Urinary System Physiology Flashcards
Kidneys
Functions include Excretion of waste H2O balance (plasma volume) Blood pressure control (renin) Acid base balance Blood cell production (erythropoietin) Vitamin D activation
Urinary system
Consists of
Kidneys
Blood supply (20% total flow)
Transport vessels (ureters, urinary bladder, urethra)
Kidney anatomy
Renal calyces
Renal cortex (outer)
Renal medulla (inner)
Renal pelvis
Nephron
Functional unit of kidney
~1million/kidney
2 types = cortical (shorter, ~85%), juxtamedullary (longer, ~15%, osmotic gradient)
Tubule portion and blood supply portion
Vascular component = renal artery, afferent arteriole, glomerulus (ball like tuft of capillaries), efferent arteriole, peritubular capillaries, vein
Tubule
Bowman’s capsule Proximal tubule Loop of henle (ascending, descending) Distal tubule Collecting duct
Basic renal processes
Glomerular filtration (fluid into tubule) Tubular reabsorption (from tubule into blood) Tubular secretion (from blood into tubule) Urine comes as a result of these three things
Sites of action
Filtration = Bowman’s capsule
Reabsorption and secretion = proximal tubule, distal tubule (hormone controlled), collecting ducts
Loop of henle = creates osmotic gradient (reabsorption)
Substance fates
Substances can be =
Filtered and secreted (some only secreted)
Filtered and reabsorbed
Filtered and partially reabsorbed
Kidney functions
Glomerular filtration = all but RBCs and proteins (too big)
Reabsorption = Na+, Cl-, Ca2+, PO4, water, glucose
Secretion = K+, H+, large organics
Glomerulus
Tuft of capillaries (fenestrated, more permeable)
Surrounded by Bowman’s capsule
Glomerular filtration
Across 3 layers of the glomerular membrane =
Glomerular capillary wall
Basement membrane (a cellular gelatinous layer of collagen and glycoproteins)
Inner layer of Bowman’s capsule (consists of podocytes that encircle the glomerulus tuft)
~160-180L/day (~125ml/min)
Moves electrolytes, water, and glucose into tubules (RBCs and most proteins are too large to be filtered)
Urine <1% of filtrate
Location in nephron, volume of fluid, osmolarity of fluid
Bowman’s capsule = 180L/day, 300 mOsM
End of proximal tubule = 54L/day, 300 mOsM
End of loop of henle = 18L/day, 100 mOsM
End of collecting duct (final urine) = ~1.5L/day, 50-1200 mOsM
Podocytes
Can change shape (control filtration) Renal failure (large slits, allows RBCs and proteins in)
Forces involved in glomerular filtration
3 main physical forces involved =
Glomerular capillary blood pressure
Plasma-colloid osmotic pressure
Bowman’s capsule hydrostatic pressure (Bowman’s capsule osmotic pressure)
Force, effect and magnitude in glomerular filtration
Glomerular capillary blood pressure = favours filtration, 55mmHg
Plasma colloid osmotic pressure = opposes filtration, 30mmHg
Bowman’s capsule hydrostatic pressure = opposes filtration, 15mmHg
Net filtration pressure (difference between force favouring filtration and forces opposing filtration) = favours filtration, 10mmHg
Glomerular filtration rate (GFR)
Depends on =
Net filtration pressure
How much glomerular surface area is available for penetration
How permeable the glomerular membrane is (podocytes slit size can change with infection)
GFR will change if the blood hydrostatic pressure changes
Auto-regulated = tubuloglomerular feedback (local (paracrine) control), hormones/autonomic (change arteriole resistance)
Arterioles control GFR
Resistance changes in renal arterioles later renal blood flow
A lower GFR if afferent arteriole constricts or if efferent arteriole dilates
A higher GFR if afferent arteriole dilates or if efferent arteriole constricts
Extrinsic control on GFR
Sympathetic control =
Long term regulation of arteriole BP
Input to afferent arterioles (baroreceptor reflex)
Lower blood pressure means lower GFR and retention of fluids
Other examples of when GFR can change
Plasma colloid osmotic pressure changes = eg) severely burned patient increases GFR, loss of proteins from blood to repair sites lowers osmotic pressure
Dehydrating diarrhea decreases GFR (loss of fluids increases osmotic pressure)
Bowman’s capsule hydrostatic pressure changes (obstructions such as kidney stone or enlarged prostate, elevates capsular hydrostatic pressure, decreases GFR)
Measuring GFR
Use inulin to measure
No reabsorption or secretion
Therefore, excretion = filtration
Movement
Trans-cellular transport = active or passive, eg) Na+ or glucose
Paracellular transport = passive only, diffusion of water, ions
Tubular reabsorption
Passive = no energy required, down electrochemical gradient or osmotic gradients Active = requires energy, moves against electrochemical gradients
Na+ reabsorption
Active process
Na+ - K+ ATPase pump in basolateral membrane is essential for Na+ reabsorption
Affects reabsorption of other substances
Na+/K+ pump creates Na+ gradients across membranes
Facilitates Na+ reabsorption
Tubule area, % of Na+ reabsorbed, role of Na+ reabsorbed
Proximal tubule = 67%, plays role in reabsorbed great glucose, amino acids, H2O, Cl-, and urea
Ascending limb of the loop of henle = 25%, plays critical role in kidneys ability to produce urine of varying concentrations
Distal and collecting tubules = 8%, variable and subject to hormonal control; plays role in regulating ECF volume
Reabsorption of other substances
Following the reabsorption of Na+ :
Water reabsorption (via osmotic gradient created)
Cl- reabsorption (via electrical gradient)
Glucose (by carries)
Glucose reabsorption
Sodium linked glucose reabsorption in the proximal tubule
Tubular maximum = point where all the glucose carriers are full, excess glucose stays in the tubules and is lost in the urine
Renal threshold
Blood glucose level where the carriers are full and glucose is seen in the urine
Eg) diabetes mellitus
Urea reabsorption
Urea = small, diffusible
Passive process
To equilibrium
50%
Reabsorption
Na+ = 99.9%, Na+/K+ ATPase pump Cl- = 99%, electrical gradient Water = 99%, osmotic gradient Glucose = 100%, carrier mediated Urea = 50%, passive
Aldosterone
Controls Na+/K+ ATPase pumps
Released if blood volume is low
High aldosterone = increased speed of pump, increased Na+ reabsorption, increased water reabsorption (decreased urine)
Eg) dehydration
Renin-angiotensin-aldosterone system
Regulates Na+ and blood pressure and blood volume
Atrial Natriuretic Peptide (ANP)
Antagonist to aldosterone
Inactivates Na+/K+ ATPase pump
Inhibits Na+ reabsorption
Secreted by atria with = increased blood pressure, increased Na+, increased stretch of atria (increased volume)
Secretion
Transfer of molecules from extra cellular fluid into tubule Active process K+ (Na+/K+ pump) H+ (acid-base balance) Large organics (biotransformed)
Counter-current mechanism
Descending loop of henle = permeable to water, impermeable to salts, filtrate becomes more concentrated
Ascending loop of henle = permeable to salts (actively reabsorbs NaCl), impermeable to water, filtrate becomes less concentrated
Vasa recta
Vessel following loop of henle
Similar osmotic gradient in blood supply
Loop of henle
Creates a large, vertical osmotic gradient in medulla
100-1200 mOsM/L
Water reabsorption
ADH causes insertion of water pores into the apical membrane
ADH = anti-diuretic hormone
Controls permeability of collecting ducts
Released if blood osmolarity high
Low ADH = impermeable to water, dilute urine (high volumes) eg) water loading
High ADH= due to high blood osmolarity, makes collecting duct permeable to water, concentrates urine (lower volumes) eg) dehydration
Dehydration
Increase ADH Increase aldosterone Decrease ANP Increase water reabsorption Decrease urine volume (more concentrated)
Behavioural mechanisms
Drinking replaces fluid loss
Low sodium stimulates salt appetite
Avoidance behaviours help prevent dehydration (eg - desert animals avoid the heat)
Water loading
Decreased ADH Decreased aldosterone Increased ANP Decreased water reabsorption Increase urine volume (more dilute)
Proximal tubule
67% of Na, Cl, and water reabsorption
100% glucose and amino acids are reabsorbed
K is secreted/reabsorbed (small amount)
Variable H secretion occurs
Organic ion secretion (not controlled)
Phosphate and electrolytes (controlled, variable reabsorption)
Urea reabsorption (to equilibrium 50%)
Distal tubule
Variable Na reabsorption (controlled by aldosterone and ANP)
Variable water reabsorption (controlled by aldosterone and ANP)
Variable K secretion/reabsorption (controlled by aldosterone)
Variable H secretion (depends on acid-base balance)
Collecting ducts
Site of water reabsorption
Controlled by ADH
Concentrated the urine
Requires osmotic gradient (loop of henle)
Variable water reabsorption (controlled by ADH)
Variable H secretion
Variable urea reabsorption (related to loop of henle)
Excretion
Excretion = filtration - reabsorption + secretion
Clearance = rate at which a solute disappears from body
Non-invasive way to measure GFR (inulin and creatine)
Renal clearance
RC = UV/P RC = renal clearance rate (ml/min) U = concentration (mg/ml) of the substance in urine V = flow rate of urine formation (ml/min; GFR) P = concentration of the same substance in plasma
Inulin/glucose clearance
Inulin clearance is equal to GFR
Glucose clearance is usually zero because of 100% reabsorption
Micturition
The urination reflex
Autonomic control of sphincters and detrusor muscle
CNS can over-ride or initiate
Muscle and innervation
Detrusor (smooth muscle) =parasympathetic (causes contraction), inhibited during filling, stimulated during micturition
Internal urethral sphincter (smooth muscle) = sympathetic (causes contraction), stimulated during filling, inhibited during micturition
External urethral sphincter (skeletal muscle) = somatic motor (causes contraction), stimulated during filling, inhibited during micturition
During filling
Bladder (detrusor) muscle is relaxed
Sphincters are contracted
During micturition
Stretch receptors increase their firing
Sphincters are relaxed
Detrusor muscle contracts
Urine flows out of bladder
Renal failure
Wide ranging consequences
Causes = infectious organisms, toxic agents, inappropriate immune responses, obstruction of urine flow, an insufficient renal blood supply
Build up of wastes to toxic levels (vomiting, diarrhea, cellular necrosis)
Loss of calcium (osteoporosis)
Na+/K+ imbalance (affects nerve and muscle)
Loss of proteins (edema)
Loss of RBCs (anemia)
Low blood pressure (decreased renin, dizziness)
Kidney stones
Crystallization of minerals in either the kidney, ureters, or bladder
Calcium
Oxalates (veggies, spinach, beets)
Dehydration (binge drinking)
Acid-base balance
Normal pH of 7.38-7.42
H+ concentration is closely regulated
Abnormal pH can alter tertiary structure of proteins and affects the nervous system
Acidosis = neurons become less excitable and CNS depression
Alkalosis = hyper excitable
pH disturbances (with K+ disturbances)
Acidosis
Metabolic acidosis = metabolic organic acid production, lactic acid (exercise), ketoacids (diabetes), diarrhea, organic acids intake (diet)
Respiratory acidosis = production of CO2 (acid production)
Alkalosis
Metabolic alkalosis = vomiting, dietary sources of bases (a few), pyloric stenosis
Respiratory alkalosis = hyperventilation
pH homeostasis
Buffers = combines with or releases H+
Ventilation = 75% of disturbances
Renal regulation = slowest of the 3 mechanisms, directly excreting or reabsorbing H+
Buffers
Fastest response (within seconds) Combines with H+ so it doesn’t affect pH Phosphate Protein - hemoglobin Bicarbonate
Respiratory compensations
pH is adjusted by changing rate and depth of breathing (reps done within minutes)
CO2 + H2O H2CO3 H+ + HCO3-
Respiratory corrections
Reflex pathway for respiratory compensation of metabolic acidosis (increase breathing)
Renal compensation: kidney
Slowest response (within hours) Can retain or eliminate H+ or H2CO3- Apical Na+ - H+ exchanger (NHE) Na+ - HCO3- symport H+ - ATPase H+ - K+ ATPase Na+ - NH4 antiport
Body’s correction for acidosis
To raise body pH =
Buffers bind to H+
Breathing increases (decreases CO2 and H+ via carbonic acid)
Kidney excretes H+ and keeps bicarbonate
Body’s correction for alkalosis
To lower pH =
Buffers release H+
Breathing slows down (retains CO2 and H+)
Kidney retains H+ and secretes bicarbonate
Intercalated cells
Type A = function in acidosis, secrete H+, reabsorb bicarbonate
Type B = function in alkalosis, secrete bicarbonate, reabsorb H+