PSL301H1 Lecture Notes - Lecture 3: Oliguria, Plasma Osmolality, Angiotensin-Converting Enzyme

17 views5 pages
Published on 12 Apr 2013
School
Department
Course
Professor
Page:
of 5
LECTURE THREE: WATER AND ELECTROLYTES
BODY FLUID COMPARTMENTS
Intracellular fluid (ICF) volume within cells: 28L
Extracellular fluid (ECF) volume of fluid outside the cell: 14L
- Plasma (aqueous: a portion of blood): 3.5 L
- Interstitial fluid (ISF) bathing the cell: 10.5L
There is a set point where your body is trying to get the fluid around a certain point. So, if everything is working normally input should
be bigger than output
ELECTROLYTES
Extracellular Ions
Concentration In Plasma
Na+
135 145 mmol/L
Cl-
96 106 mmol/L
HCO3
-
22 30 mmol/L
Ca2+
2.20 2.60 mmol/L
Intracellular Ions
Concentration In Plasma
K+
3.5 5.0 mmol/L
PO4
-
0.70 1.10 mmol/L
Anions
Since you can’t measure the concentration of ions inside, so we measure the concentration on the outside so we try and infer the
concentration on the inside
CONCENTRATION AND OSMOLALITY
Concentration is the amount of solute in given volume of solvent
- e.g. mmol/L, g/L, etc.
Osmolality is the amount of solute particles in a given weight of solvent. Independent of size and charge
- mmol/kg, mOsm/kg
- Plasma osmolality is 280 300 mOsm/kg
- (convenient way): estimated as 2[Na+] (predominant molecule) + [glucose] + [urea]
Osmolarity is the amount of solute particles in 1 kg of water
- applies to dilute aqueous solutions; e.g. urine (can use osmolarity and osmolality interchangeable)
Osmotic pressure is created when the flow is not identical and since water is easily passed through it creates and pressure
ONCOTIC PRESSURE
Capillary wall also acts as a semi-permeable membrane where water and small solutes pass freely through pores but not plasma
proteins and lipids
Proteins (esp. albumin) generate an osmotic pressure that draws water out of the ISF and into the plasma to create an oncotic
pressure
This acts to balance the hydrostatic pressure
Normally the proteins stay within the plasma but may leak following injury (e.g. burns)
Input: (Intake: 0.5 5 L/day)
Output: Renal: 0.5 4 L/day (kidney, urine)
GI: < 0.1 L/day (Poo)
Sweat, Respiration: 0.5 1 L/day: INSENSIBLE LOSS (can’t be measured easily)
DEHYDRATION AND OVERHYDRATION
Dehydration: occurs when H2O input is low yet H2O output is high is called polyuria (urinating too much). This results in your total
water system being too low
Overhydration:occurs when H2O input is high, which is called polydipsia (drinking too much water), yet H2O output is low which is
called oligouria (urinating too little)
FEATURES OF DEHYDRATION
Increased pulse (a compensation tactic)
Dry mucous membranes
Soft and sunken eyeballs
Decreased blood pressure (a compensation tactic)
Decreased urine output
Decreased skin turgor (loss of elasticity
The Loss of Water (Biochemical features):
Plasma sodium levels increase
Blood urea levels increase
Hematocrit (the volume of the RBC cell portion): thus the levels increase
FEATURES OF OVERHYDRATION
Pitting edema (a sign of overhydration): fluid builds up in the extremities
Normal pulse
Normal mucous membranes
Normal eyeballs
Normal or increased blood pressure
Normal or decreased urine output
Increased Skin Turgor
Gaining of Water (Biochemical features):
Plasma sodium levels decrease
Blood urea levels decrease
Hematocrit decrease
REGULATION OF WATER
1. Control of osmolality (direct way): there are sensors in the hypothalamus that send signals when osmolality is too high
a) Anti-diuretic hormone (ADH, or vasopressin). It is secreted from the pituitary gland and acts on the kidney’s collecting tubule
(retention of water)
b) Thirst drink water
2. Control of blood volume (indirect way):
a) Renin is secreted from kidney in response to ↓ blood flow (volume or pressure); renin is an enzyme that converts
angiotensinogen to angiotensin I
b) Angiotensin converting enzyme (ACE): ATI ATII
c) Angiotensin II stimulated vasoconstriction, Na+ resorption in kidney, and secretion of aldosterone
d) Aldosterone stimulates Na resorption in kidney
Two disorders arise from irregularities in fluid regulation:
1. Syndrome of Inappropriate ADH secretion (SIADH): the pituitary secreted too much ADH. This causes excess retention of water and
increased levels of Na+. The result is hyponatremia: is an electrolyte disturbance in which the sodium concentration in the serum is
lower than normal and sometimes fluid overload
2. Diabetes Insipidus (is an uncommon condition that occurs when the kidneys are unable to conserve water as they perform their
function of filtering blood): the pituitary is non-functional and is not secreting ADH which results in low to no water retention
SODUIM BALANCE
Intake: Diet 100 300 mmol/day
Body stress: 3700 mmol
- 25% in Bones and tissues
- 75% in ECF
Output: (the body keeps only want it needs and secreted the rest)
- Renal loss: matches intake
- Sweat: 5 mmol/day
- GI loss: 5 mmol/day
REGULATION OF SODIUM
o Aldosterone:
- secreted by adrenal gland in response to APII
- stimulates Na resorption in kidney tubules at the expense of H+, K+
o Atrial Naturetic Peptide (ANP):
- secreted by atria of heart (sends a signal to the kidney and tells it’s to secrete Na+)
- stimulates Na excretion (is often followed by water) in kidney tubules
There are two different scenarios:
1. If the flow to the kidney has decreased; the kidney secretes renin which goes through the pathway to
release aldosterone from the adrenal cortex. This results in decreased excretion is Na+ and water (meaning
they got retented) and an increased excretion of K+
2. The opposite is also possible, when there is a decreased in renin secreted, its leads to increased Na+
excretion and decreased K+ excretion
HYPERNATREMIA
Increased serum [Na] (>145 mmol/L)
1. Body water
a) water intake
b) water loss (Diabetes insipidus)
2. Water loss > Na loss (hypotonic)
a) Excessive sweating, diarrhea in children
b) Osmotic diuresis (diabetes mellitus: a person has high blood sugar more than the kidney can hold; so the
glucose pills into your urine causing a lot of water to be lose)
3. Excess Na+ Intake
a) Diet, intravenous administration
b) Hyperaldosteronism (Conn’s Syndrome: is an aldosterone-producing adenoma; Cushing’s Syndrome:
describes the signs and symptoms associated with prolonged exposure to inappropriately high levels of
the hormone cortisol)
HYPONATREMIA
Decreased serum [Na] (< 135 mmol/L)
1. Body water (requires excess H2O)
a) Syndrome of Inappropriate secretion of ADH (SIADH)
b) Renal failure (kidney unable to function)
c) Edema (where the water is essential accumulating in the tissue) (e.g. due to nephrotic syndrome,
heart failure)
d) Ingestion intravenous administration
2. Na loss > Water loss (hypertonic)
a) Diarrhea, sweating, burns
b) Diuretics: some diuretics work by secreting Na+ and K+. (They are used to treat hypertension)
3. Body Na+