Excretory System

Question 1

excretory system

Answer 1

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

Question 2

Excretory systems provide 4 functions

Answer 2

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

Question 3

4 components of the excretory system

Answer 3

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

Question 4

4th component = renal organs

Answer 4

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

Question 5

Nitrogenous waste excretion

Answer 5

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

Question 6

Renal organs have transport epithelia

Answer 6

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

Question 7

No active transporters for H2O are known to exist

Answer 7

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

Question 8

The urinary system consists of:

Answer 8

kidneys, which form urine

urine-conducting structures:
ureters
bladder
urethra

Question 9

Mammalian urinary system:

Answer 9

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

Question 10

Anatomy of a kidney

Answer 10

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

Question 11

afferent arterioles

Answer 11

When entering kidney, renal artery divides to form many small vessels
each arteriole supplies 1 nephron

Question 12

glomerulus

Answer 12

Where blood is delivered

a knot of capillaries in renal cortex

Question 13

efferent arteriole

Answer 13

Glomerular capillaries rejoin to form it

-blood that was not filtered into the tubular component leaves

Question 14

peritubular capillaries

Answer 14

which supply the kidney with blood, plus exchange molecules between the tubular system & blood

Question 15

Nephron tubular component

Answer 15

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

Question 16

Bowman’s capsule

Answer 16

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

Question 17

proximal tubule

Answer 17

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

Question 18

tubular reabsorption

Answer 18

usually ~90% of H2O & 100% of glucose are reabsorbed
Proximal Tubule

Question 19

tubular secretion

Answer 19

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

Question 20

Loop of Henle

Answer 20

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

Question 21

2 types of nephrons differ in position & LOH

Answer 21

Remember that all nephrons originate in the renal cortex, but glomeruli can lie in the “outer” cortex or “inner” cortex

Question 22

Cortical nephrons

Answer 22

have glomeruli in the outer cortex & have LOHs that barely dip into the medulla

Question 23

Juxtamedullary nephrons

Answer 23

have glomeruli in the inner cortex & LOHs that dip deep into the medulla

Question 24

vasa recta

Answer 24

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

Question 25

distal tubule

Answer 25

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

Question 26

Juxtaglomerular apparatus (JGA)

Answer 26

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

Question 27

Note that not all vertebrates have kidneys or nephrons set up like this – some examples

Answer 27

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

Question 28

Above the 4 functions of all excretory systems, mammalian kidneys have 4 additional functions:

Answer 28

1. Secretion of erythropoietin, a hormone that stimulates red blood cell production
2. 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+
3. Excretion of pheromones for sexual signaling, marking territory, etc.
4. Secretion of renin, a hormone important for salt conservation

Question 29

Glomerular filtration

Answer 29

Fluid filtered from glomerulus to Bowman’s capsule must pass through 3 layers that act like a sieve

Question 30

glomerular capillary wall: Layer 1

Answer 30

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

Question 31

glomerular capillary wall: Layer 2

Answer 31

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

Question 32

glomerular capillary wall: Layer 3

Answer 32

inner layer of Bowman’s capsule: consists of podocytes: octopus-like cells that interlace

narrow slits between podocytes = filtration slits

Question 33

Changing the glomerular filtration rate (GFR)

Answer 33

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

Question 34

The controlled changes by vasoconstriction & dilation are caused by:

Answer 34

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

Question 35

tubuloglomerular feedback

Answer 35

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

Question 36

One last way to alter GFR is to increase or decrease: (not pressure related)

Answer 36

glomerular permeability
e.g. podocytes can contract, decreasing filtration slits (less permeable, lower GFR), or relax, increasing filtration slits (more permeable, higher GFR)