Topic 6: The Excretory System Flashcards
What are te functions of the human excretory system (6)?
- regulate H2O in the body
- regulate ions in body fluids
- maintain osmolarity of body fluids
- maintain plasma volume, blood volume & pressure
- eliminate toxic wastes
- secrete hormones and enzymes
What does erythropoietin mean?
production of RBCs
What does renin mean?
Na+ retention in kidney
What is the function of the osmoregulatory systems?
homeostatic regulation of fluid compartments: cytoplasm, interstitial fluid, blood
What are the three regulations of the osmoregulatory systems (3)?
- volume regulation
- solute regulation
- waste removal
What is the primary osmoregulatory organ?
kidney, using large numbers of parallel processing units
How do human kidneys work (3)?
- collecting
- modifying
- removing
What do human kidneys collect?
very large volumes of fluid from blood plasma (~ 180 Liters/day), mostly unselectively
What do human kidneys modify?
the collected fluid via selective secretion and reabsorption (during which nearly all of the collected fluid volume is returned to the circulation)
How do the kidneys regulate volume?
can alter urine flow from low to high according to circumstances (~500 mL/ day to 2.5 L/day - or much more with high fluid intake)
How do the kidneys regulate solute levels?
- can alter the total solute load of the urine from hypoosmotic to hyperosmotic
- adjust electrolyte (Na+, K+, Ca2+) loss or retention adjust pH as needed
What does hypoosmotic mean?
~ 50-100 mOsm; less concentrated than blood or interstitial fluid
What does hyperosmotic mean?
~ 1200 mOsm; about 4 time as concentrated as blood or interstitial fluid
What type of waste do the kidneys remove?
- metabolic wastes: urea, creatinine
- environmental toxins: e.g. drugs
What produces urine?
the two kidneys in the abdominal cavity
What is a ureter?
muscular ducts that propel the urine to the bladder, which stores the urine
What supplies the two kidneys?
a large renal artery and a renal vein
What are the characteristics of the bladder?
muscular
What is at the base of the bladder?
the base of the bladder are the two sphincter muscles, internal and external, which regulate flow of urine into the urethra
What is the structure of the kidney?
- two tissue layers
- outer cortex
- inner medulla
What is the kidney made of?
- nephrons
- circulatory supply
- connective tissue
What is the composition of nephrons?
- 80% cortical nephrons (outer part of kidney)
- 20% juxtamedullary nephrons (extend deep into medulla)
What is the function of the juxtamedullary nephrons?
largely responsible for the kidney’s ability to produce a strongly hyperosmotic urine
What are the peritubular capillaries?
sites of secretion and reabsorption that surrounds the nephrons
What are the medullary peritubular capillaries in juxtamedullary nephrons called?
vasa recta
What is the function of the afferent arteriole?
carries blood to the glomerulus
What is the function of the glomerulus?
a tuft of capillaries that filters a protein-free plasma into the tubular component
What is the function of the efferent arteriole?
carries blood from the glomerulus
What is the function of the peritubular capillaries?
supply the renal tissue; involved in exchanges with the fluid in the tubular lumen
What is the function of the Bowman’s capsule?
collects the glomerular filtrate
What is the function of the proximal tubule?
uncontrolled reabsorption and secretion of selected substances occur here
What is the function of the loop of henle?
(of juxtamedullary nephrons only; not shown) establishes in the osmotic gradient in the renal medulla that is important in the kidney’s ability to produce urine of varying concentration
What s the function of the distal tubule and collecting duct?
variable, controlled reabsorption of Na+ and H2O and secretion of K+ and H+ occur here; fluid leaving the collecting duct is urine, which enter the renal pelvis
What is the function of the juxtaglomerular apparatus?
produces substances involved in the control of kidney function
Where does urine production begin?
at Bowman’s capsule which surrounds the glomerulus
Where does urine composition modification proceed?
through the proximal tubule, loop of Henle, distal tubule, and collecting duct
What happens during urine composition modification?
substances are exchanged between the nephron, interstitial fluid, and blood
Does the composition change once urine leaves the collecting duct?
No
What are the three steps to urine formation?
- glomerular filtration
- tubular reabsorption
- tubular secretion
What happens during filtration?
- large plasma proteins are not passed from the blood: nondiscriminate filtration into Bowman’s capsule
- about 180 liters per day
- entire volume of plasma is filtered 65 times per day
What happens during tubular absorption?
- selective movement of ions, molecules and water back into the blood 99% of the water that enters the tubules is moved back into the blood
- 178.5 (0.99 x 180) so that is only about 1.5 liters of urine is produced per day
What happens during tubular secretion?
selective movement of material from the blood into the tubule
What are the driving forces for filtration?
hydrostatic (blood) pressure in the capillaries (PH) - fluid (hydrostatic) pressure in Bowman’s capsule (Pfluid) - colloid osmotic pressure in plasma (pi)
What is the difference between GFR and NFP?
GFR is similar to NFP in ‘regular’ capillaries but the glomerulus is much ‘leakier’ than typical capillaries
What does GFR mean?
glomerular filtration rate
What changes the GFR?
changing total renal blood pressure, or the diameter of afferent and/or efferent glomerular arterioles
What is the GFR in upstream (afferent) vasoconstriction?
increase resistance, decreased flow and pressure: reduced GFR
What is the GFR in upstream (afferent) vasodilation?
decreased resistance, increased flow and pressure: increased GFR
How is GFR regulated locally?
- myogenic responses (smooth muscle constriction/ dilation in afferent arterioles)
- paracrine signals (tubuloglomerular feedback)
What happens during myogenic responses?
- smooth muscle cells have an intrinsic property whereby they will contract in response to being stretched. So, if systemic blood pressure goes up, the arterioles will be stretched more and they will respond by contracting
- this constriction reduces blood flow into the glomerulus and thus the blood pressure in the glomerular capillaries, lowering GFR back to normal
- the opposite happens if systemic blood pressure falls
What is tubuloglomerular feedback?
a negative feedback loop that maintains GFR through paracrine signals
What are the processes of tubuloglomerular feedback?
- GFR increases
- flow through tubule increases
- flow past macula dense increases
- paracrine form macula dense to afferent arteriole
- afferent arteriole constricts
- resistance in afferent arteriole increases
- hydrostatic pressure in glomerulus decreases
- GFR decreases
About _ of the blood volume entering the glomerulus is filtered into the tubule - about _ of that is reabsorbed (adjustable)
- 20%
- 95%
Where does reabsorbed fluid return to?
from the filtrate is returned directly of indirectly via interstitial fluid to the blood exiting the kidney
What does the ability to excrete urine of varying concentrations depend on?
the existence of an osmotic gradient in the renal medulla
What happens when you drink too much water?
want to excrete hypotonic urine
What happens when you drink too little water?
want to excrete hypertonic urine
What is the usual tonicity of body fluids?
300 mOsm
What is the tonicity of the medulla?
600 to 1200
What is the function of the descending limb?
highly permeable to water, impermeable to solutes
What is the function of the ascending limb?
actively transports Na+, Cl- and K+ out of the tubular lumen into the ECF; impermeable to water, thus salt leaves without water following
What is countercurrent multiplication?
a countercurrent exchanger coupled with active transport of solutes = a countercurrent multiplier
What is the hypothetical pattern of blood flow?
if the blood flow straight through, blood would be hypertonic on leaving. the vertical osmotic gradient would be lost because the salt would be continuously flushed away
What is the actual pattern of blood flow?
blood equilibrates with the interstitial fluid at each incremental horizontal level in both the descending limb and the ascending limb of the vasa recta, so blood is isotonic as it enters and leaves the medulla
When is ADH released?
when osmolarity is high or blood pressure is low
How is blood pressure sensed by the kidney?
- atrial stretch receptors
- carotid and aortic baroreceptors
How is osmotic pressure sensed by the kidney?
hypothalamus
What triggers the release of ADH?
osmoreceptors in hypothalamus trigger release of ADH
How does ADH work?
by inserting Aquaporin-2 into the apical membrane
What is the function of ADH?
increases the permeability of the collecting duct to water (and therefore the amount of filtrate that is reabsorbed)
What happens when ADH is at high concentrations?
- collecting duct is freely permeable to water
- water exits urine osmotically (high osmolarity in renal medulla); is removed by vasa recta
What happens when ADH is at low concentrations?
- collecting duct is not permeable to water
- water is not removed form urine, so low initial osmolarity of filtrate does not change as filtrate passes through collecting duct
What are diuretics?
drugs that increase urinary water loss
What is the function of Loop diuretics (Lasix)?
- inhibit Na+ uptake in the ascending limb
- reduces osmotic gradient and ability to reabsorb water in collecting duct
What is the function of ethanol?
inhibits pituitary ADH secretion and therefore water reabsorption in the collecting duct
What is the function of glucose?
in diabetics, excess (unreabsorbed) glucose increases filtrate osmolarity, reducing osmotic reabsorption of water in collected duct
What is the function of caffeine?
inhibits ADH release by pituitary
What are the two systems of renal regulation of blood volume and pressure?
- renin-angiotensin-aldosterone system (RAAS)
- atrial natriuretic peptide (ANP) system
What is the function of the renin-angiotensin-aldosterone system (RAAS)?
increases Na+ retention and increases blood pressure
What is the function of the atrial natriuretic peptide (ANP) system?
- decreases Na+ retention and decreases blood pressure
- stimulates adrenal cortical cells to produce aldosterone - stimulates Na_ reabsorption
- stimulates the hypothalamus to secrete ADH (aka AVP … arginine vasopressin) Increases thirst. These lead to an increase in water retension
What happens during the renin-angiotensin-aldosterone system?
- liver chronically produces angiotensinogen (inactive)
- granule cells of juxtaglomerular apparatus produce renin in response to drop in blood pressure in afferent arteriole
- renin converts angiotensinogen to ANG1
- as ANG 1 passes through the lungs, it is converted to ANG II by angiotensin-converting enzyme (ACE)
- ANG II effects to lead an increase in blood pressure
What is the function of ACE inhibitors?
deployed to control hypertension by blocking ANG II production
What happens during renin-angiotensin-aldosterone system (RAAS)?
decreased blood pressure directly and indirectly results in increased renin secretion
What does natriuresis refer to?
the excretion of large amounts of Na+ in the urine
What happens during the ANP system?
atrial natriuretic peptide (ANP) secrete by cells in the walls of the atria of the heart in response to an increase in blood pressure. brain natriuretic peptide (BNP) secreted from ventricles
