• Organ system that produces, stores, and carries urine
• Includes two kidneys, two ureters, the urinary bladder, two
sphincter muscles, and the urethra.
• Humans produce about 1.5 liters of urine over 24 hours,
although this amount may vary according to the
• Increased fluid intake generally increases urine production.
• Increased perspiration and respiration may decrease the
amount of fluid excreted through the kidneys.
• Some medications interfere directly or indirectly with urine
production, such as diuretics.
– It is paired organ (weight about 300 g)
– Compound from two parts cortex
(isotonic urine) and medulla
– Cortex: Glomerular apparatus
– Medulla: Divided: Outer and Inner
• Consists of about 1 million filtering
units termed nephrons (basic
structural and functional unit)
• The kidney plays a crucial role in
regulating electrolytes in the human
blood (e.g. Na+, K+, Ca2+).
• It clears urea, a nitrogenous waste
product from the metabolism of
• The major function of the renal
pelvis is to act as a funnel for
urine flowing to the ureter.
• The renal pelvis represents the
funnel-like dilated proximal part of
• It is the point of convergence of
two or three major calices.
• Each renal papilla is surrounded
by a branch of the renal pelvis
called a calyx.
• Urine is collected in the renal pelvis (or
pyelum), which connects to the ureters,
which carry urine to the bladder.
• The ureters are about 200 to 250 mm long.
• Smooth muscular tissue in the walls of the
ureters peristaltically force the urine
• The urinary bladder is a hollow
muscular organ shaped like a
• It is located in the pelvic fossa and
held in place by ligaments
attached to the pelvic bones.
• The bladder stores urine - up to
500 ml of urine comfortably for 2
to 5 hours.
• Sphincters (circular muscles)
regulate the flow of urine from the
– Internal urethral sphincter =
in the beginning of urethra
smooth muscle – not under our
– External urethral sphincter =
skeletal muscle – we can
• The detrusor muscle is a layer of the urinary
bladder wall, made up of smooth muscle
fibers arranged in inner and outer
longitudinal layers and a middle circular
• Contraction of the detrusor muscle causes
the bladder to expel urine through the
• Problems with this muscle can lead to
• The urethra has an excretory function in both sexes,
to pass urine to the outside, and also a reproductive
function in the male, as a passage for sperm.
• The external urethral sphincter is a striated smooth
muscle that allows voluntary control over urination.
• Urethral sphincters:
• In males the internal and external urethral sphincters
are more powerful, able to retain urine for twice as
long as females
• The process of disposing
urine from the urinary bladder
through the urethra to the
outside of the body.
• The process of urination is
usually under voluntary
• Urinary incontinence is
inability to control urination,
and is more common in
women than men.
• Urinary retention refers to the
inability to urinate.
• Nocturnal Enuresis =
incontinence during the night
• Activated when the urinary bladder wall is stretched; it results
• This reflex occurs in the spinal cord, specifically in the sacral
region that is modified by the higher centers in the brain: the
pons and cerebrum.
• The presence of urine in the bladder stimulates the stretch
receptors, which produces action potential.
• The action potentials are carried by sensory neurons to the
sacral segments of the spinal cord through the pelvic nerves,
the parasympathetic fibers carry the action potentials to the
urinary bladder in the pelvic nerves.
• The pressure in the urinary bladder increases rapidly once its
volume exceeds approximately 400-500 ml.
• A nephron (1-1.2 millions) is the
basic structural and functional
unit of the kidney.
• Its chief function is to regulate
water and soluble substances
by filtering the blood,
reabsorbing what is needed and
excreting the rest as urine.
• Each nephron is composed of
an initial filtering component (the
renal corpuscle) and a tubule
specialized for reabsorption and
excretion (the renal tubule).
• The renal corpuscle filters out
large solutes from the blood,
delivering water and small
solutes to the renal tubule for
– Glomerular apparatus
– Proximal tubule
– Loop of Henle
– Distal tubule
– Collecting ducts
• Types of nephrons:
– Cortical nephrons (glomerular
apparatus belong the surface and Loop
of Henle only to the outer part of the
– Intermedial nephrons (in the middle)
– Juxtamedullary nephrons (glomerular
apparatus deep in cortex near the
medulla and Loop of Henle is going
deep to the inner part of the medulla)
• The filtration surface is 1.5
• Amount of the solution,
which is filtered in glomerular
apparatus is around 180-200
L per day.
• The rest (97 %) has to be
reabsorbed in the tubules
back to the body, so the final
volume of urine is around
(1.5 - 2 L per day).
• Depends on:
– Pressure gradient across the filtration slit
(endothelium, basal membrane, epithelium =
– Blood circulation throughout the kidneys
– Permeability of the filtration barrier
– Filtration surface
• The solution after filtration is very similar like
plasma, but should be WITHOUT PROTEINS
• The ability of kidneys to clear plasma from different
GLOMERULAR FILTRATION RATE (GFR)
• describes the flow rate of filtered fluid through the kidney.
• can be measured by measuring the excretion and plasma
level of a substance that freely filtered through the glomeruli
and neither secreted nor reabsorbed by the tubules, such
as INULIN (polymer of fructose).
GFR = U x V/P
U = concentration of inulin in urine
V = volume of the urine
P = concentration of inulin in plasma
• Normal GFR is around 125 ml/min (7.5 l/h)
cells are cells that
and secrete the
• Specialized smooth
muscle cells in the
wall of the
that are in contact
with distal tubule.
Function -- EExxccrreettiioonn ooff wwaassttee pprroodduuccttss
• The kidneys
excrete a variety
of waste products
uric acid (from
FFuunnccttiioonn -- HHoommeeoossttaassiiss
• Acid-Base Balance
– The kidneys regulate the pH,
mineral ion concentration, and
water composition of the blood.
– Urine, on the other hand,
becomes either acidic at pH 5 or
alkaline at pH 8.
• Water Balance
• Plasma Volume
• A steroid hormone (mineralocorticoid) synthesized
from cholesterol by the enzyme aldosterone synthase.
• It is formed in the outer-section (zona glomerulosa) of
the adrenal cortex of the adrenal gland.
• It helps regulate the body's electrolyte balance by
acting on the mineralocorticoid receptor (MR).
• It diminishes the excretion of Na+ ions and therefore
water, and stimulates the excretion of K+ ions by the
• Aldosterone is synthesized in reaction to increases of
angiotensin II or plasma potassium, which are present
in proportion to sodium deficiencies.
CCoonnttrrooll ooff aallddoosstteerroonnee
• The role of baroreceptors
– Baroreceptors in the human body detect the
pressure of blood flowing though them, and can
send messages to the central nervous system to
increase or decrease total peripheral resistance
and cardiac output.
• The role of the juxtaglomerular apparatus
• The role of sympathetic nerves
• The role of the renin-angiotensin system
• A human hormone that is mainly released when the body is
low on water.
• It causes the kidneys to conserve water by concentrating the
• If there is not enough water in the body
– The osmotic activity of the EC solution is increased →
stimulation of the OSMOTIC RECEPTORS in the
hypothalamus → stimulation of posterior lobe of the
pituitary gland → activation of VASOPRESSIN → increase
of the permeability of collecting ductus for the water →
reabsorption → HYPERTONIC URINE
• If there is too much water in the body
– The increase volume stimulates VOLUME RECEPTORS in
the heart and big veins and arteries → decrease of the
activation of VASOPRESSIN → decrease of the
permeability of collecting ductus for the water → water is
not reabsorbed → ISO- or HYPOOSMOTIC URINE
RReenniinn--aannggiiootteennssiinn ssyysstteemm 11
• A hormone system that helps regulate long-term
blood pressure and blood volume in the body.
• The system can be activated when there is a loss of blood
volume or a drop in blood pressure (such as in a hemorrhage
• If the perfusion of the juxtaglomerular apparatus in the kidneys
decreases, then the juxtaglomerular cells release the
enzymatic hormone renin.
– from VOLUME RECEPTORS in afferent arteriole → decrease in
perfusion → decrease in tonus of afferent arteriole
– from CHEMORECEPTORS in macula densa → decrease of NaCl in
macula densa cells
RReenniinn--aannggiiootteennssiinn ssyysstteemm 33
• Renin activates the renin-angiotensin system
by cleaving angiotensinogen, produced in the
liver, to yield angiotensin I, which is further
converted into angiotensin II by specialized
cells of the lung capillaries.
• Angiotensin II then constricts blood vessels,
increases the secretion of ADH and
aldosterone, and stimulates the hypothalamus
to activate the thirst reflex, all these actions
leading to increased blood pressure.
• Also known as angiotensinogenase, is a circulating
enzyme released mainly by juxtaglomerular cells of
the kidneys in response to low blood volume or low
body NaCl content.
• Actions of renin:
– Vasoconstriction in efferent arteriole (increase of glomerular
– Peripheral vasoconstriction (increase in blood pressure)
– Secretion of aldosterone (reabsorption of Na+ and water)
• A prostaglandin is any member of a group of lipid
compounds that are derived from fatty acids and have
important functions in the animal body.
• Every prostaglandin contains 20 carbon atoms, including a
• Hormone-like substances
– Increase of perfusion
– Decrease of water reabsorption
– Decrease of active Na+ transport in tubules
• PTH is secreted by the parathyroid glands
– regulation of calcium and phosphates excretion by
– increase of Ca2+ reabsorption in distal tubule and
– inhibition of phosphates reabsorption in proximal
and distal tubules (increase of their excretion)
– decrease in natrium and bicarbonates
reabsorption = decrease in water reabsorption