Endocrine System, Heart, and Vascular System: Structured Study Guide

Endocrine System (Chapter 16)

General Functions of the Endocrine System

The endocrine system regulates physiological processes through the secretion of hormones. It maintains homeostasis, influences growth and development, and coordinates responses to environmental changes.

• Hormone: A chemical messenger secreted by endocrine glands/tissues, transported by blood to target cells.

• Endocrine gland: A ductless gland that releases hormones directly into the bloodstream (e.g., pituitary, thyroid).

• Endocrine tissue (organ): Tissues/organs with hormone-secreting cells (e.g., pancreas, ovaries).

• Target cell: Cell with specific receptors for a hormone, enabling response.

• Paracrine: Signaling where cells secrete factors affecting nearby cells.

• Autocrine: Signaling where cells secrete factors affecting themselves.

• Nervous vs. Endocrine System: Nervous system uses electrical impulses for rapid, short-term responses; endocrine system uses hormones for slower, longer-lasting effects. Nervous signals travel via neurons; endocrine signals travel via blood. Target specificity is determined by receptor presence.

• Tropic hormones: Hormones that stimulate other endocrine glands to release their hormones (e.g., TSH stimulates thyroid).

Chemical Classification of Hormones and Mechanism of Action

Hormones are classified as steroid or peptide, each with distinct production, storage, and action mechanisms.

• Steroid hormones: Derived from cholesterol, produced in smooth ER, not stored, released by diffusion, transported bound to plasma proteins.

• Peptide hormones: Synthesized from amino acids, stored in vesicles, released by exocytosis, transported freely in plasma.

• Receptor locations: Steroid hormone receptors are intracellular; peptide hormone receptors are on the plasma membrane.

• Mechanisms of action: Steroid hormones alter gene expression; peptide hormones activate second messenger pathways (e.g., cAMP).

• Speed: Peptide hormone effects are rapid; steroid hormone effects are slower.

Control of Hormone Secretion

Hormone secretion is regulated by various stimuli and feedback mechanisms.

• Stimuli: Hormonal (other hormones), humoral (blood levels of substances), neural (nervous input).

• Feedback: Negative feedback inhibits further secretion; positive feedback amplifies secretion.

Endocrine Control by Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland are central regulators of endocrine function, connected anatomically and functionally.

• Hypothalamus: Located in the brain, controls pituitary hormone release.

• Anterior pituitary: Connected via the hypothalamic-hypophyseal portal system; releases hormones under hypothalamic control.

• Posterior pituitary: Stores and releases hormones produced by the hypothalamus.

• Negative feedback: Peripheral gland hormones inhibit hypothalamic and pituitary hormone release.

• Major anterior pituitary hormones: ACTH, TSH, GH, FSH, LH, PRL; each has specific targets and effects.

• Hypothalamic neurohormones: Regulate anterior pituitary hormone release (e.g., TRH, CRH).

• Posterior pituitary hormones: ADH (antidiuretic hormone) and oxytocin; targets include kidneys and uterus.

Endocrine Structures and Their Hormones

Several glands and tissues secrete hormones with diverse effects.

• Thyroid gland: Located in the neck; secretes T3, T4, calcitonin; regulates metabolism and calcium.

• Parathyroid glands: Posterior to thyroid; secrete PTH; regulate calcium.

• Adrenal cortex: On kidneys; secretes cortisol, aldosterone; regulates stress and electrolyte balance.

• Pancreas: Abdominal organ; secretes insulin, glucagon; regulates blood glucose.

• Thymus gland: Mediastinum; secretes thymosin; involved in immune development.

• Other tissues: Melatonin (pineal), leptin (adipose), ANP (heart), erythropoietin (kidney), renin (kidney), testosterone (testes), estrogen/progesterone (ovaries).

Hormonal Response to Stress

The body adapts to stress via a coordinated hormonal response.

• General adaptation syndrome: Involves alarm, resistance, and exhaustion phases.

• Hormones: CRH (hypothalamus), ACTH (anterior pituitary), cortisol (adrenal cortex), epinephrine/norepinephrine (adrenal medulla), glucagon (pancreas).

• Actions: Mobilize energy, increase heart rate, suppress non-essential functions.

Predictions Related to Disruption of Homeostasis

Disruptions in endocrine function can lead to significant physiological consequences.

• Example: Lack of dietary iodine leads to decreased thyroid hormone, causing goiter and hypothyroidism.

• Disorders: Hyper/hypothyroidism, diabetes mellitus/insipidus, adrenal excess/insufficiency affect target organs (e.g., metabolism, water balance).

Heart (Chapter 17)

Gross and Microscopic Anatomy of the Heart

The heart is a muscular organ located in the thoracic cavity, responsible for pumping blood throughout the body.

• Position: Mediastinum, between lungs.

• Pericardium: Fibrous (outer), parietal and visceral serous layers, pericardial cavity with serous fluid.

• Atria vs. Ventricles: Atria receive blood; ventricles pump blood out.

• External features: Four chambers, coronary sulcus, anterior/posterior interventricular sulci, apex, base.

• Internal structures: Chambers, septa, valves (AV, semilunar), papillary muscles, chordae tendineae, fibrous skeleton, venous/arterial openings.

• Coronary circulation: Left/right coronary arteries, branches, cardiac veins, coronary sinus.

• Heart wall layers: Epicardium (outer), myocardium (muscle), endocardium (inner).

• Microscopic anatomy: Myocardium contains intercalated discs for electrical coupling.

Physiology of Cardiac Muscle Contraction

Cardiac muscle contraction is initiated by action potentials, differing from skeletal muscle.

• Autorhythmic cells: Initiate action potentials spontaneously.

• Contractile cells: Respond to action potentials, contract to pump blood.

• Action potential phases: Depolarization, plateau (Ca2+ influx), repolarization.

• Plateau phase: Prolongs contraction, prevents tetanus.

• Refractory period: Longer in cardiac muscle, prevents arrhythmias.

• Autonomic innervation: Modulates pacemaker and contractile cell activity.

• Calcium: Determines contractility; more Ca2+ increases force.

Electrical Conduction System and Electrocardiogram

The heart's electrical system coordinates contractions, monitored by ECG.

• Conduction pathway: SA node → AV node → Bundle of His → bundle branches → Purkinje fibers.

• SA node: Primary pacemaker due to fastest depolarization rate.

• Coordinated contractions: Ensured by conduction system.

• ECG waveforms: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

Blood Flow Through the Heart

Blood flows through the heart in a specific sequence, passing through valves and chambers.

• Right side: Receives oxygen-poor blood, pumps to lungs.

• Left side: Receives oxygen-rich blood, pumps to body.

• Valves: Ensure unidirectional flow.

Cardiac Cycle

The cardiac cycle consists of systole (contraction) and diastole (relaxation), with distinct phases.

• Phases: Ventricular filling, isovolumetric contraction, ventricular ejection, isovolumetric relaxation.

• ECG relation: Electrical events precede mechanical events.

• Atrial systole: Completes ventricular filling.

• Valve events: Opening/closing linked to pressure changes.

• Heart sounds: S1 (AV valves close), S2 (semilunar valves close).

• Pressure/volume: Left and right ventricles differ in pressure but similar volume changes.

Regulation of Cardiac Output, Stroke Volume, and Heart Rate

Cardiac output is determined by heart rate and stroke volume, influenced by multiple factors.

• Cardiac output (CO): Volume of blood pumped per minute; units: mL/min.

• Formula: $CO = HR \times SV$

• Stroke volume (SV): Volume ejected per beat; $SV = EDV - ESV$

• Left ventricular ejection fraction: $\text{LVEF} = \frac{SV}{EDV} \times 100\%$

• Venous return: Blood returning to heart; affects preload.

• Preload: Degree of stretch before contraction; increases EDV.

• Afterload: Resistance heart must overcome; increases ESV.

• Frank-Starling Law: Increased EDV leads to increased SV.

• Inotropic agents: Affect contractility; positive (increase), negative (decrease).

• Chronotropic agents: Affect HR; positive (increase), negative (decrease).

• Autonomic nervous system: Sympathetic increases CO; parasympathetic decreases CO.

Vascular System (Chapter 18)

Anatomy and Functional Roles of Blood Vessels

Blood vessels transport blood and are classified by structure and function.

• Artery: Carries blood away from heart.

• Vein: Carries blood toward heart.

• Capillary: Site of exchange between blood and tissues.

• Three tunics: Intima (endothelium), media (smooth muscle), externa (connective tissue).

• Comparisons: Arteries have thicker media, smaller lumen; veins have thinner walls, larger lumen; capillaries are single-layered.

• Types: Elastic arteries (conducting), muscular arteries (distributing), arterioles, capillaries, venules, veins.

• Vasoconstriction: Narrowing of vessel; Vasodilation: widening.

• Capillary types: Continuous (muscle, skin), fenestrated (kidney, intestine), sinusoidal (liver, spleen).

• Venous reservoir: Veins store blood.

• Anastomosis: Vessel interconnections; e.g., Circle of Willis.

Systemic and Pulmonary Circuits

The circulatory system consists of two main circuits with distinct functions.

• Systemic circuit: Delivers oxygen-rich blood to body.

• Pulmonary circuit: Delivers oxygen-poor blood to lungs.

• Major arteries/veins: Pulmonary arteries/veins; aorta, vena cava.

• Hepatic portal system: Blood from GI tract passes through liver before returning to heart.

Blood Pressure and Functional Interrelationships

Blood pressure is determined by cardiac output, peripheral resistance, and vessel properties.

• Blood flow: Movement of blood; Blood pressure: force exerted by blood; Peripheral resistance: opposition to flow.

• Equation: $\text{Flow} = \frac{\Delta P}{R}$

• Arterioles: Regulate tissue blood flow and systemic pressure.

• Factors: Local (metabolic), hormonal (e.g., angiotensin II), neural (sympathetic).

• Vessel diameter: Smaller diameter increases resistance.

• Pressure changes: Highest in arteries, lowest in veins.

• Pulse pressure: $PP = \text{Systolic} - \text{Diastolic}$

• Mean arterial pressure: $MAP = \text{Diastolic} + \frac{1}{3}(\text{Systolic} - \text{Diastolic})$

• Determinants: CO, TPR, HR, SV, preload.

• Capillary types: Continuous, fenestrated, sinusoidal.

• Capillary exchange: Diffusion, filtration, reabsorption.

• Forces: Hydrostatic and osmotic pressures.

• Net filtration pressure: Determines fluid movement; excess leads to edema.

• Lymphatic system: Prevents edema by returning excess fluid.

• Venous return: Aided by muscular compression and respiratory pump.

• Local control: Myogenic autoregulation, precapillary sphincters.

• Chemicals: Vasodilators (NO, histamine), vasoconstrictors (endothelin, angiotensin II).

• Baroreceptor reflex: Senses pressure changes, adjusts heart rate and vessel tone.

• Autonomic nervous system: Regulates blood pressure and volume.

Application of Homeostatic Mechanisms

The cardiovascular system uses feedback mechanisms to maintain blood pressure.

• Example: Baroreceptor reflex increases HR and vasoconstriction when BP drops.

Predictions Related to Disruption of Homeostasis

Disruptions in cardiovascular function can have significant effects.

• Example: Left ventricular failure leads to pulmonary edema.

• Reverse prediction: Pulmonary edema may result from heart failure, valve disease, or increased pulmonary pressure.

Hormone	Source	Target(s)	Main Effect
Thyroid hormones (T3, T4)	Thyroid gland	Most cells	Increase metabolism
Parathyroid hormone (PTH)	Parathyroid glands	Bones, kidneys	Increase blood calcium
Cortisol	Adrenal cortex	Many tissues	Stress response, increase glucose
Insulin	Pancreas	Most cells	Decrease blood glucose
ADH	Posterior pituitary	Kidneys	Increase water reabsorption
Oxytocin	Posterior pituitary	Uterus, mammary glands	Uterine contraction, milk ejection
Melatonin	Pineal gland	Brain	Regulate sleep
Leptin	Adipose tissue	Hypothalamus	Regulate appetite
ANP	Heart	Kidneys	Decrease blood pressure
Erythropoietin	Kidney	Bone marrow	Increase RBC production
Testosterone	Testes	Many tissues	Male characteristics
Estrogen/Progesterone	Ovaries	Many tissues	Female characteristics