Endocrine System

Characteristics of the Endocrine System

The endocrine system is a network of glands that secrete hormones directly into the bloodstream to regulate various bodily functions. Unlike the nervous system, which uses electrical signals, the endocrine system relies on chemical messengers.

• Endocrine vs. Exocrine: Endocrine glands release hormones into the blood; exocrine glands secrete substances through ducts to external or internal surfaces.

• Paracrine/Autocrine: Paracrine signaling affects nearby cells; autocrine signaling affects the same cell that releases the signal.

• Comparison with Nervous System: The nervous system acts rapidly via electrical impulses; the endocrine system acts more slowly but has longer-lasting effects via hormones.

Major Endocrine Glands and Hormones

Several major glands comprise the endocrine system, each releasing specific hormones that regulate body functions.

• Hypothalamus: Controls pituitary gland, links nervous and endocrine systems.

• Pituitary Gland: Releases growth hormone, thyroid-stimulating hormone, etc.

• Thyroid: Produces thyroxine, regulates metabolism.

• Adrenal Glands: Release cortisol, adrenaline.

• Gonads (Ovaries/Testes): Produce sex hormones (estrogen, testosterone).

Additional info: Hormone release is often regulated by feedback mechanisms, such as negative feedback.

Hormone Action and Target Cells

Hormones exert effects by binding to specific receptors on target cells, initiating cellular responses.

• Hormone-Receptor Interaction: Specificity is determined by receptor presence on target cells.

• Steroid vs. Non-Steroid Hormones: Steroid hormones (e.g., cortisol) are lipid-soluble and act on intracellular receptors; non-steroid hormones (e.g., insulin) are water-soluble and act on cell surface receptors.

Renin-Angiotensin System

The renin-angiotensin system regulates blood pressure and fluid balance.

• Renin: Released by kidneys in response to low blood pressure.

• Angiotensin II: Causes vasoconstriction and stimulates aldosterone release.

Upregulation/Downregulation

Cells can increase (upregulate) or decrease (downregulate) the number of hormone receptors in response to hormone levels.

Blood

Characteristics and Composition of Blood

Blood is a connective tissue composed of plasma and formed elements (cells and platelets).

• Plasma: Liquid component, contains water, proteins, nutrients, hormones.

• Formed Elements: Red blood cells (RBCs), white blood cells (WBCs), platelets.

Centrifuged Sample % & Blood Composition

Blood can be separated by centrifugation into plasma, buffy coat (WBCs and platelets), and RBCs.

• Hematocrit (Hct): Percentage of blood volume occupied by RBCs.

Blood Cells: Characteristics and Functions

• Red Blood Cells (Erythrocytes): Transport oxygen via hemoglobin.

• White Blood Cells (Leukocytes): Immune defense; several types (neutrophils, lymphocytes, monocytes, eosinophils, basophils).

• Platelets (Thrombocytes): Involved in blood clotting.

Lifecycle of a Red Blood Cell

RBCs are produced in bone marrow, circulate for about 120 days, and are removed by the spleen and liver.

Hemoglobin/Hematocrit

Hemoglobin is the oxygen-carrying protein in RBCs. Hematocrit is a measure of RBC proportion in blood.

Cardiovascular System

Cardiovascular Functions

The cardiovascular system transports nutrients, gases, hormones, and waste products throughout the body.

• Pulmonary Circuit: Carries blood between heart and lungs for oxygenation.

• Systemic Circuit: Delivers oxygenated blood to tissues and returns deoxygenated blood to the heart.

Heart Anatomy

The heart is a muscular organ with four chambers (two atria, two ventricles) and valves that ensure unidirectional blood flow.

• Chambers: Right atrium, right ventricle, left atrium, left ventricle.

• Valves: Tricuspid, pulmonary, mitral, aortic.

• Conduction System: Includes SA node, AV node, bundle of His, Purkinje fibers.

Wall/Layers of the Heart

• Epicardium: Outer layer.

• Myocardium: Muscular middle layer.

• Endocardium: Inner lining.

Preload and Afterload

Preload is the initial stretching of cardiac myocytes prior to contraction; afterload is the pressure the heart must work against to eject blood.

Factors Affecting Metabolic Rate of the Heart

• Heart rate

• Contractility

• Afterload

Cardiac Cycle

The cardiac cycle consists of systole (contraction) and diastole (relaxation) phases, ensuring blood flow through the heart and to the body.

Baroreceptor Reflex

Baroreceptors detect changes in blood pressure and help regulate it via autonomic nervous system responses.

Capillary Structure & Permeability

Capillaries are thin-walled vessels that allow exchange of gases, nutrients, and waste between blood and tissues.

Blood Pressure Equation

Blood pressure is determined by cardiac output and total peripheral resistance.

• Equation:

• Q: Cardiac output (volume of blood pumped per minute)

• TPR: Total peripheral resistance (resistance to blood flow in vessels)

Venous Blood Flow/Venous Pooling

Venous return is aided by muscle contractions and valves; pooling can occur when blood accumulates in veins, especially in lower limbs.

Major Arteries/Veins

Major arteries include the aorta, carotid, and femoral arteries; major veins include the vena cava, jugular, and femoral veins.

Summary Table: Blood Cell Types and Functions

Summary Table: Heart Wall Layers

Additional info: These notes provide a concise overview of key Anatomy & Physiology concepts for exam preparation, including definitions, examples, and essential equations.