Unit 10 - Urinary Physiology Flashcards
Kidney Function
- regulate water, fluid and electrolyte balance (H+, K+, NA+, electrolyte and water balance)
- regulate blood volume and blood pressure
- regulate acid base balance (H+ & HCO3-)
- Excretion of foreign compounds (drugs, presticides, food additives) and waste products (urea, uric acid, creatinine)
- Secrete hormones
Urinary System consists of
Urine forming organs ( 2 kidneys)
Structures carry urine from kidneys to outside for elimination (2 ureters L slightly longer than R, 1 urinary bladder, 1 urethra) 2 adrenal glands
Renal Structures
Renal cortex: filters blood
Renal medulla: contain 8-12 renal pyramids, empties into cuplike calyx, calyces channel urine into renal pelvis.
Renal pelvis: urine travels from renal pelvis to bladder for storage
Process of urine
Renal pyramid projects into a minor calyx
3 Minor calyces form 1 major calyx
Minor calyx accepts tip of pyramid where urine is dripping off papillary duct in pyramid.
Major calyx empty into pelvis which is at top of ureter
Urine enters ureter
Variability between kidneys
Renal papula
apex of the pyramid and projecting into the calyx
Nephron
Functional unit of the kidney
Filter blood to remove water and substances
Then necessary fluid and electrolytes reabsorbed back into blood
removes unnecessary molecules
approximately 1 million nephrons/kidney
each nephron has two components (vascular & tubular)
Vascular Component
From aorta, each renal artery subdivided into afferent arterioles
1 afferent arteriole supplies 1 nephron -> glomerular capillaries
Glomerular capillaries rejoin to form efferent arteriole
Forms peritubular capillaries
Wrap around tubular structures
Supply renal tissue with blood
Vasa recta
Series of straight capillaries in medulla
Slow rate of blood flow allows concentration of urine
Juxtaglomerular Apparatus JGA
between glomerulus and DCT to regulate function of nephron
Contains:
1. the macula densa, at start of DCT (Sensitive to concentration of NaCl of fluid flowing by
2. Juxtaglomerular cells that secrete renin
3. Extraglomerular mesangial cells (unknown function)
Tubular Component
Blood from afferent arteriole into glomerulus
Fluid that is formed -> proximal convoluted tubule (PCT)
Down descending limb of Henle, which dips into medulla
Urine from DCT to collecting duct->renal pelvis->bladder->elimination
Glomerulus
network of capillaries that sit in bowman’s capsule
Distal Convoluted tubule DCT
ascending limb of Henle returns to cortex
Three steps of Renal Process
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
Glomerular Filtration
First step to form urine
20% of blood that enters glomerulus is filtered and goes into BC
Filtered blood components (most of the water, most/all of the salts, glucose and urea
80% of unfiltered blood then travels in efferent arteriole
GFR
Males- 125ml/min
Females- 115ml/min
180 L/day
Three physical forces affect GFR
- Glomerular capillary blood pressure
- Plasma-colloid osmotic pressure
- Bowman’s capsule hydrostatic pressure
Glomerular capillary blood pressure
Pressure exerted by blood in glomerular capillaries
afferent arteriole diameter>efferent diameter
therefore, pressure of blood inside glomerulus is increased
this increased blood pressure forces the components of blood out of glomerular capillaries
elevated pressure favours fluid entering bowman’s capsule (favours GFR)
Plasma-colloid osmotic pressure
caused by more proteins in glomerulus than is BC
water moves from high conc. BC to low conc glomerulus (opposes filtration)
Bowman’s capsule hydrostatic pressure
pressure that pushes fluid out of BC (opposes filtration)
Net filtration pressure (change force favouring filtration, 1 - (2+3)
Regulation of GFR
two major control mechanisms
1. Autoregulation (prevent spontaneous GFR changes) myogenic mechanism, tubuloglomerular feedback TGF
Neuron control aimed at long-term regulation of BP ( SNS effect on afferent arterioles)
Myogenic mechanism (autoregulation)
Increase in BP (afferent arteriole automatically constricts -> decreased dismeter and flow into glomerulus -> less blood filtered and lover GFR)
Decrease in BP (afferent arteriole automatically dilates -> increased diameter and flow into glomerulus -> more blood filtered higher GFR
Tubuloglomerular feedback mechanism
Increased BP increases GFR
Elevation of BP and GFR-> more fluid and salt filtered -> macula densa release ATP and adenosine -> afferent arteriole contricts -> lowers glomerular blood flow and lowers GFR
Reduction of BP and GFR -> less fluid and salt filtered -> less ATP released and adenosine made -> afferent arteriole dilates -> raise glomerular blood flow and raise GFR
Regulatory mechanisms
myogenic and TGF regulations work together
prevent inappropriate fluctuations in GFR and dangerous imbalances of fluid, electrolytes and waste (lose too much water, electrolytes if GFR too high, retain wastes if GFR too low)
Work within MAP 80-180 mm Hg ( changes outside range cause GFR to increase/decrease)
Neural Regulation of GFR
Kidney’s blood vessels also regulated by sympathetic fibers
If blood voume decreased (hemorrhage) -> BP reduced -> baroreceptors -> CV control centre in brain stem -> raise sympathetic stimulation -> vasoconstriction of afferent arterioles that reduces GFR -> less fluid filtered -> lowers urine output -> concerves fluid to help increase BP
Hormonal Regulation of GFR
Atrial natriuretic peptide ANP
Atrial natruoretic peptide ANP
incease in BO -> atria stretch and release ANP
Inhibits Na+ reabsorption
Promotes (natriuretic more Na excreted, Diuretic more H2O excreted, reduces BP hypotensive effect)
increase GFR -> more urine formed -> less blood volume -> reduces BP
Renin-Angiotensin-Aldosterone-System RAAS
starts lower BP -> juxtaglomerular cells secrete renin
Follow flow chart
at end, angiotensin II reduces GFR (potent vasoconstrictor that narrows both afferent and efferent arterioles reducing GFR, less urine formed -> more blood volume -> increase BP)
Tubular Reabsorption
involves the transfer of substances from tubulat lumen into peritubular capillaries ( move substances from filtered fluid back into blood, body needs these substances)
Most substances reabsorbed
Reabsorbed substances must cross what five barriers
- leave tubular fluid by crossing luminal membrane of tubular cell
- pass through cytosol from one side of tubular cell to the other
- cross basolateral membrane of the tubular cell to enter interstitial fluid
- diffuse through interstitial fluid
- penetrate capillary wall to enter blood plasma
Renal threshold
maximum ability of kidney to reabsorb a specific substance
required specific carrier for each substance
Any substance beyond Tm is not reabsorbed
the glucose carrier mechanism can reabsorb a maximum of 375 mg of glucose per minute (Tm)
Glycosuria
when 375 mg/ml reached and carriers fully saturated, excess glucose excreted in urine
Renal threshold
point at which glucose begins to spill into the urine
Tubular secretion
transfer of selective substances from peritubular capillaries (blood) back into tubular lumen (to urine)
Tubular secretion is important for two reasons
H+
K+
H+
important in regulating acid-base balance
when body fluids are too acid -> more H+ is secreted
when body fluids too alkaline -> less H+ secreted
Secreted in proximal, distal, and collecting tubules
K+
secreted only in DCT and collecting tubules under control of aldosterone
normal plasma K+ concentration maintains normal membrane excitability in muscles and nerves
Tubular secretion
organic ions
secreted only in the proximal tubule (efficient elimination of foreign compounds from body)
certain drugs (penicillin)
environmental pollutants such as pesticides
Regulation of urine concentration
countercurrent mechanism
urine concentration mechanism completed in loop of henle
The concentrtion of tubular fluid (urine) is progressively increasing in the descending limb and progressively decreasing in the ascending limb
benefits of countercurrent mechanism
effective water reabsorption
final adjustment of urine concentration
Regulation of Urine Concentration
Descending limb
Ascending limb
Descending limb
thin epithelium
water flows out into interstitial fluid
no Na+ reabsorption
Ascending limb
thick epithelium
impermeable to water
actively transports NaCl out of the tubular lumen into interstitial fluid
Vasopressin ADH
DCT and collecting tubule not permeable to water
become permeable if ADH present
released when ECF too concentrated (need more water for body_ -> conc urine
inhibited when ECF too dilute (need less water for body) -> urine dilute
Renal endocrine functions
erythropoietin : increased release with hypoxia and decreased circulating red cell mass
Excretion
the end product of the renal process (excess ions, H2O, molcules, toxins, NH4+ and extra urea [nitrogenous wastes]) urine output (1.0-1.5L/day, high volume less concentrated uring, low volume more concentrated urine, depending on body's state of hydration)
Micturition
urine in bladder stimulates stretch receptors
PNS neurons stimulate stretch receptors to signal smooth muscle in bladder wall
contraction of bladder urine out of the body
micturition reflex (relaxation of external urethral sphincter muscle allowing urine to pass through urethra and out of the body)
under voluntary control but connot be delayed indefinitely
