Internal anatomy
1) Three distinct regions
a) the renal cortex – the most superficial region, light in color with a granular appearance
(composed of microscopic filtration units called Bowman’s capsules- part of
nephron)
b) the renal medulla- darker, reddish brown, deep to the cortex contains cone shaped
tissue masses called medullary, or renal pyramids. The broad base faces toward the
cortex and the apex or papilla point inwardly. Pyramids composed of microscopic
urine collecting tubules of the nephron. Renal columns (inward extensions of the
cortex) separate the pyramids. Each pyramid and column forms a lobe of the kidney.
c) the renal pelvis- flat funnel shaped tube continuous with the ureter leaving the hilus
Formed from minor calyces forming into major calyces forming into the renal pelvis
Calyces collect urine draining from the papilla and transport it to the ureter. Walls of the
calyces, pelvis, and ureter contain smooth muscle to propel urine by peristalsis to the
bladder.
The nephron
a) The basic functional unit of the kidney
b) consists of four units
1)---renal corpuscle (Bowman’s capsule) containing the glomerulus (a capillary knot: functions in production of a filtrate (blood arrives via an afferent arteriole and leaves
by an efferent arteriole)—filtration occurs as blood pressure forces fluid and
dissolved substances into the renal corpuscle.
2)
---proximal convoluted tubule: functions in reabsorption of water, ions, and all
organic nutrients,
3)
----loop of Henle: functions in further reabsorption of water (descending limb) and
both sodium and chloride ions (ascending limb).
----distal convoluted tubule: functions in secretion of ions,acids,drugs, toxins and
variable reabsorption of water and sodium ions (under hormonal control)
c) each nephron empties into a collecting duct which functions in variable reabsorption
of water and reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate
ions
d) each collecting duct empties into the papillary duct which delivers a substance called urine into the minor calyx and into the ureter
The Renal Corpuscle
a) Bowman’s capsule lined with capsular epithelium
b) Glomerular capillaries covered by glomerular epithelium
c) both are separated by a glomerular space (which receives the filtrate)
d) glomerular epithelium consists of cells called podocytes (cells with long cellular
processes which wrap around individual capillaries (glomerular epithelium separated by
endothelium of the capillaries by a common basement membrane)
e) glomerular capillaries are “fenestrated” (a window) –the endothelial cells contain pores
f) for a substance to enter into the the glomerular space it must pass through the endothelial
pores, the fibers of the basement membrane, and the slits between adjacent podocytes
(all three produce a filtration membrane preventing passage of blood cells and “most”
plasma protein, but permit passage of water, metabolic wastes, ions, glucose, fatty acids,
amino acids, vitamins into the proximal tubule-reabsorption will occur if not in excess in
the tubules)
The Proximal Convoluted Tubule (PCT)
a) receives the filtrate which is now called the tubular fluid and the cells of the PCT
absorb organic nutrients, plasma proteins, and ions and release them into the interstitial
fluid surrounding the tubule. Because of a difference in concentration of solutes water
diffuses out of tubule by osmosis into the interstitial fluid decreasing the volume of the tubular fluid
The Loop of Henle
a) ascending limb has a thick segment which is impermeable to water and solutes (consists of large bulky cuboidal epithelial cells) ,but transports sodium and chloride ions out of the tubular fluid into the interstitial fluid
b) the descending limb has a thin segment (composed of thin simple squamous epithelium) which is permeable to water will absorb water due to the high sodium and chloride
concentrations transported into the interstitial fluid by the ascending limb
out of the tubule fluid.
The Distal Convoluted Tubule(DCT)
a) begins where the ascending loop of Henle bends and comes in close contact with the
glomerulus where it passes between the efferent and afferent arterioles
b) functions in reabsorption of sodium ions, secretion of ions and acids
c) cells of the DCT are unusually tall with their nuclei clustered together form a region
called the macula densa. They along with the juxtaglomerular cells in the wall of the
afferent arteriole. Both of these form the juxtaglomerular apparatus (an endocrine
structure that secretes the hormones rennin and erythropoietin.
The Ureters
a) muscular tubes that carry the urine from the kidneys to the urinary bladder
by peristalsis
b) retroperitoneal and enter the bladder without entering the peritoneal cavity
The Urinary Bladder
a) muscular, distensible sac that stores urine prior to micturition (urination)
b) in males, it is located between the rectum and the pubic symphysis
c) in females it is located inferior to the uterus and anterior to the vagina
d) full bladder can hold up to a liter of urine
e) triangular area bounded by urethral openings and the entrance into the urethra called
the trigone of the bladder; urethral entrance located at the apex of the triangle at the lowest
point of the bladder
f) neck of the urinary bladder (area surrounding the entrance into the urethra) contains
a muscular sphincter which extends down into the proximal portions of the urethra,
called the internal urethral sphincter provides involuntary control over the discharge of
urine into the urethra from the bladder
g) bladder is lined with a transitional epithelium (which can tolerate a tremendous amount of stretching)
h) the bladder wall contains both longitudinal and circular muscle formed into the powerful
detrusor muscle of the bladder (contraction compresses urinary bladder and expels contents to the urethra
The Urethra
a) in females it is very short (about 1 inch)
b) in males it is longer ( 7-8 inches)
c) in both sexes the urethra passes through the urogenital diaphragm (a circular band of
skeletal muscle forming the external urethral sphincter (under voluntary control)
Micturition Reflex and Urination
a) urge to urinate occurs when the bladder contains about 200 ml of urine
b) The micturition reflex:
1)stretch receptors in the bladder are stimulated; impulse carried to CNS by the
parasympathetic nervous system
2) motor neurons then stimulate the smooth muscles in the bladder wall providing
a sustained contraction of the bladder elevating fluid pressures inside the bladder
resulting in the urge to urinate
3)urination cannot occur unless both internal and external sphincters are relaxed
---we control time and place of urination by voluntarily relaxing the external
sphincter (when this sphincter relaxes so does the internal sphincter)
---if the external sphincter is not relaxed , then the internal sphincter remains closed,
and the bladder gradually relaxes (temporarily removing the urge to urinate)
4)a further increase in bladder volume begins the cycle again within one hour
(each increase in urine volume stimulates the stretch receptors more making the
sensation more acute---even painful)
5)once the volume of the bladder exceeds 500 ml the micturition reflex will generate
enough pressure to forcefully open the internal sphincter which leads to a reflexive
relaxation in the external sphincter and urination occurs involuntarily
--normally after micturition less than 10 ml of urine remains in the bladder
