The Urinary System
Your body releases wastes that are filtered from your blood and tissue fluids as part of your urine. In particular, urine helps you flush out nitrogenous wastes — unnecessary, excess materials containing nitrogen that result from the breakdown of proteins and nucleic acids.
Also, because your urinary system releases fluid from your body, it plays an important role in maintaining the proper fluid balance in the body. The structures of your urinary system work like this:
You have two bean-shaped kidneys, one on each side of your back, just below your ribs. Your kidneys produce urine. The adrenal glands that sit on top of your kidneys are endocrine glands that release hormones into the bloodstream.
Urine leaves your kidneys and travels through thin muscular tubes called ureters. Smooth muscle in the ureter pushes the urine along through peristalsis.
Urine arrives at your urinary bladder, a muscular bag that lies in the pelvis behind your pubic bones. Your bladder stretches as it fills with urine, which leads to the signal to urinate.
Urine leaves your bladder and exits the body through your urethra. In females, the urethra is short (about 1½ inches long) and lies close to the vagina’s front wall. In males, the urethra is about 8 inches long and passes through the prostate gland and penis.
Your kidneys are the workhorses of your urinary system. Blood arrives at the kidney through a large artery called the renal artery and then passes into a network of smaller and smaller blood vessels until it reaches a system of capillaries that are intimately entwined within the kidney’s structure.
Fluid from your capillaries is forced by your blood pressure into the kidney, where it’s filtered to remove wastes. After the fluids are filtered, the kidney returns the clean portions back to the circulatory system, and blood exits the kidneys via the renal vein.
The function of the kidney, and the entire urinary system, can be broken down into four important components:
Filtration of body fluids by passing them through the kidney.
Reabsorption of useful materials like water and electrolytes (charged ions such as Na+ and K+) from the kidney back to the blood.
Secretion of specific waste materials from the blood into the kidney.
Excretion of wastes in urine.
For questions 1–4, use the terms that follow to identify the parts of the human urinary system shown in the figure.
The following are the answers to the practice questions: