ANS 331: Renal System
I. The Kidneys
1. Functions
A. Excretion of metabolic waste products
B. Regulation of the volume and composition of
extracellular fluids
2. Produces Urine
A. Formed from blood
B. Varies in composition
II. Anatomy of the Renal System
1. Kidney
A. Pair of Organs
a. Bean-Shaped--Most Species
b. Heart-Shaped--Horses
c. Lobulated--Cattle
B. Retroperitoneal
a. Located outside of abdominal cavity
suspended by peritoneum
C. Blood Supply
a. Renal Artery
i. Brings blood to Kidney
ii. Arises directly from aorta
b. Renal Vein
i. Brings blood away from Kidney
ii. Empties into the Caudal Vena Cava
D. Divisions
a. Cortex
i. Outer Portion
ii. Contains upper parts of nephrons
--Glomeruli and Tubules
b. Medulla
i. Inner Portion
ii. Contains lower parts of nephrons
--Loops of Henle and Collecting Ducts
c. Renal Pelvis
i. Drains kidney and connects ureters
d. Renal Hilus
i. Indented area on concave edge
ii. Ureter, Blood Vessels, Nerves
and Lymphatics enter or leave
2. Ureter
A. Smooth muscle lined tube
B. Conveys urine from renal pelvis to bladder
C. Enter bladder through ureterovesicular junction
a. Acts as a valve
b. Prevents backflow
3. Urinary Bladder
A. Hollow muscular organ (Smooth Muscle)
B. Stores Urine
a. Stretches
C. Lined with Transitional Epithelium
a. Flatten when filling occurs
4. Urethra
A. Caudal continuation of the neck of the bladder
B. Conveys urine from bladder to exterior
C. Urine release into urethra controlled
by external sphincter
a. Skeletal muscle
III. Microstructure of the Kidney: The Nephron
A. The functional unit of the kidney
a. Numbers range from 250,000 (cat) to 4 million (cow)
per kidney
b. Components
i. Glomerulus
ii. Bowman's Capsule
iii. Proximal Tubules
iv. Loop of Henle
v. Distal Tubules
B. Glomerulus
a. Tufts of Capillaries
b. Afferent arterioles
i. Conduct blood to glomerulus
c. Efferent arterioles
i. Conduct blood away from glomerulus
ii. Divide into peritubular capillaries
--Surround nephron tubules
--Vasa Recta:
Peritubular capillaries
surrounding loops of Henle
d. Function:
Filtration of Blood
C. Bowman's Capsule
a. Collect Glomerular Filtrate
b. Conduct filtrate into proximal tubules
D. Proximal Tubules
a. Conduct filtrate into Loop of Henle
b. Very convoluted
(Surrounded by Peritubular Capillaries)
c. Reabsorb, Secrete, & Excrete
--Simple cuboidal or columnar epithelium
E. Loop of Henle
a. Play important role in concentration of urine
b. Divisions
i. Descending limb
--thin segment
--simple squamous epithelium
ii. Ascending limb
--thick segments
--simple cuboidal epithelium
--returns to sight near glomerulus
of origin
c. Surrounded by Vasa Recta
F. Distal tubules
a. Convey urine from Loops of Henle
b. Also convoluted (Surrounded by peritubular
capillaries)
c. Reabsorb, Secrete, and Excrete
--Simple
d. Feed Collecting tubules and ducts
G. Juxtaglomerular Apparatus
a. Junction between distal tube and glomerulus
b. Contain unique cell types
i. Macula Densa in tubules
ii. Juxtaglomerular cells in arterioles
c. Function:
i. Regulate renal blood flow
ii. Regulate glomerular filtration
iii. Secrete Renin
--Involved in production of angiotensin II
H. Two Types of Nephrons
a. Cortical
i. Short loops of Henle
ii. Only go into Outer medulla
b. Juxtamedullary
i. Long loops of Henle
ii. Can go clear to renal pelvis
IV. Formation of Urine
1. General
A. Involves Three Processes
a. Glomerular Filtration
b. Tubular Reabsorption
c. Tubular Secretion
2. Distribution of Blood at the Glomerulus
A. Renal Blood Flow (RBF)
--Rate at which blood flows to the kidney
B. Renal Plasma Flow (RPF)
--Part of RBF that is Plasma
C. Glomerular Filtration Rate (GFR)
--Rate at which glomerular filtrate will be
formed from plasma
D. Filtration Factor (FF)
a. GFR/RPF = FF
b. % of filtrate that is plasma flowing
through glomerulus
E. RBF, RPF and GFR are all measured in ml/min
F. RBF directly related to final output of urine
3. Glomerular Filtration
A. Glomerulus is a high hydrostatic pressure system
a. Favors filtration
B. Peritubular Capillaries are a low hydrostatic
pressure system
a. Favor reabsorption
C. Slit pores at Glomerulus slightly larger than
normal capillaries
a. Filter larger molecules
b. Most proteins still too large
D. Pressures favor direction towards Bowman's Capsule
a. No Colloidal Osmotic Pressure on Bowman's
Capsule side
E. GFR can be changed (changes in glomerular pressure)
a. Vasodilation and/or constriction of
efferent or afferent arterioles
F. Two autoregulation mechanisms of GFR control
a. Macula densa
i. Sensitive to changes in ionic
concentrations
--Na+ and Cl-
ii. Lower concentrations signal
afferent arterioles to dilate
--Increase blood flow
and hydrostatic pressure
--Increase GFR
b. Juxtaglomerular cells
i. Secrete Renin
--Released during low blood
pressure
ii. Initiates Angiotensin II
--Causes Efferent arteriole
to constrict
--Increases GFR
--Increases water reabsorption
by tubules to peritubular
capillaries
4. Tubular Reabsorption and Secretion
A. Reabsorption Path
a. Tubular Lumen
b. Tubular Epithelium
c. Interstitial Fluid
d. Capillary
B. Secretion Path
a. Just the opposite
C. Reabsorbed Molecules
a. Amino Acids & Glucose
--Important for body function
b. Use carrier proteins coupled with Na+
to enter tubular cells
--Symporter
--Na+ is actively transported to lumen
--Causes concentration gradient for Na+
c. Water reabsorption follows osmotic gradient
from lumen to epithelium to capillaries
d. Other diffusible substances follow water
by diffusion because they increase
in concentration in the lumen
--Proximal Tubule Reabsorption
65% of H2O, Na+, Cl-, and
100% of Glucose and Amino Acids
D. Secreted Molecules
a. H+ is secreted throughout tubules
--Antiporter with HCO3-
b. K+ is secreted in distal
and collecting tubules
--Antiporter with Na+
c. Ammonia is secreted
--Depends on acid-base balance
5. Transport Maximum (TM)
A. Maximum Rate in which a substance can be absorbed
B. Diabetes mellitus
a. Excess Glucose Filtered
b. Exceed TM
c. Decreases Osmotic Gradient for H2O
reabsorption
d. Excess water loss through urine (diuresis)
6. Countercurrent Mechanism
A. Kidney function: To Control the osmolality of
the body fluids
a. Osmolality: Osmotic concentration, the
characteristic of a solution
determined by the ionic
concentration of a dissolved
substance per unit solvent
i. Solvent: Water
ii. Solutes: Na+ and Urea
b. The kidney excretes excess H2O in urine
when body fluids are too dilute.
c. The kidney excretes excess solutes when
the body fluids are too concentrated.
B. Players
a. Medulla
--Low to High Concentration gradient from
Outer to Inner
b. Loop of Henle
--From Juxtamedullary capillaries that dip
deep into the renal medulla
c. Vasa Recta
--Peritubular capillaries that surround
Loop of Henle
d. Distal Tubule and Collecting Ducts
--Reabsorb some Water, NaCl, and Urea
C. Two types of nephrons
a. Cortical
i. Short Loops of Henle
ii. Only go into outer medulla
b. Juxtamedullary
i. Long Loops of Henle
ii. Can go clear to renal pelvis
D. Species differences:
a. Desert Kangaroo Rat
i. Only long loops of Henle
ii. Can live on Metabolic H2O
b. Humans, Cattle, & Swine
i. Few long loops (1/3 to 1/5 of total)
ii. Void large amounts of dilute urine
c. Dogs, Cats, Camels, Sheep, and Goats
i. Many long loops of Henle
ii. Produce a relatively concentrated
urine
E. Mechanism
a. Hyperosmolality of the medullary
interstitial fluid
i. Create a very high osmotic pressure
for H2O osmosis into the kidney
interstitium.
ii. Three mechanisms
1. Active Transport of Na+ in
ascending limp of loop
of Henle--Chloride pump
2. Active Transport of Na+ in
collecting duct
--Sodium Pump
3. Passive diffusion of Urea
from Collecting duct
b. Countercurrent Multiplier--Loop of Henle
i. The countercurrent (hairpin loop)
arrangement of the nephron allows
urine a chance to flow through a
region of high osmolarity permitting
urine concentration by passive
reabsorption of H2O.
ii. Active transport of NaCl from the
thick ascending limb of the loop
of Henle is responsible for
increased hyperosmolality.
iii. Continual flow of new NaCl into
the loop of Henle allows it to
become a countercurrent multiplier.
c. Countercurrent Exchanger--Vasa Recta
i. Without a special medullary vascular
system the flow of blood through the
interstitium would rapidly remove
the excess solutes in the medulla
and keep the concentration from
rising too high.
ii. The vasa recta have two
characteristics which maintain this
high solute concentration:
--Blood flow through the vasa recta
is very sluggish.
--Hairpin loop structure and high
permeability of vasa recta
allow exchange between arms
of loop.
V. Hormones and Kidney Function
1. Antidiuretic Hormone
A. The shift of the kidney from excreting excess H2O
to excess solute is controlled by
Antidiuretic Hormone (ADH).
a. ADH (also called Vasopressin) is synthesized
by the hypothalamus and secreted by the
posterior pituitary (or neurohypophysis).
b. Low Blood [ADH] = increased H20 excretion
or a dilute urine.
c. High Blood [ADH] = increased solute
excretion or a concentrated urine.
B. Concentration of Urine
a. ADH allows H2O reabsorption by the late
distal tubules and collecting ducts
thus facilitating the concentration of urine.
b. ADH acts by facilitating the formation of
"water channels" on the luminal membrane of
tubule and ductal epithelial cells.
i. The H2O is then pulled by osmosis
into the highly concentrated
interstitial fluid.
ii. The deeper the collecting ducts go
into the medulla the stronger the
osmotic pull and the more
concentrated the collecting duct
fluid gets until it reaches the
1200 mOsm/l concentration equal to
the osmolality of the medullary
interstitium near the papilla.
c. ADH also increases the permeability of
the collecting ducts to urea.
This urea adds to the hyperosmolality of
the medullary interstitium.
d. In addition, ADH has been reported to
decrease the blood flow of the vasa recta,
further playing a role in the concentration
of urine.
C. Excretion of a dilute urine.
a. Dilution of urine is accomplished by
reabsorption of solutes, without a further
reabsorption of H2O.
i. Solutes are absorbed in the thick
ascending limp of the loop of Henle,
distal tubules, and the collecting
ducts.
ii. The thick ascending limb of the
loop of Henle and early distal
tubules are called the diluting
segment of the nephron because
they are impermeable to H2O.
iii. Without ADH the late distal tubules
and collecting ducts nephron are
impermeable to H2O.
D. Other Factors Affecting ADH Release
a. ADH is stimulated by decreased blood volume
which allows body to conserve H2O in
the face of a severe hemorrhage
--Volume receptors in blood vessels
b. ADH is inhibited by alcohol which leads
to excess diuresis when consuming alcohol.
c. Diabetes insipidus is a disease caused by
the lack of ADH.
d. Cold environments inhibit ADH which leads
to increase urine production and water
intake
e. Na+ concentrations regulate ADH
--Osmoreceptors in hypothalamus
2. Angiotensin II
A. Causes Efferent arteriole to constrict
--Increases GFR
B. Increases water reabsorption by tubules to
peritubular capillaries
C. Increases vasoconstriction of peripheral
blood vessels
D. Causes the secretion of Aldosterone
3. Aldosterone
A. Produced in adrenal cortex (Mineralocorticoid)
A. Involved in K+ regulation
a. Causes secretion in distal tubules,
collecting tubules and ducts
B. Antiporter causes Na+ reabsorption
a. Causes H2O reabsorption
--Increase blood volume & pressure
4. Parathyroid Hormone
A. Causes a reabsorption of Ca++
B. Secretion of Phosphorus
C. Controls formation of Vitamin D
a. Activated in kidney
b. Vitamin D promotes Ca++ absorption from
intestine
VI. Micturition (Urination)
1. Transfer of Urine to the Urinary Bladder
A. From Glomerulus to Renal Pelvis
--Hydrostatic pressure
B. From Renal Pelvis to Bladder
--Peristalsis of the ureter
2. Micturition Reflexes
A. Micturition: Emptying of Bladder
a. Involuntary Control
b.. Voluntary Control
B. Involuntary Control of Bladder
a. Stretch receptors in bladder wall
i. Want to contract
b. Reflex center in Brain Stem
i. Prevent Contraction of Wall
ii. Prevent Relaxation of Sphincter
c. Parasympathetic
C. Voluntary Control of Bladder
a. Cortex aroused upon filling
b. Allows micturition when appropriate
D. Urethral Reflex
a. Involuntary
b. Allows for complete emptying of bladder
c. Parasympathetic
E. Sympathetic Control
a. Closure of sphincter during ejaculation
3. Characteristics of Mammalian Urine
A. Composition
a. Varies
b. Similar to extracellular fluid
c. Dependent on whether substance is conserved
or excreted
B. Color
a. Yellow color
i. Derived from bilirubin
ii. Forms urobilin at intestine
iii. Oxidizes to urochrome
(yellow pigment)
C. Odor
a. Species Specific
b. Dependent on diet
D. Consistency
a. Watery in most species
b. Horse: Thicker and more syrupy
i. Mucous secreted in Renal Pelvis
ii. Also contains Carbonates
and Phosphates
--Easily precipitate upon standing
E. Nitrogenous Component
a. Urea
i. Formed in liver from ammonia
--Decreases toxicity of ammonia
F. Output
a. Varies
b. Terms
i. Polyuria
--Increased urine output
ii. Oliguria
--Decreased urine output
iii. Anuria
--No urine output
iv. Continence
--Normal condition of storing urine
v. Incontinence
--Leaking or dribbling of urine
uncontrollably
--Usually neural problem
--Prostate problems
vi. Dysuria
--Painful urination
V. Renal Clearance
1. Measurement of the kidney's ability to remove substances
from the plasma
2. Formula:
Cx=UxV/Px
Cx = Clearance of substance x (ml/min)
Ux = Concentration of x in urine (mg/ml)
V = Rate of urine formation (ml/min)
Px = Concentration of x in plasma (mg/ml)
3. Pharmacological and Diagnostic Tool
VI. Maintenance of Acid-Base Balance
1. Relationship of pH to H+ Concentration
A. Normal = pH 7.4
B. Severe Acidosis = pH 7.1
a. H+ concentration doubled
b. High meat diet more acid
C. Severe Alkalosis = pH 7.7
a. H+ concentration halved
b. High vegetable diet more basic
2. Mechanism of H+ Secretion by the Kidneys
A. Na+/H+ antiporter secretes H+
a. Occurs along whole nephron
b. 85% in Proximal tubules
B. Secreted H+ combines with:
a. Bicarbonate
b. Phosphate
c. Ammonia
3. Role of Respiratory System
A. Respiratory System also controls pH
of extracellular fluids
B. Bicarbonate from hydration reaction binds CO2
C. Releases H+ with CO2 at alveoli
D. Increased H+ and CO2 in extracellular fluid cause
increase ventilation
4. Chemical Buffer Systems and Mechanisms
A. System can buffer acids and bases in extracellular
fluids
B. Types:
a. Bicarbonate
Acid buffer: HCl + NaHCO3 --> H2CO3 + NaCl
Base buffer: NaOH + H2CO3 --> NaHCO3 + H2O
b. Phosphate
Acid buffer: HCl + Na2HPO4 --> NaH2PO4 + NaCl
Base buffer: NaOH + NaH2PO4 --> Na2HPO4 + H2O
c. Proteins
i. Amino-end (basic) buffer acids
ii. Carboxy-end (acidic) buffer bases
iii. Hemoglobin most abundant buffer
in body
--anemic animals get acidosis
~~~~~Revised 11/5/97~~~~~ TAW