 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 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 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 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 Mg 2+, SO 4 2-, and Ca 2+ >>>1 (lowers internal ionic composition)

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 environment  Urinary system  Kidneys  Urine formation  Renal pelvis  Ureter  Urinary bladder  Urethra

III.Osmoregulation in the terrestrial environment  Structure of the mammalian kidney 1)Cortex (outer layer) In contact with the renal capsule Possesses many capillaries 2)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

Mammalian kidney (Eckert, Fig )

III.Osmoregulation in the terrestrial environment  The nephron  Functional unit of the kidney  Two major components of the nephron: 1)Vascular component (glomerulus) A tuft or ball of capillaries Filters fluid from blood as it passes through 2)Tubular component Filtered fluid from from the glomerulus (ultrafiltrate) passes to the tubular component and is converted to urine

III.Osmoregulation in the terrestrial environment  Renal circulation (2 capillary beds) 1)Glomerular capillaries High pressure (50-60 mm Hg) Allows for rapid filtration 2)Peritubular capillaries Low pressure (10 mm Hg) Allows for reabsorption Some vessels form the vasa recta

III.Osmoregulation in the terrestrial environment  Blood flow through the kidney Afferent arterioles  Glomerular capillaries (ultrafiltration)  Efferent arterioles  Peritubular capillaries (wrapped around nephrons)  Renal tubules  Renal venules  Renal vein

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 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 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

The nephron (Sherwood, Fig. 14-3)

III.Osmoregulation in the terrestrial environment  Types of nephrons 1)Cortical nephrons Glomeruli in the outer cortex Descending limb of the loop of Henle enters partially into the medulla No vasa recta 2)Juxtamedullary nephrons Glomeruli lie in the inner cortex Descending limb enters entire length of medulla Abundant in desert species Vasa recta present

Cortical and juxtamedullary nephrons (Eckert, Fig 14-18)

III.Osmoregulation in the terrestrial environment  Processes contributing to urine formation 1)Glomerular filtration 2)Reabsorption from renal tubules into the peritubular capillaries 3)Secretion of substances from peritubular capillaries into the renal tubules Rate of urinary = Filtration – Reabsorprtion+Secretion excretion rate rate rate

(Silverthorn, Fig. 18-3) Processes contributing to urine formation

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 environment  Glomerular capillary membrane  Three major layers: 1)Endothelium 2)Basement membrane 3)Podocytes (epithelial cells)

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

(Silverthorn, Fig. 18-4) Structure of the glomerulus

Structure of the podocytes (Silverthorn, Fig. 18-4)

III.Osmoregulation in the terrestrial environment  Forces involved in glomerular filtration 1)P G : glomerular hydrostatic pressure; promotes filtration (60 mm Hg) 2)P B : hydrostatic pressure in Bowman’s capsule; opposes filtration (18 mm Hg) 3)  G : colloidal osmotic pressure of the glomerular capillary; opposes filtration (32 mm Hg) 4)  B : colloid osmotic pressure of the Bowman’s capsule; promotes filtration (0 mm Hg)

III.Osmoregulation in the terrestrial environment  GFR depends largely on two factors 1)Net filtration pressure 2)Filtration coefficient  Surface area of glomerular capillaries  Permeability of glomerular capillary-Bowman’s capsule interface

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)

Creation of high filtration pressure at the renal glomerulus (Eckert, Fig )

Control of GFR by modulating arteriolar resistance (Silverthorn Fig. 18-8)

Effect of vasoconstriction of the afferent arteriole on GFR (Silverthorn, Fig. 18-8)

Effect of vasoconstriction of the efferent arteriole on GFR (Silverthorn, Fig. 18-8)

III.Osmoregulation in the terrestrial environment  Mechanisms controlling GFR 1)Intrinsic (autoregulation) Myogenic response of the arteriolar smooth muscle 2)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)

Juxtaglomerular apparatus (Eckert, Fig )

III.Osmoregulation in the terrestrial environment 3)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

(Sherwood, Fig ) Nervous control of podocyte contraction

III.Osmoregulation in the terrestrial environment 4)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)

Tubuloglomerular Feedback (Fig , Silverthorn)