Biology 12

4.2 The Kidney

112 MHR • Unit 2 Homeostasis

Humans have two fist-sized kidneys, which are

found in the lower back on either side of the spine.

The kidneys release their waste product (urine)

into tubes called ureters, which carry the fluid to

the urinary bladder where it is temporarily stored

(see Figure 4.8). The bladder can hold a maximum

of about 600 mL of fluid. When there is about

250 mL of urine in the bladder, we become aware

of it, and at 500 mL we become very uncomfortable.

Drainage from the bladder is controlled by two

rings of muscles called sphincters. One sphincter

is involuntarily controlled by the brain. During

childhood we learn to voluntarily control the other.

Urine exits the bladder through a tube called the

urethra. In males, the urethra is approximately

20 cm long and merges with the vas deferens from

the testes to form a single urogenital tract. In females,

Figure 4.8The human urinary system

renal veins

kidney

vena cava

ureters

urinary

bladder

urethra

abdominal

aorta

renal arteries

the urethra is about 4 cm long and the reproductive

and urinary tracts have separate openings.

Kidney Functions and Structure

The kidney’s principal function is to filter the

blood in order to remove cellular waste products

from the body. The essential connection between

the kidney and the blood is illustrated by the fact

that at any given time, 20 percent of the body’s

blood is in the kidneys. Although most people have

two kidneys, the human body is capable of

functioning with only one. If one kidney ceases

to work or if a single kidney is transplanted into

a patient, the functioning kidney increases in size

to handle the increased workload.

Although the kidney has other important

functions, this organ is usually associated with the

excretion of cellular waste. The main metabolic

wastes are urea, uric acid, and creatinine, all of

which have nitrogen as a major component. Figure

4.9 shows the formula by which urea is produced

in the liver from the breakdown of excess amino

acids that are the building blocks of proteins. The

amine group (NH 2 ) is removed to release the rest

of the amino acid molecule, which can then be

converted to carbohydrates or fats. Although the

amine group can combine with a hydrogen ion to

form toxic ammonia, the ammonia is transformed

in the liver into the less toxic urea before being

released into the bloodstream.

Figure 4.9The liver combines ammonia with carbon

dioxide to form urea and water.

2 NH 3 CO 2 NH 2

ammonia carbon

dioxide

urea

H 2 NC

O

++H 2 O

EXPECTATIONS

Explain the role of the kidney in maintaining water and ion balance.

Design and carry out an experiment to investigate the physiological effects

experienced by people who consume coffee.

Describe issues and present informed opinions about problems related to

kidney functions and kidney transplants.

Describe the contribution made to knowledge and technology in the field

of homeostasis by Dr. Gordon Murray’s development of the kidney dialysis

machine.