How do hormones coordinate slow, widespread and sustained responses across the body?
Describe how endocrine glands secrete hormones that regulate target cells, using thermoregulation and blood glucose regulation as examples
A focused answer to the WACE Year 12 Human Biology Unit 3 dot point on the endocrine system. How hormones reach target cells, steroid versus protein hormone action, and worked negative-feedback loops for blood glucose and thyroxine.
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What this dot point is asking
WACE wants you to understand the endocrine system as the body's chemical messenger network, and to contrast it with the faster nervous system. Endocrine glands are ductless, so they release their hormones directly into the bloodstream rather than down a tube. The blood carries each hormone everywhere, but only target cells respond, because only they carry the specific receptor that fits that hormone. This is the lock-and-key principle that explains how a hormone can travel everywhere yet act selectively.
Properties of endocrine control
Compared with nervous control, hormonal control is slow to start, because the hormone must travel in the blood; widespread, because the blood reaches the whole body; and long-lasting, because hormones persist until broken down. The nervous system, by contrast, is fast, targeted and brief. Many homeostatic systems use both: the nervous system for the rapid initial response and the endocrine system for the sustained adjustment.
Two types of hormone and how they act
You need the difference between protein (peptide and amino-acid-derived) hormones and steroid hormones, because their mechanisms differ.
Protein hormones, such as insulin and adrenaline, are water soluble and cannot cross the lipid cell membrane. They bind to receptors on the outside of the target cell membrane and trigger a second messenger inside the cell that activates existing enzymes. The effect is fast.
Steroid hormones, such as the sex hormones and cortisol, are lipid soluble and pass straight through the membrane. They bind to receptors inside the cell and act on the DNA to switch genes on or off, changing which proteins the cell makes. The effect is slower but longer lasting.
The pituitary and the hypothalamus
The hypothalamus links the nervous and endocrine systems. It controls the pituitary gland, often called the master gland, which releases hormones that control other glands. For thyroxine, the hypothalamus releases TRH, which makes the anterior pituitary release TSH, which makes the thyroid release thyroxine. Thyroxine raises metabolic rate and heat production, and rising thyroxine inhibits the hypothalamus and pituitary, a negative feedback loop that holds thyroxine steady.
Blood glucose regulation (the key WACE example)
Blood glucose is controlled by two antagonistic hormones from the islets of Langerhans in the pancreas.
When blood glucose rises after a meal, beta cells secrete insulin. Insulin makes liver and muscle cells take up glucose and convert it to glycogen, and makes cells use more glucose, so blood glucose falls.
When blood glucose falls, alpha cells secrete glucagon. Glucagon makes the liver break glycogen down into glucose and release it into the blood, so blood glucose rises.
How this maps to the exam
Expect to be given a stimulus and asked to write the hormonal negative-feedback loop, name the gland, the hormone, the target organ and the effect. Blood glucose and thyroxine are the most examined loops. You may also be asked to compare nervous and endocrine control, so keep the fast-versus-slow contrast ready.