Urinary System Flashcards
what are the functions of the kidney?
- regulation of blood:
- ion concentrations
- osmolarity
- volume
- pressure
- pH
- glucose
- production of hormones
- excretion of wastes
what is the kidney?
- bean-shaped
- 10-12 cm long
- 5-7 cm wide
- 3 cm thick
- 135-150 grams
- blood vessels and ureter attached at hilum
where is the kidney located?
location:
- superior lumbar region (T12-L3)
- on posterior abdominal wall
- retroperitoneal
- held in place by adipose tissue
- partially protected by floating ribs
internal anatomy of kidney
parenchyma
- renal cortex
- superficial layer
- contains nephrons
- renal medulla
- deeper layer
- 8-18 cone-shaped renal pyramids separated by renal columns
- nephron
- renal column
- renal pyramid
internal anatomy of kidney cont’d
drainage system
- renal sinus
- cavity within kidney
- contains renal pelvis, calyces, fat
- minor calyx
- collect urine from papillary ducts
- major calyx
- renal pelvis
- ureter
what is a nephron (kidney)?
- basic functional unit of the kidney
- filter blood, return useful substances, remove unneeded substances
- one million/kidney
- number constant from birth
- growth of kidney due to an increase in the size of individual nephrons
- no replacement if damaged
- no symptoms of dysfunction until function declines by 75%
- loss of one kidney leads to enlargement of the other until it can filter 80% of normal rate of two kidney
- consists of:
- network of blood vessels
- renal tubule
blood supply of kidney
renal a. > segmental a. >
interlobar a. > arcuate a. >
interlobular a. > afferent arteriole >
nephron > glomerular capillaries >
efferent arteriole > peritubular capillaries >
vasa recta > interlobular v. >
arcuate v. > interlobar v. >
segmental v. > renal v.
the nephron - renal corpuscle
- glomerulus
- capillary net
- glomerular capsule
- double-walled epithelial cup that collects filtrate
- surrounds capsular space
- parietal layer
- simple squamous epithelium
- capsular space
- receives filtrate
- visceral layer
- podocytes surround glomerular capillaries
the nephron - tubule components
renal tubule
- proximal convoluted tubule
- distal convoluted tubule
- DCTs drain urine to collecting ducts and papillary ducts
- nephron loop
- extends into medulla
what is the juxtaglomerular apparatus?
- located where afferent arteriole contacts thick ascending limb of the nephron loop
- two parts:
- macula densa
- crowded columnar cells in tubule wall
- juxtaglomerular cells
- modified smooth muscle cells in the wall of the afferent arteriole
- secrete renin
- macula densa
cortical nephrons
- 80-85% are cortical nephrons
- renal corpuscle in the outer renal cortex
- short nephron loops penetrate only to superficial medulla, receive blood supply from peritubular capillaries
juxtamedullary nephrons
- 15-20% are juxtamedullary nephrons
- renal corpuscle deep in the renal cortex, close to the medulla
- long nephron loops extend deeply into the medulla and receive blood supply from peritubular capillaries and vasa recta.
- long nephron loops of juxtamedullary nephrons enable kidneys to excrete very dilute or very concentrated urine (regulated by ADH)
overview of renal physiology
- urine formation is result of 3 processes
- glomerular filtration
- tubular reabsorption
- tubular secretion
what is glomerular filtration?
- the glomerulus is a capillary net and is subject to the same forces as systemic capillaries
- fluids and dissolved substances move through the glomerular membrane to the glomerular capsule
- non-selective process, separates particles by size
- small particles pass from blood, larger particles normally don’t
- hydrostatic pressure is ABP
- filtrate (fraction of plasma in afferent arterioles that passes to glomerular capsule) is normally 16-20%
- enhanced by:
- large surface area
- thin, extremely permeable filtration membrane
- high glomerular b.p. due to smaller diameter of efferent arterioles compared to afferent arterioles
what makes up the filtration membrane?
- fenestrated endothelium of glomerular capillary
- prevents passage of blood cells
- basal lamina of glomerular capillary
- prevents filtration of larger proteins
- slit membrane between pedicels
- prevents filtration of medium-sized proteins
what forces affect glomerular filtration?
- glomerular hydrostatic pressure
- capsular hydrostatic pressure
- blood (colloid) osmotic pressure
- capsular osmotic pressure
what is glomerular hydrostatic pressure (GHP)?
- afferent arteriole diameter is larger than efferent arteriole
- more blood enters the glomerulus than leaves it
- results in pressure (GHP) that moves substances by filtration from the glomerulus to the glomerular capsule
- increase GHP > increase filtrate
- 55 mmHg
what is capsular hydrostatic pressure (CHP)?
- the pressure exerted by fluid in the glomerular capsule
- opposes movement of filtrate from blood
- increase CHP > decrease filtrate
- 15 mmHg
what is blood colloid osmotic pressure (BCOP)?
- osmotic pressure exerted by plasma proteins
- opposes movement of filtrate from blood
- decrease BCOP > increase filtrate
- 30 mmHg
what is capsular osmotic pressure (COP)?
- osmotic pressure exerted by particles in the capsular filtrate
- usually negligible
- increase COP > increase filtrate
- 0 mmHg
what is net filtration pressure (NFP)?
- the net (effective) pressure between the glomerulus and glomerular capsule
- causes filtrate formation
- NFP = (GHP + COP) - (CHP + BCOP)
- NFP = 10 mmHg
what is glomerular filtration rate (GFR)?
- the rate at which filtrate is formed in all renal corpuscles of both kidneys
- generally about 125 mL/min
- GFR = 180 L/day
- constant GFR required for homeostasis
- too high = fluid passes through tubules quickly, useful substances lost
- too low = insufficient removal of waste products
- changes in NFP affects GFR
- filtration stops if GHP drops 45 mmHg
- normal function requires MAP + 80-180 mmHg
what are regulations of glomerular filtration rate?
- renal autoregulation
- myogenic mechanism
- tubuloglomerular feedback
- neural regulation
- hormone regulation
- angiotensin II
- atrial natriuretic peptide (ANP)
renal autoregulation - myogenic mechanism
major stimulus:
- increased stretching of smooth muscle fibres in afferent arteriole walls due to increased blood pressure
mechanism and site of action:
- stretched smooth muscle fibres contract, thereby narrowing the lumen of afferent arterioles
effect of GFR:
- decrease
renal autoregulation - tubuloglomerular feedback
major stimulus:
- rapid delivery of sodium and chloride to the macula densa due to high systemic blood pressure
mechanism and site of action:
- decreased release of nitric oxide (NO) by juxtaglomerular apparatus causes constriction of afferent arterioles
effect in GFR:
- decrease
neural regulation
major stimulus:
- increase in the activity level of renal sympathetic nerves releases norepinephrine
mechanism and site of action:
- contraction of afferent arterioles through activation of alpha 1 receptors and increased release of renin
effect on GFR:
- decrease
hormone regulation - angiotensin II
major stimulus:
- decreased blood volume or blood pressure stimulates the production of angiotensin II
mechanism and site of action:
- constriction of afferent and efferent arterioles
effect in GFR:
- decrease
hormone regulation - atrial natriuretic peptide (ANP)
major stimulus:
- stretching of the atria of the heart stimulates the secretion of ANP.
mechanism and site of action:
- relaxation of mesangial cells in glomerulus increases the capillary surface area available for filtration
effect on GFR:
- increase
what is tubular reabsorption?
- return of useful substances from filtrate in tubules to blood in peritubular capillaries
- 99% of filtrate returns to blood
- solutes reabsorbed by active, passive processes
- water follows by osmosis
- small proteins by pinocytosis
what is tubular secretion?
- movement of substances from the blood in the peritubular capillaries to tubules
- opposite direction to reabsorption
- most secretion takes place in DCT and collecting duct
- H+, NH4+, creatinine, K+ secreted
- secretion of H+ helps control pH
reabsorption routes
- paracellular reabsorption
- movement between cells
- 50% of material reabsorbed enters between cells by diffusion
- transcellular absorption
- movement through cells by active transport
- substances are carried through apical and basolateral membranes by transport proteins
transport mechanisms
- Na+ reabsorption
- important because
- Na+ is most abundant ion in filtrate
- water follows solute
- several different transport systems move Na+
- important because
- no Na+/K+ pump in apical membrane
- low Na+ concentration in ICF so Na+ enters cell
- Na+/K+ pump in basolateral membrane
- pumps Na+ from cell to blood in PTC
- water reabsorption
- by osmosis
- obligatory water reabsorption when water “obliged” to follow solutes (occurs in PCT and DNL)
- facultative water reabsorption occurs in collecting duct under the control of ADH
reabsorption in proximal convoluted tubule (PCT)
- majority of water and solute reabsorption occurs here
- most processes involve Na+
- Na+ symporters move Na+ together with other solutes
- Na+/H+ antiporters exchange Na+ for H+
- Na+ symporters normally reabsorb all glucose, amino acids, water-soluble vitamins, other nutrients in the first half of the PCT
- intracellular sodium levels are kept low by Na+/K+ pump
- PCT cells continuously produce H+ and HCO3-
- H+ secreted, HCO3- is reabsorbed
- important buffering system
- Na+/H+ antiporters reabsorb Na+ into tubule cells, secrete H+ to tubule
passive reabsorption in proximal convoluted tubule (PCT)
- in the second half of PCT, substances move along electrochemical gradients made in the first half of PCT by symporters and antiporters
- CL-, K+, Ca2+, Mg2+ and urea passively diffuse to the peritubular capillaries
- water follows solute by osmosis
secretion in the PCT
- ammonia (NH3) and urea enter tubules by:
- filtration to glomerular capsule
- secretion to PCT
by the time we get to the nephron loop…
- 65% of filtered water, many nutrients are reabsorbed from PCT
- symporters in nephron loop:
- Na+, K+, Cl- reabsorbed to tubule cells by symporters
- most K+ leaks back to NL > -ve charge in interstitial fluid, blood
- > cations diffuse to blood
reabsorption in DCT
- 80% of filtered water now reabsorbed
- symporters return more Na+, Cl- to PTC (blood)
- Ca2+ reabsorption regulated by PTH
- by the end of DCT, 95% of water, solutes are returned to blood
reabsorption and secretion in collecting duct
relies on:
- principal cells
- reabsorb Na+ and secrete K+
- intercalated cells
- reabsorb K+, HCO3-, secrete H+
what are principal cells?
- Na+ enters through leakage channels
- Na+ pump maintains low Na+ in cytosol
- K+ secretion adjusted with K+ intake
- aldosterone increase Na+ water reabsorption, K+ secretion by stimulating the synthesis of new pumps and channels
what are intercalated cells?
- proton pumps secrete H+ to the tubule against the concentration gradient (urine can be 1000x more acidic than blood)
- antiporters move HCO3- to blood
hormonal regulation - renin-angiotensin-aldosterone
- decrease GFR by a vasoconstricting afferent arteriole
- enhances Na+ reabsorption
- aldosterone causes principal cells to reabsorb more Na+, Cl-
- increase water reabsorption > increase blood volume
hormonal regulation - atrial natriuretic hormone (peptide)
- decrease Na+, water reabsorption in PCT
- suppresses aldosterone, ADH secretion
- increase Na+ excretion > increase urine output, decrease blood volume
hormonal regulation - ADH
- increase the permeability of principal cells > increase facultative water reabsorption
- increase the rate of water absorption
dilute or concentrated urine?
- kidneys regulate water loss in urine to maintain homeostatsis of body fluids despite available fluid intake
- ADH controls whether dilute or concentrated urine is formed
urine formation (summary)
- filtration
- reabsoption (majority) in PCT
- reabsoption (some) and secretion in NL & DCT
- reabsoption and secretion (relies on principal and intercalated cells) in collecting duct
what are physical characteristics of urine?
- volume 1 - 2 L/day (depends on intake)
- amber colour (but varies)
- sterile, clear when fresh
- aromatic when fresh, > ammonia-like upon standing
- pH 4.6 - 8.0, average 6.0
- specific gravity 1.001 - 1.035
what are chemical characteristics of urine?
- 95% H20
- Na+, Cl-, K+, HCO3-
- urea, uric acid
- creatinine
anatomy of ureter
- 25-30 cm long, 1-10 mm diameter
- extend retroperitoneally from renal pelvis, enter posterior wall of bladder
- transport urine by peristalsis
- physiological valve closes uretal openings when bladder fills
what is the urinary bladder?
- hollow, distensible, muscular organ
- capacity 700-800 mL
- posterior to pubinc symphysis
- anterior to vagina, inferior to uterus in female
- shape depends on fullness
- three openings:
- 2 urethral orifices
- 1 urethral orifice
- outline the trigone
anatomy of urinary bladder?
- L/R ureter
- rugae
- mucosa
- detrusor muscle
- serosa
- internal urethral sphincter
- external urehtral sphincter
- urethra
how does the bladder empty urine (micturition)?
controlled by 2 urethral sphincters:
- internal urethral sphincter
- smooth muscle
- ANS control
- external urethral sphincter
- skeletal muscle
- voluntary control
what is the micturition reflex?
- 200 mL urine accumulate in the bladder
- strecth receptors in the bladder wall are stimulated
- afferent impulses pass to the sacral spinal cord
- motor relflex impulses from the sacral spinal cord cause:
- contraction of the detrusor muscle
- relaxation of the internal urethral sphincter
- urine enters the proximal urethra
- sensory impulses to the brain results in conscious awareness of bladder fullness
what is micturition?
- voluntary relaxation of the external urethral sphincter
- increase intra-abdominal pressure forces urine from bladder
what is the urethra?
- thin-walled tube from urinary bldder to the urethral opening
- in females:
- 3-4 cm
- conveys urine only
- in males
- 15-20 cm
- conveys urine and semen
