LECTURE 10 (Urine formation by kidneys) Flashcards
(41 cards)
What are the different functions of the kidneys?
- Excretion of metabolic waste products + foreign chemicals
[urea, creatinine, uric acid, bilirubin, metabolites of various hormones] - Regulation of water + electrolyte balances
[for homeostasis + to alter ECF volume only SLIGHTLY] - Regulation of body fluid osmolality + electrolyte concentrations
- Regulation of arterial pressure
[excrete Na2+ and H2O + hormones and vasoactive factors/substances] - Regulation of acid-base balance
[excrete acids + regulate body fluid buffer stores] - Secretion, metabolism + excretion of hormones
[ERYTHROPOEITIN for production of RBC by hematopoetic stem cells + production of 1-25-dihydroxyvitamin D3 (CALCITRIOL) which is essential for Ca2+ deposition in bone and reabsorption by GI tract] - Gluconeogenesis
[synthesise glucose from amino acids during prolonged fasting]
Describe the anatomy of the kidneys
- Two kidneys lie on posterior wall of abdomen outside peritoneal cavity
- Medial side -> HILIUM (renal artery + vein, lymphatics, nerve supply + ureter pass through)
- Outer cortex + inner medulla
[Inner medulla -> separated into 8-10 renal pyramids] - Base of each pyramid is from border between cortex and medulla + terminates in PAPILLA which projects into RENAL PELVIS
- Outer border is divided into open-ended pouches called “MAJOR CALYCES” which divide into “MINOR CALYCES” (collects urine from each papilla)
Describe the Renal blood supply
Renal artery branches into:
- Interlobar arteries
- Arcuate arteries
- Interlobular arteries/Radial arteries
- Afferent arterioles
which lead to Glomerular capillaries
Distal ends of capillaries coalesce to form the EFFERENT ARTERIOLE which form PERITUBULAR CAPILLARIES around renal tubules -> Efferent arteriole separates the two capillary beds + regulates hydrostatic pressure in both -> High hydrostatic pressure in Glomerular capillaries cause FILTRATION + low hydrostatic pressure in Peritubular capillaries causes REABSORPTION -> Peritubular capillaries empty into: Interlobular vein, Arcuate vein, Interlobular vein, Renal vein
Describe the Nephron
1) Glomerular capillaries (where large amounts of fluid filtered from blood) encased in BOWMAN’S CAPSULE -> fluid flows into Bowman’s capsule then into PROXIMAL TUBULE (lies in cortex of kidney)
2) Fluid flows into LOOP OF HENLE (dips into renal medulla) consisting of descending + ascending limb -> descending limb and lower end of ascending limb is “thin segment of loop of Henle” + ascending limb is “thick segment of ascending limb” (back to cortex)
3) Fluid flows to MACULA DENSA then into DISTAL TUBULE (lies in cortex) -> CONNECTING TUBULE -> CORTICAL COLLECTING TUBULE -> CORTICAL COLLECTING DUCT
4) 8-10 Cortical collecting ducts run into medulla becoming the MEDULLARY COLLECTING DUCT -> merge to form RENAL PAPILLAE
What is the difference between Cortical and Juxtamedullary Nephrons?
Cortical nephrons (glomeruli located in outer cortex) = Short loops of Henle that penetrate a short distance into the medulla + entire tubular system is surrounded by PERITUBULAR CAPILLARIES
Juxtamedullary nephrons (glomeruli deep in renal cortex near medulla) = Long loops of Henle that dip deeply into medulla + long efferent arterioles extend from from glomeruli into outer medulla forming “VASA RECTA” (return towards cortex + empty into cortical veins)
What happens in Micturition/Urination?
1) Bladder fills progressively until tension in walls rises above a threshold
2) Micturition reflex empties bladder/causes desire to urinate -> An autonomic spinal cord reflex but can be inhibited/facilitated by enters in cerebral cortex/brain stem
Describe the anatomy of the bladder
- Body where urine collects + neck which passes inferiorly and anteriorly into UROGENITAL TRIANGLE and connects with urethra
- DETRUSOR MUSCLE -> contracts to empty bladder
[smooth muscle cells fuse so action potential can spread causing unison contraction] - On posterior wall of bladder -> “TRIGONE”
[lower apex is the bladder neck which opens into POSTERIOR URETHRA + uppermost apex are two URETHRAL OPENINGS; Trigone mucosa is smooth compared to rest of bladder folded into “RUGAE” - Bladder neck composed of detrusor muscle + elastic tissue -> “INTERNAL SPHINCTER” (smooth muscle)
- Urethra passes through UROGENITAL DIAPHRAGM containing “EXTERNAL SPHINCTER OF BLADDER” (skeletal muscle)
Describe the innervation of the bladder
- PELVIC NERVES connect the spinal cord through the SACRAL PLEXUS (S2 and S3)
- Sensory fibers = detect degree of stretch in bladder wall
[stretch signals from posterior urethra initiate bladder emptying] - Parasympathetic fibers/Motor nerves = terminate on ganglion cells in wall of bladder -> short post-ganglionic nerves then innervate the detrusor muscle
- SKELETAL MOTOR FIBERS transmitted through PUDENDAL NERVE to external bladder sphincter -> somatic nerve fibers that control skeletal muscle of sphincter
- Sympathetic innervation from sympathetic chain through the HYPOGASTRIC NERVES -> connecting mainly with L2
Describe the transport of urine from the Kidney into the bladder
- Urine flowing from the collecting ducts into the renal calyces stretches the calyces + increases pacemaker activity -> initiates peristaltic contractions that spread to RENAL PELVIS + downward along length of ureter -> force urine from renal pelvis towards the bladder
- Peristaltic contractions in ureter are enhanced by parasympathetic stimulation and inhibited by sympathetic stimulation
- DETRUSOR MUSCLE compresses ureter preventing back flow of urine when pressure builds up in bladder -> each peristaltic wave along ureter increases the pressure within ureter so region passing through bladder opens + allows urine to flow into bladder
What is Vesicoureteral reflux?
When the distance that the ureter courses through the bladder wall is less than normal so contractions of bladder during micturition does not always lead to complete occlusion of ureter -> Urine in bladder is propelled backwards into ureter
MANIFESTATIONS:
- enlargement of ureters
- increase pressures in renal medulla + calyces
What is the Ureterorenal reflex?
When a ureter becomes blocked, intense reflex constriction occurs + severe pain -> pain causes sympathetic reflex back to kidney to constrict renal arterioles -> decreases urine output from kidney
EXPLANATION: prevents excessive flow of fluid into the pelvis of a kidney with a blocked ureter
What happens in the Micturition reflex?
1) Progressive and rapid increase of pressure
2) A period of sustained pressure
3) Return of the pressure to the basal tone of bladder
MICTURITION CONTRACTIONS “dashed spikes” appear as a result of stretch reflex by sensory street receptors on bladder wall -> Sensory signals conducted to SACRAL SEGMENTS of cord via PELVIC NERVES then back to bladder through the PARASYMPATHETIC NERVE FIBERS -> when bladder is partially filled, micturition contractions relax spontaneously but become more frequent + greater when filling again -> once reflex is powerful enough, another reflex passes through PUDENDAL NERVES to the EXTERNAL SPHINCTER to inhibit it -> If inhibition is greater in brain than voluntary constrictor signals to external sphincter, urination will occur
How do the higher centers in the brain control micturition?
- Keep micturition reflex partially inhibited, except when micturition is desired
- Can prevent micturition, even if micturition reflex occurs, by tonic contraction of external bladder sphincter until a convenient time
- When it is time to urinate, cortical centers facilitate the sacral micturition enters to help initiate a micturition reflex + inhibit external urinary sphincter so that urination can occur
What happens if the sensory nerve fibers are destroyed?
Overflow incontinence
EXPLANATION:
Micturition reflex cannot occur if sensory nerve fibers from bladder to spinal cord are destroyed, preventing transmission of stretch signals from bladder -> Instead of emptying periodically, bladder fills to capacity + overflows a few drops at a time through the urethra
COMMON CAUSE:
Crush injury to the sacral region of the spinal cord
What happens if there is spinal cord damage above the sacral region?
Micturition reflexes can still occur but are no longer controlled by the brain
During the first few days/weeks reflexes are suppressed due to “spinal shock” -> but, if bladder is emptied periodically by catheterisation to prevent bladder injury by overstretching of bladder -> excitability of micturition reflex gradually increases until micturition reflexes return -> some patients control urination by stimulating the skin in genital region
What happens if there is lack of inhibitory signals from the brain?
Frequent urination due to uncontrolled micturition reflex
EXPLANATION:
Partial damage in spinal cord/brain cell interrupts inhibitory signals from brain -> frequent + relatively uncontrolled micturition
What are the three renal processes that determine the rates at which different substances are excreted in the urine?
1) Glomerular filtration
2) Reabsorption of substances from the renal tubules into the blood
3) Secretion of substances from the blood into the renal tubules
What is the equation for Urinary excretion rate?
Urinary excretion rate = Filtration rate - Reabsorption rate + Secretion rate
What is the filtration, reabsorption and secretion of different substances?
- End products of metabolism, urea, creatinine, uric acid, drugs = excretion rate is high + poorly reabsorbed
- Electrolytes = highly reabsorbed + small amounts in urine
- Nutritional substances, amino acids, glucose = completely reabsorbed + do not appear in urine
Why are large amounts of solutes filtered and then reabsorbed by the kidneys?
- High GFR allows kidneys to rapidly remove waste products from the body that depend mainly on glomerular filtration for excretion
- High GFR allows all body fluids to be filtered + processed by kidneys many times each day
- High GFR allows kidneys to precisely + rapidly control the volume and composition of body fluids
Describe Glomerular filtration
- The first step in urine formation
- Glomerular capillaries are relatively impermeable to proteins -> glomerular filtrate is “protein free” + goes into Bowman’s capsule
- GFR is determined by balance of hydrostatic and colloid osmotic forces acting across capillary membrane + CAPILLARY FILTRATION COEFFICIENT (product of permeability + surface area of capillaries)
[High rate of filtration bc high glomerular hydrostatic pressure + large Kf] - Filtration fraction = GFR/Renal plasma flow
Describe the Glomerular capillary membrane
- Contains endothelium, basement membrane and a layer of epithelial cells (podocytes)
- FUNCTION: prevents filtration of plasma proteins + large negatively charged ions
- Endothelium = perforated by thousands of “fenestrae” + fixed negative charge
- Basement membrane = collagen + proteoglycan fibrillae that have large spaces through which large amounts of water + small solutes can filter
- Podocytes = separated by “slit pores” where glomerular filtrate moves + negative charge
[Glomerular capillary membrane is thicker than other capillaries BUT is more porous -> filters fluid at high rate]
What determines the GFR?
- Sum of hydrostatic and colloid osmotic forces across the glomerular membrane which gives the “net filtration pressure”
- Glomerular filtration coefficient (Kf)
GFR = Kf X Net filtration pressure
GFR=Kf X(PG -PB -πG +πB)
[PG = glomerular hydrostatic pressure which promotes filtration, PB = hydrostatic pressure in Bowman’s capsule which opposes filtration, πG = glomerular colloid osmotic pressure which opposes filtration, πB = Bowman’s colloid osmotic pressure which promotes filtration]
What is Kf?
A measure of the product of the hydraulic conductivity + surface area of capillaries
CLINICAL CORRELATION:
Increased glomerular capillary filtration coefficient increases GFR
