BIOL 2420 Lecture Notes - Lecture 5: Extracellular Fluid, Osmotic Concentration, Sodium Chloride
26 Apr 2018
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Unit 8 – Lecture 5
Sodium Balance and ECF Volume
- The average American ingest about 9 grams of salt per day (NaCl)
- Normal plasma Na+ concentration is 135-145 milliosmoles Na+ per liter of plasma
o Na+ distributes freely between plasma and interstitial fluid
o This value also represents our ECF Na+ concentration
o Clinically, it is simple to find ECF values for ions by drawing a blood sample and
analyzing the plasma portion
- If we add NaCl to the body to increase the ECF concentration to 155 milliosmoles Na+/L,
how much water would we have to add to keep the ECF Na+ concentration at
140mOsM?
o One form of an equation is:
▪ 155 mosmol/x liters = 140 mosmol/liter x = 1.1 liters
o we would have to add 0.1 liters of water for each liter of ECF volume to
compensate for the addition of the Na+
▪ assume normal ECF volume is 14 liters, we would have to add 1.4L (a 10%
gain)
- suppose that instead of adding water to keep plasma concentrations constant, we add
the NaCl but don’t drink any water
o assume normal total body osmolarity is 300 mOsM and that the volume of fluid in
the body is 42L, the addition of 155 milliosmoles of Na+ and 155 milliosmoles of
Cl- would increase total body osmolarity to 307 mOsM (a substantial increase)
o because NaCl is a non-penetrating solute, it would stay in the ECF
▪ higher osmolarity in the ECF would draw water from the cells, shrinking
them and disrupting normal cell function
- out homeostatic mechanisms usually maintain mass balance
o anything extra that comes into the body is excreted
o figure to the right shows a generalized homeostatic pathway for sodium balance
in response to salt ingestion
- the addition of NaCl to the body raises osmolarity
- triggering two responses
1. vasopressin secretion
▪ causes the kidneys to conserve water and concentrate urine
2. thirst
▪ prompts us to drink water or other fluids
▪ increased fluid intake decreases osmolarity, but the combination of salt
and water intake increases both ECF volume and blood pressure
▪ triggering another series of control pathways
• brings ECF volume, blood pressure, and total-body osmolarity back
into the normal range by excreting extra salt and water
- the kidneys are responsible for most Na+ excretion, and normally only a small amount of
Na+ leaves the body in feces and perspiration
find more resources at oneclass.com
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Document Summary
The average american ingest about 9 grams of salt per day (nacl) If we add nacl to the body to increase the ecf concentration to 155 milliosmoles na+/l, how much water would we have to add to keep the ecf na+ concentration at. Cl- would increase total body osmolarity to 307 mosm (a substantial increase: because nacl is a non-penetrating solute, it would stay in the ecf, higher osmolarity in the ecf would draw water from the cells, shrinking. Na+ leaves the body in feces and perspiration in situations such as vomiting, diarrhea, and heavy sweating, we may lose significant amounts of na+ and cl- through non-renal routes. Aldosterone is a steroid hormone synthesized in the adrenal cortex, the outer portion of the adrenal gland that sits atop each kidney. It is secreted into the blood and transported on a protein carrier to its target.
