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Urinary System
Chapter 18 2013
Introduction
• The function of the urinary system include
– Eliminating organic waste products
– Regulating plasma concentration of ions
– Regulating blood volume and pressure by
adjusting the volume of water lost and releasing
hormones
– Stabilizing blood pH and
– Conserving nutrients.
Organization
• The urinary system includes the kidneys, the ureters,
the urinary bladder and the urethra.
• The kidneys produce urine (a fluid containing water,
ions, and soluble compounds), during urination urine
is forced out of the body.
The Kidneys
• Both kidneys lie in a retroperitoneal position.
• A fibrous renal capsule surrounds each kidney.
• The hilus provides entry for the renal artery and exit
for the renal vein and ureter.
• The ureter is connected to the renal pelvis. This
chamber branches into two major calyces, each
connected to four or five minor calyces which
enclose the renal papillae.
• Urine production begins in nephrons.
Anatomy of the Kidney
The Nephron
• The nephron is the basic functional unit in the kidney
• It includes the renal corpuscle and a renal tubule,
which empties into the collecting system through a
collecting duct.
• From the renal corpuscle, the filtrate travels through
the proximal convoluted tubule, the loop of Henle,
and the distal convoluted tubule.
• As the filtrate travels along the tubule, its
composition gradually changes. The changes that
occur and the urine that results depends on the
specialized activities under way in each segment fo
the nephron.
• Each nephron empties into a collecting duct which
eventually delivers the fluid, now called urine, into
the renal pelvis.
Functions of the Nephron
• Each part of the nephron has a role in converting the
filtrate into urine.
• The renal corpuscle is the site of filtration, but this
passive process which permits of passage of organic
wastes also permits the passage of glucose, fatty
acids and amino acids and water. These must be
reclaimed.
The Nephron
Blood Supply to the Kidneys
• In healthy individuals, about 1200ml of blood flows
through the kidneys each minute – 25% of the
cardiac output.
• Each kidney receives blood from a renal artery that
originates from the abdominal aorta.
The Functions of the Nephrons and Collecting
Tubules of the Kidney
Region
Primary Function
Renal Corpuscle
Filtration of plasma to initiate urine
formation
PCT
Reabsorption of ions, organic
molecules vitamins, water
Loop of Henle
Descending: reabsorption of H2O
from filtrate
Ascending: reabsorption of ions
creating a concentration gradient–
allows kidney to make concentrated
urine.
DCT
Reabsorption of sodium ions,
secretion of acids ammonia, drugs
Collecting duct
Reabsorption of water and sodium
and bicarbonate ions.
Principles of Urine Production
• Primary purpose is to maintain homeostasis by
regulating the volume and composition of the blood.
• Involves the excretion and elimination of dissolved
solutes, specifically the following:
– Urea: produced by the breakdown of amino acids.
– Creatinine: produced through the breakdown of
creatinephosphate by skeletal muscles.
– Uric acid: produced by the breakdown of RNA
• These products must be secreted in solution, so
there is water loss.
• Kidneys minimize this by producing a urine that is 4
to 5 times more concentrated than normal body
fluids.
• Kidneys also ensure that urine does not contain
useful organic substances such as sugars and amino
acids
Kidneys use three processes
• Filtration: blood pressure forces water and solutes
across filtration membrane.
• Reabsorption: selectively removing water and solute
molecules from the filtrate through osmosis and
carrier proteins.
• Secretion: Active transport of solutes across the
membrane into the filtrate.
• All three processes must be carefully balanced. If
both kidneys fail, death will occur within a few days.
Summary of Urine Formation
Control of Kidney Function
• Regulated in three ways.
– 1. by local, automatic adjustments in blood
pressure.
– 2. through activities of the sympathetic division of
the autonomic nervous system.
– 3. through the effects of hormones.
• Aldosterone: a hormone that regulates level of
sodium in the body. The higher the aldosterone
levels, the more sodium ions are reclaimed and the
more potassium ions are lost.
• Antidiuretic hormone (ADH): regulates how much
water is reabsorbed in the kidneys. If levels are low,
little water reabsorption will occur. If levels are high,
very little urine is produced but the concentration of
solutes will be 4 to 5 times that of extracellular
fluids.
Transport, Storage and Elimination
• Filtrate modification and urine production end when
the fluid enters the renal pelvis. The urinary tract (
the ureters, urinary bladder, and urethra) is
responsible for the transport, storage and
elimination of the urine.
• Ureters: a pair of muscular tubes that carry urine from the
kidneys to the urinary bladder.
• Urinary Bladder: a hollow muscular organ that stores urine
prior to urination. Can hold about 1 liter of urine.
• Urethra: : extends from the neck of the urinary bladder to the
exterior. In females it is short – about 3 cm. In males it
extends from the bladder to the tip of the penis – about 20
cm.
Micturition Reflex and Urination
• Urine reaches the bladder by peristaltic contractions
of the ureters.
• Stretch receptors in the wall of the bladder make us
aware of the fluid pressure in the bladder when
about 200 ml of urine are present.
• The external sphincter muscle is under voluntary
control. This control continues until the bladder
contains about 500 ml. of urine. At that point,
urination may happen even if you don’t want it to.
The Nephron
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