(The images are of human models and cat dissections.)



The human body is composed of about 75 trillion cells. These cells produce wastes and these wastesbuild up in your body. Several organ systems devote time and energy to eliminating the wastes. The lungs expel carbon dioxide, heat, and a little water. The skin expels heat, water, carbon dioxide, urea, and some salts. The GI tract expels materials that can't be digested; the bile released from the liver helps eliminate wastes from the bloodstream. Last, but not least, the kidneys excrete water, nitrogenous wastes from protein catabolism (urea, uric acid, ammonia), some bacterial toxins, many drugs, H+, and excess electrolytes (salts). The urinary system is composed of 2 kidneys, 2 ureters, 1 bladder, and 1 urethra.

The kidneys perform three functions:
a) filter blood
Blood enters the kidney and, before leaving, fluid (known as filtrate) is separated from the blood.
b) reabsoprtion
Most of the filtrate is composed of water and salts that your body needs; most of the filtrate will be reabsorbed into the bloodstream leaving a high concentration of wastes in what becomes the urine.
c) secretion
The kidneys can pump additional wastes (ammonia, potassium, H ions) into the urine that were not present in the blood which was filtered.


Ureters are 10-12" long tubules which connect the kidneys to the bladder. They enter the bladder obliquely and as the bladder fills, it compresses the openings to prevent backflow. The mucous membranes lining the ureter secretes a mucus to prevent the composition of the urine from disrupting cell homeostasis.


The bladder is a hollow, muscular organ that serves as a temporary storage site for urine. It is smaller in females due to the space occupied by the uterus.

The urethra transports urine from the bladder and expels it from the body. It is shorter in females (3.8 cm) and exits through the external urethral orifice between the clitoris and vagina. It is longer in males (20 cm) and exits through the penis.


a) urine salts may crystallize to form renal calculi (kidney stones) in any portion of the tract; high Ca (or low parathyroid activity), low water, or pH imbalance may cause
--surgery and shock waves can treat
b) diuretics increase the loss of water, usually by interfering with the reabsorption of ions (primarily Na)
--caffeine inhibits Na reabsorption; alcohol inhibits ADH
c) diabetes insipidis: excretion of high volumes of dilute urine; either not enough ADH or principal cells have reduced sensitivity to it
d) urine analysis: abnormal components present in urine can be indicative of various disorders:

--albumin: kidney trauma, hypertension, heavy metals, glomerulonephritis
--ketone bodies: undernutrition; is indicative of diabetes mellitus when present with glucose
--glucose: diabetes mellitus
--red blood cells: bleeding from infections or kidney stones
--bilirubin: liver dysfunction
--white blood cells: urinary tract infection or inflammation