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Urinary System
Exercises 40 and 41
Organs of the Urinary System
2 kidneys
Urinary bladder
2 ureters
Primary function is to filter blood, and
reabsorb water and needed substances
back into blood. Remaining water and
wastes are removed in urine.
Regulate blood volume and composition
Regulate blood pH
Regulate blood pressure
Contribute to metabolism
– Calcitriol
– Erythropoietin
– Gluconeogenesis
Location of kidneys
Between T12 and L3
Partially protected by 11th and 12th ribs
Right kidney is lower than left
Kidneys and ureters are retroperitoneal
Protected by three layers:
– Renal capsule
– Adipose capsule
– Renal fascia
• Renal hilum – ureter, blood and lymphatic
• Renal sinus
• Renal cortex
• Renal medulla
– Renal pyramids
– Renal papilla
• Renal columns
Functional unit of kidney
Number constant from birth
Can function on 2/3 of one kidney
Forms the urine
Duct system
Minor calyces
Major calyces
Renal pelvis
Urine is not changed after it leaves the
collecting duct
Kidneys = 1% of body mass
Receive 15-30 % of blood flow
Blood supplied to kidney by renal artery
Afferent arteriole divides into ball shaped
capillary network called the glomerulus.
• Capillaries rejoin to form efferent arteriole,
then peritubular capillaries
• Vasa recta
Functions of the Nephron
• Filtration
• Tubular reabsorption
• Tubular secretion
Structure of a nephron
• Renal corpuscle
– Glomerulus
– Glomerular capsule or Bowman’s capsule
• Renal tubule
– Proximal convoluted tubule
– Nephron loop or loop of Henle
– Distal convoluted tubule
– Collecting duct
Bowman’s capsule
• Outer layer - simple squamous epithelium
• Inner layer – modified simple squamous epi.
– Cells called podocytes
– Projections are pedicels
• Endothelium of capillary and podocytes form
the endothelial-capsular (filtration) membrane.
Filtration membrane
1. Fenestrated endothelium – prevents
the passage of cells
2. Basement membrane of glomerulus –
prevents passage of large proteins
3. Slit membrane (between pedicels) –
prevents the passage of medium sized
• Wall of renal tubules made up of single
layer of epithelial cells and a basement
• Proximal convoluted tubule – cells are
cuboidal and have microvilli.
• About 65 % of water and 100 % of some
solutes are reabsorbed here.
Renal Physiology
• Glomerular filtration – most substances
easily filtered
• Filtrate = 180 L/ day but urine = 1-2 L /day
• Filter is thin
• Glomerular capillaries are long
• Capillary blood pressure is high
(afferent arteriole diameter > efferent arteriole)
• Filtration fraction is the percentage of
plasma that becomes filtrate, usually 1620 %.
• Glomerular Filtration Rate (GFR) is the
amount of filtrate formed in both kidneys
per minute; normally about 125 ml / min.
Renal autoregulation
• JGA senses Na+, Cl- and water
• When these ↓, JGA dilates afferent
arteriole, ↑ blood pressure in glomerulus
Hormonal regulation
• When BP ↓, JGA secretes renin which
converts angiotensinogen to angiotensin I,
angiotensin I is converted to angiotensin II
in the lungs. Angiotensin II is a potent
vasoconstrictor. It also stimulates the
release of aldosterone, and stimulates
the thirst center in the hypothalamus and
release of ADH from the posterior pituitary
Tubular reabsorption
• Volume of fluid that enters the proximal
convoluted tubule in ½ an hour > total
plasma volume.
• Reabsorb water, glucose, amino acids,
urea and ions, esp. Na+, Cl- and K+
• Na+ diffuses into cells, but is actively
pumped out into interstitial fluid
• Promotes reabsorption of water
DCT and collecting ducts
• By end of DCT 90 % of water and solutes
• Aldosterone causes principal cells to synthesize
sodium pumps. Also causes potassium to be
secreted into urine.
• ADH stimulates cells to put water channels into
membranes. ADH is responsible for facultative
(responding to need) water reabsorption.
Tubular secretion
• Two main functions:
• Secretion of substances to eliminate them
from body – ions, nitrogen-containing
waste (urea, creatinine) and certain drugs
• Secretion of H+ helps to maintain blood
Urinary bladder
• Posterior to pubic symphysis
• Stores urine prior to micturition
• Trigone – between openings of ureters
and internal urethral orifice – mucosa is
firmly bound to muscularis.
• Muscosa, submucosa, muscularis
(detrusor muscle), and serous coat.
Evaluation of kidney function
• Blood Urea Nitrogen (BUN) – urea is a
breakdown product of proteins
– Influenced by diet
• Creatinine – end product of muscle
– Normally constant
– Can determine creatinine clearance
Normal urine
• Transparent yellow
• Contains ions : Na+, Cl-, K+ ; nitrogenous
wastes: ammonia, creatinine, urea and
uric acid; and foreign chemicals: drugs,
pesticides, food additives, etc.
• Suspended solids in trace amounts:
bacteria, blood cells, casts.
Not normally present
• Albumin
• Glucose, Acetone– diabetes mellitus
• Bile – liver disease, obstruction of bile
ducts, hemolytic disease
• pH 4.6 -8.0
Specific gravity
• 1.001 -1.035
• Damage to tubules , s.g. fixed at 1.010 =
• Diabetes insipidus (lack of ADH) – 1.003
• Diabetes mellitus – “sugar diabetes”
– 1.030 osmotic diuresis
• Emesis or fever –
– 1.040