CONCEPTUAL LIFE SCIENCE

EXCRETION

Excretion means eliminating or getting rid of wastes. The body produces a variety of different wastes and has a variety of means of excretion. These are summarized in Table XII-1.

Table XII-1. Organs that perform excretion.

Organ / Product
Skin / Water, Salt
Liver / Bile
Lungs / CO2, Water
Colon /

Undigested Food Waste

Kidneys / Urine*

*Urine contains the waste products creatinine, urea and uric acid. Creatinine and urea come from protein metabolism while uric acid comes from nucleic acid metabolism.

THE HUMAN URINARY SYSTEM

The human urinary system consists of two kidneys. They are located in the rear

of the abdominal cavity up against the inner wall of the rear of the cavity. Each kidney has a blood supply. The left renal artery supplies the left kidney (renal is an adjective referring to the kidney) and the left renal vein returns blood from the left kidney to the inferior vena cava. Similarly, the right renal artery supplies the right kidney and the right renal vein returns blood from the right kidney to the inferior vena cava.

The kidney has two portions. The outer portion is known as the cortex (cortex means outer portion) and the inner portion is called the medulla (medulla means inner portion). On the inner curve of the kidney is a space called the renal pelvis. Urine collects in the renal pelvis and exits the kidney via the ureter. The ureter from each kidney drains into the urinary bladder located in the pelvic region of the human. The bladder drains via a tube called the urethra.

The nephron

The nephron is the functional unit of the kidney. There are about 1 million nephrons in each kidney. The nephron has a long collecting tube that terminates in a hollow, cup-like structure called Bowman’s capsule. Inside Bowman’s capsule is a network of very tiny arteries called the glomerulus.

Filtration of the blood occurs in the glomerulus. The key is that the diameter of the arteriole entering the glomerulus is larger than the diameter of the arteriole leaving the glomerulus. An arteriole is a small artery. The difference in diameter causes a

12-1

12-2

pressure in the glomerulus that forces water, salt, nitrogenous wastes (creatinine, urea, uric acid), sugars, amino acids, ions and other materials out of the bloodstream and into Bowman’s capsule. After this material enters Bowman’s capsule, it travels through the collecting tubule. The collecting tubule is in intimate contact with capillaries of the bloodstream. The materials that the body wants to keep such as glucose, amino acids and other beneficial molecules are reabsorbed by the collecting tubule cells and sent back into the bloodstream. The waste materials, some water and excess salts remain in the collecting tubule and travel via the ureter to the bladder.

Sometimes materials are not excreted properly. Gout is a disease characterized by the retention of uric acid in the blood. The kidney does not excrete it properly. In patients with gout, the body has to put the material someplace so it puts it in the connective tissue, often just behind the big toe. This makes it very difficult and painful to walk if not properly treated.