Endocrine and Cardiovascular Systems: Study Notes

The Endocrine System

Definitions and Key Concepts

The endocrine system is a network of glands that secrete hormones to regulate bodily functions. Understanding the terminology is essential for mastering this system.

• Endocrine: Refers to glands that release hormones directly into the bloodstream to act on distant target organs.

• Exocrine: Glands that secrete substances through ducts to an epithelial surface (e.g., sweat glands).

• Neuroendocrine: Cells that receive neuronal input and, as a consequence, release hormones into the blood.

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

• Paracrine: Hormones act on neighboring cells in the same tissue.

Peptide Hormones vs. Steroid Hormones

Hormones can be classified based on their chemical structure and mechanism of action.

• Peptide Hormones: Composed of amino acids; generally hydrophilic and bind to cell surface receptors.

• Steroid Hormones: Derived from cholesterol; lipophilic and can cross cell membranes to bind intracellular receptors.

• Polarity: Peptide hormones are polar; steroid hormones are nonpolar.

• Membrane Interaction: Peptide hormones interact with membrane-bound receptors; steroid hormones interact with intracellular receptors.

Signal Transduction Pathways

Hormones exert their effects through specific signaling pathways.

• G Protein Activation Pathways: Involve Gs, Gq, and Gi proteins that activate or inhibit downstream effectors.

• cAMP Activation: Cyclic AMP acts as a second messenger in many hormone pathways.

• DAG/IP3 Pathway: Diacylglycerol (DAG) and inositol triphosphate (IP3) are second messengers that mobilize calcium and activate protein kinase C.

• Phosphodiesterase: Enzyme that degrades cAMP, thus regulating signal duration.

• Steroid Hormone Receptor Pathway: Steroid hormones bind intracellular receptors, influencing gene transcription.

Types of Hormone Release

Hormones can be released in different patterns, affecting their physiological impact.

• Humoral: Release in response to changes in blood levels of ions or nutrients.

• Neural: Release triggered by nerve fibers.

• Hormonal: Release in response to other hormones.

• Affinity: Strength of hormone-receptor binding.

• Half-life: Time required for half the hormone to be removed from the bloodstream.

• Pervasiveness: Degree to which a hormone affects multiple tissues.

• Synergism: When two hormones amplify each other's effects.

• Antagonism: When one hormone opposes the action of another.

Endocrine Organs and Glands

The endocrine system consists of several major glands, each with specific functions.

• Hypothalamus: Regulates pituitary gland and links nervous and endocrine systems.

• Infundibulum: Stalk connecting hypothalamus to pituitary gland.

• Anterior Pituitary: Produces hormones such as growth hormone, ACTH, TSH, etc.

• Posterior Pituitary: Stores and releases oxytocin and ADH produced by the hypothalamus.

• Thyroid Gland: Produces thyroid hormones (T3, T4) that regulate metabolism.

• Parathyroid Gland: Regulates calcium levels via parathyroid hormone (PTH).

• Adrenal Gland: Cortex produces corticosteroids; medulla produces catecholamines (epinephrine, norepinephrine).

• Pineal Gland: Produces melatonin, regulating circadian rhythms.

Thyroid Hormone: Production, Regulation, and Transport

Thyroid hormones are critical for metabolic regulation.

• Production: Synthesized in the thyroid gland from iodine and tyrosine.

• Regulation: Controlled by TSH from the anterior pituitary.

• Transport: Bound to plasma proteins in the blood.

Indirect/Direct Effects on Kidneys

Some hormones act directly or indirectly on the kidneys to regulate fluid and electrolyte balance.

• Direct: ADH increases water reabsorption.

• Indirect: Aldosterone increases sodium reabsorption, affecting water balance.

Other Endocrine Organs

• Pancreas: Produces insulin and glucagon to regulate blood glucose levels.

The Blood

Functions and Components of Blood

Blood is a connective tissue with multiple vital functions.

• Functions: Transport of gases, nutrients, waste; regulation of pH and temperature; protection against pathogens.

• Contents: Plasma (liquid matrix), erythrocytes (RBCs), leukocytes (WBCs), platelets.

Anatomy and Production of Erythrocytes

• Anatomy: Biconcave, anucleate cells specialized for oxygen transport via hemoglobin.

• Production (Erythropoiesis): Occurs in red bone marrow; stimulated by erythropoietin (EPO) from the kidneys.

• Stages of Erythropoiesis: Hemocytoblast → Proerythroblast → Erythroblast → Reticulocyte → Erythrocyte.

Regulation and Destruction of Erythrocytes

• Regulation: EPO increases RBC production in response to hypoxia.

• Destruction: Old RBCs are removed by macrophages in the spleen and liver.

Blood Disorders

• Anemia: Reduced oxygen-carrying capacity of blood.

• Polycythemia: Excess RBCs increase blood viscosity.

Blood Cell Identification and Platelets

• RBC vs. WBC: RBCs are anucleate and more numerous; WBCs have nuclei and are involved in immunity.

• Platelets: Cell fragments involved in clotting.

Hemostasis and Bleeding Disorders

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

• Bleeding Disorders: Conditions such as hemophilia impair normal clotting.

Lab Practical Skills

• Identify histology features, blood typing, and blood centrifugation.

The Cardiovascular System: The Heart

Heart Structure

• Layers of the Heart: Epicardium (outer), myocardium (muscular), endocardium (inner).

• Heart Wall: Composed of the three layers above.

• Chambers: Two atria (upper), two ventricles (lower).

• Valves: Atrioventricular (tricuspid, bicuspid/mitral) and semilunar (pulmonary, aortic) valves prevent backflow.

Blood Flow and Cardiac Histology

• Flow of Blood: Right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta → body.

• Intercalated Discs: Specialized connections between cardiac muscle cells for synchronized contraction.

Cardiac Muscle and Cell Depolarization

• Cardiac vs. Skeletal Muscle: Cardiac muscle is striated, involuntary, and has intercalated discs; skeletal muscle is voluntary and multinucleated.

• Depolarization: Cardiac cells depolarize via action potentials, leading to contraction.

• Action Potential Phases: Involves rapid depolarization (Na+ influx), plateau (Ca2+ influx), and repolarization (K+ efflux).

• Excitation: Electrical impulse triggers contraction.

• Intrinsic Conduction: SA node → AV node → Bundle of His → Purkinje fibers.

• Extrinsic Innervation: Autonomic nervous system modulates heart rate and force.

Cardiac Cycle and Heart Rate

• Phases of Cardiac Cycle: Atrial systole, ventricular systole, diastole.

• Stroke Volume: Amount of blood pumped per beat.

• Heart Rate: Number of beats per minute.

Heart Disorders

• Includes arrhythmias, myocardial infarction, heart failure, etc.

Lab Practical Skills

• Identify histology related to the heart, structures in the sheep heart, and the heart model.