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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
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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.