Industrial chemical processes often involve resource intensive separation steps. While most biochemical synthetic methods themselves do not need separation steps, other physiological processes do. One example is the filtering of blood which removes potentially harmful organic and inorganic materials. Water, as well as useful minerals, glucose, and proteins are separated from the harmful materials and recycled back into the blood stream. If the water and useful compounds were excreted along with the harmful compounds, the organism would have to consume gallons of water and many grams of minerals on a constant, daily basis to recover the loss.
The filtration aspect of the kidney can be thought of as a collection of hundreds of thousands to over a million independent assembly lines carrying out the same intricate task. The assembly line here is the nephron, a tubular structure with specialized regions along its length. The great quantity of useful water that is excreted is reabsorbed back into the blood as the nephron first descends into the deeper tissues of the kidney. This part of the nephron is the descending limb of its “loop of Henle.” The cells lining the descending limb are permeable to water and impermeable to ions. As the limb descends through progressively saltier (hypertonic) regions of the kidney, water in the relatively un-salty (hypotonic) filtrate passively flows out of the nephron’s lumen (interior space) and into the surrounding salty tissues via osmosis. The concentrated filtrate then travels up the ascending limb of the loop of Henle, which ascends back towards the surface of the kidney through progressively less salty (increasingly hypotonic) interstitial regions. In contrast with the descending limb, the cells that line the lumen of the ascending limb are permeable to ions and impermeable to water. Useful ions in the filtrate are reabsorbed as they flow down their concentration gradient into the surrounding kidney tissues through protein channels, while water is excluded.
Further along the nephron, protein pumps actively transport useful ions and substances between the lumen and the kidney cells and vice versa. Active transport requires an energy input by depleting ATP molecules. More water is also absorbed, so that by the time the filtrate reaches the bladder as fully formed urine, it contains only one percent of the volume of the early filtrate.
Nephron showing movement of ions, water, glucose, and proteins
from the tubules to the bloodstream. Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
“Tubular reabsorption is the movement of fluid and solutes from the tubular system into the peritubular capillaries. This process allows the body to retain fluid and desired solutes. At a glomerular filtration rate of 125 ml/min., the kidneys produce 180 liters of filtrate daily. Yet the average urine output is only 1,000 to 1,500 ml. Through reabsorption, 99% of the glomerular filtrate is returned to the bloodstream. The proximal tubule is the major site of reabsorption in the tubular system… it reabsorbs, and thus, returns to the plasma, up to 100% of the filtered solutes that the body does not routinely wish to discard, such as glucose, amino acids, and bicarbonate and reabsorbs a large percentage of solutes, such as sodium, potassium, chloride, calcium, and magnesium, and water…reabsorption involves both passive and active transport mechanisms. Passive transport includes osmosis and diffusion while active transport mechanisms, such as primary and secondary transport and endocytosis, require the use of energy to move substances against an electrochemical gradient. Reabsorption of fluid and solutes is regulated to meet the body’s physiological needs.” (Chmielewski 2003:187-188)