Fluid balance Flashcards
Which organ regulates fluid?
The kidneys are essential for regulating the volume and composition of bodily fluids.
Which hormone directly control water excretion in the kidneys?
Vasopressin/ADH
Where is ADH made?
ADH is a peptide hormone secreted mostly by the hypothalamus.
ADH is also produced by the posterior pituitary gland.
What does ADH do?
ADH causes the insertion of water channels into cell membranes of the collecting ducts.
This allows water reabsorption to occur.
Without ADH, little water is reabsorbed in the collecting ducts and dilute urine is excreted.
What influences the secretion of ADH?
- Special receptors in the hypothalamus that are sensitive to increasing plasma osmolarity (when the plasma gets too concentrated).
- Stretch receptors in the atria inhibit ADH secretion if the blood volume is too great.
- Stretch receptors in the aorta and carotid bodies which stimulate ADH secretion when BP falls
Why is osmolarity control important?
The osmolarity of bodily fluids is tightly regulated.
Cells may shrink, swell or get damaged if osmolarity is uncontrolled.
Regulation of osmolarity must be integrated with regulation of volume.
Plasma osmolarity is controlled by aldosterone.
What is aldosterone?
This is a steroid hormone produced by the adrenal cortex.
The secretion is controlled 2 ways:
-Adrenal cortex directly senses plasma osmolarity, aldosterone secretion is inhibited if the osmolarity increases above normal.
-Kidneys sense low BP, RAAS system leads to the secretion of aldosterone.
What are the disorders of ADH secretion?
- Diabetes Insipidus
- Syndrome of inappropriate ADH secretion (SIADH)
What is diabetes insipidus?
This is a rare condition.
Either too little ADH is produced by the pituitary gland (cranial diabetes insipidus)
The kidney becomes insensitive to ADH (nephrogenic diabetes insipidus)
Large quantities of dilute urine produced -polyuria
To compensate people drink lots of water-polydipsia
Which electrolyte imbalance can occur in people that don’t drink enough water?
Hypernatraemia
What is SIADH?
This is much more common than diabetes insipidus.
Many stimuli may override osmolality control and cause the release of high amounts of ADH.
Urine has high sodium content while hyponatraemia develops as water is retained.
Diagnosis made by checking paired serum osmolality and urine osmolality.
Causes of SIADH?
Lung diseases- cancer, pneumonia
Brain lesions- tumour, head injury and bleed, stroke
Drugs- carbamazepine, SSRIs
Miscellaneous causes
Assessment of fluid balance
Pulse BP Skin turgor Mucous membranes JVP Pitting oedema Urine volume
Diagnosis of SIADH
Serum sodium <133 mmol/L Low serum osmolality Urine sodium >20 mmol/L Normal renal, adrenal, thyroid and pituitary function. Exclusion of relevant drug therapy Response to fluid restriction
What is hyponatraemia?
Serum Na+ below the reference range i.e. <133 mmol/L
This is one of the commonest electrolyte abnormalities found in hospitalised patients.
Signs and symptoms of hyponatraemia
Headache
Dizziness
Nausea
Coma (if severe)
Classification of hyponatraemia
In a patient with low serum sodium, the result should be interpreted in relation to the patient’s hydration status:
- Hypervolaemia
- Hypovolaemia
- Euvolaemia
Why does hypervolaemia cause hyponatraemia?
- Patients with increased extracellular fluid and oedema have an excess of both sodium and water.
- The primary problem is a reduced circulating blood volume due to heart failure or low serum albumin (due to cirrhosis or nephrotic syndrome)
- ADH secretion is stimulated as well as aldosterone
- Net water gain relative to sodium
- Hypervolaemia is clinically obvious, further investigation is not required.
- Treatment often involves the use of diuretics and fluid restriction.
Clinical features of hypervolaemia
Tachycardia/ bounding pulse Raised JVP Pulmonary oedema or pleural effusion Ascites Peripheral oedema
Why does hypovolaemia cause hyponatraemia?
Sodium loss may be renal or extrarenal.
If the cause is not obvious, random urine sodium can be helpful.
Patients require sodium and water replacement ideally orally but sometimes IV.
Isotonic saline is usually given but hypertonic saline is required in severe, acute secretions.
What causes extrarenal loss of sodium?
Urine sodium <20 mmol/L Causes include: Vomiting Diarrhoea Excess sweating Fistulae Small bowel obstruction Trauma Heat exposure
What causes renal loss of sodium?
Urine sodium >20 mmol/L Caused by: Diuretics Self-losing nephritis Adrenal insufficiency (mineralocorticoid deficiency)
Clinical features of hypovolaemia
Cool peripheries/prolonged capillary refill Tachycardia/ can be weak, thread pulse Postural hypotension Confusion Dry mucous membranes Reduced skin turgor
Why does euvolaemia cause hyponatraemia?
The primary problem is with water excretion. The main causes are: Excess hypotonic fluid replacement Psychogenic polydipsia Hypothyroidism (rare) Glucocorticoid deficiency SIADH Drugs including carbamazepine, chlorpropamide.
What is hypernatraemia?
This is defined as serum sodium greater than 146 mmol/L
What causes hypernatraemia?
- This occurs as a result of either sodium gain or much more commonly water loss.
- It is much less common than hyponatraemia.
- This usually occurs in elderly patients who have dementia which has impaired their sense of thirst or patients in medically induced comas in ITU.
Classification of hypernatraemia
In a patient with an elevated serum sodium, the result should always be interpreted in relation to the patient’s hydration status:
Hypervolaemia
Hypovolaemia
Euvolaemia
Why does euvolaemia cause hypernatraemia?
As a result of pure water loss and the clinical cause is usually obvious.
However, if there is clinical doubt, it can be helpful to measure the plasma and urine osmolality.
Extrarenal or renal cause
Extrarenal loss of water in hypernatraemia?
Inadequate water intake in CVA, confusion, skin loss & low intake- fever, burns
Renal loss of water in hypernatraemia?
Diabetes insipidus
Why does hypervolaemia cause hypernatraemia?
This form is relatively rare and biochemistry investigation doesn’t have a major role in most cases.
Causes include:
Iatrogenic- IV sodium bicarbonate infusion
Salt ingestion- Accidental, seawater drowning
Mineralocorticoid excess
Why does hypovolaemia cause hypernatraemia?
Loss of both sodium and water but loss of water is greater. -Measurement of urine and plasma osmolality can be helpful. -Extrarenal loss causes include: GI- vomiting, diarrhoea, fistula Excessive sweating Renal loss: Hyperosmolar diabetic coma Renal failure
