Download Chapter 25 - Austin Community College

Chapter 25
The Urinary System
Consists of two kidneys, two ureters, one urinary
bladder and a urethra.
Functions:
1. Regulates plasma concentrations of Na+, K+ , Cl-,
Ca++ & Mg++, as well as glucose, AA & creatinine.
2. Endocrine function: regulates blood volume and
blood pressure; produces erythropoietin and
releases renin. Renin is produced by the Juxtaglomerular cells (JG) near afferent/efferent arterioles
and regulates synthesis of Angiotensin-II.
3. Stabilizes blood pH in concert with respiratory
system and breathing (CO2).
4. Eliminates organic waste products (e.g. ammonia
as urea and uric acid, toxic substances and drugs).
5. Synthesizes calcitrol from Vitamin D and helps
regulate Ca++ levels.
The Urinary System
The Kidneys
Location: retroperitoneal in superior lateral abdominal cavity
one on each side of body behind liver and stomach.
Structure: Two bean shaped organs ~ 12cm x 6cm x 3cm:
GW ~ 150 gms.
Surrounding Tissue Layers:
– renal capsule – strong fibrous outer covering of
kidney provides a barrier to infection from
surrounding organs.
– adipose capsule – middle layer of fat protects kidney
from concussive blows and attaches kidney to
posterior wall of abdomen.
renal fascia – outermost layer of fibrous CT; surrounds
kidney and adrenal glands; anchors them to
surrounding structures.
Retroperitonel position
The kidneys
Kidneys makeup ~ 0.4% Body Weight but receive
~21% of Cardiac Output (5 l/min).
Gross internal appearance of kidneys:
Cortex (light pinkish in natural state) granular
outer layer.
Medulla (darker reddish brown) inner layer with
cone shaped tissue masses (renal pyramids).
Pelvis (inner core), conical shaped funnels that
lead to the hilus and ureter. Often filled with adipose.
Blood flow to kidney
Approximately 125 ml of blood is cleared by
kidneys each minute =~ 140-180 l/day.
Vascular supply is via renal artery from aorta
and return is via renal vein to inferior vena
cava.
Path is from aorta → renal artery → segmental
arteries → interlobar arteries → arcuate arteries →
inter-lobular arteries → afferent arteriole → glomerulus
→ efferent arterioles → peritubular capillaries or vasa
recta → interlobular veins → arcuate veins →
interlobar veins → renal veins → inferior vena cava.
Blood flow
to kidney
Renal innervation
The nerve supply to the kidneys is almost
entirely sympathetically mediated.
Nerves from the superior mesenteric ganglion
enter at the renal hilus and regulate blood flow
to the kidneys.
If arterial blood flow drops the sympathetic
nerves stimulate the release of renin which lead
to synthesis of Angiotensin II (Ag-II), a very
potent vasoconstrictor that will increase the
blood pressure and consequently increase the
blood flow.
The Nephron
Consists of: Renal corpulse + renal tubule
Afferent and efferent arterioles → glomerular
capillaries which filter the blood plasma.
Renal corpuscle = Glomerulus + Bowman’s
capsule (glomerular capsule) - filters blood
Renal tubule = Proximal and distal convoluted
tubules plus loop of Henle.
- Descending and ascending limbs of loop of
Henle exhibit thick and thin segments.
The Nephron
Renal corpuscle +
Renal tubule
How urine is formed
1.
Glomerular formation: blood plasma is filtered into
glomerular space = glomerular filtrate. Lacks cells
and proteins. Contains water + solutes (glucose, AA, FA,
vitamins, nitrogenous watses, and electrolytes).
Tubular reabsorption of useful solutes (e.g. glucose, AA,
electrolytes, etc.) from filtrate → back into the blood.
3. Tubular Secretion of waste products (e.g. H+, drugs,
toxic agents) from blood into filtrate for elimination from
the body as urine.
2.
Water conservation: water is reabsorbed all along the
nephron especially in collecting duct. (Helps maintain
vascular volume and pressure).
Steps in urine formation
Kidney (Renal) nephrons
The Nephron is structural and functional unit of kidney
Cortical nephrons – ~ 2.1 million/ kidney;
The glomerulus is higher in the cortex with short loop
of Henle dipping into the medulla.
– Account for 85% of kidney nephrons
– Dilute the urine but do not concentrate it
Juxtamedullary nephrons – ~ 0.4 million/ kidney.
The glomerulus is near cortical/medullary border in
cortex and loop of Henle is very long descending deep
into the medulla.
– Account for 15% of kidney nephrons
– Functions in concentrating of urine
Renal nephrons
Cortical nephron
Juxtamedullary
Medullary border
Cortical and juxtamedullary nephrons
Renal corpuscles for both
nephrons are all in the
cortex.
Loop of Henle dips into
the medulla for both
nephrons.
Cortical nephrons loops
are shorter and dip slightly
into the medulla.
Juxtamedullary nephron
loops dip deeper into the
medulla.
SEM of Glomeruli
TEM of a glomerular capillary shows podocyte
with pedicels and filtration slits and pores.
Nephron histology
Bowman’s capsule- inner (visceral) layer of podocytes wrapped
around capillaries and outer (parietal) layer of simple squamous
epithelium
Proximal convoluted tubules (PCT)- consists of simple cuboidal
epithelium with microvilli (brushed border).
Distal convoulted tubule (DCT)- consists of simple cuboidal
epithelium and no microvilli; (Affected by aldosterone and PTH).
Loop of Henle – “U” shaped with the thick segment of cuboidal
epithelium and thin segment of simple squamous epithelium.
Collecting ducts are not part of nephron but empty into papillary
duct and then minor calyx. They are lined with simple cuboidal
epithelium and 2 types of cells (intercalated and principal cells).
Effected by 2 hormones ANP & ADH
Glomerular capsule with parietal and visceral
layers; afferent and efferent arterioles, JGA etc.
Cortical nephron
-short loop of Henle
with peritubular
capillaries
Functions to form
urine.
Juxtamedullary
nephron
-Glomerulus is
In cortical layer.
Vasa recta goes
deep into med-
ulla.
-Function is to
concentrate urine
and conserve water.
Juxtaglomerular apparatus
Occurs at vascular pole of renal capsule.
Consists of three cell types:
1. Macula densa group of sensory epithelial cells in the
distal convoluted tubule, next to afferent arteriole MD
monitors flow and urine composition. Communicates
with the JG cells.
Juxtaglomerular (JG) cells = enlarged sm.m. cells on
afferent and efferent arterioles. When stimulated
they constrict to reduce flow through the glomerulus.
JG cells also secrete renin into blood → AG-II.
3. Mesangial cells located between afferent and
efferent arterioles and may modulate communication
between the two arterioles.
See next slide for clarification.
2.
Glomerulus with Juxtaglomerular apparatus
(Saladin 25.9)
Symp.
Nerve
JG
cells
Macula
densa
Afferent
arteriole
DCT
DCT
Histologic slide of a Glomerular capsule with
parietal and visceral layers; afferent and
efferent arterioles, Macula densa, JG cells, etc.
Nephron fctns.
1. Filtration
2. Reabsorption
3. Secretion
Summary of urine flow
Afferent arteriole enters Glomerulus →
glomerular filtrate → Proximal convoluted
tubule → Descending loop of Henle →
Ascending loop of Henle → Distal
convoluted tubule → Collecting duct → →
→ Papillary duct → Minor calyx → Major
calyx → Renal pelvis → Ureter → Urinary
bladder → → Urethra → Outside.
Summary of urine flow
Ureters
Muscular tube lined with transitional epithelium. Wall consists of
two layers of smooth muscle (inner longitudinal and outer circular
layer). Urine moves by peristalsis “milking action” from renal
pelvis to bladder.
Enter bladder from posterior
and below. Small flap of
mucosa acts as a valve.
~ 25 cm long; 1.7 cm diam.
Urinary Bladder
Muscular storage bag lined with transitional epithelium, with
lamina propria underneath and detrusor muscle in wall. Detrusor
muscle is 3 separate layers of smooth muscle.
Rugae present when empty
Vol. can hold up to 800 ml.
when full.
Two ureters and urethra form
“trigone” at base of bladder.
Urethra
Histology: First part of tract
is transitional, → pseudostratified columnar →
stratified squamous epithel.
at external orifice.
Males 3 areas:
1. Prostatic urethra
2. Membranous urethra
3. Spongy urethra in penis.
♂ urethra serves for urine
excretion and sperm
ejaculation.
♀ urethra for urine excretion
only.
Bladder and urethra of female
Urethra
Urine release is controlled by an internal and
external urethral sphincter.
Internal sphincter is a thickened extension of
the detrusor muscle and is involuntarily
controlled.
External sphincter is composed of skeletal
muscle and is under voluntary control. It is
located where the urethra penetrates the pelvic
floor.