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Lecture 35

BIOL 1119 Lecture 35: Class 38

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BIOL 1119
Christopher Richardson

Class 38 Electrolytes - Functions: o Chemically reactive in metabolism o Determine cell membrane potentials o Affect osmolality of body fluids o Affect body’s water content and distribution - Major cations: Na+, K+, Ca+2, H+ - Major anions: Cl-, HCO3-, PO4-3 Sodium Functions: - Responsible for the resting membrane potentials of cells, and its inflow is an essential event in nerve and muscle function - The principle cation of the ECF, for sodium, along with chloride, are about 85- 90% of ECF solutes - Sodium gradients across the plasma membrane also provide the potential energy involved in co-transport of glucose, chloride, potassium, and calcium: up against that concentration: depend on sodium gradient for their source of energy even though sodium enters through ion channel: bc of Na-K pumps: sodium behaves as if it never entered a cell in the first place - Na acts like a non-penetrating solute: cannot pass through cell membranes Sodium Homeostasis: - Sodium excreted is sodium filtered minus sodium reabsorbed. - Sodium is not secreted - More reabsorbed = less excreted - No receptors in body to detect total sodium levels - Short term regulation of total body sodium is mainly related to changes in BP - Thus, regulation of BP simultaneously regulates total body sodium - Low total body sodium and low plasma volumes: lowers BP o Directly lowers GFR ▪ Decreases sodium and water excreted in urine: less urine produced o Activates symp output via baroreceptors: constricts afferent arterioles to lower GFR ▪ Reduces sodium and water loss: helps prevent further decrease in blood volume and BP - Long term regulation of sodium is through control of tubular sodium reabsorption o The major factor determining the rate of tubular sodium reabsorption in kidney is aldosterone (salt retaining hormone) - Either low BP, causing a lower GFR, or lower sodium intake means less sodium filtered, which means less sodium flow in tubular fluid o Low Na+ flow is detected by macula densa o Renin leads to increased plasma angiotensin II, which stimulates aldosterone secretion from adrenal cortex - Aldosterone stimulates sodium reabsorption from large intestine and ducts from sweat glands - Unlike ADH, aldosterone does not change ECF sodium concentration - Increased blood volume and increased BP inhibits renin-angiotensin-aldosterone mechanism o Aldosterone increases Na reabsorption in DCT and cortical CD by increasing Na-K pumps in basolateral side of tubule epithelial cell and adding Na and K channels in luminal side of tubule epithelial cells o AGII can also directly increase Na reabsorption by increasing Na-H antiport activity in PCT o Unlike ADH, aldosterone does not change ECF sodium concentration bc water follows sodium reabsorption in the cortical CD into the tissue fluid and then into blood - Increase blood volume and BP causes ANP release, which inhibits aldosterone and renin release - Thus, increase in urine volume, decrease in blood volume, and decrease in BP - Eventually increased blood volume and increased blood pressure inhibits the renin-AGII-aldosterone mechanisms, and the kidneys reabsorb almost no sodium beyond the PCT - True imbalances in sodium concentration are relatively rare bc changes in sodium are almost always accompanied by proportionate change in water volume Fluid Compartments - body water is distributed among certain fluid compartments separated by selectively permeable membranes, which differ in chemical composition - fluid compartments o 65% ICF o 35% ECF ▪ 25% tissue fluid (most) ▪ 8% blood plasma, lymph (2 ndmost) ▪ 2% transcellular fluid (cerebrospinal, synovial, peritoneal, and other types) Water and Electrolyte Movement - fluid is continually exchanged between compartments by way of capillary walls and plasma membranes - bc water moves so easily, osmotic gradients between ICF and ECF do not last for long - osmosis from one compartment to another is determined by the relative concentration of solutes in each compartment. - as osmolarity in tissue fluid rises, water leaves cell (and vice versa) – at equilibrium the osmolarities of ECF and ICF are same - electrolytes are the most abundant solute particles particularly sodium in ECF and potass
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