Chapter 16: Urinary System and Excretion

EXCRETION EXCRETION – Is the removal of metabolic waste products from the body. Do NOT confuse this with the elimination of undigested wastes. Failure to remove these metabolic wastes would result in a build up of toxins in the body, eventually leading to death. Our focus will be on the URINARY SYSTEM

Urinary System Urinary Organs The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The bean-shaped kidneys are at the back of the abdominal wall beneath the peritoneum, protected by the lower rib cage. The renal artery and renal vein along with ureters exit the kidney at the hilum. The peritoneum is the lining of the abdominal cavity. Because of their location, the kidneys are somewhat prone to damage by blows to the back.

Two urethral sphincters control the release of urine. The kidneys produce urine which is conducted by two muscular tubes called ureters to the urinary bladder where it is stored before being released through the urethra. Two urethral sphincters control the release of urine. In females, the urethra is 4 cm long; in males, the urethra is 20 cm long and conveys both urine and sperm during ejaculation. How much urine do you think your bladder could hold before you wet yourself? 200 ml 600 ml 1000 ml 1.8 liters Because the female urethra is short, it makes bacterial invasion of the urethra easier and consequently, females are more prone to urinary tract infections. The Health Focus (page 304) describes urinary tract infections.

The urinary system Urine is found only within the kidneys, the ureters, the urinary bladder, and the urethra. About what % of the blood that leaves the left ventricle will be delivered to the kidneys? 20% C) 50% 75% D)3%

Urination As the bladder fills with urine, sensory impulses travel to the spinal cord where motor nerve impulses return and cause the bladder to contract and sphincters to relax. With maturation, the brain controls this reflex and delays urination, the release of urine, until a suitable time.

Urination As the bladder fills with urine, sensory impulses go to the spinal cord and then to the brain. The brain can override the urge to urinate. When urination occurs, motor nerve impulses cause the bladder to contract and an internal sphincter to open. Nerve impulses also cause an external sphincter to open.

Kidney Structure The kidneys filter wastes from the blood, and thus the renal arteries branch extensively into smaller arteries and then arterioles inside each kidney. Many venules unite to form small veins, which merge to become the renal vein.

Microscopically, each contains over one million nephrons. There are three regions to a kidney: an outer renal cortex, an inner renal medulla, and a central space called the renal pelvis. Microscopically, each contains over one million nephrons. The nephrons produce urine which flows into a collecting duct; several collecting ducts merge and drain urine into the renal pelvis. A number of nephrons share the same collecting duct.

Functions of the Urinary System Excretion refers to the elimination of metabolic wastes that were cell metabolites; this is the function of the urinary system. Kidneys play a role in homeostasis of the blood by excreting metabolic wastes, and by maintaining the normal water-salt and acid-base balances of blood. The metabolic wastes removed from the bloodstream by the kidneys are primarily those containing nitrogen.

Excretion of Metabolic Wastes Kidneys excrete nitrogenous wastes, including urea, uric acid, and creatinine. Urea is a by-product of amino acid metabolism. Deamination of AA’s. The metabolic breakdown of creatine phosphate in muscles releases creatinine. Uric acid is produced from breakdown of nucleotides. Collection of uric acid in joints causes gout.

Maintenance of Water-Salt Balance Kidneys maintain the water-salt balance of the body which, in turn, regulates blood pressure. Salts, such as NaCl, in the blood cause osmosis into the blood; the more salts, the greater the blood volume and also blood pressure. Kidneys also maintain correct levels of potassium, bicarbonate, and calcium ions in blood.

Maintenance of Acid-Base Balance The kidneys regulate the acid-base balance of the blood. Kidneys help keep the blood pH within normal limits by excreting hydrogen ions (H+) and reabsorbing bicarbonate ions (HCO3-) as needed. Urine usually has a pH of 6 or lower because our diet often contains acidic foods. If you are stung by a jellyfish, what have some people recommended that you should do? But you shouldn’t do it, as the pH is helpful, but the other chemicals can cause more irritation. So what household substance found at the beach food concession would do a better job?

Secretion of Hormones Kidneys secrete or activate several hormones: They secrete the hormone erythropoietin to stimulate red blood cell production, They activate vitamin D to the hormone calcitriol needed for calcium reabsorption during digestion, and They release renin, a substance that leads to the secretion of aldosterone. Why does it take so long to climb Mount Everest? Thin Air ! The kidneys release renin, a substance that leads to the secretion of the hormone aldosterone from the adrenal cortex. Aldosterone promotes the reabsorption of sodium ions by the kidney.

PRACTICE QUIZ _____/12 E I F J K G L H