17-2
Aim: How does the urinary system function in maintaining homeostasis?
**Every living organism produces chemical wastes that are not useful to the
body.**
(A) EXCRETION
 Excretion: the process by which metabolic wastes are
removed from the body
 It is vital for maintaining the body’s internal environment
 Metabolic wastes include:
o Excess water and salts
o Carbon dioxide from cellular respiration
o Nitrogenous compounds
o Urea
 The skin, lungs, and kidneys along with their associated organs
make up the excretory system
 Skin: excretes excess water and salts and a small
amount of urea
 Kidneys: excrete nitrogenous compounds—also
regulate the excretion of water which is necessary to
dissolve wastes
 Lungs: excrete most of the carbon dioxide
(B) KIDNEYS
 Main organ of the excretory system
 We have two bean shaped kidneys, one on each side of the
spinal cord
 Together they regulate the chemical composition of blood
 The kidneys are made up of nephrons: the basic functional
unit of the kidneys
 The nephron is an independent filtering unit
 filter water and solutes from the blood
 Each kidney contains about 1 million
 Each nephron has its own blood supply and its own
collecting tubule which leads to the ureter
 As blood enters a nephron through an arteriole,
impurities are filtered out and emptied into the
collecting tubule
 The purified blood leaves the nephron through a
venule
 The process of blood purification involves two
separate processes—filtration and reabsorption
(C) FILTRATION
 When blood enters a nephron, it flows into a network of
capillaries known as a glomerulus
 The glomerulus is encased in the upper end of the nephron by
a cup-shaped structure called the Bowman’s Capsule
 Materials from blood are forced out of the glomerulus and into
the Bowman’s capsule during a process called filtration
 The materials that are filtered from the blood are known as
filtrate
 The filtrate contains water, urea, glucose, salts, amino acids,
and vitamins
(D) REABSORPTION
 Most of the materials removed from the blood at Bowman’s
capsule makes its way back into the blood by a process known
as reabsorption
 Approximately 99% of the water that is filtered into the
Bowman’s capsule is reabsorbed into the blood
 Reabsorption begins when the filtrate passes from the
Bowman’s capsule into the renal tubules—ling, narrow tubules
connected to the Bowman’s capsules
 As the filtrate passes through the renal tubules, they extract
from the filtrate a variety of useful molecules, including
glucose, ions, and some water
 These substances reenter the bloodstream through capillaries
that wrap around the tubule
 This arrangement prevents these molecules from being
eliminated from the body in the urine
(E) URINE FORMATION
 The urine that is excreted from the body is formed from the
water , urea, and various salts that are left after the
absorption and secretion processes
 Ureters: tubes that carry the urine from the kidney to the
urinary bladder
 The collecting duct removes much of the water from the
filtrate that passes through it
(F) ELIMINATION OF URINE
 The ureters have smooth muscle in their walls—the
contractions of this muscle move the urine through the ureters
 The ureters direct the urine to the urinary bladder: a hollow,
muscular sac that stores urine—it gradually expands as it fills
 Muscular contractions of the bladder force urine out of the
body
 Urine leaves the bladder and exits the body through a tube
called the urethra
 A healthy adult eliminates from about 1.5 L to 2.3 L of urine a
day—depending on the volume of fluid he or she consumes
 In females, the urethra lies in front of the vagina and is only
about 2.5 cm long.—there is no connection between the
urethra and the reproductive system in females.
 In males, the urethra passes through the penis—both the urine
and the sperm exit the body through the urethra
 Urination: the elimination of urine from the body through the
urethra
 Nerve impulses cause contraction of the bladder’s muscular
walls—the bladder then empties its contents through the
urethra
 In older children and adults, the brain overrides this urination
reflex, which delays the release of urine until a convenient
time