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Questions and Answers
What is the primary treatment for nephrogenic diabetes insipidus?
Cerebral edema can occur due to acute or extreme hyponatremia.
True
What test is used to differentiate between primary polydipsia and diabetes insipidus?
Fluid deprivation test
Inappropriate ADH secretion leads to _____ hyponatremia.
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Match the conditions with their key treatments:
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What is the maximum fluid allowance per day for a patient under fluid restriction due to inappropriate ADH secretion?
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What is the consequence of a supervised water deprivation lasting more than 8 hours?
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The urine osmolality of a person with central diabetes insipidus will show a significant increase after administration of DDAVP.
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What is a consequence of cellular dehydration?
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Orthostatic decreased blood pressure occurs when standing.
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What is one treatment method for addressing severe shrinkage in the body?
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The primary ion responsible for extracellular fluid volume is ______.
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What does an increase in jugular venous pressure indicate?
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Match the treatment to its purpose:
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What can happen due to brain swelling?
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Which method is used to measure electrolytes like sodium in the body?
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What is the normal range for plasma osmolality?
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Urea is considered an effective osmole.
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What hormone is produced in the hypothalamus and stored in the posterior pituitary?
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The calculated osmolality formula is (2 x [Na] + [] + []) of plasma.
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Match the following disorders with their respective causes:
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What is the primary function of Antidiuretic hormone (ADH)?
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Osmol gap is defined as measured osmolality minus calculated osmolality.
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List two effective osmoles mentioned.
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The daily obligatory losses of water are approximately _____ mL.
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Which condition is characterized by concentrated urine and euvolemic hypotonic hyponatremia?
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The minimum urine output is calculated by dividing obligatory osmoles excreted by maximum urine osmolality.
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What stimulates the release of ADH?
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The main source of water from oxidative metabolism is approximately _____ mL.
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Match the following osmotically active substances with their classification:
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What does ADH primarily regulate in the body?
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Hyponatraemia is associated with dehydration.
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What is the role of baroreceptors in the regulation of sodium?
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The clinical presentation of hypernatraemia often includes __________.
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Which of the following hormones is associated with natriuresis?
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Intravascular volume is primarily sensed by osmoreceptors.
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What is the major compartment affected by dehydration?
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The term __________ describes weight gain due to excess fluid in the body.
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Match the following conditions with their associated symptoms:
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Which of the following correctly represents the sodium level associated with hypernatremia?
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Study Notes
Osmolality and Tonicity
- Osmolality: the concentration of osmotically active particles in a solution. Measured by freezing point thermodynamics (or boiling point) and expressed as mOsm/kg.
- Plasma Osmolality: 275-295 mOsm/kg.
- Urine Osmolality: 50-1200 mOsm/kg.
- Calculated Osmolality: Determined by (2 x [Na] + [urea] + [glucose]) of plasma.
- Osmol Gap (OG): Measured osmolality - Calculated osmolality.
- Normal OG < 10: Osmolality is primarily generated by ions like Cl-, K+, Ca2+, serum proteins, and lipids.
- OG > 10: Indicates the presence of exogenous substances that are osmotically active, such as alcohols, sugars, lipids, and proteins.
- Tonicity: Determines water movement across compartments. Influenced by effective osmoles, which create an osmotic gradient.
- Effective osmoles: Na+, K+, glucose (in diabetics), and urea (at extremely high levels). These solutes cannot easily cross cell membranes.
- Ineffective osmoles: Glucose (in normal individuals), urea, and alcohol. These solutes readily cross cell membranes.
- Implications:
- Changes in Na+ concentration affect water movement and lead to hyper/hyponatremia manifestations.
- Iso-osmolar solutions may not be isotonic, like D5 dextrose 5% - this can result in net water movement as the dextrose is metabolized.
- Urea contributes to osmolality but not tonicity. A falsely high measured osmolality due to urea should not be treated with hypotonic intravenous fluids.
Water Metabolism
- Daily water requirement (~1500 mL):
- Obligatory losses: 1500mL (Skin: 500mL, Lungs: 400mL, Gut: 100mL, Urine: 500mL).
- Sources: 1500mL (Water from oxidative metabolism: 400mL, Diet: 1100mL).
- Minimum Urine Output: Obligatory osmoles excreted / Maximum urine osmolality = 600/1200 = 0.5L.
Response to Water Depletion
- Antidiuretic Hormone (ADH) / Vasopressin (AVP):
- Produced in the hypothalamus and stored in the posterior pituitary.
- Stimulus: Osmolality increase (1% change), non-osmotic stimuli (blood volume decrease (10%), stress, nausea).
- In severe hypovolemia, ADH is secreted to preserve plasma volume at the expense of osmolality and Na+ regulation.
- Action:
- Water reabsorption: Through AQP-2 channels at the collecting duct and distal convoluted tubule, mediated by V2 receptors.
- Vasoconstriction: Via V1a receptors, leading to increased peripheral vascular resistance and blood pressure (in hypovolemia).
- Negative feedback loop: ADH levels are minimal when plasma osmolality is below 280 mOsm/kg.
Thirst
- Osmolality detected by osmoreceptors in the hypothalamus, stimulating the hypothalamic thirst center (outside of the blood-brain barrier) and activating the median preoptic nucleus.
Disorders of Water Metabolism
Diabetes Insipidus
- Causes:
- Cranial: 50% idiopathic - dysfunction of the hypothalamus or pituitary gland.
- Primary hypothalamic (more common): Tumors, vascular incidents, head injury, etc.
- Nephrogenic: Kidney cannot respond to ADH (tubular injury).
- Familial: Inherited.
- Metabolic: Low potassium, high calcium.
- Lithium toxicity
- Cranial: 50% idiopathic - dysfunction of the hypothalamus or pituitary gland.
- Manifestations: Polyuria (dilute urine - >3L/24h), Polydipsia. The patient is often euvolemic with normal Na+ levels (or hyponatremia if insufficient water intake)
- Investigation: Fluid deprivation test.
- Treatment:
- Treat underlying cause.
- Cranial DI: DDAVP (desmopressin).
- Nephrogenic DI: Thiazides (increase Na+ excretion), amiloride (decrease K+ loss), create mild hypovolemia to encourage salt and water uptake in the proximal tubule.
Syndrome of Inappropriate ADH Secretion (SIADH)
- Causes:
- CNS: Encephalitis, stroke.
- Lung: Pneumonia, tuberculosis.
- Drugs: SSRIs, carbamazepine, morphine.
- Surgery: Small cell lung cancer.
- Ectopic: Small cell lung cancer, prostate cancer, thymoma.
- Manifestations: Concentrated urine, euvolemic, hypotonic hyponatremia (dilutional hyponatremia). If acute or extreme, cerebral edema can develop.
- Investigation: Urine Na+ and urine osmolality.
- Treatment:
- Treat underlying cause.
- Fluid restriction: 1200mL/day
- Salt: PO administration, hypertonic IV (rare).
Water and Sodium Regulation
- Sensing Parameter:
- Water: Osmolality, sensed by osmoreceptors in the hypothalamus.
- Sodium: Intravascular volume, sensed by baroreceptors and chemoreceptors, leading to vasoconstriction.
- Endocrine Regulator:
- Water: ADH - regulates water reabsorption in the kidneys.
- Sodium: RAAS (Renin-Angiotensin-Aldosterone System), ANP (Atrial Natriuretic Peptide).
- Major Compartment:
- Water: ECF and ICF.
- Sodium: ECF.
- Manifestations:
- Water: Dehydration (more chronic).
- Sodium: Hypovolemia (more acute).
- Parameter Affected:
- Water: ECF sodium content/total sodium content.
- Sodium: ECF volume.
- Clinical Presentations:
- Water: Hypernatremia - lethargy, coma due to brain cell dehydration.
- Sodium:
- Hyponatremia:
- Mild: Nausea, lethargy.
- Severe: Seizures, coma, cerebral edema.
- Hypervolemia: Edema, weight gain, increased jugular venous pressure, increased right atrial pressure, shortness of breath (pulmonary edema), decreased lung compliance, decreased arterial blood volume, vasoconstriction.
- Hypovolemia:
- Poor skin turgor, tachycardia, orthostatic hypotension, decreased capillary refill.
- Hyponatremia:
- Implications in Treatment:
- Water: Free water, e.g., water restriction or D5W (dextrose 5% in water) for hypernatremia.
- Sodium: Sodium restriction for hypervolemia and normal saline (NS) for hypovolemia.
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Description
Test your understanding of osmolality and tonicity concepts with this quiz. Dive into the details of plasma and urine osmolality, the osmol gap, and the role of effective osmoles in regulating water movement. Perfect for students studying physiology or biochemistry.
