Download Functional Human Physiology for the Exercise and Sport Sciences

Functional Human Physiology
for Exercise and Sport Sciences
The Urinary System: Renal Function
Jennifer L. Doherty, MS, ATC
Department of Health, Physical Education, and
Recreation
Florida International University
Functions of the Urinary System
 The kidneys remove metabolic wastes
from the blood and excrete them to the
outside of the body in the form of urine
 The careful regulation of renal activity
keeps blood composition and body fluids
within normal limits
The kidneys also…

Maintain electrolyte and acid-base balance in body
fluids


Regulate plasma pH by regulating the concentration of bicarbonate
ions and hydrogen ions
Regulate the volume, composition, and pH of blood


Regulate plasma osmolarity and chemical composition
Assist in the regulation of BP


Regulate plasma volume and produces renin to regulate BP
Assist in the regulation of RBC production


Regulate RBC production by producing erythropoietin to stimulate RBC
formation in bone marrow
Assist in the regulation of Ca++ absorption

Metabolize vitamin D to its active form, which affects the rate of Ca++
absorption from the small intestines
Anatomy of the Urinary System
Structures of the urinary system
 Kidneys (2)
 Form urine


Renal arteries and veins
Ureters (2)
 Tubes for transport of urine from the kidneys to the bladder

Urinary bladder (1)
 Storage reservoir for urine

Urethra (1)
 Transport tube for urine to the outside of the body
Microscopic Anatomy of the Kidneys

The nephron is the
structural and functional
unit of the kidney.


Each kidney contains about
one million nephrons
Nephrons consist of:





The renal corpuscle
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct

Empties urine into the minor
calyx
The Nephron: Renal Corpuscle

The renal corpuscle
consists of…
 Glomerulus
 Glomerular Capsule
The Nephron: Renal Corpuscle

The Glomerulus is the
filtering unit of the
nephron
 Tangled cluster of
capillary beds lying
between the afferent and
efferent arterioles
 Contained within the
glomerular capsule
The Nephron: Renal Corpuscle

The Glomerular
Capsule (Bowman’s
capsule) is a thin
walled, cup-shaped
structure surrounding
the glomerulus
 It leads to the renal
tubule
 It receives fluid that filters
through the glomerulus
Basic Renal Exchange Processes
 Nephrons function to…
 Remove wastes from the blood
 Regulate water and electrolyte concentrations
 Urine is the end product of these functions
 The following 3 exchange processes occur
within the nephrons
 Glomerular Filtration
 Reabsorption
 Secretion
Glomerular Filtration
 This is the beginning of urine formation
 Glomerular capillaries are extremely permeable
compared to systemic capillaries
 Hydrostatic and Osmotic Pressure Gradients
 Greater inside glomerular capillaries
 Forces water and dissolved solutes to leave the blood
plasma in the glomerular capillaries and cross the
glomerular membrane into the glomerular (Bowman’s)
capsule
 Filtrate
 Water and dissolved solutes in the glomerulus
Glomerular Filtration
The glomerular membrane
 Separates glomerular capillary blood from
the glomerular capsule space
 Contains many small pores that allow
almost all materials to pass through the
membrane
 Exceptions: formed elements
Glomerular Filtration
Glomerular filtrate
 Contains water and dissolved solutes that
have been filtered from the blood plasma in
the glomerular capillaries and collected by
the glomerular capsule

Similar to tissue fluid containing water, glucose, amino acids,
urea, uric acid, creatine, creatinine, sodium, chloride,
potassium, calcium, bicarbonate, phosphate, and sulfate ions
 Will be processed by the renal tubules to
form urine
Glomerular Filtration
 A nonselective, passive process
 Water and dissolved solutes from the blood
plasma in glomerular capillaries are forced
through the glomerular membrane by
hydrostatic and osmotic pressure gradients
 Water and dissolved solutes travel down
their pressure gradients
Glomerular Filtration
Glomerular Filtration Pressure
(Net Filtration Pressure)
 The net force acting to move materials out of the
glomerulus (glomerular capillaries) and into the
glomerular capsule
 Filtration pressure is much higher in the glomerular
capillaries compared to systemic capillaries
because of:
 The high permeability of the glomerular membrane
 It is more permeable than systemic capillary membranes
 High glomerular blood pressure (60 mmHg)
 It is higher compared to systemic capillary blood pressure
(41 mmHg)
Glomerular Filtration
Starling Forces
 Represent the overall
effect of all the forces
operating at the
glomerular membrane
Glomerular Filtration: Starling
Forces
 Forces favoring filtration are the forces driving fluid
and solutes out of the glomerular capillaries
 Glomerular capillary hydrostatic pressure
 PGC = 60 mmHg
 The primary force pushing water and solutes out of
the glomerular capillaries
 Osmotic pressure in the glomerular (Bowman’s)
capsule
 πBC = 0 mmHg
 Negligible since few plasma proteins are normally
present in the glomerular capsule
Glomerular Filtration: Starling
Forces
 Forces opposing filtration are the forces driving
fluid and solutes back into the glomerular
capillaries
 Glomerular (Bowman’s) capsule hydrostatic
pressure
 PBC = 15 mmHg
 Exerted by the fluids within the glomerular capsule
 Osmotic pressure in the glomerular capillaries
 πGC = 29 mmHg
 Due to the plasma proteins in glomerular blood
Glomerular Filtration Pressure
 Glomerular Filtration Pressure Equation

Filtration pressure = (forces favoring filtration) - (forces opposing filtration)
 Forces favoring filtration

(glomerular capillary hydrostatic pressure + capsular osmotic pressure)
 Forces opposing filtratrion

(capsular hydrostatic pressure + glomerular capillary osmotic pressure)
 Values

(60 mmHg + 0 mmHg) - (15 mmHg + 29 mmHg) = 16 mmHg
Glomerular Filtration Rate (GFR)
 GFR = the amount of filtrate produced in the
kidneys per minute
 Normal values:
 125 ml/min (180 L/day)
 GFR varies with the filtration pressure
 All the factors that affect glomerular filtration
pressure will affect the GFR




Glomerular capillary osmotic pressure
Glomerular capillary hydrostatic pressure
Glomerular capsule osmotic pressure
Glomerular capsule hydrostatic pressure
Glomerular Filtration Rate (GFR)
 Glomerular capillary hydrostatic pressure
 ↑ glomerular capillary hydrostatic pressure = ↑ GFR
 Glomerular capsule osmotic pressure
 ↑ glomerular capsule osmotic pressure = ↑ GFR
 Glomerular capillary osmotic pressure
 ↑ glomerular capillary osmotic pressure = ↓ GFR
 Glomerular capsule hydrostatic pressure
 ↑ glomerular capsule hydrostatic pressure = ↓ GFR
Glomerular Filtration Rate (GFR)
 GFR varies with the rate of blood flow
through the glomerular capillaries
 To maintain a high GFR, blood must flow quickly
through glomerular capillaries
 Vasocontriction or vasodilation in the
glomerular arterioles elicit changes in the
glomerular filtration pressure
 Changes in the glomerular filtration pressure effect
GFR
Glomerular Filtration Rate (GFR)
 Vasoconstriction of the afferent arterioles or
vasodilation of the efferent arterioles
 ↓ glomerular capillary hydrostatic pressure
 ↓ GFR
 Vasodilation of the afferent arterioles or
vasoconstriction of the efferent arterioles
 ↑ glomerular capillary hydrostatic pressure
 ↑ GFR
Regulation of GFR
 GFR remains relatively constant
 May be ↑ or ↓ according to the body’s need
 Mechanisms of regulation:
 Intrinsic control (Autoregulation)
 Myogenic regulation
 Tubuloglomerular feedback
 Extrinsic control
 Renal blood flow
 Exercise
Regulation of GFR: Intrinsic Control
 The ability of the kidney to maintain a
constant blood flow when arterial BP is
changing
 The ability of the kidneys to maintain a
relatively constant GFR when mean arterial
pressure is changing
 This mechanism is effective over the
"normal" range of arterial BP
 80 - 120 mmHg
Regulation of GFR: Intrinsic Control
Myogenic Regulation
 Related to the inherent property of smooth muscle
to contract when stretched
 ↑ mean arterial pressure = ↑ stretch of smooth muscle in
the afferent arteriole walls stimulating vasoconstriction
 Vasoconstriction of the arterioles causes a
decrease in glomerular capillary hydrostatic
pressure
 This protects the delicate glomerular capillaries from high
mean arterial pressures
 Myogenic regulation is especially effective in the
afferent arteriole
Regulation of GFR: Intrinsic Control
Tubuloglomerular Feedback
 Negative feedback system
 GFR is regulated by changes in flow of
tubular fluid past the macula densa
 Specialized cluster of epithelial cells in the distal
convoluted tubule near the afferent and efferent
arterioles
 Changes in Na+ and Cl- concentration in the
filtrate are detected by osmoreceptors in the
macula densa
Tubuloglomerular Feedback
Macula Densa Cells
 Respond to changes in the Na+Cl- concentration in
the filtrate in the distal convoluted tubule
 ↓ Na+Cl- concentration = afferent arteriole
vasodilation
 Vasodilation of the afferent arteriole results in:
 ↑ blood flow to the glomerular capillaries
 ↑ glomerular filtration pressure
 ↑ GFR
 The opposite is also true
Tubuloglomerular Feedback
Juxtaglomerular Cells
 Contain mechanoreceptors that stimulate the
juxtaglomerular cells to release renin in response
to changes in mean arterial pressure
 Renin is an enzyme that catalyzes a cascade of
reactions in the bloodstream
 Renin converts angiotensinogen → angiotensin I
 Angiotensin converting enzyme (ACE) converts
angiotensin I → angiotensin II
 Angiotensin II is the most powerful vasoconstrictor in the body
 Increases mean arterial blood pressure
Tubuloglomerular Feedback

Activation of the juxtaglomerular cells to release renin occurs
when there is a decrease in mean arterial pressure


Usually when mean arterial pressure is less than 80 mmHg
Direct activation of juxtaglomerular cells


Achieved via the mechanoreceptors sending impulse through the
sympathetic nervous system
Indirect activation of juxtaglomerular cells


Achieved via the macula densa cells which detect changes in
Na+Cl- concentrations in the filtrate
Macula densa cells cause vasoconstriction or vasodilation, which
alters mean arterial pressure as detected by the juxtaglomerular
cells
Regulation of GFR: Extrinsic Control
Renal Blood Flow
 The sympathetic nervous system is able to
override autoregulation of the kidneys
 ↑ sympathetic input = ↓ GFR
 Sympathetic input causes vasoconstriction
of both afferent and efferent arterioles,
thereby decreasing GFR
Regulation of GFR: Extrinsic Control
Exercise
 Exercise results in increased sympathetic
nerve impulses
 Sympathetic nerve impulses stimulate the
adrenal medulla to release epinephrine,
which stimulates…
 Release of renin = ↑ mean arterial pressure
 Vasoconstriction of the afferent arteriole = ↓ GFR
Reabsorption
 The process of reclaiming fluid and solutes
from the filtrate in the renal tubules
 Reabsorption occurs in the peritubular
capillaries
 Solutes and water move from the lumen of
the renal tubules back into the plasma
 If reabsorption did not occur, a person
would lose 1L of fluid in the urine in 8 min
Solute Reabsorption
 Substances are selectively reabsorbed from
the filtrate
 Peritubular capillaries are specially adapted
for the process of reabsorption
 Under very low BP
 Walls are very permeable
 Reabsorption occurs throughout the renal
tubule; however, most reabsorption occurs
in the proximal convoluted tubule
Solute Reabsorption: Proximal
Convoluted Tubule
 Most reabsorption occurs in the proximal
convoluted tubule


Proximal convoluted tubule contains epithelial cells with microvilli
Microvilli increase the surface area within the renal tubules
Solute Reabsorption: Proximal
Convoluted Tubule
 Solutes are moved from the tubule lumen, across
the apical membrane, into the epithelial cells lining
the tubule walls
Solute Reabsorption: Proximal
Convoluted Tubule
 Solutes then move out of the epithelial cells lining
the tubule walls, across the basolateral
membrane, into the peritubular space
Solute Reabsorption: Proximal
Convoluted Tubule
 From the peritubular space, solutes easily diffuse
into the peritubular capillaries
Regional Specialization of the Renal
Tubules: Proximal Tubule

Na+ ions are actively reabsorbed by active transport
 Requires ATP
 Reabsorption of Na+ establishes an electrical gradient for
reabsorption of negatively charged ions
 As positively charged Na+ ions are transported out of the filtrate,
negatively charged ions accompany them via passive diffusion
1) Cl2) Bicarbonate (HCO3-)

Reabsorption of Na+ also establishes an osmotic gradient for
the reabsorption of water
 Water is passively reabsorbed by osmosis and returned to the
systemic circulation by the peritubular capillaries
Regional Specialization of the Renal
Tubules : Proximal Tubule
 The mechanisms of reabsorption in the proximal
tubule are so efficient that 70% of water and Na+
filtered is reabsorbed before the tubular fluid
reaches the loop of Henle
 Water and Na+ reabsorption are regulated by several
hormones
 At the end of the proximal convoluted tubule, the
filtrate and the blood in the peritubular capillaries
are isotonic (electrically neutral)
Regional Specialization of the Renal
Tubules : Distal Tubule
 Major function is regulation of Na+ and Clconcentration of the filtrate
 The primary site of aldosterone activity
 Aldosterone increases Na+ reabsorption and K+
secretion
 In the presence of aldosterone, the distal
convoluted tubule will actively reabsorb Na+ and
Cl When Na+ is reabsorbed, water reabsorption also
occurs
 Results in increased blood volume and BP
Transport Maximum
 There are different modes of transport that may be
used to reabsorb substances in particular
segments of the renal tubule
 Solutes are transported from filtrate to plasma
across the tubular epithelium by carrier proteins or
pumps
 Carrier proteins may become “saturated”

When solute concentration is high enough, all carrier proteins and
pumps are occupied
 When all carrier proteins and pumps are occupied,
the system is operating at Transport Maximum
Transport Maximum
Glucose
 Reabsorbed via active transport
 Renal tubule epithelial cells contain special
protein transporters that remove glucose from
the tubular filtrate
 When the plasma and filtrate concentration of
glucose are high enough to saturate all carrier
sites, the excess glucose will end up in the
urine
 This is the renal plasma threshold for glucose
Transport Maximum
 When the concentration of a substance in the
filtrate exceeds its renal plasma threshold, the
excess is excreted in the urine
 Example: Diabetes Mellitus (Type I insulin dependent)
 Excess glucose in the urine provides an osmotic gradient
 Glucose in the filtrate will draw water into the renal tubule
by osmosis and increases the urine volume
 This is called osmotic diuresis
 In chronic conditions osmotic diuresis can lead to kidney
damage
Reabsorption: Summary
 Some substances are not reabsorbed at all
 Found in the urine
 Some substances are reabsorbed incompletely
 Found in the urine
 Some substances lack carriers, are not lipid
soluble, or are too large to pass through the
membrane pores of the tubular cells
 Found in the urine
 The concentration of substances that remain in the
filtrate increases as water is reabsorbed
Secretion
 The movement of solutes from the blood in the
peritubular capillaries into the lumen of the renal
tubules
 Secretion occurs primarily in the proximal and
distal convoluted tubules
 Substances secreted by the kidneys into urine are:






H+
K+
Urea
Creatinine
Ammonia (NH3+)
Histamine
Secretion
Functions of the secretion process:
 Helps maintain normal blood concentrations
of certain electrolytes
 Eliminates excess K+
 Helps maintain normal pH of body fluids
 Eliminates undesirable substances that
have been reabsorbed
 Urea
 Uric acid
Secretion
Active Secretion
 Some substances are secreted actively in
the proximal and distal convoluted tubules
 These substances include various organic
compounds and H+ ions
 Acidosis (decreased pH) is controlled by the
renal tubule cells
 Actively secrete H+ ions
 Actively retain bicarbonate and K+ ions
Secretion
Passive Secretion
 K+ ions are secreted passively in the
 Distal convoluted tubule
 Collecting duct
 K+ ions are attracted to the negative charge
that develops in the lumen of the renal
tubule
Excretion
 The elimination of solute and water from the
body in the form of urine
 Micturition = the process of urination
 Substances that enter the lumen of the
renal tubules are excreted unless they are
reabsorbed
 Substances may enter the renal tubules by
either filtration or secretion
Excretion
Excretion Rate
 The rate in which a substance is excreted
depends on…
 The quantity of a certain solute that is filtered at
the glomerulus per unit time (filtered load)
 The rate at which a solute is secreted
 The rate at which a solute is reabsorbed
Regulation of Urine Concentration
and Volume
 While the function of the proximal convoluted
tubule is to reabsorb most of the water and Na+ out
of the tubular filtrate
 The role of the Loop of Henle is to adjust the
concentration and volume of urine
 The juxtamedullary nephrons are particularly suited to
adjust the concentration and volume of urine because
their nephron loops descend deep into the renal medulla
 By the end of the proximal convoluted tubule, the
filtrate volume has decreased by 80 - 85% and the
remaining fluid is isotonic
The Loop of Henle
 Descending limb
 Descends toward the medulla of the kidney
 The fluid surrounding the nephron loop is
called the medullary interstitial fluid
 As the nephron loop decends into the medulla of the
kidney, the medullary interstitial fluid becomes more
concentrated
The Loop of Henle
 The osmolarity of the medullary interstitial fluid
increases from 200 mOsm in the renal cortex
to about 1200 mOsm in the deepest parts of
the renal medulla
 The increase in osmolarity is due to Na+ ions
that are concentrated in the renal medulla by
the countercurrent multiplier
Countercurrent Multiplier
 Involves interactions between the flow of
filtrate through the Loop of Henle and the
flow of blood through the adjacent blood
vessels, the vasa recta
 Countercurrent refers to the opposite direction of
flow in the Loop of Henle and the vasa recta
Countercurrent Multiplier
 The concentration of filtrate leaving the
proximal convoluted tubule and entering the
nephron loop is isotonic
 Filtrate contains concentrations equal to the
blood plasma and the interstitial fluid of the renal
cortex
 Urine that is excreted must be more
concentrated than blood plasma
 The kidney achieves this goal through the
countercurrent multiplier effect
The Countercurrent Multiplier Effect



Refers to filtrate flowing in opposite directions in the
descending and ascending limbs of the Loop of
Henle
Because of this countercurrent flow, small
differences in the concentration of the filtrate in the
descending and ascending limbs results in a large
medullary interstitial fluid concentration gradient
The medullary interstitial fluid concentration gradient
is established by the following mechanisms:
 Permeability of the descending loop
 Permeability of the ascending loop
 Permeability of the vasa recta
The Countercurrent Multiplier Effect
Descending limb of the nephron loop
 Very permeable to water

A relatively high Na+ concentration remains in the filtrate as it
reaches the ascending limb
Ascending limb of the nephron loop
 Selectively permeable

Na+ is removed from the filtrate but water is not

Na+ becomes concentrated in the medullary interstitial fluid
The vasa recta
 Highly permeable to Na+ and Cl Responsible for maintaining Na+ and Cl- concentration in the
renal medulla
The Countercurrent Multiplier Effect
 Increased concentration of the medullary
interstitial fluid increases the osmotic
pressure in the renal medulla
 More Na+ remains in the medullary interstitial fluid
 As a result of the countercurrent multiplier
effect, small differences in the osmolarity of
the filtrate in the ascending and descending
nephron loops creates a large medullary
concentration gradient
 The medullary concentration gradient serves
as the driving force for urine concentration
The Collecting Duct
 By now, the filtrate is called urine
 The major function of the collecting duct is
regulation of water reabsorption
 Therefore, it is the primary site of antidiuretic
hormone (ADH) activity
 ADH from the posterior pituitary gland
increases the permeability of the…
 The distal convoluted tubule
 The collecting duct
The Collecting Duct
 No ADH present
 The epithelial cells of the collecting duct are
relatively impermeable to water
 Produces dilute urine
 ADH present
 The pores in the collecting duct enlarge and
increase the permeability to water
 Results in greater reabsorption of water
1) Reduced urine volume
2) Urine is very concentrated
Micturition
 Voiding or eliminating urine
 Expulsion of urine from the bladder
 Involves contraction of the detrusor muscle and
relaxation of the external urethral sphincter
 Distension of the bladder stimulates stretch
receptors in the wall of the bladder
 The stretch receptors stimulate the micturition reflex
center located in the sacral region of the spinal cord
Micturition
Miturition Reflex
 The micturition reflex center sends
parasympathetic motor impulses to the detrusor
muscle
 Parasympathetic impulses to the detrusor muscle
stimulates rhythmic contraction and relaxation of the
internal urethral sphincter
 As the bladder fills, its internal pressure increases
and opens the internal urethral sphincter
 A second reflex relaxes the external urethral
sphincter
 Contraction of the external urethral sphincter may be
voluntarily controlled
Micturition
 The desire to urinate is stimulated by
distention of the bladder
 Occurs when the bladder fills with ~150 ml of urine
 Urine volume of ~300 ml or more leads to
sensations of uncomfortable fullness
 At maximum capacity, the bladder may hold
~500 – 600 ml (1 pint) of urine
 Following urination, less than ~10 ml of urine
usually remain in the bladder
Micturition
 Nerve centers in the brain stem and cerebral
cortex aid in the control of urination
 When the need to urinate is sensed, the
micturition impulse can be temporarily
inhibited through cerebral cortex and
midbrain control
 Voluntarily relaxation of the external sphincter
allows urination to occur
Urine Composition
 Urine is ~ 95 % water

Also contains urea, uric acid, and creatinine
 Urine may contain trace amounts of…


Amino acids
Electrolytes
 Urine is usually acidic with a pH of ~6

Urine is more acidic than blood plasma and intracellular fluid
 Urine volume is ~0.6 - 2.5 L/day

The glomerular capillaries filter about 180 L/day