Urinary System and Renal
Physiology
Functions of the Urinary System
1) Regulate plasma ionic composition (Na+, K+, etc)
2) Regulate plasma volume
3) Regulate plasma osmolarity (solute
concentration)
4) Regulate plasma pH
5) Remove metabolic waste products and foreign
substances from plasma
6) Hormone secretion
a) Erythropoietin- increase red blood cell production
b) Renin- role in aldosterone production (Na+ balance)—
more later
Blood Supply to Kidneys
➢Kidneys are primarily a blood (plasma) filter
1) Receive 20% of cardiac output at rest
a) Very large flow of blood
b) Less than 1% of body weight
c) Utilize 16% of ATP usage by body
-- ATP used for active transport
– Blood supply via renal arteries
Microscopic Anatomy of the
Kidney
Uriniferous Tubules = functional units
✓ About 1 million per kidney
Uriniferous tubules consist of:
1) Nephrons- each with 2 main parts
a) Renal corpuscle
i.
ii.
Glomerulus
Glomerular (Bowman’s) Capsule
b) Renal tubule- long tube divided into several specialized
regions
2) Collecting Ducts
Renal Corpuscle of Nephron
1) Glomerulus = twisted ball of blood capillaries
a) Where plasma is filtered from blood and enters into
tubule system
b) Blood enters glomerulus from afferent arterioles, some
of the plasma is filtered, rest of blood leaves
glomerulus through efferent arterioles
c) Efferent arterioles then form another capillary network
which surrounds the tubular portion of nephron =
peritubular capillaries
2) Glomerular (Bowman’s) capsule = inflow end of
renal tubule
a) Where filtered plasma enters into tubule
Renal Corpuscle
Peritubular Capillaries
Renal Tubules of Nephron
1) Proximal Convoluted Tubule
2) Loop of Henle
a) Descending limb
b) Thin ascending limb
c) Thick ascending limb
3) Distal Convoluted Tubule
4) Collecting Duct
Anatomy of a Nephron
Figure 19.3
Four Basic Processes in the
Formation of Urine
1) Glomerular filtration – filtration of plasma from
glomerulus into Bowman’s capsule
2) Reabsorption – movement of substances from
tubules back into blood (peritubular capillaries)
3) Secretion – movement of substances from
peritubular capillaries directly into tubules
4) Excretion – from tubules out of body
Basic Renal Processes
Figure 19.7
1) Primary function of the kidneys is to filter blood
2) Filtrate has a similar composition to plasma minus
the plasma proteins
3) Initially, composition of ions and small molecules
same as plasma (changes as filtrate passes through
renal tubules)
4) Only about 20% of plasma passing through each
kidney is filtered
a)
However, still results in large volume filtered each day
Glomerular Filtration (GFR)
Rate of filtration is known as
Glomerular Filtration Rate = GFR
GFR = 125 mL/min or 180 liters/day
Regulation of GFR
1) 180 liters fluid filtered/day
a) Only 1.5 liters urine excreted/day (<1%)
b) >99% of filtered fluid is reabsorbed
2) Small increase in GFR → large increase
volume fluid filtered and excreted
3) Because of this, GFR is highly regulated
➢GFR is primarily dependent on 2 factors:
1. Blood pressure in glomerular capillaries
a) Forces plasma out of capillaries into Bowman’s space
b) Primary force affecting GFR
2. Osmotic force of plasma (due to plasma
proteins)
a) Water tends to remain in capillaries unless forced out
b) Decreasing osmotic force increases GFR
Glomerular Blood Pressure (BP) is
Primary Force Producing Filtration
1) Increase BP
2) Decrease BP
increase in filtration
decrease in filtration
• Glomerular BP affected by blood flow through
afferent arterioles
1) Dilate afferent arteriole– increases blood flow into
glomerulus
increases BP in glomerular capillaries
2) Constrict afferent arteriole– decrease blood flow into
glomerulus
decrease BP
1) Afferent arterioles (and to lesser extent, efferent
arterioles) are controlled by both 1) local messenger
molecules within the kidney and 2) by the sympathetic
nervous system
✓ Tubuloglomerular Feedback (local control)
a)
If there is a decrease in fluid in kidney tubules, vasodilating
molecules are released by tubule cells; dilate afferent arterioles
increases BP in glomerulus and increases GFR (more fluid filtered)
b) If increased fluid in tubules, vasoconstricting molecules are released
which constrict afferent arterioles and decreases GFR (less filtered)
➢ Maintains a relatively constant GFR and fluid level in
tubules
Sympathetic Control of GFR
1) Decreases in BP can decrease GFR
a) Baroreceptor reflex increases sympathetic
stimulation of afferent arterioles
b) Causes vasocontriction
c) Reduces blood flow into glomerular capillaries
which decreases GFR (less filtered)
d) Conserves fluid in plasma to increase blood
volume and increase BP
-- Example: blood loss due to hemorrhage
Reabsorption
Movement of substances from renal tubules into
peritubular capillaries (substances returned to
the blood)
1) Most occurs in proximal convoluted tubule
2) Some in distal convoluted tubule
3) Barrier for reabsorption
a) Epithelial cells of renal tubules
b) Endothelial cells of capillary (minimal)
Reabsorption Barrier
Figure 19.13
Solute and Water Reabsorption
Dependent on transport proteins and membrane
permeability to water
Figure 19.14c
Rates of Reabsorption of Water
and Select Solutes
(Don’t memorize values from Table 19.1)
lymphocyte
Table 19.1
Reabsorption in Proximal
Convoluted Tubule (PCT)
Molecules reabsorbed include (but not limited to):
1) Glucose
Normally, 100% reabsorbed from tubule in PCT
2) Amino acids
3) Sodium
4) Calcium
5) Urea (from protein degradation)
6) Water: approx 65% of filtered water is reabsorbed in PCT
✓ Water is reabsorbed due to osmotic gradient produced by the
reabsorption of other solutes
Example:
Reabsorption of Glucose
and Amino Acids
1) Glucose and amino acids are co-transported with
sodium (secondary active transporters) into PCT cells
2) These transporters can be saturated so that above a
certain filtrate concentration, no further re-absorption
occurs
remainder lost in urine
3) Rate of transport when carrier proteins are saturated =
transport maximum or Tmax
Example: Glucose Reabsorption
1) Normal plasma glucose = 100 mg/100 ml plasma
2) Freely filtered at glomerulus, so concentration in filtrate is
same as plasma concentration
3) Normally 100% actively reabsorbed
in proximal convoluted tubule
4) Normally, zero glucose in urine
5) Tmax for glucose = 375 mg/100 ml filtrate (normal plasma
glucose is well below the Tmax )
Diabetes mellitus
1) During diabetes mellitus, plasma glucose can be
as high as 600 mg/100 ml plasma
2) Glucose concentration now exceeds Tmax
3) In this case, an amount of glucose = 225 mg/100
ml (600 mg/100 ml – 375 mg/100 ml) will
remain in PCT and be excreted in urine
a) Makes the urine sweet: mellitus = “sweet”
Descending Loop of Henle
1) Mostly involved in water reabsorption
2) 15% of filtered water reabsorbed
3) Little solute reabsorption
a) Concentrates the fluid in tubule
Ascending Loop of Henle
1) Mostly involved with sodium and chloride
reabsorption; no (little) water reabsorption
2) Thin portion of loop: passive sodium channels on
tubule cells (depends on concentration gradient)
3) Thick portion: secondary active transporter for
sodium, potassium, and chloride
a)
Known as the NK2C transporter (transports 1 Na+, 1 K+, and 2 Clfrom filtrate into the tubule cells
Na+ pumped out of
cell and
reabsorbed into
peritubular
capillaries (blood)
Na+
Impermeable
to water
Distal Convoluted Tubule (DCT)
➢ Involved with regulated sodium reabsorption:
✓Dependent on the hormone Aldosterone
1) Aldosterone secreted by cells in adrenal gland cortex
2) Part of RAAS system
a) Renin, Angiotensin, Aldosterone System
3) Renin = hormone secreted by kidney in response to low
plasma sodium or low plasma/extracellular fluid volume
Renin-Angiotensin-Aldosterone
System
*In kidney; in walls of afferent arterioles
= granular cells
To kidney
ACE = Angiotensin Converting Enzyme
Figure 20.15
✓ Aldosterone increases reabsorption of sodium from
Distal Convoluted Tubule (DCT)
1) Aldosterone increases the number of sodium
channels and Na+/K+ ATPase transporters (pumps)
• Able to transport more sodium from tubule and return it to
peritubular capillaries
Na+ channels
Collecting Duct
1) Some reabsorption of sodium (dependent on aldosterone)
2) Mostly involved with water reabsorption
3) Water reabsorption in collecting duct dependent on the
hormone anti-diuretic hormone (ADH, vasopressin)
4) ADH inserts water channels (aquaporins) into membrane of
collecting duct cells
1) Allows water to move from tubule into cells and then back into
peritubular capillaries
2) Without ADH, water cannot pass through the membrane of these cells
1) ADH produced in hypothalamus (released from
posterior pituitary gland)
2) ADH secreted in response to low extracellular fluid
(ECF) volume and/or increased ECF osmolarity
3) Allows water reabsorption to increase plasma/ECF
fluid volume (i.e. conserve body water)
4) Without ADH, water remains in tubules and is
excreted in urine (lost from body)- more later
Nephron Secretion
1) Solute moves from peritubular capillaries
into tubules
2) Barriers same as for reabsorption
3) Transport mechanisms same but opposite
direction
– Secreted Substances:
a) Potassium
b) Hydrogen ions (helps regulate pH)
c) Penicillin
• Highly efficient- penicillin has short half life in plasma
Formation of Concentrated
vs. Dilute Urine
➢Kidneys regulate volume of water in body
1) Increased volume of extracellular fluid
excrete large volume of dilute urine
2) Decreased volume
excrete small volume
of concentrated urine to conserve body water
Osmosis
Water diffuses down concentration gradient
1) Water moves from low [solute] to high
[solute]
2) Water reabsorption follows solute
reabsorption
Water Reabsorption
1) Proximal tubules
a) 70% filtered water is reabsorbed
b) Not regulated
2) Desending Loop of Henle
a) 15% filtered water is reabsorbed
b) Not regulated
3) Distal tubules and Collecting ducts
a) Most remaining water is reabsorbed
b) Regulated by ADH
Medullary Osmotic Gradient for
Water Reabsorption
1) Gradient critical for water reabsorption
2) Osmolarity of body fluids = 300 mOsm
3) Osmolarity of interstitial fluid of renal
medulla varies with depth
a) Lower osmolarity near cortex
b) Greater osmolarity deep in medulla
Osmotic Gradients – cont.
1) Fluid in distal tubule - 100 mOsm
2) interstitial fluid of medulla – gradient up to
1400 mOsm
➢Large osmotic gradient in interstitial fluid
favors reabsorption of water from tubule by
osmosis
Osmotic Gradients of Late Distal
Tubule and Collecting Ducts
If epithelium impermeable to
water, then no reabsorption
occurs
If permeable, water will
move by osmosis and be
reabsorbed back into blood
Figure 20.9a
Water Reabsorption in Distal
Tubules and Collecting Ducts
1)
Dependent on epithelium permeability to
water
2)
Water permeability dependent on water
channels
a) Aquaporin-2: present in apical membrane only
when ADH present in blood
Permeable Membrane in
Presence of ADH
1) ADH stimulates insertion of
water channels (aquaporin-2)
into apical membrane.
a)
Water can permeate membrane
and be reabsorbed by osmosis
2) Excrete small volume of
concentrated urine
3) Max osmolarity urine = 1400
mOsm
Figure 20.9b
Without ADH,
1) No water reabsorption; water remains
in tubule
2) Excrete large volume of dilute urine
3) Reduces total body volume