Lecture 3 ExPhys

The Endocrine System

  • The endocrine system is a complex network involved in regulating various bodily functions through hormones.

Key Components of the Endocrine System

  • Hormones: Chemical messengers released into the bloodstream to regulate physiological processes.

  • **Glands and Tissues:
      - Hypothalamus
      - Pituitary Gland
      - Thyroid Gland
      - Parathyroid Glands
      - Thymus Gland
      - Adrenal Glands
      - Pancreas
      - Kidneys
      - Adipose Tissue
      - Ovaries (in females)
      - Testes (in males)

Role of the Endocrine System in Exercise

  • The endocrine system plays a major role in helping the body adapt to stress during exercise.

  • It is defined as all tissues or glands that secrete hormones.

  • Hormones travel through the blood to target organs after secretion.

Hormones Secreted from the Brain

Pituitary Gland

  • Location: Brain

  • Function: Hormone production regulated by hypothalamic neurons affecting both anterior and posterior lobes.

  • Blood Supply:
      - Supplied by the Superior Hypophyseal Artery (Anterior Lobe)
      - Supplied by the Inferior Hypophyseal Artery (Posterior Lobe)
      - Drains through Hypophyseal Veins.

Growth Hormone (GH)

  • Secreted From: Anterior Pituitary Gland

  • Target Organ: All cells in the body.

  • Major Functions:
      - Promotes development and enlargement of all body tissues.   - Increases protein synthesis and fatty acid (FFA) metabolism while decreasing carbohydrate (CHO) usage.

  • Used in Sports:
      - Increases muscle mass, improves energy, and enhances exercise capacity.

Antidiuretic Hormone (ADH) / Vasopressin

  • Secreted From: Posterior Pituitary

  • Target Organ: Kidneys.

  • Function: Controls water excretion from the urinary system and increases blood pressure.

  • Application in Exercise: Aids in hydration and maintaining blood pressure.

Thyroid Hormones

T3 (Triiodothyronine) and T4 (Thyroxine)

  • Secreted From: Thyroid

  • Target Organs: All cells in the body.

  • Functions:
      - Increase the rate of cellular metabolism.   - Increase heart rate and cardiac contractility.

  • Relevance in Exercise: Related to disorders like hypothyroidism and hyperthyroidism.

Hormones of the Kidneys

Adrenal Glands:

  • Location: On top of the kidneys (medulla and cortex)

Epinephrine

  • Secreted From: Adrenal Medulla.

  • Target Organ: Most cells in the body.

  • Functions:
      - Increases heart rate and contractility.   - Increases skeletal muscle blood flow, stimulates lipolysis, and breakdown of glycogen.

  • Exercise Impact: Release is significant even in anticipation of exercising.

  • Question: Which types of exercise might not benefit from an epinephrine rush?

Norepinephrine

  • Secreted From: Adrenal Medulla.

  • Target Organ: Most cells in the body.

  • Functions:
      - Increases blood pressure and stimulates lipolysis.

  • Exercise Relation: Functions similarly to epinephrine.

Aldosterone

  • Secreted From: Adrenal Cortex.

  • Target Organs: Kidneys.

  • Functions:
      - Increases sodium retention.

  • Importance in Exercise: Relevant for hydration, sweating, and muscle contraction.

Cortisol

  • Secreted From: Adrenal Cortex.

  • Target Organ: Most cells in the body.

  • Functions:
      - Controls metabolism of carbohydrates, fats, and proteins.

  • Exercise Impact: Enhances metabolic performance.

  • Question: Can excessive cortisol production have negative implications?

Hormones of the Pancreas

Insulin

  • Secreted From: Pancreas.

  • Target Organs: All body cells, especially muscle tissue.

  • Functions:
      - Regulates blood glucose levels and enhances glucose uptake by cells.

  • Relevance in Exercise: Important in metabolism; misuse found in some athletes exploiting anabolic steroids with insulin to prevent muscle breakdown.

Glucagon

  • Secreted From: Pancreas.

  • Target Organs: All body cells, particularly fat cells and muscle.

  • Functions:
      - Increases blood glucose by promoting breakdown of fat, protein, and glycogen.

Gonadal Hormones

Testosterone

  • Secreted From: Testes.

  • Target Organ: Sex organs and muscles.

  • Functions:
      - Development of male characteristics and muscle growth.

  • Exercise Relation: Muscle development, anabolic steroids, testosterone replacement therapy (TRT).

Estrogen

  • Secreted From: Ovaries.

  • Target Organs: Sex organs, muscle tissue, and adipose tissue.

  • Functions:
      - Development of female characteristics, muscle growth/strength, regulation of the menstrual cycle, and increases in fat storage.

  • Exercise Impact: Influences metabolism in females, potential decrease in tendon/ligament elasticity leading to injuries; issues related to hormonal birth control use.

Regulation of Carbohydrate Metabolism During Exercise

  • Methods of Increasing Blood Glucose:
      1. Remaining glucose in the bloodstream post-consumption.
      2. Glycogen breakdown from liver and muscles (glycogenolysis).   - Glucose can also be derived from lactic acid, amino acids, and glycerol (gluconeogenesis).

  • Balance: Plasma glucose concentration depends on glucose uptake by exercising muscles versus release by the liver.

  • Key Hormones:   - Glucagon   - Epinephrine   - Norepinephrine   - Cortisol

Regulation at Rest and During Exercise

  • At Rest: Glucose release from the liver is promoted by glucagon, supporting liver glycogen breakdown and glucose formation from amino acids.

  • During Exercise:
      - Increased glucagon and catecholamines (epinephrine & norepinephrine).   - These hormones collectively enhance glycogenolysis.   - Roles of cortisol: Stimulates protein breakdown and supports gluconeogenesis by freeing amino acids.   - Exercise intensity impacts catecholamine release and the balance of blood glucose will rise until glycogen stores are depleted.

Glucose Uptake by Muscles

  • Muscles do not directly absorb glucose from blood. Insulin regulates the transport of glucose from the bloodstream into muscle cells.

Regulation of Fat Metabolism During Exercise

  • Post depletion of carbohydrate stores, the body utilizes fats for fuel, necessitating the detachment of fatty acids (FFAs) from triglycerides.

  • Lipolysis is modulated by:
      - Epinephrine   - Norepinephrine   - Cortisol   - Growth hormone   - Low levels of insulin.

  • Exercise Effect: General exercise increases lipolysis and mobilization of FFAs. Increased FFAs in blood promotes uptake by tissues.

  • Insulin levels need to decrease during prolonged exercise to lower glucose uptake.

Adipose Tissue as an Endocrine Organ

  • Adipose tissue consists of adipocytes which release various hormones affecting inflammation and appetite.

  • Key Hormone: Leptin, secreted by fat cells, decreases hunger by acting on hypothalamic receptors.