Urinary review Flashcards
What is the function of the kidneys?
Filter small waste molecules and recover completely filtered molecules that are essential
Control plasma osmolarity by filtering and variably recovering water
Control plasma volume by filtering and variably recovering salts
Control plasma pH by filtering and variably recovering hydrogen carbonate and production of new hydrogen carbonate to enter ECF
Where does filtration occur?
Glomerulus
What drives filtration?
Gradient of pressure
Hydrostatic pressure in glomerular capillaries (55mmHg) is much higher than other capillaries. Special capillaries between 2 resistance vessels
What controls the pressure in the capillaries?
Contraction of smooth muscle
Where do efferent arterioles drain to?
Second set of peri-tubular capillaries
What substrates are not normally filtered?
Cells Proteins (basement membrane - negatively charged molecules cross less readily)
When do proteins appear in urine?
If basement membrane damaged
How is glomerular filtration rate estimated?
Clearance of substances neither reabsorbed or secreted (usually creatinine)
Approx 125ml/min
What is clearance?
The volume of plasma that contains the amount of a substance excreted in the urine in one minute:
Urine concentration x urine volume/plasma concentration
What is the clearance of a substance completely reabsorbed?
0
What is the clearance if a substance is both filtered and completely secreted by the tubules?
= renal flow
All that enters the renal arteries appears in the urine
Where does most reabsorption occur and how is it driven?
Proximal convoluted tubule 67% of water and most salts 100% glucose and amino acids 90% of hydrogen carbonate Active transport required, mostly driven by active transport of Na+
What is the characteristic of epithelial cells that enables them to transport material across them?
Different properties on each surface - basolateral and luminal membrane
WHat is the role of sodium pumps?
Extrude sodium across basolateral membrane
Sodium enters across luminal membrane down concentration gradient
Energy from sodium movement drives reabsorption of other substances such as glucose
Water follows electrolytes osmotically
How is HCO3- recovered?
Coupled to Na H pump
Reacts with H+ to form H2O and CO2
Reformed in the tubular cells
How is osmolarity controlled?
Variable ingestionand excretion of water
If osmolarity rises, water conserved by producing concentrated urine - anti-diuresis, thirst stimulates
If osmolarity fals, water lost by producing lots of dilute urine - diuresis
How is dilute urine produced?
Water unable to be directly pumped
Take solute out and leave water behind
This occurs in the loop of Henle and distal convoluted tubule
Can reduce osmolarity to around 50 mosmol/L with up to 18L of urine produced a day
How is a concentrated urine produced?
Remove water, leave excreted solutes behind
Need osmotic gradient to make water
Have to create a space with a much higher osmolarity than ECF.
Water follows Na reabsorption in DCT
Main solute entering collecting duct is urea
Water drawn out of collecting duct by osmotic gradient in medulla
Needs insertion of aquaporins, stimulated by ADH from posterior pituitary
Released in response to detection of increased osmolarity by hypothalamic osmoreceptors
How is an osmotic gradient to produce concentrated urine created?
Renal medulla has a much higher osmolarity than ECF - up to 1400 mosmol/L
Created by active counter-current multiplication in loop of Henle and passive concentration of urea
Loop of hence :
Solute extruded from ascending limb draws water out of descending limb
Fluid in descending limb becomes progressively more concentrated as it flows down
Over successive segments generates 1200 momol
What are the features of the loop of Henle?
Descending: - no active transport - permeable to water, salts and urea Thin ascending: - impermeable to water Thick ascending: - Impermeable to water - actively extrudes Na/K/Cl via co-transporter (200mosmol gradient)
What is the role of urea in osmolarity in the loop of henle?
Urea drag out of collecting duct
Enter descending limb
Remains in filtrate
Till drawn out again from collecting duct
Steadily accumulates in medulla
Contributing a large fraction of the high omolartiy
How is the gradient in the medulla maintained?
Blood flow to medulla via vasa recta
Counter-current flow preserves osmotic gradient by passive counter-current
What is volume control dependent on?
Sodium concentration
How is sodium balance controlled?
Most recovered in pct and loop of hence
Control by variable reabsorption in act by ENaC
Falls in volume stimulate sodium retention via effects on arterial pressure
Rises in volume stimulates sodium excretion via effects on venous pressure
What stimulates sodium retention?
Aldosterone:
Fall in perfusion pressure of afferent arteriole
Detected by junta-glomerular apparatus
Renin released
Angiotensiogen -> angiotenin I -> angiotensin II by ACE
Angiotensin II acts on adrenal cortex -> aldosterone released and sodium retained increasing volume and BP
How is sodium release increased?
Increase in venous pressure
Senesed in atrium
ANP released
Stimulates extra Na loss
How is pH controlled?
Plasma pH depends on ratio of [HCO3-] to pCO2 (20:1)
Kidney controls [HCO3-]
In most condition s by both recovering all filtered HCO3- and making more
Renal production of HCO3- for ECF requires H+ excretion into urine
Where pH is buffered by phosphate and ammonia
How is HCO3- created in the proximal tubule?
Glutamine broken down to produce alpha ketoglutarate (which makes HCO3-) and ammonium
HCO3- into ECF
NH4+ into lumen
How is HCO3- created in the distal tubule?
Usually HCO3- all recovered
Na gradient insufficient to drive H+ secretion
Need active secretion of H+ into lumen
Where it is buffered by filtered phosphate and excreted ammonia
H+ generated from metabolic CO2
Producing HCO3- which enters ECF
How does acid base balance affect plasma K+?
Acidaemia -> hyperkalaemia
Alkalaemia -> hypokalaemia
What is micturation?
The desire to urinate
What does voiding of urine require?
Contraction of detrusor muscle
Relxation of sphincter muscles
How is urination initiated, coordinated and actioned?
Initiated by cortex, coordinated by the brainstem
