Renal System Physiology

Front 1

Formation of dilute urine
• when would this happen??

Back 1

filtrate at top of ascending limb is dilute due to salt removal
• in absence of ADH ( hormone) collecting ducts remain impermeable to water & very dilute urine produced
• osmolarity of urine can be as low as 65 mOsm

Front 2

Formation of Concentrated Urine

Back 2

ADH from hypothalamus; amount secreted depends on body hydration
• ADH acts at collecting ducts  increases number of water channels in principal cells
• collecting ducts extend into medullary area > filtrate can again attain an osmolarity up to 1200 mOsm

Front 3

Facultative Water Reabsorption (ADH)

Back 3

constant low level of ADH secretion  ***increased by any event which raises plasma osmolarity above 300 mOsm (eg: )
ADH important for formation of concentrated urine, but ADH action depends on medullary gradient & presence of urea also critical (consequences for severely malnourished individuals??)

Front 4

define diuretic & provide some examples

Back 4

enhances urinary output
• any substance that is:
 not reabsorbed
 exceeds renal reabsorption ability eg:
What is effect of alcohol on urine formation and why??
• caffeine and most prescribed diuretic drugs inhibit Na+ reabsorption

Front 5

define renal clearance & express this parameter in terms of U, V & P

Back 5

volume of plasma from which a substance is 100% cleared per unit time
RC = UV/P
U: [substance(mg/ml)] in urine
V: flow rate of urine formation (ml/min)
P: [substance(mg/ml)] in plasma

Front 6

physical characteristics of urine

Back 6

Colour & transparency:
• clear/pale to deep yellow (urochrome = pigment from )
• deepness of yellow indicates ???
• some drugs/vitamins can alter colour of urine; cloudiness can indicate ???
Odour:
• will develop ammonia odor if left to stand due to bacterial metabolism of urea
• odor can be altered by some drugs & vegetables, also diseases (prime example is: )
pH:
• usually ~ 6, but can vary (~4.5-~8)
(i) what is an acid-ash diet?[acidic urine]
(ii) what is an alkaline-ash diet? [alkaline urine] – 2 other causes of alkaline urine?

specific gravity:
• usually 1.001 to 1.035

Front 7

Chemical Composition of urine

Back 7

95% water, 5% solutes
• solute in highest concentration is:
• also uric acid (from ) & creatinine (from )
• in decreasing order: , Na+, K+, phosphate, sulfate, creatinine, uric acid
• much less, but more variable levels of: Ca++, Mg++, HCO3-
• v high concentrations of any constituent may indicate pathology

Front 8

Incontinence:

Back 8

inability to control micturition voluntarily > emotional problems, pressure of pregnancy, nervous system problems

Front 9

Urinary retention

Back 9

bladder unable to expel urine; e.g. after general anaesthetic; can also result from prostate hypertrophy

Front 10

define renal failure; indicate potential causes & options for treatment

Back 10

not enough functioning nephrons  filtrate formation reduced or stopped:
√ repeated damaging kidney infections
√ physical injuries to kidneys
√ prolonged pressure on skeletal muscle
√ inadequate blood delivery to tubules
• nitrogenous wastes accumulate & blood becomes acidic  diarrhea, vomiting, edema, labored breathing, cardiac irregularities, convulsions, coma, death
• usu 3 times/week; 4-8 h session
• symptoms apparent when ~75% of renal function lost

Front 11

Peritoneal Dialysis:

Back 11

Works by using the peritoneal membrane; can be done at home or at work
does not require weekly hospital visits
dialysate is infused into the peritoneal cavity through a catheter; dialysate made of made up mostly of salts and sugar, encourages filtration through the peritoneum
Extra fluid and wastes is drawn from blood into dialysate

Front 12

Two types of dialysis

Back 12

Continuous Ambulatory Peritoneal Dialysis (CAPD): usu 4-5 times/day. Pt puts dialysate (about two litres) into peritoneal cavity through catheter - dialysate stays there for 4-5 hours before it is drained back into the bag and thrown away. There is a new bag of dialysate for each exchange.

Continuous Cycling Peritoneal Dialysis (CCPD): usu is done at home using a special machine called a cycler. This is similar to CAPD except that a number of cycles (exchanges) occur. Each cycle usually lasts 1-1/2 hours and exchanges are done during the night while pt sleeps.

Front 13

Demonstrate understanding of the role of the kidneys in fluid & electrolyte balance (Ch. 26)
4.4.1 define the fluid compartments

Back 13

ntracellular fluid compartment (ICF) – within cells 60% total body fluid
Extracellular fluid compartment (ECF) – 40% body fluid; 2 areas
(a) plasma: ~20% ECF
(b) interstitial space: ~80% ECF

Front 14

differences between electrolytes & non-electrolytes and of the key electrolytes in ICF & ECF

Back 14

electrolytes have greater osmotic power than non-electrolytes – Why?
• in ECF: chief cation is sodium and chief anion is chloride
• in ICF: chief cation is potassium and chief anion is phosphate
• Na+ and K+ are opposite, when comparing ECF & ICF  ATP-dependent Na/K pumps keep intracellular [Na+] low and maintain high intracellular [K+]

Front 15

define water balance

Back 15

water intake must = water output
Intake: liquids, foods, cellular metabolism
Output: ~60% via kidneys; also lungs, skin, sweat, feces
Increased plasma osmolality (usu 285-300 mOsm):
thirsty  increase water intake
ADH stimulates renal water reabsorption
Decreased plasma osmolality:
thirst not stimulated
ADH secretion not stimulated

Front 16

escribe the thirst mechanism

Back 16

 in plasma volume of > 10% or 1-2%  in plasma osmolarity decrease dry mouth
osmoreceptors of hypothalamic thirst centre lose water to hypertonic ECF > become irritable & depolarize
dampening of thirst begins once mucosa of mouth & throat moistened > prevents overdrinking while water moves to ECF

Front 17

three Disorders of water balance

Back 17

(i) Dehydration:

(ii) Hypotonic hydration:

(iii) Edema:

Front 18

justify the role of sodium as the only solute exerting significant osmotic pressure and describe the influences on sodium balance of: aldosterone, cardiovascular baroreceptors, ADH and ANF

Back 18

salt content of body can vary due to: diet, loss via sweating, vomiting, etc
Sodium
• 90-95% of ECF solute; NaCl & NaHCO3 contribute 280/300 mOsm solute concentration; the only solute exerting significant osmotic pressure
• regulating Na+ balance one of most important functions of kidneys
• sodium content of body may change, but its concentration in ECF remains stable due to immediate adjustments in water volume

Front 19

Factors influencing [Na+]

Back 19

Aldosterone
• most influential agent wrt kidney; acts slowly
• even without aldosterone, 65% Na+ in filtrate reabsorbed in PCT & 25% in loop of Henle; aldosterone secretion essential to life
• high aldosterone: virtually all remaining Na+ (chloride co-transported) actively reabsorbed by DCT & collecting ducts

Front 20

2 pathways to aldosterone secretion:

Back 20

(i) renin-angiotensin system
(ii) direct effect of high K+ or low Na+

Front 21

renin secretion in response to

Back 21

(i) symp ns
(ii) decreased filtrate osmolarity
(iii) decreased stretch (bp)

Front 22

levels leading to angiotensin secretion

Back 22

angiotensinogen (from liver)
(renin)
angiotensin I
(ACE)
angiotensin II

Front 23

Addison’s disease

Back 23

hyposecretion of aldosterone excess loss of Na+ & water in urine

Front 24

influence of sodium balance on Cardiovascular Baroreceptors

Back 24

pressure diuresis:
when blood volume and/or pressure , symp input to kidney   result is dilation of afferent arterioles &  GFR --- overall result is???
• not that effective – why????
• [Na+] determines blood volume

Front 25

influence of sodium balance on ADH

Back 25

water reabsorption in collecting ducts relies on ADH secretion
osmoreceptors in hypothalamus:
(i) low [Na+] = excess fluid  reduced ADH secretion & dilute urine
(ii) high [Na+] = decreased blood volume  ADH & reduced urine volume

Front 26

influence of sodium balance on Atrial Natriuretic Factor

Back 26

released by certain cardiac cells when bp 
• potent diuretic & natriuretic hormone:
(i) inhibits Na+ reabsorption by DCT & collecting duct
(ii)  release of ADH, renin & aldosterone
(iii) induces vasodilation
overall effect is to reduce bp

Front 27

other hormones influenced by sodium balance

Back 27

Other Hormones
Estradiol
Progesterone
Cortisol

Front 28

outline the main features of potassium balance

Back 28

high [K+] in ECF is toxic
√ reduces membrane excitability
√ involved in acid-base balance
• usu 10% K+ in filtrate lost (90% )
• if [K+] in ECF high, tubular secretion (principal cells of cortical collecting ducts)
• dietary K+ important

Front 29

2 factors determine rate & extent of K+ excretion:

Back 29

(i) plasma [K+]
(ii) [aldosterone]

Front 30

outline the main features of calcium and phosphate balance

Back 30

99% of body’s calcium found in bone
• Ca++ important for: , , -- hence must be closely regulated
• 2 hormones:
(i) PTH: primary, from parathyroid glands; increases blood Ca++
(ii) calcitonin: parafollicular cells of thyroid gland; decreases blood calcium
• usu ~98% filtered Ca++ is reabsorbed

Front 31

PTH:

Back 31

stimulus for secretion is drop in blood Ca++
bone (osteoclasts)
small intestine
kidneys – opposite effects on Ca++ vs phosphate
• most (~75%) of filtered phosphate reabsorbed in proximal tubules by AT
no PTH: phosphate reabsorbed to Tm
high PTH: reduced AT of phosphate

Front 32

calcitonin

Back 32

stimulus for secretion is rising blood Ca++

Front 33

describe the 3 mechanisms of acid-base balance used in the body; describe in detail the 3rd mechanism – the role of the kidneys

Back 33

very important: activity of functional ptns depends on pH (7.4 arterial, 7.35 venous, 7.0 intracellular)
What is alkalosis? What is acidosis?

Front 34

Sources of acid:

Back 34

Breakdown of phosphorus-containing ptns  phosphoric acid
Anaerobic metabolism of glucose 
Fat metabolism  fatty acids & ketone bodies
Loading & transport of CO2 as bicarbonate  H+ ions
HCl released during digestion stays in GI tract but must be buffered

Front 35

blood is regulated by

Back 35

Blood [H+] is regulated by:
Chemical buffer systems: (H2CO3  HCO3-, Na2HPO4  NaH2PO4, protein buffers)
pro:
con:
Respiratory centre in brain stem:
pro:
con:
Renal mechanisms:
pro:
con:
• renal mechanism involves:
(1) excreting bicarbonate (= gaining H+)
(2) reabsorbing or generating new bicarbonate (= getting rid of H+)

Front 36

H+ Secretion:

Back 36

tubule & collecting duct respond to pH of ECF & alter rate of H+ secretion
Secreted H+ comes from carbonic acid
For each H+ actively secreted into tubule lumen, one Na+ is reabsorbed, maintaining electrochemical balance
H+ can combine with HCO3- producing CO2 & H2O; CO2 returns to tubule cell & promotes more H+ secretion

Front 37

Bicarbonate reabsorption:

Back 37

Problem: tubule cells almost completely impermeable to bicarbonate in filtrate
• need to constantly replenish body’s stores of bicarbonate
(why?? how is it lost??)
• bicarbonate ions are reabsorbed indirectly from bicarbonate made in tubule cells
• Na+ accompanies HCO3- into peritubular capillaries

Front 38

Generating new bicarbonate ions:

Back 38

bicarb reabsorption just recycles bicarb already present in body
• new H+ enters via diet  must generate new bicarb to maintain pH
(i) phosphate buffer system
(ii) NH4+ excretion

Front 39

Phosphate buffer system:

Back 39

weak base is HPO42-

components freely filter into tubules; usually ~75% reabsorbed, except during acidosis

type A intercalated cells actively secrete H+ (new HCO3- generated as byproduct; HCO3-/Cl- antiport)

Front 40

NH4+ Excretion:

Back 40

generated in PCT cells by metabolism of glutamine
• HCO3- moves into blood; NH4+ is a very weak acid – lost via urine

Front 41

Bicarbonate ion excretion:

Back 41

more rare occurrence; type B intercalated cells; reverse of reabsorption