PHYS: Filtration and Urination

Question 1

glomerular filtration

Answer 1

• blood enters via afferent arteriole
• high pressure passively and non-selectively pushes small molecules e.g. water, glucose, AAs through very large fenestrations in capillaries = now called filtrate = goes to bowman’s capsule and PCT
• large molecules e.g. proteins, blood cells do not cross = transported back into general circulation via efferent arteriole

Question 2

tubular reabsorption

Answer 2

• materials from filtrate are selectively reabsorbed back into blood via peritubular capillaries
• water, Na+ and glucose are fully reabsorbed
• waste products e.g. urea are poorly reabsorbed

Question 3

tubular secretion

Answer 3

• solutes move from peritubular capillaries into DCT to be excreted into urine
• e.g. drugs, H+ ions

Question 4

3 layers of glomerular filtration barrier

Answer 4

• simple squamous fenestrated capillary endothelium
• non-cellular basement membrane
• simple epithelium of Bowman’s capsule (contains podocytes with filtration slits, bridged by a diaphragm)

Question 5

forces that drive and oppose glomerular filtration

Answer 5

• FAVOURS filtration: glomerular hydrostatic pressure (in blood)
• OPPOSES filtration: hydrostatic pressure in Bowman’s capsule and plasma osmotic (colloid) pressure - since there is more proteins and less water in plasma (therefore glomerulus) than bowman’s capsule = water wants to move out of Bowman’s capsule back into glomerulus

Question 6

net glomerular filtration pressure

Answer 6

• GFP = hydrostatic pressure - (capsular hydrostatic pressure + osmotic pressure)
• NORMAL = 55 - (15 + 30) = +10 mmHg = positive filtration

Question 7

interpretation of GFR

Answer 7

• 60-120 = normal
• 15-60 = kidney disease
• 0-15 = kidney failure

Question 8

how is GFR measured?

Answer 8

• renal clearance - volume of plasma that is completely cleared of a substance by the kidney per unit time
• often measured using clearance of creatinine (waste product of creatine) - freely filtered but not reabsorbed or secreted (i.e. passes straight into urine following lack of reabsorption), AND not synthesised or metabolised by the kidney

Question 9

two most important determinants of GFR

Answer 9

• renal blood flow
• glomerular capillary pressure

Question 10

what pathologies can increase or decrease GFR?

Answer 10

• increase: kidney stones blocking renal pelvis, increase in plasma proteins
• decrease: blockage/stenosis of efferent arteriole, diarrhoea

Question 11

how does the bladder pressure remain low during filling?

Answer 11

• highly compliant due to transitional urothelium: 5-7 layers of cuboidal/columnar cells when relaxed and 2-3 layers of squamous cells when stretched
• rugae (internal folds) also helps facilitate stretch
• therefore able to increase in volume without increasing pressure

Question 12

how is the bladder tissue protected from the waste in urine?

Answer 12

• urothelium has specialised impermeable apical layer containing tight junctions and glycoproteins

Question 13

change in bladder shape during filling

Answer 13

• becomes spherical and then pear-shaped as it fills
• very full bladder may be palpable above the pelvic brim

Question 14

guarding reflex + voiding suppression process

Answer 14

• normal involuntary reflex to hold urine (regulated at spinal cord level)
• afferent signals from stretch receptors travel via spinothalamic and DCML pathways to reach pontine micturition centres
• when bladder reaches a critical level of distension, brain is alerted: periacqueductal grey (PAG) helps to voluntarily suppress urination via connections with all the nerves

Question 15

when can we get incontinence?

Answer 15

• parasympathetic nerve damage
• reduced bladder compliance = intravesical pressure increases = urge to void faster
• sphincter damage

Question 16

urge incontinence

Answer 16

• sudden, intense urge to urinate followed by involuntary voiding
• can be caused by UTI or neurological conditions

Question 17

when can we get urinary retention? what can it result in?

Answer 17

• obstruction of bladder outflow e.g. prostatic hypertrophy, cystocele
• nerve damage affecting sphincter tone
• damage of afferent nerves or poor detrusor muscle contractility (rare)
• can result in overflow incontinence if intravesical pressure gets large enough to overcome increased outflow resistance
• can also predispose to infection, bladder stones, retrograde flow (hydroureter, hydronephrosis etc)

Question 18

cystocele

Answer 18

• bladder prolapses posteriorly and inferiorly due to weakness of pelvic floor muscles
• can cause urinary retention due to compression of bladder

Question 19

what can cause damage/dysfunction to pelvic splanchnics?

Answer 19

• damaged during prostatectomy or abdominal surgeries
• pelvic trauma
• excessive compression
• diabetic neuropathy

Question 20

stress incontinence

Answer 20

• increased abdominal pressure under stress (weak pelvic floor muscles e.g. childbirth)
• loss of smooth and skeletal muscle tone