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 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 Mg2+, SO4

2-, and Ca2+ >>>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 kidney1) 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. 14-17)

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 (cortexmedulla) and an ascending limb (medullacortex)

• 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 formation1) 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 filtration1) PG: glomerular hydrostatic pressure; promotes

filtration (60 mm Hg)

2) PB: 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. 14-20)

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 GFR1) 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. 14-24)

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. 14-15)

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. 18-10, Silverthorn)