The Urinary Tract
• Consists of four main parts paired kidneys & ureters, a single
bladder & urethra
• The urinary tract is a pathway for the elimination of
metabolic by-products and toxic and other non-essential
molecules all dissolved in a small amount of water (urine)
Kidney Structure & Function
• Excretion: removal of metabolic by-products, toxic
molecules & other non-essential molecules
• Conservation of water, as necessary
• Maintenance of acid-base balance in the blood
The Ureters & Kidney Stones
• The ureters carry the urine
formed in the kidneys to the
bladder
• Ureters are fibromuscular
tubes, lined by transitional
epithelium.
• Their narrow structure makes
them prone to being
obstructed by mineralized
concentrations (“stones”) from
the kidneys.
• If the stones are too large to
pass naturally, they can be
crushed using shockwave
therapy.
Urinary Bladder & Urethra
• The urinary bladder is also a fibromuscular structure and the
mucosa is lined with transitional epithelium
• The bladder can hold as little as 50mL of urine or up to 7001000mL without injury
• As it distends, it rises into the abdominal cavity and bulges
posteriorly
• The urethra is a fibromuscular glandular structure also lined
with transitional epithelium
• The male urethra is larger (20cm) than the female (4cm)
– Males are more prone to urethritis (swelling & irritation of
urethra due to viral or bacterial infection; painful urination,
blood may be in the urine)
– Females are more prone to cystitis – bladder inflammation,
usually bacterial (UTI)
Kidney Structure I
• The kidneys are about the size of
a clenched fist and lie against
the back abdominal wall just
above the waistline
• The kidney consists of filtering
capsules, tubules, and blood
vessels tightly packed together
into what is called the
parenchyma.
• The outer covering of the kidney,
called the capsule, is thin but
tough and fibrous
• When cut open, two regions
appear and outer cortex and
inner medulla.
• A microscopic view will reveal
the unit of kidney function, the
nephron
Kidney Structure II
• The cortex consists of
convoluted tubules and
filtering capsules
• The apex of each medullary
pyramid forms a papilla
which fits into a cupshaped funnel called the
minor calyx
• These funnels open up into a major calaces which then open
into the renal pelvis in the area called the renal sinus. The
renal pelvis narrow to form the proximal ureter, sharing the
area with the renal artery and vein
Renal Blood Flow
• Renal blood flow (the amount of blood that
flows through the kidney) is app 1300mL per
minute (through both kidneys)
• About 125-130mL of plasma is filtered into the
renal tubular systems each minute.
• Less that 1% of the filtered plasma (~0.7mL) is
actually excreted as urine, demonstrating the
important role kidneys play in water
conservation.
The Nephron
• The functional unit of
the kidney is a
nephron; There are
about 1 million per
kidney
• A uriniferous tubule
consists of a nephron
and a collecting
tubule
Uriniferous Tubule
Nephron Structures & Functions
• Renal Artery: carries blood to the kidney
• Glomerulus: cluster of capillary like vessels where filtrate
leaves blood
• Bowman’s (renal) capsul: surrounds the glomerulus;
accepts/collects filtrate
• Proximal Convoluted Tubule: where tubular reabsorption
begins; NaCl, HCO3-, H2O, glucose & Aas, poisons & H+ filtered
(secreted for removal)
• Loop of Henle: H2O reabsorbed going down, NaCl
reabsorbed going up; first passively than actively
• Distal Convoluted Tubule: more NaCl and bicarbonate
reabsorbed; some drugs & poisons filtered
• Collecting Tubule: carries urine to ureter; also further
reabsorption of H2O, NaCl & urea
A closer look at the Renal Capsule
The Loop of Henle
Summary Video Clip
Urine
• Under normal resting conditions, the
kidneys, comprising less than 0.5% of
the body weight, receive 25% of the
cardiac output.
• Of the 1300mL of blood that enter
through the renal arteries every minute,
1290-1299mL leave through the renal
veins. The remaining 1-2mL leave as
urine via the ureter.
• Urine is primarily water, salt, small amounts of acid and a
variety of waste products such as urea, however urine
composition and volume change to compensate for any
fluctuation in volume or composition of body fluids.
• Kidneys are the “guardians” of the internal environment,
reworking the body fluids 15x/day (hydration, pH etc.)
Normal Characteristics of Urine:
Color & Transparency
• Ranges from yellow to amber depending on urochrome
(pigment resulting from hemoglobin destruction)
concentration
• The more concentrated the urine, the deeper the yellow
color
• Abnormal coloration (pink, brown, smoky tinge)may result
from eating certain foods (beets) or presence of bile
pigments or blood in the urine
• Some drugs and vitamins alter urine color (riboflavin
causes the bright “neon” yellow/green)
• Cloudy urine may indicate infection in the urinary tract
Normal Characteristics of Urine: Odor & pH
• Fresh urine should be only slightly aromatic
• Left to stand, an ammonia odor will develop as bacteria
metabolize urea solutes
• Some drugs and vegetables alter odor (asparagus)
• Diseases may also alter odor (diabetes mellitus can
produce a fruity odor due to acetone content)
• Normally urine is slightly acidic (pH ~6)
• Metabolic or dietary changes can alter range (~4.5-8)
– Diets high in protein and whole wheat lowers pH
– Diets high in vegetables raise pH4
– Prolonged vomiting or urinary tract infection creates alkaline
urine
Normal Characteristics of Urine:
Specific Gravity
• Because urine is water plus solutes, it is more dense
and weighs more than distilled water.
• Specific gravity is the comparison of the weight of a
substance to the weight of an equal volume of distilled
water.
• The specific gravity of distilled water is 1.0, the specific
gravity of urine ranges from 1.001 to 1.035, depending
on solute concentration
• In extremely concentrated urine, solutes will
precipitate out of solution
Normal Characteristics of Urine:
Chemical Compostion
• Approximately 95% of the volume of urine is water, the
remaining 5% is solutes
• The largest component of urine, by weight, is urea, derived
from the normal break down of amino acids
• Other nitrogenous wastes include uric acid (from nucleic acid
metabolism) and creatinine (from CP metabolism)
• Normal solute constituents in order of decreasing
concentration are: urea, sodium, potassium, phosphate and
sulfate ions. Much smaller and variable amounts of calcium,
magnesium & bicarbonate ions may also be found.
• Abnormally high concentration of any solute may indicate
pathology.
• Certain diseases cause dramatic changes in composition:
glucose, blood proteins, RBCs, hemoglobin, WBCs (pus) or bile
pigments. Such substances are a sign of disease and useful in
diagnosis.