Hormones

Overview of the Endocrine System

  • The endocrine system is made up of glands and organs that produce chemical regulators of body functions known as hormones.

  • Major Glands

    • Pituitary

    • Thyroid

    • Adrenal

    • Pineal

  • Major Organs

    • Pancreas

    • Ovaries

    • Testis

Homeostasis

  • Homeostasis is maintained through numerous feedback systems.

  • A feedback system involves a controlled condition in the body that is constantly monitored, evaluated, changed, remonitored, and reevaluated.

  • Any disruption that alters a controlled condition is referred to as a stimulus.

Components of a Feedback System

  • Receptor

    • A body structure that monitors changes in a controlled condition.

    • Sends input information (via nerve or chemical signals) to the control center.

  • Control Center

    • Typically the brain, responsible for setting the acceptable range of values for the controlled condition.

    • Evaluates input from receptors and generates output commands.

  • Effector

    • Receives output commands from the control center and produces a response that alters the controlled condition.

    • Almost any organ or tissue can function as an effector.

Nervous System Vs. Endocrine System

  • Neurons release neurotransmitters at synapses or neuromuscular junctions.

    • Effectors include other neurons, muscles, or glands.

  • Nervous System

    • Has a rapid response time.

    • Releases hormones into the bloodstream (general circulation).

    • Effectors include almost any type of body cell, leading to broad impacts on homeostasis and metabolism.

  • Endocrine System

    • Characterized by slower and longer-lasting responses because hormones remain in the blood for longer periods.

Hormones

  • Definition: Hormones are chemicals produced by endocrine glands or tissues.

  • Functions of Hormones:

    • Help maintain daily homeostasis.

    • Involved in growth and development.

    • Influence reproductive processes.

    • Participate in circadian (daily) rhythms.

  • Hormones are transported in the bloodstream.

  • Hormones act only on target cells, which have receptor sites specific for each hormone:

    • Target cells possess thousands of receptors specific to a hormone and may respond to more than one hormone.

Types of Hormones

  1. Water-soluble Hormones:

    • Comprised of amino acids (proteins).

    • Examples:

      • Epinephrine

      • Norepinephrine

      • Antidiuretic hormone (ADH)

      • Insulin

  2. Lipid-soluble Hormones:

    • Comprised of steroids (rings of cholesterol).

    • Examples:

      • Testosterone

      • Estrogen

      • Thyroid hormones T3 and T4.

Mechanism of Action for Water-soluble and Lipid-soluble Hormones

  • Water-soluble Hormones:

    1. The hormone is released from the cell.

    2. The hormone attaches to a receptor site located on the exterior of the cell membrane.

    3. The hormone-receptor complex initiates changes that trigger enzyme activation in the cytoplasm.

  • Lipid-soluble Hormones:

    1. The hormone diffuses from the cell and across the cell membrane into a target cell, binding to an internal receptor molecule.

    2. This hormone-receptor complex initiates gene expression leading to protein synthesis within the cell.

Mechanism of Insulin Action

  • Insulin is a hormone secreted by the pancreas when blood sugar is high.

  • After carbohydrate consumption, simple sugars are absorbed by the intestines and enter the bloodstream.

  • Blood sugar levels ideally remain between 70 to 130 mg of glucose per dL of blood.

  • If blood sugar is too high, insulin works to lower it by facilitating cellular glucose uptake.

  • In cases of high blood sugar, insulin promotes the following actions:

    • Glucose enters cells, decreasing blood sugar levels and restoring homeostasis.

Insulin's Functionality

  • Insulin is water-soluble (a protein) and must bind to a cell surface receptor to exert its effects.

  • Upon binding, the receptor changes shape, initiating a signal transduction pathway leading towards the nucleus of the target cell, where it influences DNA.

  • The effector molecule involved turns on genes that produce glucose transporters (GLUTs) allowing glucose entry into cells.

Effects of Glucagon

  • When blood sugar levels are too low, the pancreas releases glucagon.

  • Glucagon works oppositely to insulin, stimulating the release of glucose from cells into the bloodstream, thereby aiding in homeostasis.

Summary of Hormonal Actions for Maintaining Blood Sugar Levels

  • Insulin and glucagon function oppositely to regulate blood sugar, ensuring both high and low levels are managed efficiently in the body.

Diabetes Types

  1. Type 1 Diabetes:

    • Low insulin levels due to autoimmune destruction of pancreatic beta cells (up to 90% loss).

    • Common in individuals under age 20.

    • Symptoms include hyperglycemia and glucose spilling into urine.

    • Metabolism shifts to fatty acid use for ATP, leading to toxic waste buildup.

    • Treatment involves insulin replacement therapy.

  2. Type 2 Diabetes:

    • Normal insulin levels but target cells develop resistance; they do not respond to insulin effectively.

    • More common than Type 1 and usually occurs after age 35.

    • Management includes dietary control, exercise, weight loss, and potentially insulin-sensitizing drugs.

Control of Hormone Secretion

  • Hormones are released in short bursts regulated by:

    • Nervous system signals

    • Changes in blood substance concentrations (e.g., blood sugar affecting insulin)

    • Other hormones signaling hormone release (e.g., thyroid-stimulating hormone triggering thyroid hormones).

Structure and Function of Glands

  • Hypothalamus: regulates the pituitary gland, controlling various bodily functions.

  • Pituitary Gland: about the size of a small grape, it has anterior and posterior lobes.

    • Anterior pituitary hormones include:

    1. Human Growth Hormone (hGH): stimulates growth-related activities in the body.

    2. Thyroid Stimulating Hormone (TSH): stimulates thyroid hormone production from the thyroid gland.

    3. Follicle Stimulating Hormone (FSH): induces egg production in females and stimulates sperm production in males.

    4. Luteinizing Hormone (LH): assists in ovulation in females and testosterone production in males.

    5. Prolactin: initiates and sustains milk production in mammary glands.

Stress Response

  • The stress response is characterized by three stages:

    1. Alarm Phase

    • Mobilizes the body's resources quickly for immediate physical activity; involves the release of epinephrine and norepinephrine leading to heightened energy availability.

    1. Resistance Phase

    • Prolonged response compared to the alarm phase involving cortisol release to aid in energy production.

    1. Exhaustion Phase

    • Occurs when the body's resources are depleted, potentially leading to health deterioration, such as muscle wasting and immune suppression.

Adrenal Glands and Hormones

  • Adrenal Gland Structure

    • Each gland has two regions:

    • Adrenal Cortex:

      1. Mineralocorticoids (e.g., Aldosterone) regulate blood mineral ion homeostasis.

      2. Glucocorticoids (e.g., Cortisol) regulate glucose metabolism and stress responses.

      3. Androgens contribute to sex drive and secondary sexual characteristics.

    • Adrenal Medulla: produces epinephrine and norepinephrine, primarily during stress responses (fight-or-flight).

Summary

  • Hormones are critical regulators of homeostasis, influencing multiple body functions and allowing responses to changes in both the internal and external environments. Their interactions and feedback mechanisms ensure that bodily functions remain balanced and that stressors are managed effectively.