Renal Physiology: Salt and Water (Zoysa) Flashcards
Describe the Fluid Compartments
Total body water represents 60% body composition (1kg of ideal body weight exclude fat is 1L water). This can be divided into 2/3 intracellular fluid (ICF), and 1/3 extracellular fluid (ECF)
- ICF have many separate components, all within the cell membrane, all have a similar composition
- ECF have many separate components, all external to cell membrane, all have a similar composition (1/4 plasma of intravascular fluid, 3/4 interstitial fluid, transcellular fluid (~1L)).
Define Molarity and osmotic concentration
Molarity refers to the number of moles per litre. Avogadro’s constant is 6.022´1023 molecules.
Osmotic concentration is measure of solute concentration. It is number of Osm of solute per litre of solution. It’s tightly regulated.
What is Plasma osmolarity?
Define Hper and Hypo-osmolarity
Plasma osmolarity is 285-295mOsm/L. It is regulated by balance of salt and water.
- Hyperosmolarity is defined by t_oo much cation and too little water_
- Hyposmolarity is defined by too little cation and too much water
Define Tonicity
Tonicity refers to what happens to a cell in solution
- If the cells take up water from a solution (i.e. swell), then solution is hypotonic
- If the cells lose water from a solution (i.e. shrink), then solution is hypertonic
- If the cells do not change its size, then solution is isotonic
What is the normal GFR?
Glomerular filtration rate is 120mL/min. Therefore, more than 170L/day filtered (120mL´60min´24hr)!
Describe the absorption of water and salt along the Nephron
-
At glomerulus,
- free filtration of salt and water occurs (from afferent arteriole to Bowman’s space). There is also protein filtration, note that kidney typically excretes 200-300mg protein per day (albumin 40g/L).
-
At proximal tubule,
- 65-75% sodium and water are reabsorbed (most occurs here).
-
At loop of Henle,
- water is freely reabsorbed in descending limb, while 15-20% sodium is reabsorbed in ascending limb.
- At distal convoluted tubule,
- 5% sodium is reabsorbed.
- At collecting duct,
- water reabsorption is controlled by ADH, while 5% sodium is reabsorbed.
Describe the ADH
Antidiuretic Hormone (ADH)/Arginine Vasopressin (AVP)
ADH is made in hypothalamus, secreted from pituitary.
- If there is decreased BP or increased osmolarity, there is increased ADH production
- This increases water reabsorption (increase aquaporin-2 channels in apical membrane)
- This leads to increased BP and reduced osmolarity
What stimulates the release of Aldosterone?
Aldosterone is a mineralocorticoid.
- It is stimulated by increased potassium and angiotensin II.
- It acts on _distal convoluted tubule a_nd _collecting ducts t_o increase Na+ reabsorption and K+ excretion.
Describe the role of Renin/what releases it
Juxtaglomerular apparatus senses decreased renal perfusion and secretes renin.
Renin increases angiotensin I (and angiotensin II).
Describe the role of Angiotensin II
Angiotensin II
Angiotensin II is the most powerful sodium-retaining hormone. I_t is stimulated by low BP and/or low ECF._
- Increased angiotensin II can lead to
- (1) vasoconstriction,
- (2) ADH secretion,
- (3) aldosterone secretion,
- (4) thirst,
- (5) efferent arteriole constriction (increase perfusion pressure to increase GFR),
- (6) sodium reabsorption in proximal tubules.
- Conversely, decreased angiotensin can lowers GFR by contraction of mesangial cells, thus reducing area for glomerular filtration.
Define disorders of sodium
Disorders can be caused by either an abnormality of sodium or an abnormality of water (or both).
- Hypernatramia [Na+] >145mmol/L
- Normal [Na+] 135-145mmol/L
- Hyponatraemia [Na+] <135mmol/L
What is hypernatraemia caused by?
Hypernatremia is caused by:
- Impaired thirst or level of consciousness (e.g. d_iabetes insipidus_)
- No access to fluid
- Loss of fluid (e.g. burns/diarrhoea/blood loss)
- Solute diuresis (typically in hyperglycemia e.g. HONK/ DKA)
- Central diabetes insipidus*
- Nephrogenic diabetes insipidus*
- Diuretics*
- Hypergylcaemia*
- Lack of water/dehydration*
Describe Diabetes Insipidus
- Diabetes insipidus (die-uh-BEE-teze in-SIP-uh-dus) is an uncommon disorder that causes an imbalance of water in the body. This imbalance leads to intense thirst even after drinking fluids (polydipsia), and excretion of large amounts of urine (polyuria).*
- While the names diabetes insipidus and diabetes mellitus sound similar, they’re not related. Diabetes mellitus — which can occur as type 1 or type 2 — is the more common form of diabetes.*
Diabetes insipidus is reduction in amount or efficacy in ADH, which leads to polyuria and water loss; _dilute urin_e (<200mOsm/kg).
It can be central or nephrogenic:
- Central is caused by traumatic brain injury (50%).
- Nephrogenic is problem with aquaporin channels in kidney (due to partial or complete resistance to ADH)
Therefore, patient can’t drink enough to keep up with losses. This leads to elevated plasma osmolality, hypernatremia, and dehydration.
What is Pseudohyponataremia?
How can you tell if someone is pseudohyponatraemic?
Pseudohyponatremia is low serum [Na+] due to measurement errors.
- This can be caused by hyperglycemia **, hyperlipidemia, hyperparaproteinemia, which interferes with [Na+] measurement.
- Hyperglycemia is high serum concentration of glucose. It causes osmotic shift of water from cells to bloodstream. This may occur with infusions of mannitol.
Therefore, it is important to check serum osmolarity. If serum _osmolarity is normal (_with low serum [Na+]), the patient has pseudohyponatremia.
Describe Hyponatremia
Hyponatremia can be caused by either
(1) excessive loss of sodium;
or/and
(2) excessive water retention.
- It is important to check urine osmolality (<100mOsm/kg is very very diluted urine); also check volume status.
- This is consistent with polydipsia (excessive thirst) and water intoxication.
Differential diagnosis is psychotropic drugs, schizophrenia, beer potomania
