Study Guide: Endocrine Control of Growth, Metabolism, and Cardiac Physiology

Chapter 23 – Endocrine Control of Growth and Metabolism

Cortisol: Secretion, Effects, and Clinical Relevance

Cortisol is a glucocorticoid hormone produced by the adrenal cortex, playing a crucial role in metabolism, immune response, and stress adaptation.

• Secretion Control: Cortisol secretion is regulated by the hypothalamic-pituitary-adrenal (HPA) axis. The hypothalamus releases corticotropin-releasing hormone (CRH), stimulating the anterior pituitary to secrete adrenocorticotropic hormone (ACTH), which in turn stimulates cortisol release from the adrenal cortex.

• Physiological Effects: Cortisol increases blood glucose via gluconeogenesis, suppresses the immune system, and promotes the breakdown of fats, proteins, and carbohydrates.

• Health Implications of Excess: Chronically high cortisol can lead to Cushing's syndrome, characterized by hyperglycemia, muscle wasting, immune suppression, and central obesity.

• Therapeutic Use: Cortisol and its synthetic analogs are used to treat inflammation and autoimmune diseases. However, exogenous administration can suppress endogenous production and cause side effects.

• Primary vs. Secondary Hypercortisolism: Primary is due to adrenal gland pathology; secondary is due to increased ACTH from the pituitary.

• Additional info: ACTH and CRH levels can help distinguish between primary and secondary causes.

Thyroid Hormones: T4 (Thyroxine) and T3 (Triiodothyronine)

Thyroid hormones regulate metabolism, growth, and development.

• Role: T4 and T3 increase basal metabolic rate, stimulate protein synthesis, and are essential for normal growth and neural development.

• Hyperthyroidism: Characterized by weight loss, heat intolerance, and increased heart rate.

• Hypothyroidism: Characterized by weight gain, cold intolerance, and slowed metabolism.

Growth Hormone (GH): Regulation and Effects

Growth hormone is secreted by the anterior pituitary and is essential for normal growth and metabolism.

• Factors Affecting Secretion: GH secretion is stimulated by growth hormone-releasing hormone (GHRH) and inhibited by somatostatin. Chronic stress can suppress GH secretion.

• Impacts of Reduced GH: Leads to stunted growth in children and altered metabolism in adults.

• Normal Growth: Depends on GH, thyroid hormones, sex steroids, adequate nutrition, and genetics.

Bone Growth and Calcium Homeostasis

Bone growth occurs at the epiphyseal plates during childhood and adolescence, requiring adequate hormones and nutrients.

• Conditions for Bone Growth: Requires GH, IGF-1, thyroid hormone, sex steroids, and sufficient calcium and vitamin D.

• Calcium Balance: Calcium is vital for bone structure, muscle contraction, nerve function, and blood clotting.

• Calcium Compartments:

  • Bone: ~99%

  • Intracellular: ~1%

  • Extracellular (plasma): ~0.1%

• Hormonal Control: Parathyroid hormone (PTH), calcitonin, and calcitriol (active vitamin D) regulate calcium levels.

• Osteoblasts vs. Osteoclasts: Osteoblasts build bone; osteoclasts resorb bone.

• Osteoporosis: A condition of decreased bone mass and increased fracture risk, often due to hormonal changes, aging, or inadequate calcium/vitamin D.

• Risk Factors for Osteoporosis: Age, female sex, low body weight, smoking, family history, low calcium/vitamin D intake, and sedentary lifestyle.

Chapter 14 – Cardiac Physiology

Cardiac Anatomy and Blood Flow

The heart is a muscular organ with four chambers that pumps blood through the pulmonary and systemic circuits.

• Oxygenated vs. Deoxygenated Blood: The left side of the heart receives oxygenated blood from the lungs and pumps it to the body; the right side receives deoxygenated blood from the body and pumps it to the lungs.

• Major Vessels:

  • Pulmonary artery: carries deoxygenated blood from the right ventricle to the lungs.

  • Pulmonary vein: carries oxygenated blood from the lungs to the left atrium.

  • Aorta: carries oxygenated blood from the left ventricle to the systemic circulation.

Cardiac Muscle Cells: Autorhythmic vs. Contractile

The heart contains two main types of cells: autorhythmic (pacemaker) cells and contractile cells.

• Autorhythmic Cells: Generate spontaneous action potentials that set the heart rate.

• Contractile Cells: Respond to action potentials by contracting and pumping blood.

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

• Plateau Function: Prolongs the action potential, preventing tetanus and allowing time for ventricular filling.

• Refractory Periods: Cardiac muscle has a longer refractory period than skeletal muscle, preventing sustained contractions.

Pacemaker Potential and Heart Rate

Pacemaker cells in the sinoatrial (SA) node initiate the heartbeat and determine heart rate.

• Pacemaker Potential: The gradual depolarization of pacemaker cells due to Na+ and Ca2+ influx.

• Heart Rate: Determined by the rate of depolarization in pacemaker cells (e.g., SA node: 70-80 bpm, AV node: 40-60 bpm).

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

Electrocardiogram (EKG/ECG) Interpretation

An EKG records the electrical activity of the heart and is used to diagnose arrhythmias and other cardiac conditions.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization

• T wave: Ventricular repolarization

• Atrial Repolarization: Not visible on EKG as it is masked by the QRS complex.

• Cardiac Arrhythmias: Abnormal heart rhythms that can cause palpitations, dizziness, or syncope.

• Heart Block: Impaired conduction between atria and ventricles, seen as prolonged PR interval or dropped beats on EKG.

• Ischemic Disease (e.g., Myocardial Infarction): Recognized by ST segment changes, T wave inversion, or Q waves on EKG.

Autonomic Regulation of Heart Rate

The autonomic nervous system modulates heart rate via sympathetic and parasympathetic pathways.

• Parasympathetic Control: Acetylcholine released from vagus nerve binds to muscarinic receptors, increasing K+ permeability and decreasing heart rate.

• Sympathetic Control: Norepinephrine increases Na+ and Ca2+ influx, increasing heart rate and contractility.

Summary Table: Key Hormones and Their Effects

Hormone	Source	Main Effects
Cortisol	Adrenal cortex	Increases blood glucose, suppresses immune system, stress adaptation
Growth Hormone (GH)	Anterior pituitary	Stimulates growth, protein synthesis, increases blood glucose
Thyroxine (T4), Triiodothyronine (T3)	Thyroid gland	Increase metabolic rate, essential for growth and development
Parathyroid Hormone (PTH)	Parathyroid glands	Increases blood calcium by stimulating bone resorption
Calcitonin	Thyroid gland	Lowers blood calcium by inhibiting bone resorption
Calcitriol (Vitamin D)	Kidney (activated form)	Increases intestinal absorption of calcium

Key Equations

• Cardiac Output: $\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

• Mean Arterial Pressure: $\text{MAP} = \text{CO} \times \text{TPR}$ where CO = cardiac output, TPR = total peripheral resistance

Additional info: This guide expands on the provided questions with academic context, definitions, and examples to support exam preparation in Anatomy & Physiology.