of solute conc. along PCT, bicarb is hyperbolically decreasing

38
Q

Differential handling of PAH, inulin and creatinine in the PCT

A

PAH - freely filtered and then 90% of remainder is secreted in PCT

Creatinine - freely filtered and then only 20% of remainder is secreted in PCT

Inulin - freely filtered and then NONE is secreted

(but conc. of all three increases along length of PCT because water is reabsorbed and they are not)

39
Q

How does Ca++ promote muscle contraction?

(2 molecule names)

(within the muscle cell, not referring to presynaptic Ca++ influx that stimulates ACh release)

A

Binds to TROPONIN C and then TROPOMYOSIN is displaced from actin so that myosin can bind

40
Q

How does muscle cell action potential result in Ca++ release from the SR?

2 components

A

causes a conformational change in the cell membrane 1) DHPR (dihydropyridine receptor) which is mechanically coupled to the sarcoplasmic reticulum-surface 2) RYR that releases Ca++

41
Q

After Troponin C binds Ca++ and tropomyosin displacement frees up myosin binding sites…

what 2 steps occur before myosin release?

and what is the status of myosin-ADP/ATP binding during this?

A
  1. CROSSBRIDGE formation - myosin binds actin with its attached ADP + Pi
  2. POWER STROKE - myosin head cocks back, pulling actin with it, which occurs via the RELEASE OF Pi from ADP
42
Q

What happens between the power stroke and myosin detachment from actin?

(2 steps)

A
  1. ADP IS RELEASED from myosin

2. ATP BINDS to myosin > myosin head releases actin (and Ca++ is resequestered in SR)

43
Q

What causes the myosin head to return from the detached-from-actin-but-still-bent state to the “cocked” high energy (90 deg angle) state?

A

ATP hydrolysis to ADP + Pi

44
Q

What is a miniature end plate potential in NMJ physio?

A

the EPP produced by a SINGLE ACh vesicle

the smallest possible depolarization that can be induced in a muscle

usually around 0.4 mV - would NOT be reduced in response to something like botulism because botulism just inhibits vesicle release, but if a vesicle is released it will still have an effect and make an mEPP

45
Q

How do the oxygen dissociation curves of MYOGLOBIN and HEMOGLOBIN differ?

A

Hemoglobin - has a SIGMOIDAL curve with a “P50” of 26 mmHg (meaning 50% saturation at pO2 26); due to increasing affinity as O2 binds each of 4 heme grps

Myoglobin - has a HYPERBOLIC curve with a P50 of 1 mmHg; is monomeric with a single heme group and higher affinity than Hb

46
Q

What is the relationship of structure + function between single hemoglobin subunits and myoglobin?

A

both alpha and beta subunits of hemoglobin are very similar in secondary/tertiary structure to myoglobin so…

they bind O2 with similar affinity (high) when in monomeric form (a HYPERBOLIC o2 dissociation curve, ie left-shifted)

*USMLE* (28 decks)

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