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The kidney- a fluid processing organ. The major function of the animal kidney is to regulate the composition of blood plasma by removing water, salts, and other solutes from the plasma in a controlled fashion

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the kidney a fluid processing organ
The kidney- a fluid processing organ
  • The major function of the animal kidney is to regulate the composition of blood plasma by removing water, salts, and other solutes from the plasma in a controlled fashion
  • Effects of the kidney on blood composition can be studying by comparing the urine composition (U) to plasma composition (P) or the U/P ratios
the kidney a fluid processing organ1
The kidney- a fluid processing organ
  • The effects of kidney function on osmotic regulation depend on the osmotic U/P ratio
    • If U/P = 1, urine is isosmotic to plasma, no effect on water or solute excretion, plasma osmotic pressure unaltered
    • If U/P < 1, urine is hyposmotic to plasma, urine contains more water relative to solutes than plasma, plasma osmotic pressure is raised
    • If U/P > 1, urine is hyperosmotic to plasma, urine contains less water relative to solutes than plasma, plasma osmotic pressure is lowered
the kidney a fluid processing organ2
The kidney- a fluid processing organ
  • The effects of of kidney function on volume regulation depends on the amount of urine produced
    • Kidneys can play a role in volume regulation without a direct role in osmotic regulation

Freshwater crabs of tropical regions

-experience both volume and osmotic challenges

-kidneys deal with volume challenge by excreting an equivalent amount of water that is gained by osmosis but are unable to produce a hypoosmotic urine

-other tissues are involved in maintaining osmotic balance

the kidney a fluid processing organ3
The kidney- a fluid processing organ
  • The effects of kidney function on ionic regulation depend on ionic U/P ratios
    • Kidneys can play a role in ionic regulation without playing a direct role in osmotic regulation

Marine teleost fish

    • hyposmotic to SW (lose water osmotically and gain ions by diffusion)
    • Produce a urine that is isosmotic to plasma (U/P=1), therefore urine production plays no direct role in osmotic regulation
    • However, urine ionic composition differs greatly from plasma , U/P ratios for Mg2+, SO42-, and Ca2+ >>>1 (lowers internal ionic composition)
iii osmoregulation in the terrestrial environment
III. Osmoregulation in the terrestrial environment
  • Functions of the mammalian kidney
      • Maintain water balance
      • Regulate concentration of ions in the ECF
      • Maintains long term arterial pressure
      • Maintains acid-base balance
      • Maintain proper ECF/ICF osmolarity
      • Excrete end products of metabolism
      • Excrete foreign compounds
      • Secrete erythropoietin and renin
      • Converts vitamin D into its active form
iii osmoregulation in the terrestrial environment1
III. Osmoregulation in the terrestrial environment
  • Urinary system
    • Kidneys  Urine formation  Renal pelvis  Ureter  Urinary bladder  Urethra
iii osmoregulation in the terrestrial environment2
III. Osmoregulation in the terrestrial environment
  • Structure of the mammalian kidney
    • Cortex (outer layer)
      • In contact with the renal capsule
      • Possesses many capillaries
    • Medulla (deeper region)
      • Composed of renal pyramids separated by renal columns
      • Renal pyramids project into minor calyces
      • Minor calyces unite to form major calyx
      • Major calyces form renal pelvis
slide8

Mammalian kidney

(Eckert, Fig. 14-17)

iii osmoregulation in the terrestrial environment3
III. Osmoregulation in the terrestrial environment
  • The nephron
    • Functional unit of the kidney
    • Two major components of the nephron:
    • Vascular component (glomerulus)
      • A tuft or ball of capillaries
      • Filters fluid from blood as it passes through
    • Tubular component
      • Filtered fluid from from the glomerulus (ultrafiltrate) passes to the tubular component and is converted to urine
iii osmoregulation in the terrestrial environment4
III. Osmoregulation in the terrestrial environment
  • Renal circulation (2 capillary beds)
    • Glomerular capillaries
      • High pressure (50-60 mm Hg)
      • Allows for rapid filtration
    • Peritubular capillaries
      • Low pressure (10 mm Hg)
      • Allows for reabsorption
      • Some vessels form the vasa recta
iii osmoregulation in the terrestrial environment5
III. Osmoregulation in the terrestrial environment
  • Blood flow through the kidney

Afferent arterioles  Glomerular capillaries (ultrafiltration)  Efferentarterioles  Peritubular capillaries (wrapped around nephrons)  Renal tubules Renal venules  Renal vein

iii osmoregulation in the terrestrial environment6
III. Osmoregulation in the terrestrial environment
  • Parts of a nephron
    • Bowman’s capsule
      • Invagination around the glomerulus which collects filtered fluid from the glomerulus
    • Juxtaglomerular apparatus
      • Specialized tubular and vascular cells lying next to the glomerulus
      • Produces renin
iii osmoregulation in the terrestrial environment7
III. Osmoregulation in the terrestrial environment
  • Proximal tubule
    • Within the cortex
    • Reabsorption of selected solutes
  • Loop of Henle
    • U-shaped loop that dips into the medulla
    • Two sections: descending limb (cortexmedulla) and an ascending limb (medullacortex)
    • Establishes an osmotic gradient in medulla
    • Allows kidney to produce urine of varying concentration
iii osmoregulation in the terrestrial environment8
III. Osmoregulation in the terrestrial environment
  • Distal tubule
    • Lies within the cortex
    • Empties into the collecting duct
    • Highly regulated reabsorption of Na+ and water
    • Secretion of H+ and K+
  • Collecting duct
    • Drains fluid from the nephrons
    • Enters medulla and empties into the renal pelvis
    • Similar functions to the distal tubule
slide15

The nephron

(Sherwood, Fig. 14-3)

iii osmoregulation in the terrestrial environment9
III. Osmoregulation in the terrestrial environment
  • Types of nephrons
    • Cortical nephrons
      • Glomeruli in the outer cortex
      • Descending limb of the loop of Henle enters partially into the medulla
      • No vasa recta
    • Juxtamedullary nephrons
      • Glomeruli lie in the inner cortex
      • Descending limb enters entire length of medulla
      • Abundant in desert species
      • Vasa recta present
slide17

Cortical and

juxtamedullary

nephrons

(Eckert, Fig 14-18)

iii osmoregulation in the terrestrial environment10
III. Osmoregulation in the terrestrial environment
  • Processes contributing to urine formation
    • Glomerular filtration
    • Reabsorption from renal tubules into the peritubular capillaries
    • Secretion of substances from peritubular capillaries into the renal tubules

Rate of urinary = Filtration – Reabsorprtion+Secretion

excretion rate rate rate

iii osmoregulation in the terrestrial environment11
III. Osmoregulation in the terrestrial environment
  • Glomerular filtration rate: the amount of fluid that filters into the Bowman’s capsule per unit time
    • In humans, about 180 l/day
    • Kidneys excrete about 1 l/day, therefore most of the filtrate is returned to the vascular system (>99% reabsorbed)
    • GFR is about 20% of renal blood flow
iii osmoregulation in the terrestrial environment12
III. Osmoregulation in the terrestrial environment
  • Glomerular capillary membrane
    • Three major layers:
      • Endothelium
      • Basement membrane
      • Podocytes (epithelial cells)
iii osmoregulation in the terrestrial environment13
III. Osmoregulation in the terrestrial environment
  • Podocytes
      • Surround outer surface of the capillary membrane; cell body with several ‘arms’ or pedicels (foot processes)
      • Narrow slits between pedicels allow for the passage of molecules based on MW and charge
      • Glomerular capillaries are fenestrated, allowing for a high filtration rate
      • Most substances except large proteins are filtered
slide23

Structure of the glomerulus

(Silverthorn, Fig. 18-4)

slide24

Structure of the podocytes

(Silverthorn, Fig. 18-4)

iii osmoregulation in the terrestrial environment14
III. Osmoregulation in the terrestrial environment
  • Forces involved in glomerular filtration
    • PG: glomerular hydrostatic pressure; promotes filtration (60 mm Hg)
    • PB: hydrostatic pressure in Bowman’s capsule; opposes filtration (18 mm Hg)
    • G: colloidal osmotic pressure of the glomerular capillary; opposes filtration (32 mm Hg)
    • B: colloid osmotic pressure of the Bowman’s capsule; promotes filtration (0 mm Hg)
iii osmoregulation in the terrestrial environment15
III. Osmoregulation in the terrestrial environment
  • GFR depends largely on two factors
    • Net filtration pressure
    • Filtration coefficient
      • Surface area of glomerular capillaries
      • Permeability of glomerular capillary-Bowman’s capsule interface
iii osmoregulation in the terrestrial environment16
III. Osmoregulation in the terrestrial environment
  • Regulation of GFR
    • Prevents extreme changes in renal excretion from occurring in response to small arterial pressure changes
    • Regulation is generally achieved by adjusting resistance to flow in the afferent arteriole
    • Afferent arteriole has large diameter and short length (low resistance)
    • Efferent arteriole and vasa recta have smaller diameter and are longer (offer higher resistance)
iii osmoregulation in the terrestrial environment17
III. Osmoregulation in the terrestrial environment
  • Mechanisms controlling GFR
    • Intrinsic (autoregulation)
      • Myogenic response of the arteriolar smooth muscle
    • Hormonal control
      • Involves the juxtaglomerular apparatus (JGA)
      • JGA- a specialized renal structure where regions of the nephron and afferent arteriole are in contact with each other
      • Macula densa and juxtaglomerular cells (granular cells)
slide33

Juxtaglomerular apparatus

(Eckert, Fig. 14-24)

iii osmoregulation in the terrestrial environment18
III. Osmoregulation in the terrestrial environment
  • Nervous control
    • Afferent arteriole innervated by sympathetic nervous system
    • Sympathetic activation causes constriction of glomerular cells and causes podocytes to contract
    • Nervous mechanism overrides autoregulatory mechanisms if there is a sharp decrease in BP
slide35

Nervous control of

podocyte contraction

(Sherwood, Fig. 14-15)

iii osmoregulation in the terrestrial environment19
III. Osmoregulation in the terrestrial environment
  • Tubuloglomerular feedback
    • Changes in fluid flow sensed by macula densa
    • Paracrine factors can either cause vasoconstriction or vasodilation
    • Endothelin (vasoconstrictor); bradykinin and nitric oxide (vasodilators)
slide37

Tubuloglomerular Feedback

(Fig. 18-10, Silverthorn)