Human Anatomy & Physiology Chapter 16 – The Endocrine System

General Characteristics What is the endocrine system? Endocrine glands produce hormones Hormones Chemical messengers Released into the blood and transported throughout the body Influence the activity of target tissues Figure 16.1

General Characteristics Endocrine glands vs. exocrine glands Hormone Chemistry Most hormones are amino acid based Amino acid derivatives Short chains of amino acids (peptides) Proteins Steroid Hormones (synthesized from cholesterol) Aldosterone and gonadal hormones

Effects of Hormones To bring about their effects, hormones alter cell activity They are like chemical “triggers” Hormones can have the following effects on a cell: Open or close membrane channels Stimulate the synthesis of new proteins Activate or inactivate enzymes Induce secretory activity Stimulate mitosis

Effects of Hormones Some of these mechanisms function through second messengers Cyclic AMP Others activate G-protein systems Affect ion channels Induce the synthesis of second messengers

Effects of Hormones Hormone chemistry determines how a hormone can communicate with its target cells Amino acid based hormones are water soluble and CANNOT move through the membrane Therefore they require membrane-bound receptors Examples from other text

Effects of Hormones G-protein systems - Figure 16.2

Effects of Hormones Awesome Animation Steroid hormones are lipid soluble and CAN move through the membrane Their receptors are located inside the cell Once bound to their receptor, the hormones stimulate transcription and the production of new proteins Figure 16.3 and from other text

Effects of Hormones Target cell specificity For a cell to be affected by a hormone, it must have a receptor for that hormone Examples Hormones are removed from the blood by the kidneys or liver Half-life (a few seconds to ~30 minutes) Onset of hormone effects varies Duration of effect ranges from seconds to several hours

Control of Hormone Release Control of most hormones is regulated by negative feedback What does that mean? There are a couple hormones regulated by positive feedback Figure from other text

Control of Hormone Release Stimulation of hormone release Humoral stimuli – changing levels of ions or nutrients in the blood Insulin and blood glucose PTH and calcium Neural stimuli – neurons stimulate the release of hormones Catecholamines (epinephrine) and the sympathetic nervous system Hormonal stimuli – hormones from one gland stimulate the release of other hormones TSH and the thyroid hormones Figure 16.4