9-1: Kidneys at Work

Question 1

Osmoregulation

Answer 1

• controlled movement of solutes btwn internal fluids and external environment
• water enters or leaves cells by osmosis
• unit of measurement is osmolarity = total solute concentration per unit volume of solvent

Question 2

Saltwater vs Freshwater fish

Answer 2

1) seawater: hypo-osmotic to seawater
- problem: water loss
- solution: replace water by drinking
2) freshwater: hyper-osmotic to fresh water
- problem: water gain
- solution: lose water in urine formation

Question 3

Terrestrial animals

Answer 3

• problem: water loss

- solutions: replace water by drinking and eating, gain metabolic water

Question 4

excretory system steps

Answer 4

• filtration
• reabsorption
• secretion
• excretion

Question 5

filtration

Answer 5

-The excretory tubule collects a filtrate from the blood. Water and solutes are forced by blood
pressure across the selectively permeable membranes
out of the blood capillaries and into the excretory tubule

Question 6

reabsorption

Answer 6

-sending valuable substances back to the blood.

Question 7

secretion

Answer 7

-Toxins and excess ions, are extracted from body fluids
and added to the excretory tubule. Cells of the transport
epithelium also secrete their products into the lumen of
the tubule.

Question 8

excretion

Answer 8

-filtrate is excreted

Question 9

kidney

Answer 9

• supplied w blood via renal artery, drained by a renal vein
• urine exits kidney thru ureter
• ureters drain into urinary bladder
• urethra releases urine outside the body

Question 10

kidney functions

Answer 10

• conservers water by concentrating urine due to:
  1) cooperation and arrangement of descending and ascending arms of loop of henle and collecting duct
  2) NaCl and urea contribution to osmolarity of interstitial fluid

Question 11

nephrons

Answer 11

-functional unit of kidney w four major parts: renal corpuscle, proximal tubule, loop of Henle and distal tubule

Question 12

types of nephrons

Answer 12

-cortical nephrons (80%): short loops of henle, located mostly in cortex renal corpuscle, proximal tubule, loop of
Henle and distal tubule. It empties into a collecting duct
-Juxta-medullary nephrons (20%) – long loops of Henle, extend into the renal medulla
-longer nephron = more concentrated urine = better water retention

Question 13

summary steps of excretions

Answer 13

1. The renal corpuscle filters blood.
2. Proximal tubule reabsorbs
   valuable nutrients, vitamins,
   ions, and water
3. The loop of Henle establishes a strong osmotic
   gradient in the tissues outside of the loop
4. In the distal tubule, ions and water are reabsorbed in a
   regulated manner, which
   helps maintain water and
   electrolyte balance.
5. More water is reabsorbed in
   the collecting duct. Also, urea
   leaves the collecting duct, and
   helps establish high osmolarity in the medulla

Question 14

blood filtration

Answer 14

• occurs in glomerulus which filters blood to create a filtrate comprising water, electrolytes, and other small substances
• larger substances can’t pass through
• blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule

Question 15

reabsorption

Answer 15

-proximal convoluted tubule reabsorbs
-Na+ actively pumped
this force is the “master gradient”
-NaCl (salt) in the filtrate enters the cells of the transport epithelium by facilitated diffusion and cotransport mechanism and then is
transferred to the interstitial fluid by active transport. This transfer of positive charge out of the tubule, drives the passive transport of Cl-
As salt moves from the filtrate to the interstitial fluid, water follows by osmosis reducing filtrate volume considerably.
-Almost all glucose is reabsorbed into the blood (actively or passively), such
as other nutrients and ions such as potassium.
-Final result: recovering water, nutrients and electrolytes, but leaving wastes in the filtrate.

Question 16

Proximal (convoluted) tubule also secretes

Answer 16

-Some toxic materials (drugs and other
poisons), as well as H+ and ammonia
are secreted into the filtrate
-Maintains pH in body fluids
-Urine that is eventually excreted is composed of both filtered and secreted substances.
-loop of henle creates osmotic gradient

Question 17

Descending limb of loop of henle:

Answer 17

-the transport epithelium is freely permeable to water, but not to salt
-reabsorption of water continues via aquaporins
-Moving downward, the filtrate continues to lose water to interstitial fluid, and the filtrate
becomes increasingly concentrated with solute

Question 18

Ascending limb of loop of henle:

Answer 18

• ascending limb’s epithelium is permeable to salt but not to water
  -There are 2 regions: a thin segment near the loop tip and a thick segment closer to the
  distal tubule
  -Na+ and Cl- diffuse out in the thin segment and are actively transported out in the thick
  segment.
  -By losing salt without giving up water, the filtrate is progressively diluted and the
  osmolarity of the medulla increases.

Question 19

Distal (convoluted) tubule regulates water and

electrolyte balance

Answer 19

-Regulates the K+ and NaCl concentrations of
body fluids by varying the amount of K+ that is
secreted into the filtrate and the amount of
NaCl that is reabsorbed from the filtrate
- Contributes to pH regulation: secreting H+ and
reabsorbing HCO3

Question 20

Collecting duct regulates water and electrolyte balance

Answer 20

• takes the filtrate through the medulla to the renal
  pelvis
  -Outer medulla: reabsorbing NaCl, concentration of urea in the interstitial fluid is low in
  the outer medulla.
  -Inner medulla: NaCl cannot pass, urea leaks, and is
  high in the inner medulla (this section is permeable
  to urea, and some urea leaks here), water is reabsorbed.

Question 21

ADH

Answer 21

-The presence/absence of aquaporins in the
collecting duct is regulated by ADH: antidiuretic hormone.
• When producing concentrated urine:
1. ADH is present;
2. Water crosses the epithelium of the duct;
3. Salt cannot cross;
4. Urea cannot cross in the cortex, but can cross in the inner medulla
• When producing dilute urine (when ADH is absent):
1. ADH is absent.
2. Water cannot leave the collecting
duct.

Question 22

Countercurrent multiplier system

Answer 22

-in the loop of Henle uses energy to actively transport NaCl from the filtrate in the upper part of the ascending limb of the loop.
- maintains a high
salt concentration inside the
kidney, enabling the kidney to
form concentrated urine.

Question 23

CMS: What is ‘multiplying’ in this system?

Answer 23

Descending limb of the loop of Henle is permeable to H2O but not to
salts; ascending limb of the loop of Henle is permeable to salts but not
to H2O.
• Together, they multiply (increase) the osmotic gradient between the
filtrate and the interstitial fluid in the space around the tubule.

Question 24

CMS: What is countercurrent in this system?

Answer 24

The descending and ascending vessels of the vasa recta (a series of
straight small capillaries in the medulla) carry blood in opposite
directions through the kidney’s osmolarity gradient.

Question 25

What prevents the capillaries of the vasa recta from dissipating the salt gradient by carrying away the high concentration
of NaCl in the medulla’s interstitial fluid?

Answer 25

• Vasa recta’s descending vessel: water leaves the blood, and NaCl is gained by diffusion
• Vasa recta’s ascending vessel: water reenters the blood, and NaCl diffuses out

Question 26

How do our kidneys
conserve water by
producing
concentrated urine?

Answer 26

-the urine is hyperosmotic to blood and
interstitial fluid elsewhere in
the body. This high osmolarity
allows the solutes remaining in
the urine to be excreted from
the body with minimal water
loss.