Lecture: Urinary System Flashcards
What is the function of the kidneys?
homeostasis: maintain proper blood volume and concentration
What are the 5 processes of the kidneys?
1) filter blood plasma
2) regulate BP, blood volume, and osmotic concentration
3) secrete renin
4) secretes erythropoietin
5) synthesize calcitrol
Define: waste.
any substance that is useless to the body or present in excess of the body’s needs
Define: metabolic waste. What are the most toxic metabolic wastes? Name 3 examples.
waste substance produced by the body; nitrogenous waste; -urea -uric acid -creatine
Define: excretion. What 4 systems is this process carried out by?
process of separating wastes from the bodily fluids and eliminating them from the body;
1) respiratory
2) integumentary
3) digestive
4) urinary
What are the principal parts of a nephron?
renal corpuscle: filters blood plasma
-glomerulus: ball of capillaries
-glomerular capsule: sphere that encloses the glomerulus;
renal tubule: duct that leads away from the glomerular capsule and ends at the tip of a medullary pyramid
-proximal convoluted tubule (PCT):
-nephron loop
-distal convoluted tubule (DCT)
-collecting duct: receives fluid from many nephrons
Differentiate between the 2 types of nephrons.
cortical: just beneath the renal capsule, close to the kidney surface
juxtamedullary: close to the medulla
Give a brief overview of the 4 steps of urine formation.
1) glomerular filtration: creates a plasmalike filtrate of the blood
2) tubular reabsorption: returning water and solutes to blood
3) tubular secretion: addition of substances to filtrate
4) water conservation: conserving water & concentrating urine
From where to where do water and some solutes in the blood plasma travel during glomerular filtration? What kind of process is this? What is it caused by?
capillaries of the glomerulus into the capsular space of the nephron;
passive and nonselective process;
caused by hydrostatic pressure
Describe the 3 barriers that constitute a filtration membrane.
1) fenestrated endothelium of the capillary
2) basement membrane: contains a negatively charged proteoglycan gel that repels certain small molecules
3) filtration slits: podocytes of the glomerular capsule have arms with numerous little extensions called foot processes which wrap around capillaries and repel large anions via their negative charges
Name 6 materials which can permeate the filtration membrane.
1) water
2) electrolytes
3) glucose
4) amino acids
5) nitrogenous wastes
6) vitamins
Name 5 materials which cannot permeate the filtration membrane.
1) calcium
2) iron
3) fatty acids
4) thyroid hormones
5) proteins
What are the 4 filtration pressures which govern glomerular filtration?
1) glomerular hydrostatic pressure
- significantly higher (60 mmHg) than other capillaries (15 mmHg)
- caused by large afferent arteriole diameter compared to small efferent arteriole diameter
2) capsular hydrostatic pressure
3) glomerular colloid osmotic pressure (COP)
- caused by proteins, etc. within blood
4) capsular COP
- is essentially 0 mmHg, unless kidney disease allows protein to filter into the capsular space
Define: glomerular filtration rate.
amount of filtrate formed per minute by both kidneys combined;
males: 125 mL/min (180 L/day);
females: 105 mL/min (151 L/day);
daily values are ~50-60 times more than total blood plasma volume;
only 1-2 L/day of urine output
What happens if the glomerular filtration rate (GFR) is too high?
- fluid flows too rapidly
- urine output rises
- dehydration
- electrolyte depletion
What happens if the glomerular filtration rate (GFR) is too low?
- fluid flows too slowly
- reabsorb wastes
Name the 3 homeostatic mechanisms which change glomerular BP from moment to moment in order to adjust GFR.
1) renal autoregulation
- kidneys alter GFR rate despite high arterial BP
2) sympathetic control
- SNS
3) renin-angiotensin mechanism
Define renal autoregulation and name its two mechanisms.
ability of nephrons to adjust their own blood flow and GFR without external (nervous or hormonal) control;
1) myogenic mechanism
2) tubuloglomerular feedback
Explain the myogenic mechanism of renal autoregulation.
- response to pressure change in renal blood vessels
- smooth muscle of afferent arteriole contracts when stretched due to high BP & relaxes when BP falls
- BP increase»_space;> stretches AA»_space;> AA constricts»_space;> decreases blood flow
Explain the tubuloglomerular feedback mechanism of renal autoregulation.
glomerulus receives feedback on the status of the downstream tubular fluid and adjusts filtration accordingly; juxtaglomerular apparatus (JGA): at end of nephron loop -macula densa cells: --sense flow or fluid composition --secrete messenger to stimulate JGC -juxtaglomerular (JG) cells --constrict or dilate afferent arteriole --secrete renin -mesangial cells --chemo-mechanoreceptor --communication between MD and JGC
Explain SNS control of glomerular BP.
sympathetic NS & adrenal epinephrine constrict afferent arterioles;
strenuous exercise or circulatory shock:
-shunts blood away from the kidneys
-indirectly triggers renin-angiotensin mechanism, stimulating the macula densa cells
-SNS directly stimulates JG cells to release renin
Explain the renin-angiotensin mechanism.
stabilizes systemic BP and ECF volume;
triggered when JG cells release renin;
angiotensin I is converted to angiotensin II by angiotensin-converting enzyme (ACE);
angiotensin II vasoconstricts:
-increases MAP
-constricts AA and EA
-stimulates aldosterone: causes renal tubules to reclaim more Na+ ions from filtrate, water follows, increasing BV
-stimulates ADH: promotes water reabsorption by collecting duct
Explain how reabsorption and secretion maintain homeostasis.
reabsorption:
- reclaims materials from nephron to blood via peritubular capillaries
secretion: removes materials form blood to nephron via peritubular capillaries
Name the 2 routes of reabsorption.
transcellular route, through cytoplasm;
paracellular route, move between cells
Name the 2 routes of reabsorption.
transcellular route, through cytoplasm;
paracellular route, move in between cells
Describe the 3 methods of reabsorption.
1) primary active transport
- ex. Na+ - K+ pump, a type of antiport
2) secondary active transport
- driven indirectly by passive ion gradients
- ex. SGLT, a type of symport
3) solvent drag
- solutes follow solvent
- gradients must be present
Differentiate symports and antiports.
symports are transport proteins that move two solutes across a membrane in the same direction;
antiports are transport proteins that exchange two solutes across a membrane in opposite directions
What part of the nephron reabsorbs most tubular fluids? What does it reabsorb?
proximal convoluted tubule;
all glucose, lactate, and AAs
Describe the 3 methods of reabsorption.
1) primary active transport
- ex. Na-K pump, a type of antiport
2) secondary active transport
- driven indirectly by passive ion gradients
- ex. SGLT, a type of symport
3) solvent drag
- solutes follow solvent
- gradients must be present
Why is sodium reabsortion so crucial to reabsorption as a whole?
it creates an osmotic and electrical gradient to drive reabsorption of water and others;
Explain the process of sodium reabsorption.
facilitated diffusion occurs from lumen of tubule into tubule cell;
-symport with glucose, AA, phosphate, and lactate
-antiport with H+
Na-K pump (antiport) pumps Na+ into ECF;
Na+ enters peritubular capillary via solvent drag with water
What 2 factors contribute to chloride reabsorption?
1) chloride follows sodium
2) water reabsorption creates gradient
Explain the process of chloride reabsorption.
Cl- moves from lumen of tubule into tubule cell via antiport exchange with sodium;
Cl- enters ECF via symport with K+;
Cl- enters peritubular capillary via solvent drag with water
What is a key difference between glucose reabsorption and sodium & chloride reabsorption?
glucose reabsorption not assisted by concentration gradient;
more glucose in tubular cell than lumen of tubule
Explain the process of glucose reabsorption.
glucose moves from lumen of tubule into tubule cell via SGLT (symport);
glucose enters ECF via facilitated diffusion;
glucose enters peritubular capillary via solvent drag with water
Define: transport maximum.
maximum rate of reabsorption; reached when the transporters are saturated
Define: renin.
an enzyme secreted by the kidneys in response to hypotension; converts the plasma protein angiotensinogen to angiotensin I, leading indirectly to a rise in BP
Define: transport maximum.
maximum rate of reabsorption, reached when the transporters are saturated;
limited # of protein carriers to transport solutes;
# specific to material carried;
transport maximum reflects # of carriers in renal tubules available:
-when no carriers are available, solute remains in tubule fluids and appears in urine
What happens to bicarbonate during reabsorption?
bicarbonate is reabsorbed;
bicarbonate reactions occurs within tubule cell
What happens to bicarbonate during reabsorption?
bicarbonate is reabsorbed;
bicarbonate reactions occurs within tubule cell
What happens to electrolytes during reabsorption?
K, Mg, PO4, diffuse along with water
What happens to nitrogenous wastes during reabsorption?
nitrogenous wastes like urea diffuse with along with water
What happens to organic solutes during reabsorption? Name examples.
lactate, AAs, peptide hormones;
symport with Na+,
leave via facilitated diffusion
Explain the process of water reabsorption.
tubular fluid is hypotonic compared to tubule cells and tissue fluid;
water moves into tubule cell and enters ECF via osmosis (water follows solutes) and diffusion through aquaporins;
water enters peritubular capillaries via solvent drag
Define: obligatory water reabsorption.
process by which water follows reabsorbed solutes and is reabsorbed at a constant rate in the proximal convoluted tubule
What are the 2 purposes of tubular secretion in the proximal convoluted tubule?
1) waste removal from blood
- urea, uric acid, bile acids, ammonia, catecholamines, creatinine
- penicillin, pollutants, morphine, aspirin
2) acid-base balance
- hydrogen
What is the primary purpose of the Loop of Henle?
generate an osmotic gradient that enables the collecting duct to concentrate the urine and conserve water
Describe the permeability characteristics of the Loop of Henle.
thin segment:
-mostly descending limb
-water can leave tubule
thick segment:
-mostly ascending limb
-impermeable to water (Na+ transport takes place but is not coupled to water movement)
-contransport of Na+, K+, and Cl- causes tubular fluid here to become very dilute
What 4 things does the Loop of Henle reabsorb?
20-25% Na+
20-25% water
35% Cl-
40% K+ (in ascending limb, secreted in descending limb)
Comment on the composition of the tubular fluid arriving at the DCT.
contains about 20% of water and 10% of salts from original filtrate;
if this were all passed as urine, it would amount to ~36 L/day, so there is more reabsorption to come
What controls DCT and CD reabsortion? Name 4 examples.
hormones:
1) aldosterone
2) natriuretic peptides
3) antidiuertic hormone (ADH)
4) parathyroid hormone (PTH)
Name the 2 types of cells present in the DCT and CD and explain their functions.
principal cells: -receptors for hormones -involved in salt and water balance intercalated cells: -reabsorb K+ and secrete H+ -maintains acid-base balance
What is another name for aldosterone?
“salt-retaining hormone”
What are the 3 conditions that aldosterone is released in response to?
directly: -decreased blood volume and BP -low Na+ concentration or high K+ concentration indirectly: -stimulating renin-angiotensin mechanism
What are the 3 actions of aldosterone?
1) acts on principal cells of DCT and cortical part of collecting duct to open Na+ channels to increase Na+ reabsorption;
2) stimulates synthesis of more Na+ transporters and and K+ channels;
3) movement of Na+ is followed by Cl- and water; therefore, urine volume is reduced and contains more K+ but less NaCl
What occurs in the absence of aldosterone?
no sodium is reabsorbed from DCT and CD, causing a catastrophic loss of Na+ via excretion in urine
Where is atrial natriuretic peptide (or factor) secreted and what is its secretion stimulated by?
- secreted by cells in atrial myocardium
- stimulated by ^ BP or ^ BV
What are the 4 actions of atrial natriuretic peptide (or factor)?
- dilates afferent arteriole (AA) and constriction of efferent arteriole (EA), increasing GFR
- inhibits aldosterone & renin secretion
- inhibits ADH secretion & action
- inhibits Na+ and water reabsorption in CD, reducing BV
What is the action of antidiuretic hormone (ADH)?
increases permeability of CD, allowing more water to be reabsorbed into blood
What is the action of parathyroid hormone (PTH)?
promotes Ca+2 reabsorption in Loop of Henle and DCT
What is the role of the collecting duct (CD)?
reabsorb water and create hypertonic (concentrated) urine
What 2 factors make the collecting duct (CD) favor reabsorption?
1) ECF osmolarity is higher in medulla than cortex
2) medullary (lower) portion of CD is more permeable to water than NaCl
Upon what doe the concentration of urine depend?
hydration state
Define: water diuresis.
hypotonic urine resulting from large water intake;
cortical CD reabsorbs more NaCl but is impermeable to water
Explain what occurs in the kidneys during dehydration.
ADH is released, causing # of aquaporins in principals cells of CD to increase, allowing more water into ECF, leaving behind more concentrated tubular fluid
Describe the rate at which ADH is released.
constant, but if there is a reduction in blood solute concentration, ADH declines
Define: countercurrent multiplier.
in juxtamedullary nephrons, flow of tubular fluid through nephron loop which functions to maintain the osmotic gradient;
reason that salinity in medulla is 4x greater than in cortex, despite the natural tendency for salt content to equilibriate
Explain the 5 “steps” of the countercurrent multiplier.
1) more salt is continually added by PCT
2) the higher the osmolarity of the ECF, the more water leaves the descending limb by osmosis
3) the more water that leaves the descending limb, the saltier the fluid is that remains in the tubule
4) the saltier the fluid in the ascending limb, the more salt the tubule pumps into the ECF
5) the more salt that is pumped out of the ascending limb, the saltier the ECF is in the renal medulla
Define: countercurrent exchange.
system formed by vasa recta that prevents it from carrying away urea and salt needed to maintain the high osmolarity (hypertonic environment) of the medulla;
blood flows in opposite directions in adjacent parallel capillaries;
as blood flows downward into medulla: water out, salt in;
as blood flows upward into cortex: salt out, water in;
net gain of water in vasa recta
