Renal 2 - Terrestrial Osmoregulation: The Kidney

Water Loss

• Greatest challenge to terrestrial organisms is water loss

• Sources of Water Loss

  – Respiratory Water Loss → breathing: when we breathe we lose water

  – Evaporative Water Loss (EWL) → ex. loss through skin

  – Excretory Water Loss → necessary removal of waste products (ex. uric acid/urea)

Excretory Water Loss

• Necessary to remove waste products

  – Nitrogen from catabolism of proteins

• Composition of urine can be modified

  – Composition, concentration, and volume

Urine Concentration Modification

•  During times of drought

  – Water is retained by production of highly concentrated urine

• During times of water loading

  – Excess water is removed by production of large amounts of dilute urine

• Concentrations can be represented by the U/P ratio

U/P Ratio

• Osmotic pressure of the urine divided by osmotic pressure of the plasma

• U/P = 1 urine is isosmotic to plasma

• U/P < 1, urine is hypo-osmotic (dilute) → water loading

• U/P > 1, urine is hyperosmotic (concentrated) → drought

Mammals U/P ratios

Concentrating Ability

• mammalian kidney

  • can increase conc in urine but is very selective so that certain things - K+, Na+, etc - are reabsorbed in blood and waste is excreted

• urine production

• regulate H2O

• regulate solute concentration

  • for there to be increased conc urine have to move solutes from an area in blood plasma thats less conc to an area that’s more conc

kidney

nephron

Bowman’s Capsule (BC)

Glomerulus

• the blood vessels adjacent to BC

• beginning of nephron and all of the vasculature

hydrostatic pressures

Ultrafiltration

• Hydrostatic Pressure outside tubule greater than in tubule lumen

• Large solutes can NOT pass

• Pressure created by Systole (contraction of heart)

ultra filter

primary urine

• isosmotic to blood plasma

• Aqueous solution first introduced into kidney tubules

• consists of H2O, urea, ions (Na, Cl, K)

• Inorganic ions and organic solutes pass into the capsular fluid

• Large plasma proteins do not pass

• Blood osmotic pressure is higher than capsular fluid

definitive urine

what is excreted

Glomerular Filtration Rate (GFR)

• Rate of production of primary urine

  • amt produced going from blood to nephrons = GFR

• ~ 120 ml/min

• Full blood volume filtered every ~30 min!!

Urine Concentration

• GFR is relatively constant

• How are organisms able to retain water in periods of drought and excrete excess water after water loading?

  • Other aspects of nephron function accomplish this

Loops of Henle - image

Loops of Henle

• Renal component that allows for concentration of urine

  • U/P Ratio

• Max urine concentration correlates with abundance of long loops of Henle

Renal Medullary Thickness

Renal Medullary Thickness - graph

Concentration in Loop of Henle

active NaCl transport on the thick segment of the ascending limb

Loops of Henle - descending thin segments

• Highly permeable to water Loops of Henle

• Moderately permeable to most solutes

Loops of Henle - ascending thin segment

• Impermeable to water

• Moderately permeable to most solutes

Loops of Henle - ascending thick segment

• Impermeable to water

• Active transport of NaCl

Active NaCl Transport

Osmotic Pressures in Loop of Henle

• single effect

• countercurrent multiplication

single effect

• horizontal/transverse osmotic pressure gradient

• initial change in pressure because of active transport

Countercurrent Multiplication

• axial/vertical osmotic pressure gradient

• osmotic pressure differences are multiplied due to fluids moving in opposite directions

  • what happens when single effect has occurred but fluid inside limbs are moving

osmotic pressure process 1

osmotic pressure process 2

Osmotic Pressure Gradient

urine concentration

Collecting Ducts

Antidiuresis – Concentrated Urine

Diuresis – Dilute Urine

Hormonal Control of Urine [ ]

• Water retention of collecting duct under hormonal control

• Antidiuretic hormone (ADH) - aka

  – Arginine vasopressin

  – Vasopressin

• Modulates permeability of collecting ducts to water (by changing amt of Aquaporin that are in the walls of the collecting duct)

  – ADH causes insertion of an aquaporin (Aquaporin-2), which is a water channel protein in membrane that allows H2O to pass through

Antidiuretic Hormone - receptors

• Release is stimulated by low levels of blood plasma

• Detected by baroreceptors (detect change in blood volume due to stretch of blood vessels)

  – Located in Pulmonary Venous System, Cardiac Atria, Aortic Arch, and carotid sinus

• Detected by osmoreceptors (detect change in blood osmolarity)

  – Located in hypothalamus

Antidiuretic Hormone

• Modulates permeability of collecting ducts to water

• ADH causes insertion (or removal) of an aquaporin

  • Aquaporin-2

Aquaporin 2 (AQP2)

ADH control of AQP2 density

urea

urea low permeability in

• distal convoluted tubule

• thick ascending segment of Loop of Henle

• cortical and outer renal medulla

urea high permeability in

• collecting duct and inner renal medulla

Urea Permeability - image

Urea Permeability

•  Urea transporter protein

  – Facilitates diffusion from collecting duct into interstitial fluid

  – Upregulated by ADH

Renal Medulla Blood Supply

• Capillaries are permeable

• As blood flows from cortex to medulla

• Plasma would

  – Lose water to interstitium

  – Take up NaCl

  – Take up Urea

The Vasa Recta - image

blood vessels that serve the deepest part of the inner RM

The Vasa Recta

• Does NOT destroy osmotic pressure gradients

  • bc it turns around and comes back towards the renal cortex

• Very Little Blood Flow

  – Only 1% - 2% of total renal blood flow

  • remaining to glomeruli to get filtered through nephron to ensure tight control

• Countercurrent Exchangers

  – Minimizes washout of solutes

the vasa recta - osmolarity