I. Introduction
A. components of the urinary system
1. two kidneys
2. two ureters
3. one urinary bladder
4. one urethra
B. kidney process blood and form urine
1. filter blood plasma (filtration)
2. return most of the water and solutes to the bloodstream (reabsorption)
3. the remaining water and solutes constitute urine
a. passes through ureters
b. stored in the urinary bladder
c. excreted from the body through the urethra
C. nephrology- the scientific study of the anatomy, physiology, and pathology of the kidneys
D. urology- the branch of
medicine that deals with the male and female urinary systems and
the male
reproductive system
E. urologist- a physician who specializes in urology
F. functions of the urinary system
1. the kidneys
-regulate blood volume and composition
-help regulate blood pressure
-synthesize glucose
-release erythropoietin
-participate in vitamin D synthesis
-excrete wastes in the urine
2. the ureters transport urine from the kidneys to the urinary bladder
3. the urinary bladder stores urine
4. the urethra discharges urine from the body
II. Overview of kidney functions
A. kidneys- do the major work of the urinary system
B. the remaining parts of the urinary system- passageways and storage areas
C. functions of the kidneys
1. regulation of blood ionic composition
a. the kidneys help regulate the blood levels of Na+, K+, Ca2+, Cl-, and HPO42-
2. regulation of blood pH
a. the kidneys excrete H+ into urine
b. bicarbonate ions (HCO3-) are conserved- an important buffer of H+ in the blood
3. regulation of blood volume
a. the kidneys conserve or eliminate water in the urine
b.
an increase in blood volume increases blood pressure; a decrease in blood
volume
decreases blood pressure
4. regulation of blood pressure
a. the kidneys secrete the enzyme renin
b. renin activates the renin-angiotensin-aldosterone pathway
c. increased renin causes an increase in blood pressure
5. maintenance of blood osmolarity
a. the kidneys maintain a relatively constant blood osmolarity (about 300 milliosmoles per liter)
b. they do this by regulating the loss of water and solutes in the urine
6. production of hormones
a. the kidneys produce two hormones
(1) calcitriol
-the active form of vitamin D
-helps regulate calcium homeostasis
(2) erythropoietin- stimulates production of RBCs
7. regulation of blood glucose level
a. like the liver, the kidneys can use the amino acid glutamine in gluconeogenesis
b. gluconeogeneisis is the synthesis of new glucose molecules
c. can release glucose into blood to maintain normal blood glucose level
8. excretion of wastes and foreign substances
a. excrete wastes by forming urine
b. examples of metabolic wastes
-ammonia and urea from the deamination of amino acids
-bilirubin from the catabolism of hemoglobin
-creatinine from the breakdown of creatin phosphate in muscle fibers
-uric acid from the catabolsim of nucleic acids
c. examples of other wastes (foreign subsances from the diet)
-drugs
-environmental toxins
III. Anatomy and Histology of the Kidneys
A. the paired kidneys are reddish, kidney-shaped organs
B. the kidneys are retroperitoneal- their position is posterior to the peritoneum of the abdominal cavity
C. the right kidney is slightly lower than the left- due to the presence of the liver
D. external anatomy of the kidneys
1. size- about 4-5 in. long, 2-3 in. wide, 1 in. thick (about the size of a bar of bath soap)
2. the concave medial border of each kidney faces the vertebral column
3. renal hilum-
a deep vertical fissure near the center of the concave border
- where the ureter, blood vessels, lymphatic vessels, and nerves emerge
4. three layers of tissue surround each kidney
a. the renal capsule
-the deep layer
-smooth, transparent sheet of dense irregular connective tissue
-serves as a barrier against trauma
-helps maintain the shape of the kidneys
-continous with the outer coat of the ureter
b. the adipose capsule
-middle layer
-a mass of fatty tissue surrounding the renal capsule
-protects kidney from trauma
-holds it firmly in place in the abdominal cavity
c. the renal fascia
-the superficial layer
-thin layer of of dense irregular connective tissue
-anchors the kidney to surrounding structures and to the abdominal wall
E. internal anatomy of the kidneys
1. two regions
a. the renal cortex
-superficial
-smooth-textured reddish area
b. the renal medulla
-deep
-reddish-brown inner region
-consists of 8 to 18 cone-shaped renal pyramids
2. renal pyramids
-the base faces the renal cortex
-the apex (renal papilla) points toward the renal hilum
3. renal columns- portions of the renal cortex that extend between renal pyramids
4. renal lobe-
a renal pyramind + its overlying area of renal cortex + 1/2 of each
adjacent
renal column
5. the parenchyma-
the functional portion of the kidney
- consists of the renal cortex and renal pyramids of the renal medulla
- contains about 1 million mircoscopic structures called nephrons
6. nephrons are the functional units of the kidney
7. urine formed
by the nephrons drains into large papillary ducts- extend through
the renal
papillae
8. papillary ducts drain into cuplike structures called minor and major calyces (singular, calyx)
9. from the major calyces, urine drains into a large cavity called the renal pelvis
10. urine leaves the renal pelvis through the ureter
11. ureters convey urine to the urinary bladder
12. flow of urine:
nephron-->collecting duct---> papillary duct---> minor calyx---> major
calyx--->
renal pelvis--->ureter----> urinary bladder----> urethra
IV. Blood Supply of the Kidneys
A. the kidneys are abundantly supplied with blood vessels- they process blood
B. the kidneys receive 20 - 25% of the resting cardiac output via the right and left renal arteries
C. in adults, blood flow through both kidneys (renal blood flow) is about 1200 ml per minute
D. See Fig.
Blood supply of the kidneys
V. The Nephron
A. the functional unit of the kidney
B. parts of a nephron
1. two parts
a. renal corpuscle- where blood plasma is filtered
b. renal tubule- into which filtered fluid passes
2. renal
corpuscle-lies within the renal cortex
-consists of two components
a. glomerulus- a capillary network
b. the glomerular (Bowman's) capsule- a double-walled epithelial cup that
surrounds the glomerulus
3. parts of a renal tubule
a. proximal convoluted tubule- lies within the renal cortex
b. loop of Henle (nephron loop)- extends into the renal medulla, makes
a hairpin turn, and
returns to the renal cortex
c. distal convoluted tubule- lies within the renal cortex
C. the distal convoluted tubules of several nephrons empty into a single collecting duct
D. the loop of Henle
1.
the ascending limb- the first part of the loop of Henle
- dips into the renal medulla
2. the ascending limb- returns to the renal cortex
E. cortical nephrons- 80-85% of
nephrons
- have short loops of Henle that lie mainly in the cortex / penetrate only
the
outer region
of the renal medulla
F. juxtamedullary nephrons- 15- 20% of
nephrons
- have renal corpuscles that lie deep in the cortex, near the medulla
- have a long loop of Henle that extemnds into the deepest regions of
the medulla
- the ascending limb of the loop of Henle consists of two portions
1. a thin ascending limb (absent in cortical or short loop nephrons)
2. a thick ascending limb
-these nephrons enable the kidneys to excrete very dilute
or very concentrated urine
VI. Histology of the Nephron and Collecting Duct
A. a single layer of epithelial
cells forms the entire wass of the glomerular capsule,
renal tubule, and
ducts
B. each part has distictive histological features
C. glomerular capsule (Bowman's capsule)
1. consists of two layers
a. visceral layer- consists of modified simple squamous epithelial cells
called podocytes
- footlike extensions of these cells (pedicels) wrap around the single layer
of
endothelial cells of the glomerular capillaries
- form the inner wall of the capsule
b. parietal layer- simple squamous epithelium
- forms the outer layer of the capsule
2. fluid filtered from the glomerular capillaries enters the capsular (Bowman's)
space- the
space between the visceral and parietal layers
D. renal tubule and collecting duct
1. proximal convoluted tubule
a. composed of simple cuboidal epithelial cells with a brush border of
microvilli on
their apical surface
b. microvilli increase surface area for reabsorption and secretion
2. the descending limb and first part of the ascending limb of the loop
of Henle
a. composed of simple squamous epithelium
3. the thick ascending limb of the loop of Henle
a. composed of simple cuboidal to low columnar cells
4. the final part of the ascending limb of the loop of Henle makes contact
with the
afferent arteriole serving the renal corpuscle
a. the columnar tubule cells in this regior are crowded together- known as the macula densa
b. alongside the macula densa, the wall of the afferent arteriole contains
modified
smooth muscle
fibers called juxtaglomerular cells
c. the macula densa + the juxtaglomerular cells = the juxtaglomerular apparatus-
helps
regulate blood pressure within the kidneys
5. the distal convoluted tubule begins a short distance past the macula densa
6. the last part of the distal convoluted tubule and the collecting ducts
contain two
types of cells
a. principal cells- have receptors for ADH and aldosterone
b. intercalated cells- play a role in blood pH
VII. Overview of Renal Physiology
A. the nephron forms urine (processes blood plasma) by means of three processes
1. filtration- movement of water and solutes from plasma in the glomerulus
into Bowman's
capsule
2. reabsorption- movement of molecules out of the tubule and into peritubular
blood
(peritubular capillaries = capillaries around the tubules)
3. secretion- movement of molecules out of the peritubular blood and into
the tubule
for excretion