Endocrine System & Blood: Study Guide for Human Anatomy and Physiology II

Endocrine System

Functions of the Endocrine System

The endocrine system is a network of glands that secrete hormones to regulate various physiological processes throughout the body.

• Regulation of metabolism: Controls the rate of chemical reactions in cells.

• Growth and development: Influences tissue growth and differentiation.

• Homeostasis: Maintains internal balance of water, electrolytes, and nutrients.

• Reproduction: Regulates sexual development and reproductive cycles.

• Response to stress and injury: Coordinates the body's adaptation to stressors.

Hormones: Definition and Types

• Hormone: A chemical messenger secreted by endocrine glands into the bloodstream, affecting distant target cells.

• Protein-based hormones: Composed of amino acids (e.g., insulin, growth hormone). Usually water-soluble and bind to cell surface receptors.

• Steroid-based hormones: Derived from cholesterol (e.g., cortisol, estrogen). Lipid-soluble and typically bind to intracellular receptors.

• Key difference: Protein hormones act via second messengers; steroid hormones directly influence gene expression.

Major Endocrine Glands and Their Secretions

• Hypothalamus: Produces regulatory hormones (releasing and inhibiting hormones) that control the pituitary gland.

• Anterior Pituitary (Adenohypophysis): Secretes TSH, ACTH, LH, FSH, PRL, GH.

• Posterior Pituitary (Neurohypophysis): Releases ADH and oxytocin (produced in hypothalamus).

• Thyroid Gland: Produces thyroxine (T4), triiodothyronine (T3), and calcitonin.

• Parathyroid Glands: Secrete parathyroid hormone (PTH).

• Adrenal Cortex: Produces aldosterone, cortisol.

• Adrenal Medulla: Secretes epinephrine (EPI) and norepinephrine (NE).

• Pineal Gland: Produces melatonin.

• Thymus: Secretes thymosin.

• Pancreas (Islets of Langerhans): Produces insulin and glucagon.

• Ovaries: Secrete estrogen and progesterone.

• Testes: Produce testosterone.

Hypothalamus and Pituitary: Anatomical and Functional Relationships

• Infundibulum: The stalk connecting the hypothalamus to the pituitary gland.

• Hypophyseal portal system: A network of blood vessels that carries hormones from the hypothalamus directly to the anterior pituitary.

• Adenohypophysis (anterior pituitary): Glandular tissue; produces and releases hormones in response to hypothalamic signals.

• Neurohypophysis (posterior pituitary): Neural tissue; stores and releases hormones made by the hypothalamus.

• Major difference: Anterior pituitary synthesizes its own hormones; posterior pituitary only releases hormones made in the hypothalamus.

Hormones: Sites of Production, Targets, and Functions

Hormone	Site of Production	Target	Function
TSH	Anterior pituitary	Thyroid gland	Stimulates thyroid hormone release
ACTH	Anterior pituitary	Adrenal cortex	Stimulates cortisol production
LH	Anterior pituitary	Gonads	Stimulates sex hormone production
FSH	Anterior pituitary	Gonads	Stimulates gamete production
PRL	Anterior pituitary	Mammary glands	Promotes milk production
GH	Anterior pituitary	Most tissues	Stimulates growth
ADH	Posterior pituitary	Kidneys	Promotes water reabsorption
Oxytocin	Posterior pituitary	Uterus, mammary glands	Stimulates contractions, milk ejection
T3, T4	Thyroid gland	Most cells	Regulate metabolism
Calcitonin	Thyroid gland	Bones	Lowers blood calcium
PTH	Parathyroid glands	Bones, kidneys	Raises blood calcium
Aldosterone	Adrenal cortex	Kidneys	Increases sodium reabsorption
Cortisol	Adrenal cortex	Most tissues	Stress response, increases glucose
Epinephrine/NE	Adrenal medulla	Many tissues	Fight-or-flight response
Insulin	Pancreas	Most cells	Lowers blood glucose
Glucagon	Pancreas	Liver	Raises blood glucose
Estrogen/Progesterone	Ovaries	Reproductive organs	Regulate female cycle
Testosterone	Testes	Reproductive organs	Regulates male traits
Melatonin	Pineal gland	Brain	Regulates sleep cycles
Thymosin	Thymus	Immune cells	Promotes T cell maturation

Organs with Secondary Endocrine Functions

• Kidneys: Produce calcitriol (vitamin D activation), erythropoietin (EPO, stimulates RBC production), and renin (regulates blood pressure).

• Heart: Secretes atrial natriuretic peptide (ANP), which lowers blood pressure.

• Adipose tissue: Produces leptin (regulates appetite) and resistin (affects insulin sensitivity).

• Thymus: Secretes thymosin (immune function).

Hormone Interactions

• Antagonistic: Opposing effects (e.g., insulin vs. glucagon).

• Synergistic: Additive effects (e.g., GH and glucocorticoids on glucose metabolism).

• Permissive: One hormone enables another to act (e.g., thyroid hormones increase the effect of epinephrine).

• Integrative: Different but complementary effects (e.g., calcitriol and PTH on calcium balance).

Stress Response: Three Phases

• Alarm phase: Immediate, fight-or-flight response (adrenal medulla releases EPI/NE).

• Resistance phase: Long-term adaptation (adrenal cortex releases cortisol).

• Exhaustion phase: Body's resources depleted, leading to decreased function and possible organ failure.

Blood

Functions of Blood

• Transport: Carries oxygen, nutrients, hormones, and wastes.

• Regulation: Maintains pH, temperature, and fluid balance.

• Protection: Defends against pathogens and prevents blood loss.

Components of Blood and Their Functions

Component	Function
Red blood cells (erythrocytes)	Transport oxygen and carbon dioxide
Hemoglobin	Protein in RBCs that binds oxygen (heme = iron-containing; globin = protein chains)
White blood cells (leukocytes)	Defend against infection
Neutrophils	Phagocytize bacteria
Eosinophils	Combat parasites, modulate allergies
Basophils	Release histamine, mediate inflammation
Monocytes	Become macrophages, phagocytize pathogens
Lymphocytes	Produce antibodies, regulate immune response
Platelets (thrombocytes)	Initiate blood clotting
Plasma	Liquid matrix; transports substances
Water	Main solvent in plasma
Plasma proteins	Maintain osmotic pressure, transport, clotting (albumin, globulin, fibrinogen)
Nutrients, wastes	Transported to and from cells

Hemostasis: Sequence of Events

1. Vascular spasm: Blood vessel constricts to reduce blood flow.

2. Platelet plug formation: Platelets adhere to exposed collagen and aggregate.

3. Coagulation: Fibrin mesh forms, stabilizing the clot. Requires calcium, vitamin K, and clotting factors.

Hemopoiesis (Blood Cell Formation)

• Occurs in red bone marrow (in adults: flat bones, proximal epiphyses of long bones).

• Stem cells differentiate into all blood cell types.

• Sequence: Hemocytoblast → myeloid/lymphoid stem cells → mature blood cells (erythrocytes, leukocytes, platelets).

Blood Typing: ABO and Rh Systems

• ABO system: Based on presence of A and/or B antigens on RBCs.

• Rh system: Presence (+) or absence (−) of Rh antigen (D antigen).

• Transfusion reactions occur if incompatible blood is given.

Laboratory Tests

• Hematocrit (VPRC or PCV): Percentage of RBCs in blood; assesses oxygen-carrying capacity.

• Other tests: White blood cell count, platelet count, hemoglobin concentration.

Movement of White Blood Cells

• Diapedesis: WBCs move out of blood vessels into tissues.

• Chemotaxis: WBCs migrate toward chemical signals at infection sites.

• Amoeboid movement: WBCs change shape to move through tissues.

Additional info: Students should also be familiar with diseases/disorders associated with each system, as indicated in the study guide.