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General Biology: Endocrine System, Blood, and Related Physiology Study Notes

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Endocrine System vs. Nervous System

Overview and Comparison

The endocrine and nervous systems are the two main regulatory systems in the human body, coordinating responses to internal and external stimuli.

  • Endocrine System: Uses chemical messengers called hormones, which are released into the bloodstream and travel throughout the body. Responses are generally slower (seconds to days) but can last longer.

  • Nervous System: Uses electrical signals to regulate the activity of muscles and glands. Responses are rapid and short-lived.

Exocrine vs. Endocrine Glands

Definitions and Differences

  • Exocrine Glands: Non-hormonal glands that use ducts to transport substances to surfaces (e.g., sweat, saliva).

  • Endocrine Glands: Ductless glands that release hormones directly into surrounding tissues or the bloodstream.

Endocrine Glands vs. Endocrine Organs

  • Endocrine Glands: Specialized for hormone production and release (e.g., pituitary, thyroid, adrenal, pineal, thymus).

  • Endocrine Organs: Organs with additional functions that also secrete hormones (e.g., pancreas, hypothalamus).

Hormones: Definition and Types

What is a Hormone?

A hormone is a chemical substance secreted by cells into the extracellular fluids that regulate the metabolic function of other cells in the body.

Types of Hormones

  • Amino Acid-Based Hormones: Water-soluble (except thyroid hormones), cannot cross the cell membrane easily, act via cell surface receptors and second messengers. Examples: amines, peptides, proteins.

  • Steroid-Based Hormones: Lipid-soluble, derived from cholesterol, can cross cell membranes and act on intracellular receptors. Examples: gonadal and adrenocortical hormones.

Hormone Actions

  • Act only on target cells with specific receptors.

  • May alter membrane permeability, stimulate synthesis of proteins/enzymes, activate/deactivate enzymes, induce secretory activity, or stimulate mitosis.

Water-Soluble vs. Lipid-Soluble Hormones

  • Water-Soluble: Bind to cell surface receptors, use second messengers.

  • Lipid-Soluble: Cross cell membrane, bind to intracellular receptors, directly affect gene expression.

Hormone Secretion Regulation

  • Stimuli can be hormonal, humoral (blood-borne), or neural.

  • Feedback mechanisms (mainly negative feedback) regulate hormone levels.

Major Endocrine Glands and Hormones

Pituitary Gland

  • Location: Sella turcica of the sphenoid bone.

  • Anterior Pituitary: Secretes hormones regulated by the hypothalamus.

  • Posterior Pituitary: Stores and releases hormones made by the hypothalamus.

Hormone

Function/Target

Growth Hormone (GH)

Stimulates growth, metabolism of fat, spares glucose; targets liver, muscle, bone, cartilage, other tissues.

Thyroid-Stimulating Hormone (TSH)

Stimulates thyroid gland to release thyroid hormones.

Adrenocorticotropic Hormone (ACTH)

Stimulates adrenal cortex to release glucocorticoids.

Follicle-Stimulating Hormone (FSH)

Stimulates ovarian follicle maturation and estrogen production; stimulates sperm production.

Luteinizing Hormone (LH)

Triggers ovulation, stimulates estrogen/progesterone/testosterone production.

Prolactin (PRL)

Promotes lactation.

Oxytocin

Stimulates uterine contractions, milk ejection.

Antidiuretic Hormone (ADH)

Stimulates kidney to reabsorb water.

Thyroid Gland

  • Location: Anterior neck, below larynx.

  • Hormones: Thyroid hormones (T3, T4) regulate metabolism, growth, and development.

Parathyroid Glands

  • Location: Posterior aspect of thyroid gland.

  • Hormone: Parathyroid Hormone (PTH) regulates calcium balance in blood.

Adrenal Glands

  • Location: On top of kidneys.

  • Adrenal Medulla: Secretes epinephrine and norepinephrine (fight-or-flight response).

  • Adrenal Cortex: Produces corticosteroids (mineralocorticoids, glucocorticoids, androgens).

Pineal Gland

  • Location: Roof of third ventricle in diencephalon.

  • Hormone: Melatonin regulates sleep-wake cycles.

Pancreas

  • Location: Behind stomach.

  • Endocrine Function: Islets of Langerhans secrete insulin (lowers blood glucose) and glucagon (raises blood glucose).

  • Exocrine Function: Acinar cells produce digestive enzymes.

Gonads

  • Ovaries: Produce estrogen and progesterone (female secondary sex characteristics, menstrual cycle, pregnancy).

  • Testes: Produce testosterone (male secondary sex characteristics, reproductive function).

Blood: Composition and Functions

Blood Components

  • Plasma: 55% of blood; mostly water, proteins, nutrients, hormones, waste.

  • Formed Elements: 45% of blood; red blood cells (RBCs), white blood cells (WBCs), platelets.

Physical Characteristics of Blood

  • Sticky, opaque, scarlet-red.

  • pH: 7.35–7.45; temperature: 38°C (100.4°F).

  • Volume: 4–6 L in adults.

Functions of Blood

  • Distribution: Transports oxygen, nutrients, hormones, and waste.

  • Regulation: Maintains body temperature, pH, and fluid volume.

  • Protection: Prevents blood loss (clotting) and infection (immune response).

Red Blood Cells (Erythrocytes) and Hemoglobin

  • Function: Transport oxygen and carbon dioxide.

  • Hemoglobin: Protein with iron that binds oxygen; each RBC contains ~250 million hemoglobin molecules.

  • Equation:

Blood Cell Formation (Hematopoiesis)

  • Occurs in red bone marrow from hematopoietic stem cells.

  • Erythropoiesis: Formation of RBCs, stimulated by erythropoietin (EPO).

  • Leukopoiesis: Formation of WBCs.

  • Thrombopoiesis: Formation of platelets.

Blood Disorders

  • Anemia: Reduced oxygen-carrying capacity of blood (e.g., hemorrhagic, iron-deficiency, pernicious, sickle-cell).

  • Polycythemia: Excess RBCs, increased blood viscosity.

  • Leukemia: Cancer of WBCs, abnormal proliferation.

  • Leukopenia: Low WBC count.

Platelets and Hemostasis

  • Platelets: Cell fragments involved in clotting.

  • Hemostasis: Process to stop bleeding, involves vascular spasm, platelet plug formation, and coagulation.

Summary Table: Major Endocrine Glands, Hormones, and Functions

Gland/Organ

Hormone(s)

Main Function(s)

Pituitary (anterior)

GH, TSH, ACTH, FSH, LH, PRL

Growth, metabolism, stress response, reproduction, lactation

Pituitary (posterior)

Oxytocin, ADH

Uterine contraction, milk ejection, water balance

Thyroid

T3, T4, Calcitonin

Metabolism, calcium regulation

Parathyroid

PTH

Calcium regulation

Adrenal cortex

Aldosterone, Cortisol, Androgens

Electrolyte balance, stress response, sex hormones

Adrenal medulla

Epinephrine, Norepinephrine

Fight-or-flight response

Pineal

Melatonin

Sleep-wake cycles

Pancreas

Insulin, Glucagon

Blood glucose regulation

Ovaries

Estrogen, Progesterone

Female sex characteristics, menstrual cycle

Testes

Testosterone

Male sex characteristics, sperm production

Additional info:

  • Some explanations and examples were expanded for clarity and completeness.

  • Tables were synthesized from the text and standard academic knowledge.

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