The document summarizes key concepts about excretion by the kidneys. It discusses glomerular filtration which forms filtrate from blood plasma, and the tubular reabsorption and secretion processes that modify the filtrate to form urine and remove wastes. Specifically, it describes the mechanisms of passive and active transport that allow reabsorption of useful solutes and water back into blood from the filtrate in different kidney tubule segments. The countercurrent mechanism and role of antidiuretic hormone in regulating urine concentration and volume are also summarized.

1. Excretion

3. Glomerular filtration
Filtrate is a plasma-derived fluid that is
processed by the renal tubules to form
urine.
Urine is the end product of excretory
activity on the filtrate, and contains mostly
metabolic wastes and unneeded substances.

4. Net filtration pressure is responsible for
filtrate formation, and is the product of the
forces acting at the glomerular bed.
Glomerular hydrostatic pressure is the
primary force pushing water and solutes out
of the blood across the filtration membrane.

5. Colloid osmotic pressure and capsular
hydrostatic pressure are forces that drive
fluids back into glomerular capillaries.
Glomerular filtration rate is the total amount
of filtrate formed per minute by the kidneys.

7. Renal autoregulation is a process by which
the kidney can adjust its own resistance to
blood flow, by maintaining a nearly
constant glomerular filtration rate.
The myogenic mechanism reflects the
general tendency of vascular smooth muscle
to contract when it is stretched.

9. The renin-angiotensin mechanism is
triggered when the juxtaglomerular cells
release renin in response to various stimuli.
Angiotensin converting enzyme (ACE)
catalyzes the reaction that converts
angiotensin I to angiotensin II.

10. Tubular reabsorption
Tubular reabsorption is a reclamation
process that prevents significant loss of
plasma volume during the filtration process.

12. Passive tubular reabsorption is the
movement of substances along their
electrochemical gradient without the use of
ATP.
Because water follows salt in the osmotic
process, a sodium-linked water flow is
established, called obligatory water
reabsorption.

13. Solvent drag explains the passive
reabsorption of lipid-soluble substances in
the filtrate, as a result of solutes following
the movement of solvent.
Secondary active transport is due to the
gradient created by the Na+/K+ pump, which
causes the reabsorption of glucose, amino
acids, lactate vitamins and most cations.

14. The transport maximum reflects the number
of carriers in the renal tubules available to
carry each substance.
Urea, creatinine, and uric acid are important
substances not reabsorbed completely by
the nephron.

15. Tubule Segment Substance Reabsorbed Mechansim
Proximal convuluted Sodium ions Active transport via sodium/potassium carrier; sets up
tubule electrochemical gradient of passive solute diffusion,
Aldosterone promotes Na+ retention
Nutrients (glucose, amino Active transport; cotransport with sodium
acids, vitamins)
Cations Active transport; cotransport with sodium
Anions Passive transport; paracellular diffusion by electrochemical
gradient
Water Osmosis
Urea Passive diffusion via solvent drag
Small proteins Pinocytosed by tubule cells, digested to amino acids
Loop of Henle
Descending loop Water Osmosis
Ascending loop Sodium, Chloride, Potassium Active transport

16. Tubule Segment Substance Reabsorbed Mechansim
Distal Convoluted Sodium Active transport; requires aldosterone
tubule Calcium PTH mediated active transport
Chloride Diffusion along electrochemical gradient; cotransport
with sodium
Water Osmosis; depends on ADH to increase porosity of
tubule epithelium
Collecting Duct Sodium, Hydrogen, Potassium, Aldosterone mediated active transport of sodium;
Bicarbonate, Chloride cotransport of hydrogen, potassium, chloride, bicarbonate
Water Osmosis; depends on ADH to increase porosity of
tubule epithelium
Pinocytosed by tubule cells, digested to amino acids
Urea Diffusion along electrochemical gradient; most remains
in medullary insterstitial space

17. Tubular secretion
Tubular secretion is another method, like
failure of tubule cells to reabsorb filtered
solutes, of clearing plasma of unwanted
substances.

19. Regulation of urine
concentration and volume
Osmolality is the number of solute particles
dissolved in one liter (1000 grams) of water
in a solution.
An osmol is equivalent of 1 mole of a non-
ionizing substance in l liter of water.
A milliosmol is equal to 0.001 osmol, and is
used to describe the solute concentration of
body fluids.

20. The countercurrent mechanism allows the
kidneys to keep the solute load of body
fluids constant at about 300 mosm (= to
blood plasma), by directing fluids in
opposite directions through adjacent
channels.

22. Phases of the countercurrent
multiplier
1) The descending limb of the loop of
Henle is relatively impermeable to
solutes and freely permeable to water.
2) The ascending limb of the loop of Henle
is impermeable to water and actively
transports sodium chloride into the
surrounding interstitial fluid.

23. 3) The collecting ducts in the deep
medullary regions are permeable to
urea.
4) The vasa recta as a countercurrent
exchanger, maintaining the osmotic
gradient while delivering the nutrient
blood supply to cells in the area.

24. The countercurrent exchanger protects the
medullary gradient set up by the loop of
Henle by preventing rapid removal of salt
from the medullary interstitial space.

25. Formation of concentrated urine
Antidiuretic hormone inhibits urine output
(diuresis), by increasing the number of
water-filled channels in the principal cells
of the collecting ducts.

28. Water reabsorption that depends on the
presence of ADH is called facultative water
reabsorption.
Diuretics are chemicals that enhance
urinary output.

29. Renal clearance
Renal clearance is the volume of plasma
from which a particular substance is
completely cleared by the kidneys in a
given time, usually 1 minute.

30. Characteristics and composition
of urine
Urochrome is a pigment that results from
the destruction of hemoglobin, and is
responsible for the color of urine.
Urine, because it is water plus solutes, has a
higher specific gravity, or weighs more than
water.
Nitrogenous wastes include urea, uric acid,
and creatinine.