Tubular fluid is a substance in the kidneys. It is a liquid that flows through small tube-shaped structures in those organs, called tubules. It is very important to the functioning of the kidneys, and thus to vital biological functions such as regulating the body's biochemistry and concentrating waste products to be eliminated through urination.
A kidney is composed of thousands of smaller units called nephrons, and it is at this level that the kidney's main functions are carried out. Tubular fluid starts out in the renal corpuscle, a sack like structure sac-like structure at the beginning of the nephron. When blood in the circulatory system reaches the renal corpuscle it passes the part of the corpuscle called the glomerulus, a dense collection of semipermeable capillaries that subject the blood to intense pressure due to the fact that the small blood vessels, called arterioles, through which blood enters the glomerulus are wider in diameter than the arterioles through which it leaves. As the blood flows through, water and various water-soluble molecules pass through its membranes and so are filtered out of the blood.
The filtered blood continues onward through the circulatory system, leaving the accumulated water and waste products remain behind in a sac-like structure in the renal corpuscle called a Bowman's capsule. This is the first stage in the tubular fluid's existence. It is solution made of water along with solutes including amino acids, the sugar glucose (C6H12O6), and sodium, potassium, and chloride ions. It also contains urea (CH4N2O), which is a byproduct of some metabolic processes and is the means by which the body eliminates ammonia (NH3) from the body, and another metabolic waste product called uric acid (C5H4N4O3).
The tubular fluid then flows into the next part of the nephron, the proximal tubule. As it passes through, all of the glucose and amino acids in the fluid, along with almost all of the uric acid and most of the ions, are reabsorbed through the tubule to be returned to the bloodstream. Most of the water, about 80%, is also reabsorbed. The remaining fluid continues into the next segment of the nephron, the loop of Henle, and then on to the distal tubule, continuing to lose more water and ions to reabsorption along the way. The fluid then passes through the collecting duct, where any final alterations to the fluids chemical composition can be made, before leaving the nephron and entering the ureter, which leads to the urinary bladder.
The vast majority of the fluid that originally gathers in the renal corpuscle will be reabsorbed as it travels through the nephron before it reaches the ureter, allowing the reabsorbed molecules to remain in the body. When the tubular fluid reaches the end of the nephron all of the amino acids and sugars, most of the uric acid, and almost all of the ions originally present will have been reabsorbed, as will most of the water. Only about half of the fluid's urea is reabsorbed, however, with the other half remaining to be excreted in the urine. The large amount of urea remaining is the cause of urine's odor, since it breaks down into ammonia outside of the body.
The amount of tubular fluid gathered and processed by the human kidneys is immense. In the course of 24 hours, around 180 liters of tubular fluid will pass through a typical human's nephrons. This is considerably more fluid than the human body actually contains, which means that the molecules in a person's bloodstream capable of being absorbed by the glomerulus pass through the nephrons as tubular fluid and then return to the bloodstream several times each day. In the course of a normal day, between 1 and 2 liters of this will be excreted.