Electrolyte Balance

Kidney Nephron by Holly Fischer
Line divides cortex (light) and medulla (dark).

The kidney maintains the balance of electrolytes in the body. Its four functions include filtration, reabsorption, secretion and synthesis.

1. Filtration occurs at the glomerulus, where the negatively charged, sieve-like glomerular membrane filters out molecules that are large and negatively charged. 
2. Reabsorption occurs in the renal tubules (the majority at the proximal convoluted tubule). Nonpolar molecules are easily reabsorbed while ions are likely to be excreted. However, sodium is actively reabsorbed into the blood by co-transporters to power the transportation of other ions. 
3. Secretion occurs in the renal tubules, where unfiltered molecules in the blood may still be secreted into the tubular lumen to be excreted or reabsorbed. 
4. The kidney produces renin, bicarbonate, prostaglandin and erythropoietin, among other molecules. 

Loop of Henle, which direction?

Water reabsorption occurs in the descending limb. Think waterfall and water splashing out of the loop. Sodium reabsorption occurs in the ascending limb, which is also impermeable to water. This creates an interstitial environment that is "very salty" or hyperosmolar, and where water permeability is increased by ADH. Therefore, ADH enables water to leave the collecting ducts and concentrates urine. 

Regulation of sodium

Ingestion

The hypothalamus induces "thirst" if sodium concentration is high. 

Excretion

There are two autoregulatory processes. In glomerular-tubular balance, the glomerular filtration rate (GFR) influences how much sodium is filtered, and therefore, how much of (the same amount of) sodium is reabsorbed. In tubulo-glomerular feedback, the macula densa senses excessive fluid flow, which is reflected in increased GFR, and constricts the afferent arterioles to decrease GFR. 

Hormone atrial natriuretic factor (ANF) responds to excessive blood volume and cardiac atria dilation, and induces sodium excretion. ADH responds to increased sodium concentrations in the blood, and conserves water to restore normal serum osmolality. 

To avoid charge buildup, sodium cations must be transported with an anion, or be exchanged with a cation, such as K⁺ or H⁺. Examples: in the late proximal tubule, Na⁺ is reabsorbed with Cl^-; in the ascending loop of Henle, the Na⁺-K⁺-2Cl^- cotransporter reabsorbs these ions into the cell. 

Collecting duct by Lennert B (edited)

Regulation of potassium

Potassium is the main intracellular cation, and the Na⁺/K⁺-ATPase pump keeps potassium in cells. Aldosterone stimulates the pump to take K⁺ into the cell and Na⁺ into the blood. In times of low plasma sodium, low blood pressure or high plasma potassium, aldosterone facilitates Na⁺ into the tubule cells (and into the blood via Na⁺/K⁺-ATPase) while K⁺ is excreted.

pH change
When blood pH decreases (and [H⁺] increases) H⁺ tends to enter cells in exchange for K⁺. When pH increases H⁺ tends to leave cells in exchange for K⁺.

Regulation of chloride
The most common extracellular anion, and is exchanged with anion bicarbonate or is carried with sodium.