Ion and Water Balance
Part 2
Chapter 10; 15.11.-19.11.2010
The Kidney
 Most animals maintain ion and water balance using
some form of internal organ
 Multiple cell types combine to produce a tubelike
structure
 Vertebrate kidneys have six roles in homeostasis
 Ion balance
 Osmotic balance
 Blood pressure
 pH balance
 Excretion of metabolic wastes and toxins
 Hormone production
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The Kidney
 Ion balance:
Sodium levels are an important determinant of
extracellular fluid osmolarity. Potassium balance is
important because changes in [K+] can alter resting
membrane potential, which affects the function of
excitable tissues such as muscles and neurons.
 Osmotic balance:
The kidneys determine the volume of urine produced,
and thereby control water balance.
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The Kidney
 Blood pressure:
By controlling blood volume, the kidney acts over the
long term to regulate blood pressure. Low blood
pressure (hypotension) compromises the delivery of
fuels to tissues with high energy demands, such as the
brain and locomotor muscle. High blood pressure
(hypertension) can compromise the integrity of the
microvasculature in vital tissues, putting the animal at
risk for a myocardial infarction, stroke, or embolism.
Many antihypertensive agents are diuretics, enahncing
the production of urine to reduce blood volume.
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The Kidney
 pH balance:
The kidney augments the respiratory system in the
control of the pH of body fluids. The kidney regulates
the pH of the extracellular fluid by retaining or
excreting H+ or HCO3-.
 Excretion of metabolic wastes and toxins:
The kidney plays an important role in the excretion of
nitrogenous wastes as well as other water soluble
toxins.
 Hormone production:
The kidney has an important role in the synthesis and
release of hormones, such as renin, which controls
blood pressure, and erythropoietin, which regulates
red blood cell synthesis.
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Kidney Structure and Function
 Mammalian kidney has two layers
 Outer (renal) cortex
 Inner (renal) medulla
 Urine leaves kidney via ureter
 Ureters empty into urinary bladder
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Figure 10.19
The Nephron
 Functional unit of the kidney
 Composed of
 Renal tubule
 Lined with transport epithelium
 Various segments with specific transport functions
 Vasculature
 Glomerulus
 Ball of capillaries
 Surrounded by Bowman’s capsule
 Capillary beds surrounding renal tubule
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The Nephron and Its Vasculature
Renal artery
Cortical nephrons have their renal corpuscle in the superficial renal cortex, while the
renal corpuscles of juxtamedullary nephrons are located near the renal medulla.
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Figure 10.20 and Figure 10.21
Urine Production
 Four processes
 Filtration
 Filtrate of blood formed at glomerulus
 Reabsorption
 Specific molecules in the filtrate removed
 Secretion
 Specific molecules added to the filtrate
 Excretion
 Urine is excreted from the body
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Filtration
 Liquid components of the blood are filtered into
Bowman’s capsule
 Water and small solutes cross glomerular wall
 Blood cells and large macromolecules are not filtered
 Glomerular capillaries are very leaky
 Podocytes with foot processes form filtration structure
 Mesangial cells control blood pressure and
filtration within glomerulus
 Filtrate flows from Bowman’s capsule into
proximal tubule
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Filtration
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Figure 10.22
Reabsorption
 Primary urine
 Initial filtrate filtered in Bowman’s capsule that is
isosmotic to blood
 Most water and salt in primary urine reabsorbed
using transport proteins and energy
 Rate of reabsorption limited by number of transporters
 Renal threshold
 Concentration of a specific solute that will overwhelm
reabsorptive capacity
 Each zone of the nephron has transporters for
specific solutes
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Reabsorption of Glucose
Glucose is reabsorbed by secondary active transport
Reabsorbed molecules taken up by the blood
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Figure 10.23
Renal Threshold
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Figure 10.24
Secretion
 Similar to reabsorption, but in reverse
 Molecules removed from blood and transported into
the filtrate
 Molecules secreted include
 K+, NH4+, H+, pharmaceuticals, and water-soluble
vitamins
 Requires transport proteins and energy
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Tubule Regions
 Different regions of the tubule have different
transport functions and permeabilities
 Proximal tubule
 Most of the solute and water reabsorption
 Loop of Henle
 Descending limb
 Ascending limb
 Distal tubule
 Reabsorption completed for most solutes
 Collecting duct
 Drains multiple nephrons
 Carries urine to renal pelvis
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Transport in Tubule Regions
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Figure 10.25
Transport in Tubule Regions
 Differences in transport and permeability due to differences
in epithelium along the tubule
cuboidal
squamous
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Figure 10.26
Transport in the Proximal Tubule
 Most reabsorption of solutes and water takes place
in proximal tubule
 Many solutes reabsorbed by Na+ cotransport
 Water follows by osmosis
 Proximal tubule also carries out secretion
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Figure 10.27
Ion and Water Transport in the Loop of Henle
 Descending limb is permeable to water
 Water is reabsorbed
 Volume of primary urine decreases
 Primary urine becomes more concentrated
 Ascending limb is impermeable to water
 Ions are reabsorbed
 Primary urine becomes dilute
 Reabsorbed ions accumulate in interstitial fluid
 An osmotic gradient created in the medulla
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Osmotic Gradient
Critical to the
water recovery is
an osmotic
gradient that
exists within the
medulla
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Figure 10.28
Transport in the Loop of Henle
a. This region of the tubule is specialized
to transport water, but it is not a major
site of transport for solutes. Aquaporins
allow water to move across epithelial
cells in relation to the osmotic difference
from the lumen to the interstitial fluid.
b. In this region, epithelial cells express
solute transporters. As a result of various
transporters in the apical and basolateral
membranes, there is a net movement of
Na+ and Cl- from the primary urine to
the interstitial fluid. On the apical
membrane, the NKCC transporter
mediates uptake of Na+, K+, and Cl- into
the cell. The basolateral membrane
transports Na+ and Cl- into the interstitial
fluid.
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Figure 10.29
Transport in the Distal Tubule
 Distal tubule can reabsorb salts and water
 Distal tubule can secrete potassium
 Transport function of distal tubule affected by
hormones
 Parathormone increases Ca2+ reabsorption
 Aldosteron increases K+ secretion
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Figure 10.30
Countercurrent Multiplier
 Loop of Henle acts
as countercurrent
multiplier
 Creates an osmotic
gradient that
facilitates
reabsorption of water
It allows the urine to be remodeled in terms of
both solute concentration and volume
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 Low osmolarity near
cortex
 High osmolarity
deep in medulla
Figure 10.28
Excretion
http://www.tutorvista.com/content/science/science-ii/excretion/excretionosmoregulation.php
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 After urine is produced, it
leaves kidney and enters
urinary bladder via ureters
 Urine temporarily stored
in bladder
 Urine leaves bladder via
urethra
 Sphincters of smooth
muscle control flow of
urine out of bladder
 Opening and closing of
sphincters controlled by a
spinal cord reflex arc
(micturition reflex) and
can be influenced by
voluntary controls
Regulation of Urinary Function
 Hormones affect kidney function
 Steroid hormones
 For example, aldosterone
 Slow response (over hours)
 Peptide hormones (from hypothalamic-pituitary axis)
 For example, vasopressin
 Rapid response
 Dietary factors that affect urine output
 Diuretics
 Stimulate excretion of water
 Antidiuretics
 Reduce excretion of water
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Glomerular Filtration Rate (GFR)
 Is determined by pressure across glomerular wall
 Glomerular filtration rate (GFR) is the volume of fluid filtered from
the renal (kidney) glomerular capillaries into the Bowman's capsule
per unit time. Central to the physiologic maintenance of GFR is the
differential basal tone of the afferent and efferent arterioles.
 Three main forces
 Glomerular capillary hydrostatic pressure
 Bowman’s capsule hydrostatic pressure
 Oncotic pressure – osmotic pressure due to protein concentration
in blood
GFR = VU/P
P - concentration of a solute in the plasma
U - concentration of a solute in the urine
V - filtrate volume
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Glomerular Filtration Rate (GFR)
The overall pressure for fluid
movements is the difference
between inward and outward
pressures. The hydrostatic
pressure gradient, the difference
between the mean blood pressure
and the hydrostatic pressure of the
lumen, favors movement into the
lumen. The oncotic pressure
gradient, due to the proteins that
remain in the plasma, opposes
movement into the lumen.
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Figure 10.31
Intrinsic Regulators of GFR
 Four intrinsic pathways
 Myogenic regulation
 Constriction/dilation of afferent arteriole
 Tubuloglomerular feedback (by altering arteriole resistance)
 Juxtaglomerular apparatus
 Macula densa cells in distal tubule
 Juxtaglomerular cells in afferent arteriole
 Macula densa cells of distal tubule control diameter of afferent
arteriole
 Mesangial control
 Altered permeability of glomerulus
 Pressure natriuresis
 Altered pressure in the renal arteries leads to the altered ability
of the tubule to recover solutes and water from the primary urine
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Juxtaglomerular Apparatus
Bowman’s capsule
Efferent
arteriole
Juxtaglomerular
apparatus
Proximal tubule
Distal tubule
Afferent
arteriole
Lumen of
Bowman’s
capsule
Glomerular capillary
Efferent
arteriole
Distal tubule
Macula
densa
Direction
of blood
flow
Loop of
Henle
Afferent
arteriole
Juxtaglomerular cells
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Figure 10.32
Intrinsic Controls of GFR
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Figure 10.33
Extrinsic Regulators of GFR
 Hormones
 Vasopressin (antidiuretic hormone, ADH)
 Renin-Angiotensin-Aldosterone (RAA) pathway
 Atrial natriuretic peptide (ANP)
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Vasopressin
 Also called antidiuretic hormone (ADH)
 Peptide hormone
 Produced in hypothalamus and released by posterior
pituitary gland
 Increases water reabsorption from the collecting duct
by increasing number of aquaporins
 Release stimulated by increasing plasma osmolarity
detected by osmoreceptors in the hypothalamus
 Release is inhibited by increasing blood pressure
detected by stretch receptors in atria and baroreceptors
in carotid and aortic bodies
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Vasopressin Increases Cell Permeability
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Figure 10.34a
Urine Concentration
 Osmotic concentration of final urine depends on
permeability (aquaporins) of the collecting duct,
which can be regulated by vasopressin
 Impermeable
 Water not reabsorbed from collecting duct
 Dilute urine (formed in ascending limb) excreted
 Permeable
 Water reabsorbed from collecting duct
 Concentrated urine (formed in collecting duct) excreted
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Aldosterone
 Hormones called mineralcorticoids control ion
excretion
 Produced by interrenal tissue in fish
 Cortisol the mineralcorticoid in fish
 Produced by adrenal cortex in tetrapods
 Aldosterone is the mineralcorticoid in tetrapods





Steroid hormone
Targets cells in distal tubule and collecting duct
Stimulates Na+ reabsorption from urine
Enhances K+ excretion
Also stimulated by increases in circulating K+
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Aldosterone Stimulates Na+ Reabsorption
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Figure 10.34b
Renin-Angiotensin-Aldosterone
(RAA) Pathway
 Juxtaglomerular cells secrete enzyme renin
 Secretion of renin controlled in three ways
 Baroreceptors in juxtaglomerular cells release renin in
response to low blood pressure
 Sympathetic neurons in cardiovascular control center of
medulla oblongata trigger renin secretion in response to
low BP
 Macula densa cells in distal tubule respond to decreases
in flow by releasing a paracrine signal that induces
juxtaglomerular cells to release renin
 Renin secreted when blood pressure or GFR lower
than normal
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Renin-Angiotensin-Aldosterone Pathway
 Renin converts angiotensinogen to angiotensin I
 Angiotensinogen an inactive protein in plasma
 Angiotensin converting enzyme (ACE) on epithelia
of blood vessels converts angiotensin I to
angiotensin II
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Renin-Angiotensin-Aldosterone Pathway
 Angiotensin II causes synthesis and release of
aldosterone from adrenal cortex
 Steroid hormone
 Targets cells in distal tubule and collecting duct
 Stimulates Na+ and water reabsorption from urine
 Enhances K+ excretion
 Also stimulated by increases in circulating K+
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Regulation of Blood Pressure
 RAA pathway helps regulate blood pressure
 Angiotensin II a vasoconstrictor
 Raises blood pressure by increasing resistance
 Aldosterone increases Na+ (and water) retention
 Raises blood pressure by increasing blood volume
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Atrial Natriuretic Peptide (ANP)
 Produced in specialized cells within the atria
 Secreted in response to stretch associated with
increase in blood volume
 ANP increases urine output and consequently
lowers blood volume and pressure
 Acts as an antagonist with RAA pathway
 Increases excretion of Na+ in urine
 Increases GFR by relaxing contractile cells that
control size of filtration slits of glomerulus
 Inhibits secretion of vasopressin
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Thirst
 The perception of thirst can arise in response to
dehydration or Na+ overload
 Detected and controlled by hypothalamus
 Osmoreceptors in circumventricular organs monitor
the osmolarity of the cerebrospinal fluid that bathes
the hypothalamus
 Receptors monitor levels of angiotensin II (also in this
region of the brain)
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Invertebrates
 Sponges use simple contractile vacuoles to expel
cellular waste, including water
 Simple animals like worm taxa have
protonephridia
 Similar to vertebrate tubule
 Fluids are taken from interstitial space
 Most developed in freshwater organisms
 Molluscs and annelids have more complex
metanephridia
 Fluid taken from blood or coelom
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Invertebrates
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Figure 10.35
Insects
 The Malpighian tubule insect equivalent to
vertebrate kidney
 Empties into hindgut
 Primary urine formed by secretion, not filtration
 Reabsorption in hindgut modifies primary urine
 Diuretic hormones increase urine formation
 CRF-related diuretic hormones
 Insect (myo)kinins
 Cardioacceleratory peptides
 Less is known about antidiuretic hormones
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Insects
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Figure 10.36
Chondrichthian Kidneys
 Sharks slightly hyperosmotic to seawater due to
high urea concentrations
 Countercurrent arrangement recovers up to 90% of
the urea from primary urine
 Final urine slightly hyposmotic relative to shark
tissues and isosmotic to seawater
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Fish Kidneys
 Role of the kidney differs in freshwater and
seawater
 Freshwater
 Ions reabsorbed from primary urine
 Excretion of very dilute urine
 Seawater
 Produce only small amounts of urine
 Most ion, water, and nitrogen excretion responsibilities
met by gills and skin
 Some marine fish lack glomeruli (aglomerular kidney)
 All fish nephrons lack a loop of Henle
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Amphibian Kidney
 Structure and function of kidney changes in
metamorphosis
 Structure
 Pronephros in larval forms
 Tubule opens into coelom
 More mammal-like nephron in adult
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Amphibian Kidney
 Function
 In aquatic life, little need for water retention
 Excretion of dilute urine
 Conserve water on land
 Reduce the GFR
 Reabsorb water from bladder
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Comparison of Vertebrate Nephrons
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Figure 10.37
Terrestrial Animals
 Major innovation was loop of Henle, allowing
production of concentrated urine
 Mammals producing more concentrated urine have
longer loop of Henle and relatively thicker medulla
 Birds and reptiles without a loop of Henle conserve
water by excreting uric acid
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Terrestrial Animals
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Figure 10.38