HUMAN BIOLOGY

Too little insulin can lead to diabetes mellitus. Because

target cells can’t take up glucose from blood, glucose builds

up in the blood (mellitus means “honey” in Greek). The

kidneys move excess sugar into the urine, water is also lost,

and the body’s water–solute balance is upset. Affected peo-

ple become dehydrated and extremely thirsty. They also

lose weight as their glucose-starved cells break down pro-

tein and fats for energy. Fat breakdown releases ketones, so

these acids build up in the blood and urine. This can lead

to dangerously low blood pressure and a condition called

metabolic acidosis—a blood pH so low (acidic) that it

may harm functioning of the brain.

About one in ten diabetics has type 1 diabetes, in which

an autoimmune response destroys pancreas beta cells. It

may be caused by a viral infection in combination with

genetic susceptibility. Symptoms of type 1 diabetes usually

appear early in life, and affected people survive with insulin

injections or insulin provided by a pump (Figure 15.11).

Type 2 diabetes is a global health crisis

In type 2 diabetes, insulin levels are near or above nor-

mal, but for any of several reasons target cells can’t respond

properly to the hormone. The beta cells break down and

steadily produce less insulin. According to the World

Health Organization, in developed countries type 2 diabe-

tes has reached crisis proportions, along with its major risk

f a c t o r, o b e s it y.

Blood containing too much sugar damages capillaries.

Over time, the blood supply to the kidneys, eyes, and lower

limbs may be so poor that tissues die and terrible complica-

tions may develop (Table 15.5).

Diabetes also correlates strongly with cardiovascular

disease. Even diabetics in their 20s and 30s are at high risk

of suffering a stroke or heart attack.

“Prediabetes” is a warning sign

As many as 20 million Americans have “prediabetes”—

slightly elevated blood sugar that increases the risk of

developing type 2 diabetes. Metabolic syndrome, the

constellation of features described in Section 11.14, is an

early indicator that someone may be at risk for diabetes.

These features include a fasting glucose measurement of

110 mg/dL or higher.

Type 2 diabetes can be controlled by a combination of

proper diet, regular exercise, and sometimes drugs that

improve insulin secretion or activity. In obese people who

develop type 2 diabetes, the disease often disappears if the

person loses significant weight.

Low blood sugar threatens the brain

In hypoglycemia, so much sugar is removed from the

blood that cells in the brain and elsewhere may suddenly

have too little fuel to function properly. Anything that

raises the blood level of insulin, such as a miscalculated

insulin injection or an insulin-secreting tumor, can cause

hypoglycemia. The result can be life-threatening insulin

shock, in which the brain essentially “stalls” as its fuel

dwindles. A person experiencing insulin shock may feel

dizzy and confused and have trouble talking. Anything

that quickly raises blood sugar, including a shot of gluca-

gon, solves the problem.

Blood sugar disorders

Figure 15.11 insulin pumps continuously monitor blood

glucose levels and supply insulin as needed. The device is

programmed to deliver insulin through a tube inserted into

the skin. It helps smooth out fluctuations in blood sugar and

so reduces the risk of complications due to excessively low

or high blood sugar. (Left: © Elizabeth Musar; Right: © Manny Hernandez/

Diabetes Hands Foundation, http://www.tudiabetes.com)

15.10

Table 15.5 Some Complications of diabetes

Eyes Changes in lens shape and vision; damage

to blood vessels in retina; blindness

Skin Increased susceptibility to bacterial and

fungal infections; patches of discoloration;

thickening of skin on the back of hands

Digestive Gum disease; delayed stomach emptying

system that causes heartburn, nausea, vomiting

Kidneys Increased risk of kidney disease and failure

Heart and Increased risk of heart attack, stroke, high

blood vessels blood pressure, and atherosclerosis

Hands and Impaired sensations of pain; formation of

feet calluses, foot ulcers; possible amputation

of a foot or leg because of necrotic tissue

that formed owing to poor circulation

the endOCrIne system 299

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