The Endocrine System

Slow Control

The nervous system sends fast electrical signals down defined wires to specific targets. The endocrine system does the same job with chemistry: glands release hormones into the bloodstream, and those hormones act on any cell carrying the matching receptor.

That difference explains the timing. A nerve impulse acts in milliseconds and stops the moment the signal stops. A hormone takes seconds to days to have its effect and lingers afterward. The endocrine system handles growth, metabolism, reproduction, fluid balance and the long term stress response.

Most of it runs on negative feedback. A hormone's own effect signals the gland to stop producing it, which holds the level in a narrow range.

The Glands and What They Release

The adrenal glands sit one on top of each kidney, and the two parts are functionally separate. The medulla is effectively an extension of the sympathetic nervous system, which is why the hormonal stress response and the nerve stress response produce the same picture.

Blood Glucose Control

This is the endocrine material that matters most in emergency care, because altered mental status from blood sugar is common and correctable.

Glucose is the fuel every cell prefers and the only fuel the brain can use readily. The body works hard to hold it in a narrow range, and two pancreatic hormones do the work in opposite directions.

Insulin, from the beta cells of the pancreas, is released when blood glucose rises. It opens the door for glucose to move from blood into cells, which lowers blood glucose and gives the cells their fuel.

Glucagon, from the alpha cells, is released when blood glucose falls. It signals the liver to release stored glucose, which raises blood glucose.

A useful way to hold it: insulin puts sugar in the cells. Glucagon tells the liver to go get sugar.

Normal fasting glucose sits roughly between 70 and 110 mg/dL.

When Glucose Control Fails

Hypoglycemia is low blood glucose, and the brain notices immediately because it has no stored supply. Onset is fast, often over minutes. The patient is pale, cool and sweaty, with a rapid pulse, and may be confused, combative, seizing or unresponsive. Signs come from two sources at once: the brain running out of fuel, and the sympathetic response triggered by the falling level.

It develops when a patient takes insulin and then does not eat, exercises unexpectedly, or is fighting an illness. A patient on a long acting insulin or an oral diabetes medication can drop again hours after being corrected, which is why a rapid recovery is not the end of the problem.

Hyperglycemia is high blood glucose. Onset is slow, usually over hours to days. Without insulin the glucose cannot get into cells, so the cells starve in the middle of plenty and start burning fat instead. The byproducts are ketoacids.

The resulting picture in diabetic ketoacidosis is distinctive:

• Deep, rapid respirations, called Kussmaul respirations, because blowing off carbon dioxide is the fastest way the body can shed acid
• A sweet or fruity smell on the breath from the ketones
• Warm, dry, flushed skin
• Excessive thirst and excessive urination, because glucose spilling into the urine pulls water with it
• Dehydration that can be severe by the time anyone calls

Both conditions can produce an altered patient. The glucose meter is what tells them apart, and every altered patient earns one.

Type 1 diabetes is a failure to produce insulin, usually beginning in childhood, and those patients depend on injected insulin. Type 2 diabetes is resistance to the insulin that is present, usually developing in adulthood, and it is managed with diet, oral medication, and sometimes insulin.

Thyroid Function

The thyroid sets the body's metabolic rate, and its failures are best understood as the metabolism running too fast or too slow.

Hyperthyroidism is too much thyroid hormone. Fast heart rate, weight loss despite eating, heat intolerance, anxiety, tremor. The extreme form, thyroid storm, produces a very high heart rate, high fever and altered mental status.

Hypothyroidism is too little. Slow heart rate, weight gain, cold intolerance, fatigue, dry skin and slowed thinking. The extreme form produces low temperature, low blood pressure and a depressed level of consciousness.

Calcium is also a thyroid region function. Calcitonin from the thyroid lowers blood calcium, while parathyroid hormone from the parathyroid glands raises it by pulling calcium out of bone. The two hold blood calcium steady, which matters because calcium is required for both muscle contraction and clotting.

The Stress Response

Put a patient under physical stress and two endocrine responses run at once.

The fast one comes from the adrenal medulla, releasing epinephrine and norepinephrine on a sympathetic signal within seconds. Heart rate up, vessels constricted, bronchioles open, glucose released.

The slower one comes from the adrenal cortex, releasing cortisol over minutes to hours. Cortisol raises blood glucose, mobilizes fat and protein for fuel, and reduces inflammation.

Aldosterone, also from the cortex, tells the kidneys to hold onto sodium, and water follows sodium. That is one of the ways the body defends blood volume when it is falling, working alongside antidiuretic hormone from the pituitary, which tells the kidneys to hold onto water directly.

Those two hormones are why a patient losing volume produces less urine. The body is keeping every drop it can.

What to Take Away

Hormones travel in blood and act slowly, in contrast to the nervous system's fast wiring. Insulin lowers blood glucose by moving it into cells, glucagon raises it by pulling it out of the liver. Hypoglycemia comes on in minutes with pale sweaty skin, hyperglycemia over days with deep breathing, a fruity odor and dehydration. Check a glucose on every altered patient. The adrenal medulla produces the fight or flight hormones, and aldosterone and antidiuretic hormone are how the body holds onto volume.