Excretory System Flashcards
excretory system
Homeostasis of internal aqueous environment (water, solutes, wastes) = crucial for life
Diet, metabolic products, loss of or excess H2O & ions create frequent imbalances
Regulation, selective retention, & excretion = purpose
Excretory systems provide 4 functions
1- maintenance of internal levels of inorganic solutes
e.g. Na+, K+, Cl-, H+, etc.
2- maintenance of internal water volume
especially important for blood pressure
3- removal of nonnutritive & harmful substances
e.g. metabolic products, toxins, hormones, etc.
4- maintenance of osmotic balance
must maintain proper water-salt balance
4 components of the excretory system
1- Lungs
regulates CO2 & acid-base balance of blood
2- Digestive system (including liver)
removes some wastes & undigested food
3- Skin & glands
e.g. can excrete ions/salts & some wastes in sweat
4th component = renal organs
4th component = renal organs
Filter body fluids, regulate water, ions, & other organic substances, then selectively reabsorb or secrete these substances
In vertebrates renal organs= kidneys, which are the main organ of the urinary system
The kidneys solve a few common problems that vertebrates face, such as nitrogenous waste excretion
Nitrogenous waste excretion
Metabolism of protein & nucleic acids produce nitrogenous wastes in the form of ammonia (quite toxic), or urea & uric acid (less toxic)
All are excreted by the kidneys to prevent buildup & subsequent poisoning
Renal organs have transport epithelia
These cells facilitate both passive & active transport of molecules
Active transport of salts is most heavily governed by the Na+/K+ pump
e.g. notice that active transport of Na+ from the kidney to the bloodstream pulls Cl- along with it passively because of the electrical gradient
No active transporters for H2O are known to exist
This means that to move H2O, we must actively move salts to draw it via passive osmosis
To move salt only, specialized water pores (aquaporins) can be removed
The urinary system consists of:
kidneys, which form urine
urine-conducting structures:
ureters
bladder
urethra
Mammalian urinary system:
Kidneys are supplied by blood flowing through renal artery & renal vein
Urine made by the kidneys drains into 2 ureters & is then stored by the bladder
Urine from bladder is emptied to exterior through urethra
Anatomy of a kidney
Smallest functional unit = nephron
- ~1 million in each human kidney
- have tubular & vascular components
- arrangement gives rise to 2 distinct regions:
- outer renal cortex
- inner renal medulla
afferent arterioles
When entering kidney, renal artery divides to form many small vessels
each arteriole supplies 1 nephron
glomerulus
Where blood is delivered
a knot of capillaries in renal cortex
efferent arteriole
Glomerular capillaries rejoin to form it
-blood that was not filtered into the tubular component leaves
peritubular capillaries
which supply the kidney with blood, plus exchange molecules between the tubular system & blood
Nephron tubular component
Fluid filtered from the glomerulus passes through nephron tubules, which have specialized segments with different functions
Bowman’s capsule, proximal tubule, loop of Henle, distal tubule, & collecting duct
Bowman’s capsule
cup-shaped invagination around the glomerulus
“Glomerular filtration” = plasma being forced from glomerulus into Bowman’s capsule
filtrate includes anything in blood except cells (e.g. erythrocytes) & large proteins
proximal tubule
Filtrate flows into itlies within renal cortex
here the substances of value (e.g. glucose, amino acids, much H2O) are returned to the peritubular capillaries
= tubular reabsorption
tubular reabsorption
usually ~90% of H2O & 100% of glucose are reabsorbed
Proximal Tubule
tubular secretion
Proximal Tubule
Transfer in opposite direction: substances from capillaries enter the proximal tubule for excretion
Only ~20% of plasma is filtered through glomerulus: secretion allows for removal of substances from the 80% plasma that’s unfiltered
substance must have a specific transporter for secretion or reabsorption
Loop of Henle
U-shaped loop that dips into the renal medulla
Descending limb dips toward/into medulla & ascending limb travels back up, running in a countercurrent fashion
important osmo-concentration process happens here – more later
2 types of nephrons differ in position & LOH
Remember that all nephrons originate in the renal cortex, but glomeruli can lie in the “outer” cortex or “inner” cortex
Cortical nephrons
have glomeruli in the outer cortex & have LOHs that barely dip into the medulla
Juxtamedullary nephrons
have glomeruli in the inner cortex & LOHs that dip deep into the medulla
vasa recta
Both nephrons have peritubular capillaries but here they also form 1 long, vascular loop running parallel to the LOH: (vasa recta)
especially important for proper osmoconcentration
distal tubule
Final tubular component in nephron Like the proximal tubule, also plays role in tubular reabsorption & secretion
Like the loop of Henle, also plays a role in osmoconcentration
Drains fluid (now urine) into collecting duct, which drains into ureters, leading to bladder
Juxtaglomerular apparatus (JGA)
combined vascular-tubular component
Ascending limb of the loop of Henle returns to the glomerular region of its own nephron & passes through the fork formed by the afferent & efferent arterioles
JGA plays role in sensing blood osmolarity & pressure, & regulates kidney function
Note that not all vertebrates have kidneys or nephrons set up like this – some examples
some have a hindgut instead of bladder
some fish are able to use the bladder for water storage as well as urine storage for excretion
reptiles & amphibians have intermediate tubules instead of LOHs
non-mammals have a cloaca instead of urethra
Above the 4 functions of all excretory systems, mammalian kidneys have 4 additional functions:
- Secretion of erythropoietin, a hormone that stimulates red blood cell production
- Conversion of Vitamin D into its active form
- regardless of its source, vitamin D must be activated in the liver & then kidneys in order to promote intestinal absorption of Ca2+ - Excretion of pheromones for sexual signaling, marking territory, etc.
- Secretion of renin, a hormone important for salt conservation
Glomerular filtration
Fluid filtered from glomerulus to Bowman’s capsule must pass through 3 layers that act like a sieve
glomerular capillary wall: Layer 1
a single layer of endothelial cells with many pores (“fenestrations”) that are very permeable to H2O & small solutes
-Remember: RBCs & large plasma proteins can’t get through
glomerular capillary wall: Layer 2
basement membrane, a gelatinous layer between the glomerulus & Bowman’s capsule
Has small pores in the matrix, allowing small molecules to pass
A small amount of the smallest plasma proteins (e.g. albumin) can get through
are picked up in proximal tubule by endocytosis, so healthy urine = protein-free
glomerular capillary wall: Layer 3
inner layer of Bowman’s capsule: consists of podocytes: octopus-like cells that interlace
narrow slits between podocytes = filtration slits
Changing the glomerular filtration rate (GFR)
In a healthy mammal, ~20% of the plasma entering the glomerulus is filtered
reabsorption is why we don’t urinate all this out!
GFR can be altered by changing blood pressure or arteriole diameter
increased BP = increased GFR
Vasoconstriction (by smooth muscle surrounding the afferent arterioles) = decreased GFR
Vasodilation = increased GFR
The controlled changes by vasoconstriction & dilation are caused by:
extrinsic & intrinsic factors:
Extrinsic = sympathetic control
-for long-term GFR regulation
Intrinsic = Autoregulation: the kidneys themselves have mechanisms to prevent spontaneous changes
- Some is done by the reaction of smooth muscle to stretch (^pressure = ^stretch = ^constriction)
- Much is done through the tubuloglomerular feedback mechanism, involving the JGA
tubuloglomerular feedback
When GFR is raised, more fluid flows through the distal tubule specialized cells (macula densa) at the JGA detect this & release paracrines causing constriction of the afferent arteriole
One last way to alter GFR is to increase or decrease: (not pressure related)
glomerular permeability
e.g. podocytes can contract, decreasing filtration slits (less permeable, lower GFR), or relax, increasing filtration slits (more permeable, higher GFR)
