Urinary System Flashcards
What does the urinary system comprise of?
Kidneys
Ureters
Urinary bladder
Urethra
Describe the anatomy of the kidneys
Retroperitoneal in upper abdomen
Highly vascularised
Surrounded by dense fibrous capsule
Outside this is a fascial pouch (renal fascia) containing the peri-renal adipose tissue
Posteriorly overlapped by the diaphragm and pleural cavity superiorly
Multilobar
Suprarenal glands (adrenal) sit on top of superior poles
Which kidney is slightly lower?
Right kidney is usually slightly lower than the left
Superior pole of the R kidney lies at the level of the 11th intercostal space and that of the L at the 11th rib
Where does the hilum of the kidney lie?
About the level of L2
What are the posterior relations of the kidneys?
Overlapped by diaphragm (at top)
Psoas major muscle (medial)
Quadratus lumborum muscle
Transversus abdominis muscle (lateral)
What is the kidney surrounded by?
Surrounded by dense fibrous capsule
Outside this is a fascial pouch (renal fascia) containing the peri-renal adipose tissue (perinephric fact)
Paranephric fat is outside the renal fascia
What are the anterior relations of the kidney?
Right= liver, hepatic flexure and hilus lies behind second part of duodenum
Left= stomach, pancreas, spleen and splenic flexure
What is the blood supply to the kidneys?
Abundant blood supply via renal arteries
- Short direct branches from abdominal aorta
Blood pressure drives ultrafiltration by glomerular capillaries
Renal veins drain into the IVC
Describe the external surface of the kidney
From top: Suprarenal gland (adrenal) Superior pole Anterior surface (with lateral margin) Inferior pole
Renal arter/vein/pelvis connected
Renal pelvis-> ureter
How does drainage from the kidneys work?
Each lobe drains through its own papilla and calyx
What do the cortex and medulla of the cortex look like and why?
Cortex= granular-looking
Because of random organisation
Medulla= striated
Because of radial arrangement of tubules and micro-vessels
Houses nephrons
What are the minor and major calyxes?
Calyx= chamber of the kidney where urine passes through
Renal pyramid into minor calyx (through renal papilla)
Minor calyxes-> major calyx
Major calyxes-> renal pelvis-> ureter
How are renal pyramids and minor calyxes separated from neighbouring ones?
Renal pyramids separated by renal columns
Renal minor calyxes separated by renal sinus (under column)
What are the ureters?
Ducts by which urine passes from the kidney to the bladder
Run vertically down posterior abdominal wall in the plane of the tips of the transverse processes of the lumbar vertebrae
Cross the pelvic brim anterior to the sacro-iliac joint and bifurcation of the common iliac arteries
Descend anteromedially to enter bladder at the level of the ischial spine (open obliquely through bladder wall)
How is urine transported in ureters?
By peristalsis in ureter smooth muscle walls
Ureters open obliquely through bladder wall
What are the 3 sites of ureteric constriction? What can cause sites of renal colic?
- Pelviureteric junction
- Where ureter crosses pelvic brim
- Where ureter traverses bladder wall
Sites of renal colic caused by kidney stones attempting to pass
What is the bladder?
Hollow muscular pelvic organ (triangular pyramid with apex pointing anteriorly and base posteriorly)
Collects urine from the kidneys before disposal by urination
Very distensible (up to 600ml urine can be held)
Lined by urothelium (transitional epithelium)
3-layered epithelium with very slow cell turnover
Large luminal cells have highly specialised low-permeability luminal membrane
Prevents dissipation of urine-plasma gradients
How many ml can the bladder hold?
The bladder is a distensible organ- can hold up to 600 mL of urine
What are the surfaces of the urinary bladder and how do they connect?
Superior surface (triangle between ureters and median umbilical ligament)
- Joins ureters at fundus (base)
- Joins median umbilical ligament (apex)
Inferolateral
- Neck underneath-> urethra
What are the urinary sphincters?
INTERNAL URETHRAL ORIFICE
Other names= sphincter visicae / internal sphincter
Location= Neck of bladder (bottom)
Musculature= Smooth
Opening= Reflex
Stimulus= Bladder wall tension (i.e. distension- filling)
Control= Parasympathetic
EXTERNAL URETHRAL ORIFICE
Other names=sphincter urethrae / external sphincter
Location= Perineum (outside opening)
Musculature= Striated
Opening= Voluntary
Stimulus= Urge to urinate (continence)
Control= Voluntary inhibition (somatic- pudendal nerve)
What does it mean ‘the external urethral orifice is under control by voluntary inhibition’?
Tone is maintained by the nerves, on urination you are inhibiting these messages (i.e. relaxing the sphincter) rather than engaging anything
Somatic control- pudendal nerve
Outline reflex and voluntary control in opening of the bladder sphincters
REFLEX Bladder fills -> + stretch receptors (in bladder wall) -> + parasympathetic nerve -> + bladder -> bladder contracts -> internal urethral sphincter mechanically opens when bladder contracts
Bladder fills
- >
- stretch receptors (in bladder wall)
- >
- motor neuron
- > external urethral sphincter opens when motor neurone is inhibited
VOLUNTARY CONTROL
Cerebral cortex
-> + motor neuron
-> external urethral sphincter remains closed when motor neurone is stimulated
How are urethra different in males and females?
FEMALE
Very short urethra (hence why women are more prone to UTIs)
MALE
Length is variable
Four major areas of the male urethra:
- Pre-prostatic
- Internal urethral orifice (bladder neck, bladder outlet) - Prostatic
- Membranous
- Spongy
- Bulbar urethra
- Penile urethra
- Navicular Fossa
- External urethral meatus
Outline the passage of urine
Kidney-> ureter-> bladder-> urethra
Urine is made in the kidneys (within each nephron)
It drains through each collecting duct into the renal pelvis (via the minor and major calices)
Travels down the ureters via peristalsis
Enters the bladder
Passes through internal urethral orifice
Travels down the urethra
Opening of the external urethral orifice results in urination
What type of muscle lines ureters?
Smooth muscle
How does food and drink become faeces, exhalation, urine and sweat?
Food and drink-> BODY and faeces (undigested residue)
BODY Regulation of osmolarity, [Na+], [K+], pH, nitrogen etc. Controls body fluid volume -> Exhalation (H2O, CO2) -> Urine (H2O, Na, K, H, urea) -> Sweat (H20, Na)
What is the function of the kidneys?
Production of urine:
- Filtration of blood plasma
- Selective reabsorption of contents to be retained
- Tubular secretion of some components
- Concentration of urine as necessary
Filtration and excretion of waste produces
- Control of osmolarity and composition of blood and urine
Electrolyte homeostasis
- Control of osmolarity of blood
BP control
- Control of volume of blood
- Responds to various blood pressure states to maintain homeostasis
Acid-base homeostasis
Endocrine function
- Signals to rest of body (hormones include renin, erythropoietin, 1,25-OH vitamin D)
Sensitive to body needs via hormones, nerves
How is urine produced in the kidney?
FILTRATION (glomerulus)
Blood passing through glomerulus is filtered
Filtrate consists of all components
What is a renal corpuscle?
A renal corpuscle is the initial blood-filtering component of a nephron
Consists of two structures: a glomerulus and a Bowman’s capsule
Also, podocytes associated with glomerulus
FOR FILTRATION
What is the blood supply of the renal corpuscle?
At vascular pole
From afferent arteriole
Exit through efferent arteriole
Glomerular capillaries at high pressure
What does the filtration barrier of the renal corpuscle consist of?
Fenestrae (“windows”) in capillary endothelium
Specialised basal lamina
Filtration slits between foot processes of podocytes
Allows passage of ions and molecules
How is filtrate drained from the renal corpuscle?
At urinary pole of corpuscle
Drains to proximal convoluted tubule
What are the functions of the proximal convoluted tubule?
FOR REABSORPTION
Reabsorption of 70% of glomerular filtrate
Na+ uptake by basolateral Na+ pump
Water and anions follow Na+
Glucose uptake by Na+/glucose co-transporter
Amino acids by Na+/amino acid co-transporter
Protein uptake by endocytosis
What are the structural features of the proximal convoluted tubule?
Cuboidal epithelium
Sealed with (fairly water-permeable) tight junctions
Membrane area increased to maximise rate of resorption
- Brush border at apical surface
- Interdigitations of lateral membrane
Contains aquaporins
- Mediate transcellular water diffusion
Prominent mitochondria reflect high energy requirement
What is the function of the loop of Henle and vasa recta?
Creation of hyper-osmotic fluid
Describe the Loop of Henle (concurrent mechanism)
DESCENDING THIN TUBULE
Passive osmotic equilibrium (aquaporins present)
Simple squamous epithelium
ASCENDING THICK LIMB
Na+ and Cl- actively pumped out of tubular fluid
Very water-impermeable tight junctions
Membranes lack aquaporins - low permeability to water
Results in hypo-osmotic tubular fluid, hyper-osmotic extracellular fluid
Cuboidal epithelium, few microvilli
High energy requirement - prominent mitochondria
VASA RECTA
Blood vessels also arranged in loop
Blood in rapid equilibrium with extracellular fluid
Loop structure stabilises hyper-osmotic [Na+]
What cell types are in the PCT, DCT, ascending and descending limbs of the loop of Henle?
PCT= cuboidal epithelium
Descending= simple squamous epithelium
Ascending= cuboidal epithelium, few microvilli
DCT= cuboidal epithelium, few microvilli
What is the difference in permeability of the ascending and descending limbs of the loop of Henle?
Descending= low/no permeability to ions (Na and Cl), moderate permeability to urea, highly permeable to water
Ascending= impermeable to water, pumps out NaCl
Why is the loop of Henle described as ‘countercurrent’?
Fluid flows in opposite direction through two adjacent parallel sections of a nephron loop (ascending and descending)
What is the function of the distal convoluted tubule (and cortical collecting duct)?
ADJUSTMENT OF ION CONTENT IN URINE
Site of osmotic re-equilibration (control by vasopressin)
Adjustment of Na+/K+/H+/NH4+ (control by aldosterone)
Cuboidal epithelium, few microvilli
Complex lateral membrane interdigitations with Na+ pumps
Numerous large mitochondria
Specialisation at macula densa, part of juxtaglomerular apparatus
What is the function of the (medullary) collecting duct?
CONCENTRATION OF URINE
Passes through medulla with its hyper-osmotic extracellular fluid
Water moves down osmotic gradient to concentrate urine
Rate of water movement depends on aquaporin-2 in apical membrane
- Content varied by exo-/endocytosis mechanism
Under control from the pituitary - hormone vasopressin
Basolateral membrane has aquaporin-3, not under control
Duct has simple cuboidal epithelium
Cell boundaries don’t interdigitate
Little active pumping so fewer mitochondria
Drains into minor calyx at papilla of medullary pyramid
Minor and major calyces and pelvis have urinary epithelium
What does the juxtaglomerular apparatus do?
Endocrine specialisation
Secretes renin to control blood pressure via angiotensin
Senses stretch in arteriole wall and [Cl-] in tubule
Cellular components are
- Macula densa of distal convoluted tubule
- Juxtaglomerular cells of afferent arteriole
- Mesangial cells
What is ‘correction’ in the kidneys?
Ascending limb= major site of correction
Minute changes enable ions to cross back into the blood to regulate urine concentration
Outline the role of the different parts of the nephron
AFFERENT ARTERIOLE
Blood to nephron
Controls perfusion
GLOMERULUS
Ball of capillaries-> filtration
Alter perfusion (respond to signals)
BOWMAN’S CAPSULE
Hollow tubular epithelium surrounding glomerulus
PCT
Major site of reabsorption (H2O, K, Na, HCO3, AAs)
DESCENDING LIMB
Major site of concentration (of urine)
Highly permeable to ions not water
DCT
Fine tuning site
Sensitive to ADH and thiazide diuretics
COLLECTING DUCT
Small amount of reuptake
EFFERENT ARTERIOLE
Takes filtered blood away away from nephron
Constriction controls GFR (assisting in ultrafiltration of the blood)
In terms of the kidney, what is reabsorption?
What goes back into the blood
So at the glomerulus, everything goes in and then throughout the nephron composition of the urine is altered depending on the body’s requirements
Outline the role of the juxtaglomerulus
The JGA has three cellular components
- Macula Densa cells ([Na+] sensor)
Columnar epithelium
Located in the DCT
Senses high NaCl delivery and secretes ATP causing afferent vasoconstriction
- Granular cells (responds to PNS and SNS changes in tone)
Present throughout JGA but most dense in afferent arteriole
Senses changes caused by PNS and SNS
β-adrenergic stimulation
Reduced renal perfusion pressure
Reduced [Na+]
Secretes renin in response to decreased perfusion
Mesangial cells (produce EPO)
Extra-glomerular cells
Form part of the supportive matrix
What is glomerular filtration?
Formation of an ultra-filtrate of plasma in the glomerulus
Passive process: fluid is ‘driven’ through the semipermeable (fenestrated) walls of the glomerular capillaries into the Bowmans capsule space by the hydrostatic pressure of heart
What is renal failure?
An abrupt fall in glomerular filtration
Abnormalities in renal circulation-> reduced glomerular filtration i.e. renal failure
What is the glomerular filtration barrier permeable to?
Fenestrated endothelium of capillaries and semipermeable Bowman’s capsule
Highly permeable to:
Fluids
Small solutes (freely-filtered- sam concentration in filtrate and plasma)
Impermeable to:
Cells
Proteins
Drugs etc. carried bound to protein
What is produced by glomerular filtration?
A clear fluid (ultrafiltrate), completely free from blood and proteins, is produced containing electrolytes and small solutes
This is ‘primary urine’
Describe the movement of filtrate in glomerulus
From capillary lumen through fenestra into basement membrane
Through filtration slits between foot processes (podocyte) into capsular space
What is the blood flow to the glomerulus?
Renal input= renal artery
Renal output= renal vein and ureter
How can the amount excreted be described in terms of amount filtered, secreted and absorbed?
Amount excreted = amount filtered + amount secreted - amount absorbed
What is the driving force of glomerular filtration?
Hydrostatic pressure in glomerular capillaries (due to blood pressure) (Pgc)
- The force of the body circulating the blood to the renal artery
What are the pressures the oppose the driving force in glomerular filtration?
Hydrostatic pressure of tubule (Pt)
- The opposing force from the tubule against the glomerulus
Osmotic pressure of plasma proteins in glomerular capillaries (πgc)
- Protein (mainly albumin) exerts a pressure pulling water back
What determines the net ultrafiltration pressure (Puf)?
DRIVING FORCE:
Hydrostatic pressure in glomerular capillaries (due to blood pressure) (Pgc)
OPPOSING PRESSURES:
Hydrostatic pressure of tubule (Pt)
Ssmotic pressure of plasma proteins in glomerular capillaries (πgc)
Puf= Pgc - Pt - πgc
Ultimately there is a net ultrafiltration pressure of 10-20mmHg
What is the usual net ultrafiltration pressure?
10-20mmHg
What is the glomerular filtration rate?
GFR = Puf x Kf
The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (mL/min)
Sum of the filtration rate of ALL functioning nephrons
Index of kidney function
What happens to GFR when there is a loss of nephrons?
E.g. in kidney disease, may reduce number of functioning glomeruli so reduced surface area and reduced Kf
Loss of nephrons-> loss of surface area-> fall in Kf-> fall in GFR
Any changes in Kf-» GFR imbalances
What is Kf?
An ultrafiltration coefficient,
Accounts for
- Membrane permeability
- Nephrons available for filtration
Any changes in Kf-» GFR imbalances
How can Kf be reduced or increased?
Kidney diseases may reduce number of functioning glomeruli = reduced surface area = reduced Kf
Dilation of glomerular arterioles by drugs/ hormones will increase Kf
What is the formula for GFR?
GFR = Puf x Kf
What does renal flow deliver?
Oxygen, nutrients and substances for excretion
How is GFR affected by Puf and Kf?
Puf= overall pressure of the filtrate, influenced by
- HP of the glomerular capillary
- Opposing HP of the tubule
- COP of the plasma proteins in the glomerulus
Kf = ultrafiltrate coefficient, accounts for
- Membrane permeability
- Nephrons available for filtration
SO….
GFR depends of Pgc, πgc, Pt and Kf
How is renal blood flow calculated?
Renal blood flow (RBF) = approx 1L/min (1/5 of cardiac output)
Renal plasma flow (RPF) = approx 0.6L/min
Filtration fraction (FF) = 0.2 (ratio between RPF and amount of filtrate filtered by glomerulus, which is normally 20%)
Glomerular filtration rate (GFR) = RPF x FF
= Approx 120ml/min (volume of filtrate formed in 1 minute)
How is GFR regulated?
GFR is not a fixed value but is subject to physiological regulation
Achieved by neural or hormonal input to the afferent/efferent arteriole resulting in changes in Puf
Mechanisms of autoregulation
What is autoregulation of the GFR?
Autoregulation ensures fluid and solute excretion remain reasonably constant (otherwise varying pressure will vary urine production and cause loss of important ions)
To decrease GFR
- Constrict afferent arteriole
- > Decreased Pgc
- > Decreased GFR
- Dilate efferent arteriole
To increase GFR
- Constrict efferent arteriole
- > Increased Pgc
- > Increased GFR
- Dilate afferent arteriole
Mechanisms to do this
- Myogenic mechanisms
- Tubuloglomerular feedback
- RAAS
How do myogenic mechanisms in autoregulation work to regulate GFR?
Vascular smooth muscle constricts when stretched-> Keeps GFR constant when blood pressure rises
Arterial pressure rises -> afferent arteriole stretches -> arteriole contracts -> (vessel resistance increases)-> blood flow reduces and GFR remains constant
How does tubuloglomerular feedback work in autoregulation to regulate GFR?
NaCl concentration in fluid sensed by macula densa in juxtaglomerular apparatus
Macula densa signals afferent arteriole and changes its resistance and so GFR
Increased NaCl-> ATP released by macula densa-> vasoconstrict afferent arteriole-> decreased filtration-> NEGATIVE FEEDBACK
What happens to GFR is there is a severe haemorrhage, obstruction in nephron tubule, reduced plasma protein concentration or small increase in blood pressure?
Severe haemorrhage= decrease GFR
Obstruction in nephron tubule= decrease
Reduced plasma protein concentration= increase GFR
Small increase in blood pressure= no effect
Why and how does a severe haemorrhage cause GFR to decrease?
Need to drop GFR to maintain volume
Haemorrhage-> blood pressure drop (MAP drop detected by carotid baroreceptors)
Sympathetic nervous system overides renal regulation
-> Innervates afferent arteriole and constricts it
GFR decreased (to maintain volume)
Why does a drop in oncotic plasma proteins increase GFR?
Increased Puf -> increased GFR
Why does a small increase in blood pressure not change GFR?
Regulated by renal autoregulation and constriction
What is renal clearance?
As substances in blood pass through the kidney they are filtered to different degrees
The extent of filtering a substance undergoes and litres of plasma produced per unit time
Urinary excretion rate over plasma concentration
Cs= (Us x V)/ Ps
Substance s
What is the formula for renal clearance?
C= (U x V)/ P
U = concentration of substance in urine P = concentration of substance in plasma V = rate of urine production
How is GFR estimated using clearance?
If a molecule is freely filtered and neither reabsorbed nor secreted in the nephron then the amount filtered equals amount excreted
Use INULIN
(Gives clearance value of 120ml/min)
Needs to be transfused
Why could inulin be used to estimate GFR using clearance?
Gold standard but not used (have to transfuse)
A plant polysaccharide
Freely filtered and neither reabsorbed nor secreted
Not toxic
Measureable in urine and plasma
Gives a clearance value of 120ml/min which is GFR for average adult
Why is creatinine used to estimate GFR using clearance?
Endogenous (unlike inulin) so don’t need to infuse it
Waste product from creatine in muscle metabolism
Amount of creatinine released is fairly constant
If renal function stable, amount creatinine in urine is stable
Low values of creatinine clearance may indicate renal failure
High plasma creatinine may indicate renal failure
How can creatinine indicate renal failure?
Low values of creatinine clearance may indicate renal failure
High plasma creatinine may indicate renal failure
i.e. Creatinine plasma concentration goes up
How is renal plasma flow (RPF) measured?
By PAH (Para aminohippurate) clearance= 625ml/min
Why is PAH used to measure RPF?
Filtered and actively secreted in one pass of the kidney
I.e. all PAH is removed from the plasma passing through the kidney so its clearance equals the renal plasma flow
What do most solutes, inulin/creatinine and PAH do in the kidneys?
Most solutes= controlled excretion, reabsorption
Inulin/creatine= GFR, no reabsorption or secretion
PAH= RPF, secretion
What diagnostic features show renal disease?
Fall in GFR (-> excretory products build up in plasma)
Raised plasma concentration of creatinine
Excretion of some substances may be impaired in renal failure (including some drugs) so need to take into account when calculating drug doses
What is tubular function?
Need to go from 180L of filtrate a day to the 0.6-2.5L of urine
On average day, consume more water and salt than we need-> need to lose this with other waste products (e.g. urea)
How is waste removed while keeping necessary components?
Controlled reabsorption and secretion
- Need to reabsorb 99% of the ultra filtrate
- Need to maintain solute balance, plasma concentration and pH
Different parts of the nephron are specialise to perform specific tasks
What is osmolarity?
A measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane
All the concentrations of the different solutes (measured in mmol/l) added together
- Each ion is “counted” separately
Osmolarity is dependent on the number of particles in a solution and NOT the nature of the particles
E.g. 1mmol/L of Na2HPO4 is the equivalent of 3 mosmoles/L
This is made up from 1 mosmol/L HPO42- and 2 mosmol/L of Na+
What effect does a solute present at equal concentrations either side of a semi-permeable membrane have on water movement?
Any solute present at equal concentrations either side of a semi-permeable membrane can have no net effect on water movement