Begins
at the kidneys and includes (in order) the paired ureters,
unpaired urinary bladder, and the urethra.
The
process of urination is called micturition.
The
urinary system is the principal system responsible for water and electrolyte
balance. It also functions to excrete urea and creatinine
(nitrogenous compounds).
Kidneys - 2 (paired) = normal
condition. The left is higher than the right due to the presence of the liver.
Has a hilum
for passage of
1. renal
artery
2. renal
vein
3. ureter
4. nerves
is located retroperitoneal. It
is embedded in fat. This fatty pouch consists of 3 layers:
1. Renal capsule - innermost layer. Is strong and fibrous. It is attached to the kidney. It
functions to prevent infection.
2. Adipose capsule - second
layer. Is a protective layer.
3. Renal fascia - third layer. Is a supportive layer. It anchors the kidney to the
peritoneum and the abdominal wall.
Kidney
structure
Has two layers
1. Outer cortex - many capillaries
2. inner
medulla - many blood vessels and tubules
a.
renal pyramids - tubules (tips = renal papilla)
b.
renal columns - separate pyramids. Transmit blood vessels
Nephron - the functional unit of the kidney. It
functions to produce urine. It is made up of many tubules and their associated
blood vessels. There are over 1 million nephrons
Glomerulus
- A tuft of capillaries with fenestrations
Glomerular (Bowman's) capsule - surrounds the glomerulus. Together they form the renal corpuscle. The
epithelium of the glomerular capillaries contains
pores called fenestrae. These allow filtrate but NOT
proteins to pass from the blood into the glomerular
capsule.
The glomerular capsule has
a parietal and a visceral layer. The parietal layer is for structure
only and does not play a role in filtration.
The visceral layer is made up of highly specialized
cells called podocytes. These cells have foot
processes (pedicels) which line the basement membrane of the glomerulus. Between the foot processes we see filtration
slits which allow the filtrate to pass into the capsular space.
Proximal convoluted tubule - Here we see cuboidal cells with a brush border (microvilli)
These cells resorb
substances from the filtrate as well as secrete substances into the filtrate.
The microvilli greatly increase the ability to resorb water.
Loop of Henle
- has both ascending and descending limbs
Descending limb - (thin segment) is simple squamous epithelium and is highly permeable to water.
Ascending limb - (thick segment) the epithelium is cuboidal or even low columnar.
Distal convoluted tubule - here the cells are cuboidal and thinner than those seen in the proximal
convoluted tubule. This shows that these cells play a role in secreting
substances into the filtrate rather than removing substances from it. Here we
also see two cell types:
1. intercalated cells (cuboidal
with microvilli) - these function in acid/base
balance of the blood.
2. principal
cells ( no microvilli) - these function in body Na+
and water balance.
Collecting duct - Not a part of the Nephron. The distal convoluted
tubules connect to collecting ducts. Passes through the renal pyramids and ends
at the renal papillae where it empties into a minor calyx.
Note
that there are two types of Nephrons:
1. Cortical nephrons:
these make up 85% of our nephrons. With the exception
of the tip of the loop of Henle these nephrons are located in the renal cortex.
2. Juxtamedullary nephrons: here the renal corpuscles are located in the
cortex, yet very near the cortex-medulla junction.
Blood
supply
Renal
arteries feed into segmental arteries, which turn into interlobar
arteries (in renal columns) which feed into arcuate
arteries (branched out at the level between the cortex and medulla).
Interlobular arteries branch off of the arcuate
arteries and run out into the cortex. From these branch the afferent arterioles
which bring blood to the glomeruli (blood filtrate
enters the urinary tubules). Blood leaving the glomerulus
enters an efferent arteriole which takes the blood to peritubular
capillaries around the convoluted tubules OR vasa
recta surrounding the ascending and descending limbs of Henle.
At this point the blood enters veins that parallel the arteries. That is,
interlobular veins to arcuate veins to interlobar veins to renal veins to the IVC.
Notice that the left renal vein is
longer. Why?
Ureters - are retroperitoneal and paired. - enter
the urinary bladder
The
wall of the ureter has 3 layers (tunics)
1. inner mucosa
- continuous with the lining of the urinary bladder. It has transitional
epithelium that secretes a protective mucous (lubrication)
2. middle layer
- Muscularis. It has an inner longitudinal and outer
circular layer of smooth muscle.
The proximal 1/3 also has an outer
longitudinal layer of smooth muscle.
3. Outer layer - fibrous coat made of loose
CT. This coat covers the ureter and anchors it in
place.
Urinary bladder - storage bag for urine. It is located behind the
pubic symphysis yet in front of the rectum in males
and in front of the uterus in females.
The
wall has 4 layers:
1. Muscoa -
innermost layer. Has transitional epithelium. Will find many
folds (rugea) except in the area known as the trigone. This is a triangular area demarcated by
three points, the two openings of the ureters and the
exit point for the urethra.
2. Submucosa
- supports the mucosa
3. Muscularis - has
three layers called detrusor muscle. This
muscle helps to form the internal urethral sphincter.
4. Serosa -
outermost layer. It is found only on the superior surface of the bladder. It is
actually a continuation of the peritoneum.
Urethra - female = short. Approx. 4 cm long
male
= longer. Consists of three regions
1. prostatic urethra - passes through the
prostate gland
2. membranous
urethra - passes through the UG diaphragm
3. Penile (spongy)
urethra - passes the length of the penis.
Urethral
wall -The
inside of the wall is lined by mucous membrane surrounded by a thick layer of smooth
muscle. We also see urethral glands which secrete mucous into the urethral
canal. There is an external urethral sphincter which is composed of voluntary
skeletal muscle.
Micturition = urination, which is a reflex action. Stretch receptors activate the detrusor muscle and relax the internal urethral sphincter. The external urethral sphincter is under voluntary control after approx. 2 years of age. This has to do with growth of the spinal cord and appropriate nerves.