reabsorption (100%) via carboxylate transporters.
The body is very sensitive to its pH level. Outside the range of pH that is compatible with life, proteins are denatured and digested, enzymes lose their ability to function, and the body is unable to sustain itself. The kidneys maintain acid-base homeostasis by regulating the pH of the blood plasma. Gains and losses of acid and base must be balanced. Acids are divided into "volatile acids" and "nonvolatile acids". See also titratable acid.
The blood is filtered by nephrons, the functional units of the kidney. Each nephron begins in a renal corpuscle, which is composed of a glomerulus enclosed in a Bowman's capsule. Cells, proteins, and other large molecules are filtered out of the glomerulus by a process of ultrafiltration, leaving an ultrafiltrate that resembles plasma (except that the ultrafiltrate has negligible plasma proteins) to enter Bowman's space. Filtration is driven by Starling forces.
Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood. It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the intestines).
Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium (the connective tissue that surrounds the nephrons), and then the transport of these substances from the interstitium into the bloodstream. These transport processes are driven by Starling forces, diffusion, and active transport.
Renal plasma threshold
The renal plasma threshold is the minimum plasma concentration of a substance that results in the excretion of that substance in the urine.
For example, the renal plasma threshold for glucose is 180 to 200 mg per 100 ml. Glycosuria (sugar in urine) results when the plasma glucose concentration reaches and exceeds the renal plasma threshold of glucose. When the plasma glucose concentration is very high, the filtered glucose can saturate the carriers and reach the transport maximum of that molecule. Any amount past the transport maximum will continue through the renal tubules and be excreted in the urine.
In some cases, reabsorption is indirect. For example, bicarbonate (HCO3-) does not have a transporter, so its reabsorption involves a series of reactions in the tubule lumen and tubular epithelium. It begins with the active secretion of a hydrogen ion (H+) into the tubule fluid via a Na/H exchanger:
In the lumen
The H+ combines with HCO3- to form carbonic acid (H2CO3)