Principles of Biology By Frank H. Osborne, Ph. D. 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 the table.

Organs that perform excretion Organ Product SkinWater, Salt LiverBile LungsCO 2, Water ColonUndigested food waste KidneysUrine*

Excretion * 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.

The Human Urinary System 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 for the right kidney.

Kidney

The Human Urinary System Regions of the Kidney 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).

The Human Urinary System Regions of the Kidney 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 pelvis. The bladder drains via 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 fine arteries called the glomerulus.

The Nephron 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 Nephron This difference in diameter causes a 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.

The Nephron After this material enters Bowman's capsule, it travels through the collecting tubule. The collecting tubule is in intimate contact with the capillaries of the bloodstream.

The Nephron 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 Nephron The waste materials, some water and excess salts remain in the collecting tubule and travel via the ureter to the bladder.

The Nephron 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. So the body stores it in the connective tissue, often behind the big toe. This makes it very difficult and painful to walk if not treated.

The End Principles of Biology Excretion