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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