2.1: Human Physiology & Homeostasis

2.1 Human Physiology & Homeostasis

The Overseas Pharmacist Readiness Assessment (OPRA) exam tests core biomedical concepts with a strong emphasis on how they translate into clinical practice and drug action. Understanding the autonomic nervous system (ANS), cardiovascular hemodynamics, and renal tubule transport is fundamental to predicting drug-drug interactions, side effects, and therapeutic choices in the Australian clinical environment.

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1. The Autonomic Nervous System (ANS)

The autonomic nervous system regulates involuntary physiological processes. It is divided into the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) divisions, which maintain homeostasis through opposing actions. Drugs commonly target receptors in these pathways.

Neurotransmitter Dynamics

• Acetylcholine (ACh): The primary neurotransmitter of the parasympathetic nervous system, all autonomic preganglionic fibers, and somatic neuromuscular junctions. It is synthesized from choline and acetyl-CoA by choline acetyltransferase (ChAT), stored in vesicles, and released via calcium-dependent exocytosis. ACh action is terminated rapidly in the synaptic cleft by acetylcholinesterase (AChE), which hydrolyzes it into choline and acetate.
• Noradrenaline (NA) & Adrenaline: The primary neurotransmitters of the sympathetic nervous system (except for sweat glands). Tyrosine is converted to DOPA by tyrosine hydroxylase (the rate-limiting step), which is then converted to dopamine and transported into vesicles. In sympathetic terminals, dopamine is converted to noradrenaline. In the adrenal medulla, noradrenaline is methylated to adrenaline. The action of catecholamines is primarily terminated by active reuptake into the presynaptic terminal (Uptake 1), where they are either recycled or degraded by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT).

Receptor Subtypes, Signaling, and Physiology

Clinical Practice Correlations & Traps

• Beta-blockers and Bronchoconstriction: Non-selective beta-blockers like propranolol block both beta-1 and beta-2 receptors. Blockade of Gs-coupled beta-2 receptors in bronchial smooth muscle prevents endogenous adrenaline from causing bronchodilation, leading to severe bronchospasm in patients with asthma or COPD. Therefore, cardioselective beta-1 blockers (e.g., metoprolol or atenolol) are preferred in patients with respiratory disease, though selectivity is lost at high doses. In Australia, this means clinicians must exercise caution when starting metoprolol in asthmatics, preferring alternative agents or closely monitoring lung function.
• Sympathetic Cholinergic Exception: Sweat glands are unique. Although they are part of the sympathetic nervous system, postganglionic sympathetic fibers innervating sweat glands release acetylcholine (ACh) instead of noradrenaline, acting on muscarinic M3 receptors. Anticholinergic drugs (e.g., oxybutynin for urinary urge incontinence or amitriptyline) block these receptors, causing dry skin, hyperthermia, and anhidrosis (dryness), which is a crucial monitoring parameter in hot Australian summers.
• Alpha-1 Blockade and Reflex Tachycardia: Selective alpha-1 blockers like prazosin cause vasodilation, resulting in a sudden drop in blood pressure. This drop triggers a baroreceptor-mediated reflex increase in sympathetic outflow, leading to tachycardia. This effect is minimized by titrating the dose slowly and advising patients to take their first dose at bedtime ("first-dose hypotension" counseling).
• Anticholinergic Burden in Older Adults: The cumulative effect of taking multiple medications with anticholinergic properties (e.g., amitriptyline for neuropathic pain, oxybutynin for bladder overactivity, and chlorphenamine for allergies) is known as the anticholinergic burden. In elderly patients, this can lead to cognitive decline, confusion, dry mouth, blurred vision, constipation, and urinary retention. Standard Australian pharmacy guidelines (like the AMH) emphasize calculating this burden to prevent adverse drug events.

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2. Cardiovascular Physiology & Blood Pressure Regulation

Systemic blood pressure (BP) is dynamically controlled to ensure organ perfusion. The fundamental equation governing blood pressure is:

BP = CO x SVR

Where CO is Cardiac Output and SVR (or TPR) is Systemic Vascular Resistance. In clinical practice, SVR is primarily determined by the contractile state of arteriole smooth muscle, regulated by alpha-1 adrenergic receptors (causing vasoconstriction) and beta-2 adrenergic receptors (causing vasodilation).

Cardiac Output and Stroke Volume Determinants

Cardiac Output is the volume of blood pumped by the heart per minute, calculated as:

CO = HR x SV

• Heart Rate (HR): Regulated by the ANS. Sympathetic activation (beta-1) increases HR (positive chronotropy), while parasympathetic activation (M2) decreases HR (negative chronotropy).
• Stroke Volume (SV): The volume of blood ejected per contraction. It is determined by three main variables:
  1. Preload: The degree of myocardial stretch at the end of diastole, determined by venous return. According to the Frank-Starling law, increased preload increases stroke volume up to a physiological limit by aligning actin and myosin filaments for more efficient cross-bridge formation.
  2. Contractility: The intrinsic strength of cardiac contraction at any given preload, increased by intracellular calcium availability (stimulated by sympathetic beta-1 activation or drugs like digoxin which inhibit the Na+/K+ ATPase, thereby increasing intracellular calcium via the Na+/Ca2+ exchanger).
  3. Afterload: The resistance the ventricles must overcome to pump blood, primarily determined by systemic arteriolar tone (vasoconstriction increases afterload; vasodilation decreases it).

The Baroreceptor Reflex

The baroreceptor reflex is the body's rapid, neural mechanism for buffering acute changes in blood pressure.

1. Detection: High-pressure baroreceptors (stretch-sensitive mechanoreceptors) are located in the carotid sinus (sends signals via the glossopharyngeal nerve, CN IX) and the aortic arch (via the vagus nerve, CN X).
2. Integration: These nerves project to the Nucleus Tractus Solitarius (NTS) in the medulla oblongata.
3. Compensatory Response to Hypotension (e.g., Orthostatic Hypotension):
   • Standing up causes blood pooling in lower extremities, reducing venous return, stroke volume, and arterial blood pressure.
   • This decreases the stretch on baroreceptors, reducing their baseline firing rate to the NTS.
   • The NTS reduces vagal (parasympathetic) outflow to the SA node and increases sympathetic discharge from the rostral ventrolateral medulla.
   • Sympathetic stimulation releases noradrenaline, causing:
     • Constriction of arterioles (alpha-1) to increase SVR.
     • Constriction of veins (alpha-1) to increase venous return (preload).
     • Increased heart rate and contractility (beta-1) to increase CO.
4. Clinical Trap: Vasodilating drugs (e.g., dihydropyridine calcium channel blockers like nifedipine, or alpha-1 blockers like prazosin) can trigger this reflex, causing reflex tachycardia. Clinicians use modified-release formulations or combine them with beta-blockers to prevent this cardiotoxic side effect.

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3. Renal Physiology & Tubule Transport

The kidneys maintain fluid and electrolyte homeostasis, regulate acid-base balance, and excrete waste. They are also the site of action for major class-1 antihypertensives and diuretics.

Glomerular Filtration & the Filtration Barrier

Filtration occurs across the glomerular capillaries into Bowman's space, driven by Starling forces:

GFR = Kf x [ (P_GC - P_BS) - (pi_GC - pi_BS) ]

• P_GC (glomerular capillary hydrostatic pressure) is the primary driver of filtration. It is controlled by the relative resistance of the afferent and efferent arterioles. Afferent arteriole constriction (e.g., induced by NSAIDs via inhibition of vasodilatory prostaglandins) decreases GFR. Efferent arteriole dilation (e.g., induced by ACE inhibitors or ARBs via blocking the vasoconstrictor action of Angiotensin II) also decreases GFR but reduces glomerular hypertension.
• The Glomerular Filtration Barrier: Comprises three layers:
  1. Fenestrated Endothelium: Restricts cells (RBCs, WBCs) but allows plasma components through.
  2. Glomerular Basement Membrane (GBM): Composed of collagen and heparan sulfate. The negative charge of heparan sulfate electrostatically repels negatively charged plasma proteins (e.g., albumin).
  3. Podocyte Foot Processes: Wrap around the GBM, creating filtration slits spanned by nephrin diaphragms.
• Clinical Pearl: In diabetic nephropathy, loss of the negative charge on the GBM leads to microalbuminuria, which is an early marker of renal damage. ACE inhibitors or ARBs are protective because they dilate the efferent arteriole, reducing intraglomerular hydrostatic pressure (P_GC), thereby reducing protein excretion and slowing disease progression.

Segmental Tubule Transport

Glomerulus -> PCT -> Loop of Henle -> DCT -> Collecting Duct

1. Proximal Convoluted Tubule (PCT)

• Reabsorbs ~67% of filtered water, sodium, potassium, and bicarbonate, and 100% of filtered glucose and amino acids.
• SGLT2 (Sodium-Glucose Cotransporter 2): Reabsorbs glucose and sodium from the lumen. SGLT2 inhibitors (e.g., dapagliflozin, empagliflozin) block this transporter, leading to glucosuria and mild natriuresis. This lowers HbA1c and reduces preload/afterload, providing strong cardiorenal protection.
• Carbonic Anhydrase: Essential for bicarbonate reabsorption. Inhibited by acetazolamide, causing urinary bicarbonate loss and metabolic acidosis.

2. Loop of Henle

• Thin Descending Limb: Highly permeable to water but impermeable to solutes. As fluid descends into the hypertonic medulla, water is reabsorbed, concentrating the tubular fluid.
• Thick Ascending Limb (TAL): Impermeable to water but actively reabsorbs solutes via the NKCC2 (Na+/K+/2Cl-) cotransporter.
• Loop Diuretics (e.g., Frusemide): Reversibly block NKCC2. This inhibits sodium and chloride reabsorption, causing massive diuresis. It also abolishes the hypertonic medullary gradient (so the collecting duct cannot reabsorb water even in the presence of ADH) and increases excretion of magnesium and calcium.

3. Distal Convoluted Tubule (DCT)

• Reabsorbs sodium and chloride via the NCC (Na+/Cl-) cotransporter.
• Thiazide Diuretics (e.g., Hydrochlorothiazide, Indapamide): Block the NCC. This increases distal sodium delivery and excretion. However, unlike loop diuretics, thiazides increase calcium reabsorption in the DCT (stimulated by PTH). This makes thiazides beneficial for hypertensive patients with concurrent osteoporosis or hypercalciuria (kidney stones).

4. Collecting Duct (CD)

• The final site of water and electrolyte fine-tuning, containing two main cell types:
  • Principal Cells: Reabsorb sodium via epithelial sodium channels (ENaC) and secrete potassium via ROMK channels.
  • Intercalated Cells: Regulate acid-base balance by secreting hydrogen ions (alpha-intercalated) or bicarbonate (beta-intercalated).

Hormonal Regulation: Aldosterone vs. ADH (Vasopressin)

1. Aldosterone:
   • Stimulus: Released from the adrenal cortex (zona glomerulosa) in response to Angiotensin II or high serum potassium.
   • Mechanism: Binds to intracellular mineralocorticoid receptors in principal cells, translocates to the nucleus, and upregulates the synthesis and insertion of ENaC channels, basolateral Na+/K+ ATPase pumps, and ROMK channels.
   • Effect: Promotes sodium and water reabsorption (volume expansion) and increases potassium and hydrogen excretion.
   • Antagonists: Spironolactone and eplerenone block the mineralocorticoid receptor, promoting natriuresis while conserving potassium (potassium-sparing diuretics), which is vital in heart failure management.
2. Antidiuretic Hormone (ADH / Vasopressin):
   • Stimulus: Synthesized in the hypothalamus and released from the posterior pituitary in response to high plasma osmolality (detected by osmoreceptors) or severe hypovolaemia (detected by baroreceptors).
   • Mechanism: Binds to Gs-coupled V2 receptors on the basolateral membrane of collecting duct principal cells. This increases intracellular cAMP, triggering the transport of vesicles containing Aquaporin-2 water channels and their insertion into the apical membrane.
   • Effect: Increases water permeability, allowing free water to be reabsorbed down the concentration gradient, which dilutes the plasma and concentrates the urine.
   • Clinical Dysfunction: In Syndrome of Inappropriate ADH (SIADH), excess ADH causes water retention and dilutional hyponatraemia, which requires fluid restriction or V2 receptor antagonists (vaptans). In Diabetes Insipidus, lack of ADH (central) or receptor resistance (nephrogenic) causes profound polyuria and hypernatraemia.