Urinary System, Lecture 1 Flashcards
Kidneys
at rest ~20% of cardiac output moving through kidneys (~1L of 5L blood volume)
functions:
- regulating water, ions, acidity, blood volume/pressure
- removing metabolic wastes and foreign chemicals (how much are retained and how much is gotten rid of from the body)
- gluconeogenesis - production of glucose under certain conditions
- source: erythropoietin (hormone), renin (enzyme) -> renin is an important regulatory enzyme coming from the kidneys
4 Components of Urinary System
glomerular filtration: leaving blood; glomerulus to capsular space
- substances are leaving the bloodstream and moving from structure called the glomerulus to capsular space (space surrounding glomerulus)
tubular reabsorption: returning to blood; renal tubule to peritubular capillaries
- anything we want to take back to bloodstream, regulation aspect, if we reabsorb more or less; tells us about the process we are trying to achieve
tubular secretion: leaving blood; peritubular capillaries to renal tubule
- another way to get rid of things, move out of bloodstream further down the tubule
urinary excretion: elimination from body; combination of above processes:
- if you get a positive value or negative value if you tells you something about how much is being excreted from the urine
◦ excreted = filtered + secreted - reabsorbed
Substance X (secretion), Substance Y (filtration) and Substance Z (reabsorption)
- in all three cases we are leaving bloodstream and moving into capsule
- all three have about the same amount of glomerular filtration
Substance X: no reabsorption; strong amount of secretion in peritubular capillaries (getting rid of a lot of it in the urine as there is some filtration (certain amount) but a ton of secretion)
- substance we want to get rid of like metabolic waste or drug -> unwanted
- almost nothing is in bloodstream
Substance Y: certain amount is filtered and reabsorption is pretty much the same as prior substance; fairly small amount that ends up in urine as most of it comes back out of the blood vessels of the kidneys
- no secretion in Substance Y
- classic one (regulatory)
Substance Z: filter a certain amount but we reabsorb all of it as none ends up coming out of the urine
- reabsorbed back into bloodstream and carried out in kidney
- see this with something like glucose as we do not want it in the urine
-> one ends up all in the urine, one ends up with some in the urine and one has nothing in urine
Glomerular Filtration
principle of filtration:
- force fluids/solutes through a membrane by pressure
- pressure gradient -> moving from high to low (pushed from glomerulus into capsule)
- fluids/solutes leaving blood and entering capsular space becomes “filtrate”
◦ filtration fraction: percentage (~20%)
◦ glomerular filtration rate (GFR): volume per unit
time (~125 mL/min or ~150 - 180 L/day)
glomerulus - specialized capillary bed
- more efficient filter than other systemic capillary beds (150-180 L/day glomerulus vs. 4 L/day systemic capillary beds):
◦ membrane: larger surface area, very permeable
(with pores (openings in these capillaries- easy for
things to move through as long as pressure is t
here); 45x “leakier” than typically systemic
capillary bed)
◦ glomerulus blood pressure higher (~60 mmHg)
than typical systemic capillary blood pressure
(arterial end of capillary bed ~35 mmHg)
- if we have more pressure we can push more with more force as we can get a bigger pressure gradient
- most of it gets reabsorbed
Glomerular Filtration (strongly regulated)
- try to maintain a relatively constant GFR despite what is happening in the rest of the body
- strong regulated mechanisms:
◦ too high - filtrate moves too quickly so not
enough time for reabsorption and too much ends
up in urinary excretion
◦ too low - filtrate nearly all reabsorbed so some
waste, harmful substances, or excesses, not
adequately eliminated by urinary excretion - kidney problems often related to poor GFR
- keep GFR constantly regular meaning despite what is happening in the body, it remains fairly regular
- regulated quite strongly cause we need enough of reabsorption and elimination
Glomerular Filtration - 4 starling forces
4 starling forces:
- blood hydrostatic pressure (PGC) ~60 mmHg
* fluid hydrostatic pressure (PCS) ~15 mmHg
* blood osmotic pressure (πGC) ~29 mmHg
* fluid osmotic pressure (πCS) ~0 mmHg
- favouring filtration: PGC and πCS
- opposing filtration: PCS and πGC
- it always a positive number in favour of filtration (just a matter of how positive) - we do not get anything in the reverse
- combined 4 forces to get net filtration pressure (NFP):
◦ NFP = PGC + πCS - PCS - πGC - forces may change but in healthy individual favour filtration over entire glomerulus surface area (positive NFP)
GFR - control - renal autoregulation
renal autoregulation - intrinsic mechanisms within kidneys to maintain a relatively constant GFR
1st autoregulatory mechanism - myogenic mechanism (quickest one)
- reponds in seconds
- stimulus: alter GFR
- response: alter smooth muscle / alter arteriole radius / alter blood hydrostatic pressure / alter NFP / alter GFR
my notes:
- the change is both stimulus and response
- GFR is increasing - is it the stimulus triggering or response to something
GFR - control - renal autoregulation (2)
2nd autoregulatory mechanism - tubuloglomerular feedback
- a little slower to respond than myogenic
- stimulus: alter GFR
- response: macular densa of JGA detect altered filtration/ JGA alter release of nitric oxide (NO) / alter afferent arteriole radius/ alter PGC / alter NFP / alter GFR
- generally not trying to get a change
GFR - control - neural
- extrinsic mechanisms - influencing GFR from outside kidney
- for GFR change must be a stronger effect than renal autoregulation can correct for
- stimulus: venous, atrial and arterial blood pressure changes monitored by baroreceptors
- response: alter sympathetic firing and levels of norepinephrine(NE) / alter afferent arteriole vasoconstriction / alter PGC / alter NFP / alter GFR
- note - direct effect (response) - alter arterial BP / alter PGC / alter NFP / alter GFR (not involving sympathetic nerves)
GFR - control - hormonal
- extrinsic mechanisms - influencing GFR from outside kidney
- for GFR change must be a stronger effect than renal autoregulation can correct for
2 main GFR altering hormones:
angiotensin ll
- released from liver inactive (angiotensinogen) ultimately converted to angiotensin ll in lung and kidney endothelium
- response: circulates and vasoconstricts afferent and efferent arterioles; greater effect on afferent arteriole so decreases GFR
atrial natriuretic peptide (ANP)
- released from heart
- response: circulates and vasodilates afferent arterioles and vasoconstricts efferent arterioles; increases GFR
- response: ANP can also increase capillary surface area of glomerulus by relaxing mesangial cells; increases GFR
