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Biology 12
13.1 The Urinary System
The urinary system is involved in excretion, which is the removal of metabolic
wastes from the body.
The urinary system produces urine and conducts it outside the body.
Functions of the Urinary System
Excretion of Metabolic Wastes
• The kidneys excrete metabolic wastes (mostly nitrogenous wastes: urea,
ammonium, creatinine, uric acid)
o Urea is formed when ammonia released during amino acid breakdown combines
with CO2
o Some ammonia (NH3) is excreted as ammonium ion (NH4+)
o Creatinine is a breakdown product of creatine phosphate, a high-energy
phosphate reserve molecule
o Uric acid is produced from the breakdown of nucleotides
− Gout can occur if too much uric acid in the blood has crystallized and
moved into the joints
Osmoregulation
• The kidneys are involved in osmoregulation (maintenance of the balance of water and
salt in the blood)
o Salts can cause osmosis (diffusion of water) into the blood, causing blood volume
and blood pressure to increase
• Kidneys also maintain levels of other ions, such as potassium (K+), bicarbonate (HCO3-),
and calcium (Ca2+), in the blood
Regulation of Acid-Base Balance
• The kidneys help regulate the acid-base balance of the blood
o Monitor and keep blood pH at 7.4 by excreting hydrogen ions (H+) and
reabsorbing bicarbonate ions (HCO3-)
Secretion of Hormones
• The kidneys help the endocrine system in hormone secretion
o Secrete renin, an enzyme that stimulates the adrenal cortex to secrete the
hormone aldosterone, which promotes the absorption of sodium ions (Na+) by
the kidneys
o Secrete the hormone erythropoietin (EPO) to simulate red blood cell production
when oxygen demand increases
o Help activate Vitamin D, a hormone-like molecule that promotes calcium (Ca2+)
absorption from the digestive tract
Biology 12
Organs of the Urinary System
The urinary system consists of the
kidneys, ureters, urinary bladder, and
urethra.
Kidneys
• Paired, bean-shaped organs that
regulate acid-base balance and
water-salt balance of blood
• Each kidney is covered by a tough
connective tissue layer called a
renal capsule
• Each kidney has a depression
(called a hilium) on the concave
side where a renal artery enters
and a renal vein and ureter exit
Ureters
• Small muscular tubes that transport urine from the kidneys to the bladder
• Wall of each ureter has three layers: inner mucosa, smooth muscle layer, outer fibrous
connective tissue
• Peristaltic contractions in the ureters cause urine to enter the bladder
Urinary Bladder
• Stores urine until it is expelled from the body
• Has three openings: two for the ureters, and one for the urethra, which drains the
bladder
• Has two sphincters that lie close to where the urethra exits the bladder
o External sphincter is under voluntary
control
Urethra
• Small tube opening that extends from the
bladder to an external opening
• Removes urine from the body
• Males: 20 cm long; urethra carries urine and
semen
• Females: 4 cm long; urethra carries urine
(not connected to reproductive system)
Urination
• When the bladder fills with about 250 mL of
urine, stretch receptors send nerve impulses
to the spinal cord
Biology 12
•
•
Motor nerve impulses from the spinal cord cause the bladder to contract and sphincters
to relax, allowing urination to occur
The brain controls this reflex in older children and adults, allowing urination to be
delayed