Page 68 - Physiology and Pathophysiology MNU 2024-2025 نظرى

Clinical pharmacy 2024/2025 Level 2 Pharm D Physiology and Pathophysiology (MD303)

+
+
1) When the membrane is permeable only to Na 2) When the membrane is permeable only to K
+
o K ions initially diffuse out of the cell down their concentration gradient
 Na+ ions initially diffuse into the cell down their concentration [from higher concentration to lower concentration]  excess +ve

gradient  excess +ve ions accumulates in the ICF along the internal ions accumulate in the ECF along the external surface of the plasma

surface of the plasma membrane. membrane.
–
–
 An excess of -ve impermeable extracellular anion (Cl ), remains o The impermeable A ions remain inside the cell  attracted to these
outside the cell along the external surface of the plasma membrane. positive charges, along the internal surface of the plasma membrane
 This creates a +ve membrane potential  negative membrane potential [the inside of the cell is negative

relative to the outside

+
o As the positively charged K ions continue to diffuse outward  an
+
• As the Na ions continue to diffuse inward, an electrical gradient
electrical gradient begins to develop & excess (+) charges are
develops.
+
accumulated on the external surface.  repel any additional K ions
• The (+) charges accumulated in the ICF begin to repel any
+
o At this point, K ions not only diffuse outward down their
+
additional Na ions & oppose the further movement of (+) charges
concentration gradient, but also diffuse into the cell down their electrical
inward.
gradient
➢ the force moving Na ions inward down their concentration gradient
+
+
➢ The force that moved K ions inward balances the force that moved
+
is balanced by the force moving Na ions outward down their
+
+
K ions outward  no further net diffusion of K .
electrical gradient  no further net diffusion of sodium
o The membrane potential at this point has reached the equilibrium o The membrane potential at this point reached the equilibrium
+
potential for Na (ENa )  +60 mV potential for K+ (EK )  –90 mV
+
+

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