3.3 Hormonal Controls

Regulation of Body Function

• Regulation of body function uses two regulatory systems to maintain homeostasis:

  • Nervous system: sends fast electrical or chemical signals through neurons; regulates quick responses to abrupt environmental change.

  • Endocrine system: sends chemical signals via the circulatory system to target cells; regulates more defined and time-consuming processes such as development.

Chemical Signaling

• Paracrine signaling: communication between adjacent cells; local regulators include growth factors, prostaglandins, nitric oxide (NO).

• Synaptic signaling: communication between nerve cells by sending neurotransmitters across synapses.

• Pheromonal signaling: communication between organisms.

Endocrine System

• Endocrine gland = ductless gland; works via circulatory system.

• Hormone: derived from Greek hormon, meaning to excite or set in motion.

• Target cell contains a specific receptor for the hormone.

• Exocrine gland: glands with ducts that convey secretions outside the body or into a lumen (e.g., sweat gland, salivary gland).

Effect of Hormones

• Hormones require specific receptors on/inside target cells.

• Different receptors can lead to different cellular responses.

• The same hormone can have different effects in different target cells due to receptor type, signal transduction machinery, and cellular context.

Type & Example of Hormones

• Protein – Insulin

• Glycoprotein – Follicle-stimulating hormone (FSH)

• Peptide – Oxytocin

• Steroid – Testosterone

• Amine (tyrosine derivative) – Thyroxine (T4)

• Eicosanoids; fatty acid derivatives – Prostaglandins

• Molecular size varies; key distinction: small, lipid-soluble hormones vs large, water-soluble hormones

Small, lipid-soluble hormone (Mode of Action)

• Penetrate through the cell membrane.

• Bind to receptors inside the cell.

• The hormone–receptor complex binds to DNA in the nucleus, initiating transcription (RNA) and translation (protein synthesis).

• Effect: protein synthesis leading to a cellular response.

• Example: sex hormones such as testosterone and estrogen.

Large, water-soluble hormone (Mode of Action)

• Cannot penetrate the cell membrane.

• Bind to membrane receptors.

• Use second messengers and signal transduction pathways.

• Effect: rapid or specialized responses (e.g., secretion, protein synthesis).

• Most hormones act this way.

Major Endocrine Glands (Glands of Vertebrates)

• Hypothalamus

• Pineal gland

• Glands of Vertebrate Pituitary gland

• Thyroid gland

• Parathyroid glands

• Thymus

• Adrenal glands

• Pancreas

• Ovary (female)

• Testis (male)

Table 45.1 Major Vertebrate Endocrine Glands and Some of Their Hormones (summary)

• Hypothalamus

  • Hormones regulated by hypothalamic signals: releasing and inhibiting hormones.

  • Hormones affecting posterior pituitary: oxytocin; vasopressin/antidiuretic hormone (ADH).

  • Chemical class: peptide/hormones released by hypothalamus.

• Posterior pituitary (neurohypophysis)

  • Hormones released: Oxytocin; Antidiuretic hormone (ADH).

  • Chemical class: peptide.

  • Source: hormones produced by hypothalamus and stored/released by posterior pituitary.

• Anterior pituitary (adenohypophysis)

  • Hormones: Growth hormone (GH); Prolactin (PRL); Follicle-stimulating hormone (FSH); Luteinizing hormone (LH); Thyroid-stimulating hormone (TSH); Adrenocorticotropic hormone (ACTH).

  • Chemical class: GH and PRL are proteins; FSH, LH, TSH are glycoproteins; ACTH is a peptide/hormone.

• Thyroid gland

  • Hormones: Triiodothyronine (T3); Thyroxine (T4) – amines derived from tyrosine; Calcitonin – peptide.

  • Actions: T3/T4 stimulate metabolism; Calcitonin lowers blood calcium.

• Parathyroid glands

  • Hormone: Parathyroid hormone (PTH) – peptide.

  • Action: Raises blood calcium level.

• Pancreas

  • Hormones: Insulin (protein); Glucagon (protein).

  • Actions: Insulin lowers blood glucose; Glucagon raises blood glucose.

• Adrenal glands

  • Adrenal medulla: Epinephrine and norepinephrine – amines.

  • Adrenal cortex: Glucocorticoids (steroids); Mineralocorticoids (steroids).

  • Actions: Glucocorticoids raise blood glucose and metabolic activity; Mineralocorticoids promote Na+ reabsorption and K+ excretion in kidneys.

• Gonads

  • Testes: Androgens (steroids) – support sperm formation; promote male secondary sex characteristics.

  • Ovaries: Estrogens and Progesterone (steroids) – promote uterine lining growth; female secondary sex characteristics.

• Pineal gland

  • Hormone: Melatonin – amine.

  • Actions: Involved in biological rhythms; promotes uterine lining growth.

• Thymus

  • Hormone: Thymosin (peptide).

  • Action: Stimulates T lymphocytes; role linked to immune function and light/dark cycles via hypothalamic input.

• Note: Some entries in older texts align poorly; the overarching pattern is hypothalamic control over pituitary hormones, and pituitary hormones controlling peripheral endocrine glands.

Neural and Endocrine Systems

• Neurohormone pathway: Some neurons secrete chemicals into the bloodstream that affect target cells similarly to endocrine signals.

• Neuroendocrine pathway: Neurons secrete chemicals into the bloodstream that affect endocrine cells, which then secrete chemicals into the bloodstream to affect target cells.

Hypothalamus–Posterior Pituitary

• Posterior pituitary derives from neural tissue.

• Neurosecretory cells in the hypothalamus produce hormones, travel down axons, and store in the posterior pituitary.

• These are called neurohormones.

• Examples: Antidiuretic hormone (ADH); Oxytocin.

Hypothalamus–Anterior Pituitary

• Neurosecretory cells in the hypothalamus secrete releasing or inhibiting hormones.

• Endocrine cells in the anterior pituitary respond by secreting hormones into the bloodstream.

• This pathway is referred to as neuroendocrine signaling.

Maintenance of Homeostasis: Antagonistic Hormones

• Calcium homeostasis uses antagonistic hormones:

  • Calcitonin (from the thyroid gland) reduces calcium levels.

  • Parathyroid hormone (PTH) (from the parathyroid glands) increases calcium levels.

Negative Feedback in Endocrine Axes

• Thyroid axis example:

  • Hypothalamus secretes TRH (TSH-releasing hormone).

  • Anterior pituitary secretes TSH (thyroid-stimulating hormone).

  • Thyroid gland secretes T3 and T4.

  • When blood levels of T3, T4, and TSH are too high, TRH secretion from the hypothalamus is inhibited (negative feedback).

Maintenance of Calcium Level (Regulatory Diagram Summary)

• Stimulus: rising blood Ca^{2+} levels triggers calcitonin release from the thyroid gland.

• Calcitonin acts to decrease Ca^{2+} levels by

  • Increasing Ca^{2+] deposition in bones,

  • Increasing Ca^{2+} uptake in the kidneys,

  • Decreasing Ca^{2+} uptake in the intestines (relative balance depending on context).

• Stimulus: falling blood Ca^{2+} levels triggers PTH release from the parathyroid glands.

• PTH acts to raise Ca^{2+} levels by

  • Increasing Ca^{2+} release from bones,

  • Reducing Ca^{2+} excretion in kidneys,

  • Increasing Ca^{2+} absorption from the gut (via vitamin D activation).

Maintenance of Blood Glucose

• Insulin (from β-cells of the pancreas): lowers blood glucose level.

  • Stimulus: rising blood glucose after a carbohydrate-rich meal.

  • Action: drives uptake of glucose by body cells; encourages storage as glycogen in liver; reduces blood glucose toward a set point.

• Glucagon (from α-cells of the pancreas): raises blood glucose level.

  • Stimulus: falling blood glucose level.

  • Action: liver breaks down glycogen to glucose and releases it into the bloodstream.

• In turn, higher glucose levels suppress glucagon release and lower glucose levels promote insulin release; the system maintains blood glucose homeostasis.

Diabetes Mellitus (Hyperglycemia Disorder)

• Etymology: Diabetes (Latin) = siphon, honeyed discharge; Mellitus (Latin) = honey.

• Symptoms: excessive urination (polyuria) and excessive thirst (polydipsia).

• Pathophysiology: high blood glucose levels; sugar appears in urine.

• Severe consequence: fat becomes a major fuel source, leading to production of acidic metabolites and potentially life-threatening acidosis (ketoacidosis).

Diabetes Mellitus: Type I vs Type II

• Type I DM (insulin-dependent): autoimmune autoimmune disorder; usually appears in childhood; treatment often requires insulin injections.

• Type II DM (non-insulin-dependent): decreased responsiveness to insulin; usually occurs after age 40; risk increases with age; accounts for over 90% of diabetes cases.

Stress Response: Short-term vs Long-term

• Short-term (acute) stress response:

  • Increases blood glucose, blood pressure, breathing, and metabolic rate.

  • Increases alertness.

• Long-term (chronic) stress response:

  • Retains Na+ and H2O; increases blood volume and pressure.

  • Increases blood glucose; suppresses immune response.

• Hormone & Stress: the endocrine system mediates stress responses, often via the hypothalamic–pituitary–adrenal axis and sympathetic pathways.

Interacting Regulatory Systems Maintain Homeostasis

• Multiple regulatory systems operate in parallel, including:

  • Kidneys, liver, and other organs.

• Feedback mechanisms involve the nervous system and the endocrine system.

Antidiuretic Hormone (ADH)

• Stimulus: blood osmolarity (concentration) detected by osmoreceptors in the hypothalamus.

• Hypothalamus senses increased osmolarity; ADH is released from the posterior pituitary.

• Increased water reabsorption in the collecting ducts of the kidney reduces blood osmolarity and helps prevent dehydration.

• Thirst also increases to restore water balance; drinking further reduces blood osmolarity toward the set point.

• Flow: Osmolarity increases → osmoreceptors stimulate hypothalamus → ADH release → increased H2O reabsorption → lowered osmolarity; conversely, if osmolarity is low, ADH release is reduced and water loss increases.

Renin–Angiotensin–Aldosterone System (RAAS)

• Stimulus: changes in blood pressure and blood volume detected by the juxtaglomerular apparatus in the kidneys.

• Response: increased Na^{+} and H2O reabsorption in the proximal tubule, leading to higher blood volume and pressure; aldosterone acts on distal tubules to promote Na+ reabsorption and K+ excretion (mechanism involves angiotensin II).

• Outcome: helps regulate blood pressure and fluid balance, especially in response to decreased renal perfusion or dehydration.