renal 7

Question 1

what does the renin-angiotensin pathway begin with?

Answer 1

renin secretion
three stimuli:
i. low blood pressure in renal arterioles causes granular cells to secrete renin
ii. sympathetic neurons activated by CVCC when blood pressure decreases terminate on granular cells and stimulate renin secretion
iii. paracrine feedback (prostaglandins) from macula densa cells signal to the granular cells to secrete renin

Question 2

what is renin’s main role?

Answer 2

-to convert an inactive plasma protein, angiotensinogen, into angiotensin I

Question 3

how is angiotensin converted? (second step in renin-angiotensin pathway)

Answer 3

-angiotensin I is then converted to angiotensin II by an enzyme produced in blood vessel endothelium (especially in the lungs) known as angiotensin converting enzyme (ACE)
-ANGII then travels to the adrenal cortex and stimulates production of aldosterone

Question 4

what are the effects of ANGII (angiotensin II)?

Answer 4

1. increases vasopressin secretion: ANG receptors in hypothalamus initiate this reflex
2. stimulates thirst
3. one of the most potent vasoconstrictors in the body
4. ANGII receptors activated in CVCC increase sympathetic output to heart and blood vessels
5. increases proximal tubule Na reabsorption: stimulate an apical Na/H exchanger, increases water reabsorption

all of these mechanisms help to restore blood pressure

Question 5

what are some treatments to treat hypertension?

Answer 5

pharmaceutical companies now use ACE inhibitors for the treatment of hypertension
-preventing the conversion of ANGI to ANGII leads to relaxation of the vasculature and lower blood pressure

other options include:
-AT receptor antagonist
-renin inhibitors

Question 6

what happened when they discovered what aldosterone and vasopressin do?

Answer 6

-after discovering aldosterone and vasopressin increase Na and water absorption scientists speculated there may be hormones that cause a loss of Na (natriuresis) and water (diuresis)
-in 1981, Canadian scientist discovered atrial natriuretic peptide (ANP)

Question 7

what is was ANP and BNP discovered to do?

Answer 7

-ANP: a peptide hormone produced and secreted by specialized myocardial cells primarily in the atria of the heart
-increased blood volume causes increased stretch of the atria, causing the specialized myocardial cells to release ANP
-a second type of natriuretic peptide is produced in the cardiac ventricles and in some neurons in the brain: brain natriuretic peptide (BNP) (less of a physiological role, used as a biological marker)

Question 8

what is the ANP receptor?

Answer 8

-an enzymatic membrane bound receptor acting though cGMP second messenger system

Question 9

what is ANP?

Answer 9

-increased blood volume stretches atrial wall during filling
-atrial myocytes release ANP in response to stretch

Question 10

what happens in the kidneys, hypothalamus, adrenal cortex and medulla when atrial myocytes respond to stretch?

Answer 10

kidney
-relaxes afferent arterioles (increases GFR)
-reduces renin release from granular cells (reduces aldosterone and ANGII)
-reduces Na reabsorption at the collecting duct
-Hypothalamus: reduces AVP release, inhibit thirst
-adrenal cortex: inhibits aldosterone release
-medulla: acts on the CVCC to decrease blood pressure

Question 11

what happens to potassium?

Answer 11

-complicated by the fact that K is reabsorbed and secreted
-reabsorbed in proximal tubule and ascending limb of loop of henle
-secreted at distal tubule and cortical collecting duct

Question 12

what happens when there is normal K, low K and high K?

Answer 12

-at normal K excretion=10-20% of filtered load (reabsorption is less than filtered)
-low K decreases aldosterone release, reducing secretion which reduces excretion (2%)
-high K stimulates aldosterone release directly and increase secretion and thus excretion (10-150%)

Question 13

what plays a critical role in K homeostasis?

Answer 13

-aldosterone also plays a critical role in K homeostasis (enhances secretion in P cells)
-plasma K needs to be maintained within a narrow range (3.5-5mM)
-alterations in body K levels affects the resting membrane potential of all cells

Question 14

where is potassium particularly important?

Answer 14

-in excitable tissues such as heart or skeletal muscles

Question 15

what does hypokalemia cause?

Answer 15

-muscle weakness because its more difficult for hyperpolarized motor neurons and muscles to fire Ap’s (failure of respiratory and cardiac muscles is particularly worrisome)

Question 16

what does hyperkalemia cause?

Answer 16

-more dangerous, initially leads to hyperexcitability

Question 17

what can a K disturbance balance cause?

Answer 17

-hyperkalemia
-hypokalemia
-eventually cells are unable to repolarize and actually become less excitable
-can lead to life threatening arrythmias in the heart

Question 18

what can disturbances in K balance be a result from?

Answer 18

-kidney disfunction, eating disorders, loss of K in diarrhea or use of diuretics that prevent kidneys from properly reabsorbing K

Question 19

what are critical in restoring the normal state ot salt and water balance?

Answer 19

-behavioral responses
-particularly when ECF volume decreases or osmolarity deviates

Question 20

what is an example of a behavioral response for salt and water balance?

Answer 20

-drinking water is normally the only way to replace lost water and eating salt is the only way to raise the body’s Na content
-the act of drinking relieves thirst, water does not actually have to be absorbed
-unknown receptors in the mouth/pharynx respond to water by decreasing thirst and decreasing AVP release

Question 21

what is the spider web of thirst?

Answer 21

Question 22

what else can influence thirst?

Answer 22

Question 23

what are avoidance behaviors?

Answer 23

help prevent dehydration
-desert animals will avoid heat of day and become active at night
-midday nap “siesta” in tropical countries keep people indoors during the hottest part of the day