36 Urinary Structures and Functions

Much of the maintenance of proper chemical balance, or homeostasis, is the function of the kidneys, which are located towards the lower back. The process begins with waste carrying blood entering each of the two kidneys through the renal artery. Urine is produced by the nephrons in the kidneys. Once filtered, the blood exits through the renal vein. Urine leaves each kidney through a ureter. Each ureter transports the urine via peristalsis to the urinary bladder (a hollow, muscular chamber that collects and stores urine). A single urethra transports the urine from the urinary bladder to the outside of the body. This process through the smooth muscles is otherwise known as: peristalsis. An internal urethral sphincter muscle and an external urethral sphincter muscle help keep the urine in the bladder until the process of urination. The internal urethral sphincter muscle, surrounds the neck of the urinary bladder at the juncture of the bladder with the urethra. It opens reflexively when the bladder muscle contracts and builds up pressure. The external urethral sphincter muscle is part of the urogenital diaphragm and is located at the external opening of the urethra. It is under voluntary control, and as a result of the voluntary control, urination can be delayed for a time. Micturition is the entire process of urination.

The process of urination is known as micturition which begins with blood carrying various wastes that enter each of the two kidneys through the renal artery and ends when urine exits out of the body through the urethra.

Anatomy of the Kidney

There are six organs in the urinary system: two kidneys, two ureters, the urinary bladder, and the urethra. The kidneys are the body’s main purification system. They remove wastes, some of which are toxic, from the blood. The kidneys also help to regulate blood composition and volume. By manipulating blood volume, the kidneys contribute to the regulation of blood pressure. The kidneys are about the size of your fist and are shaped like beans. The nephron is the structural and functional unit of the kidney. The average number of nephrons in each kidney is about one million, although this number can vary from about half a million to two million. Nephrons are so good at what they do that it only takes about one fourth of them to be functional to meet the needs of the body. Still, a low number of functioning nephrons has been linked to a greater risk of developing kidney disease and high blood pressure (hypertension).

Kidney Location

The kidneys lie between the parietal peritoneum and the posterior abdominal wall, just above waist level. The location posterior to the parietal peritoneum means they are retroperitoneal. These reddish, paired organs are offered some protection by the eleventh and twelfth rib pairs. The right kidney lies slightly lower than the left kidney. This asymmetry occurs due to the increases space required by the liver on the right side of the abdominal cavity superior to the kidney.