Endocrine System and Blood: Structured Study Guide for Anatomy & Physiology

Endocrine System (Chapter 16)

Main Endocrine Glands and Their Functions

The endocrine system consists of glands that secrete hormones directly into the bloodstream to regulate various physiological processes. Understanding the anatomy, histology, and function of each gland is essential for grasping their roles in homeostasis.

• Pituitary Gland: Has two major lobes: anterior (adenohypophysis) and posterior (neurohypophysis). The anterior lobe develops from the mouth (Rathke's pouch), while the posterior lobe develops from the hypothalamus. The anterior pituitary produces six major hormones (e.g., GH, TSH, ACTH, FSH, LH, PRL), while the posterior pituitary releases ADH and oxytocin.

• Thyroid Gland: Located in the neck, organized into follicles. Produces two major hormones: thyroxine (T4) and triiodothyronine (T3). T3 is more biologically active; T4 is produced in greater abundance. Hormone synthesis requires iodine and thyroglobulin. Regulation involves the hypothalamic-pituitary-thyroid axis.

• Parathyroid Glands: Usually four glands located on the posterior surface of the thyroid. Produce parathyroid hormone (PTH), which regulates calcium homeostasis via negative feedback.

• Adrenal Glands: Also called suprarenal glands, located atop the kidneys. Consist of cortex (three layers: zona glomerulosa, fasciculata, reticularis) and medulla. Cortex produces mineralocorticoids, glucocorticoids, and androgens; medulla produces catecholamines (epinephrine, norepinephrine).

• Pineal Gland: Located in the brain, produces melatonin which regulates circadian rhythms.

• Pancreas: Both exocrine and endocrine functions. Endocrine cells include alpha (glucagon) and beta (insulin) cells. Dysfunction leads to diabetes mellitus.

• Gonads: Ovaries and testes produce sex hormones (estrogen, progesterone, testosterone) and are regulated by pituitary gonadotropins.

• Placenta: Produces hormones such as hCG, which is detected in pregnancy tests.

Other Glandular Secretions: Additional hormones are described in Table 16.5, including their sources and targets.

Types of Chemical Signaling

• Autocrine: Signals act on the same cell that secreted them.

• Endocrine: Hormones travel through the bloodstream to distant targets.

• Paracrine: Signals act on nearby cells.

Chemical Classes of Hormones

• Amino acid-based hormones (e.g., peptides, proteins, amines)

• Steroid hormones (derived from cholesterol)

Signal Transduction Pathways

• cAMP Second Messenger System: Hormone binds receptor → activates G protein → stimulates adenylate cyclase → increases cAMP → activates protein kinases.

Equation:

$\text{Hormone} + \text{Receptor} \rightarrow \text{G protein activation} \rightarrow \text{Adenylate cyclase} \rightarrow \text{cAMP} \rightarrow \text{Protein kinase activation}$

Mechanisms of Hormone Control

• Humoral: Changes in blood levels of ions/nutrients trigger hormone release.

• Neural: Nerve fibers stimulate hormone release.

• Hormonal: Hormones stimulate other endocrine glands to release hormones.

Clinical Manifestations

• Diseases/syndromes associated with hypo/hypersecretion of hormones (e.g., diabetes mellitus, Cushing's syndrome, hypothyroidism).

Blood (Chapter 17)

Functions and Composition of Blood

Blood is a connective tissue responsible for transport, regulation, and protection. It consists of plasma and formed elements (erythrocytes, leukocytes, platelets).

• Functions: Nutrient transport, waste removal, thermoregulation, immune defense.

• Layers after centrifugation: Plasma (top), buffy coat (middle, WBCs and platelets), erythrocytes (bottom).

• Hematocrit: Percentage of blood volume occupied by erythrocytes.

• Plasma: Composed of water, proteins (albumin, globulins, fibrinogen), nutrients, hormones, electrolytes.

• Serum: Plasma without clotting factors.

Formed Elements

• Erythrocytes (RBCs): Biconcave, lack nuclei and organelles, flexible for capillary passage. Contain hemoglobin for oxygen transport.

• Leukocytes (WBCs): Five main types: neutrophils, lymphocytes, monocytes, eosinophils, basophils. Two histological categories: granulocytes and agranulocytes.

• Platelets: Cell fragments from megakaryocytes, involved in clotting.

Hemoglobin and Gas Transport

• Hemoglobin (Hb): Protein with four subunits, each containing a heme group. Binds O2 and CO2.

• Oxyhemoglobin: Hb bound to oxygen.

• Deoxyhemoglobin: Hb after releasing oxygen.

• Carbaminohemoglobin: Hb bound to CO2.

• CO poisoning: CO binds Hb with higher affinity than O2, preventing oxygen transport.

RBC Development and Erythropoiesis

• Stages: Proerythroblast → basophilic erythroblast → polychromatic erythroblast → orthochromatic erythroblast → reticulocyte → erythrocyte.

• Hormonal control: Erythropoietin (EPO) from kidneys stimulates RBC production.

• Dietary requirements: Iron, vitamin B12, folic acid.

RBC Destruction and Heme Catabolism

• RBCs live ~120 days. Destroyed in spleen and liver.

• Heme is broken down into bilirubin and iron (byproducts).

• Bilirubin is excreted in bile; iron is recycled.

Anemias and Erythrocyte Disorders

• Anemia: Reduced oxygen-carrying capacity. Causes: blood loss, decreased production, increased destruction.

• Polycythemia: Increased RBC count; can be primary or secondary.

Leukocytes and Leukopoiesis

• Five types: neutrophils, lymphocytes, monocytes, eosinophils, basophils.

• Leukopoiesis: Formation of WBCs from stem cells.

• Leukemia: Cancer of WBCs; classified by cell lineage and progression.

Platelets and Hemostasis

• Platelets: Also called thrombocytes; function in clot formation.

• Hemostasis: Three steps: vascular spasm, platelet plug formation, coagulation.

• Coagulation: Involves clotting factors, intrinsic and extrinsic pathways, convergence at factor X.

• Clot retraction: Platelet contraction pulls wound edges together.

• Vessel repair: Growth factors stimulate tissue repair.

Blood Disorders

• Thromboembolic disorders: Thrombosis (clot in vessel), embolism (clot travels).

• Bleeding disorders: Thrombocytopenia, hemophilia, DIC.

Blood Groups and Transfusion

• ABO blood groups: Determined by antigens (A, B) on RBCs; antibodies in plasma.

• Transfusion reactions: Occur if mismatched blood is given.

• Blood typing: Identifies blood group for safe transfusion.

Table: Major Endocrine Glands, Hormones, and Functions

Gland	Major Hormones	Main Functions
Pituitary (anterior)	GH, TSH, ACTH, FSH, LH, PRL	Growth, metabolism, reproduction
Pituitary (posterior)	ADH, Oxytocin	Water balance, uterine contraction
Thyroid	T3, T4, Calcitonin	Metabolism, calcium regulation
Parathyroid	PTH	Calcium homeostasis
Adrenal cortex	Aldosterone, Cortisol, Androgens	Salt balance, stress response, sex traits
Adrenal medulla	Epinephrine, Norepinephrine	Fight-or-flight response
Pineal	Melatonin	Circadian rhythm
Pancreas	Insulin, Glucagon	Blood glucose regulation
Gonads	Estrogen, Progesterone, Testosterone	Reproduction, secondary sex traits
Placenta	hCG	Pregnancy maintenance

Additional info: This guide covers the major topics and subtopics outlined in the provided study notes, expanding brief points into full academic explanations and including a summary table for reference.