Endocrine

Endocrine System

Objectives

1. List the functions of the endocrine system.
   • The endocrine system regulates metabolism, growth, and development, tissue function, sexual function, reproduction, sleep, and mood, among other things.
2. Discuss the role and function of hormones in the body, including:
   • Define hormone.
     • Hormones are chemical messengers produced by glands in the endocrine system that travel through the bloodstream to target organs, initiating various physiological responses.
   • Explain the process by which hormones bind to the receptor sites of specific tissues (targets).
     • Hormones bind to specific receptors on target tissues using a lock-and-key mechanism, ensuring that only cells with the correct receptors respond to the hormone.
   • Explain the three mechanisms that control the secretion of hormones.
     • Secretion is governed by feedback control loops, biorhythms, and the central nervous system's regulation. Each of these mechanisms coordinates the release of hormones to maintain homeostasis.
3. Discuss the pituitary gland, including:
   • Describe the relationship of the hypothalamus to the pituitary gland.
     • The hypothalamus controls the pituitary gland through releasing and inhibiting hormones that regulate its functions.
   • Describe the location, regulation, and hormones of the pituitary gland.
     • The pituitary gland is located at the base of the brain and is divided into an anterior lobe (adenohypophysis) and a posterior lobe (neurohypophysis). It produces various hormones that control other endocrine glands and various bodily functions, such as growth, reproduction, and stress response.
4. Identify the other major endocrine glands and their hormones, and explain the effects of hyposecretion and hypersecretion.
   • Major endocrine glands include the thyroid gland (produces thyroxine), adrenal glands (produce cortisol, epinephrine), pancreas (produces insulin), and gonads (testosterone, estrogen).
   • Hyposecretion can lead to disorders such as diabetes and hypothyroidism, whereas hypersecretion can result in conditions like Cushing's syndrome or hyperthyroidism.

Key Terms

• adenohypophysis
• adrenal glands
• biorhythms
• catecholamines
• endocrine glands
• hormone
• hypothalamus
• lock-and-key mechanism
• negative feedback control
• neurohypophysis
• organ-specific hormones
• pancreas
• parathyroid glands
• pineal gland
• positive feedback control
• receptor
• second messenger
• steroids
• thymus gland
• thyroid gland
• tropic hormones

Introduction to Endocrine System

The nervous system and the endocrine system are the two chief communicating and coordinating systems in the body.

• They regulate almost all organ systems.
• The nervous system communicates through electrical signals called nerve impulses, achieving rapid, short-term effects.
• The endocrine system communicates through chemical signals called hormones, responding more slowly and exerting longer-lasting effects.
• The hormones regulate metabolic processes involving carbohydrates, proteins, and fats.
• Hormones play important roles in growth and reproduction, water and electrolyte balance, and responses to hunger, thirst, temperature changes, infection, trauma, and stress.
• The study of the endocrine system is called endocrinology.

Endocrine Glands

The endocrine system is composed of endocrine glands widely distributed throughout the body.

• Endocrine glands secrete hormones directly into the bloodstream instead of through ducts, which allows for systemic effects on various target tissues and organs.
• For instance, the pancreas secretes insulin, which regulates blood glucose levels throughout the body.
• Many hormones are also secreted by other organs, such as the stomach (gastrin), kidney (erythropoietin), and heart (atrial natriuretic peptide).
• Unlike endocrine glands, exocrine glands (e.g., sweat glands) use ducts to transport their secretions to the body’s surface.

Classification of Hormones

Hormones are classified as either protein-based (peptides and amino-acid derivatives) or steroid-based, with specific origins in various glands.

• Most hormones, except for those from the adrenal cortex and sex glands, are protein-based.
• The adrenal cortex and sex glands secrete steroid hormones, which are lipid-soluble and can pass through cellular membranes to exert hormonal effects.

Targets

1. Each hormone targets specific tissues or organs.
   • Nearby target tissues or those located distantly from the secreting gland may respond.
   • For example, insulin targets liver, muscle, and fat cells.
2. Hormones can influence multiple target organs:
   • Thyroid hormone and insulin have wide-reaching effects.
   • Parathyroid hormone (PTH) has specific targets, primarily affecting bone and kidney cells.

Hormone Receptors

1. Hormones attach to specific receptors on target cells.
   • Receptors can be either membrane-bound, responding to water-soluble hormones, or intracellular, responding to lipid-soluble hormones.
2. Lock-and-Key Mechanism:
   • Hormones must fit their receptor sites precisely for effective signaling to occur. (e.g., insulin only influences cells with insulin receptors.)
3. Types of Receptors:
   • Membrane Receptors: Associated with protein hormones and utilize second messengers (e.g., cAMP) to elicit cellular responses.
   • Intracellular Receptors: Associated with steroid hormones, affecting gene expression and cellular function.

Control of Hormone Secretion

1. Feedback Control Loops:
   • Maintain homeostasis. Active regulation is critical to preventing excess or insufficient hormone levels.
   • Example: ACTH regulates cortisol secretion through feedback loops.
2. Biorhythms:
   • Natural cycles that influence hormone levels, such as the diurnal cycle affecting cortisol.
3. Central Nervous System Control:
   • Stress responses involve direct central nervous system inputs to endocrine glands, such as the adrenal glands