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Lecture 3

# LMP LECTURE THREE.docx

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School
University of Toronto St. George
Department
Physiology
Course
PSL301H1
Professor
all
Semester
Winter

Description
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 - HCO 3 22 – 30 mmol/L Ca2+ 2.20 – 2.60 mmol/L Intracellular Ions Concentration In Plasma + K - 3.5 – 5.0 mmol/L PO 4 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 H O input is low yet H O output is high is called polyuria (urinating too much). This results in your total 2 2 water system being too low  Overhydration:occurs when H O 2nput is high, which is called polydipsia (drinking too much water), yet H O 2utput 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 SODIU
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