Cells and large proteins are too large to cross the capillary membranes of the glomerulus, and cannot be filtered into Bowman's capsule. After this initial step of filtration, urine in the nephron is only affected by secretion and reabsorbtion of water, ions, and other small molecules. Finally, each kidney is composed of about one million nephrons.

The kidney-nephron system is one of the few portal systems in the human body. Portal systems link two arteriole or capillary systems together, in this case the afferent and efferent arterioles. The efferent arterioles leave the glomerulus after it has been filtered into Bowman's capsule, and follow along the nephron to pick up any reabsorbed material from the nephrons via the vasa recta capillary bed. Both the glomerulus and vasa recta are groups of capillaries, which qualifies the renal circulation as a portal system.

The proximal convoluted tubule is incredible for its ability to reabsorb glucose at levels of nearly 100%. This function is due to specialized proteins that help transport the glucose out of the filtrate. Damage to the proximal convoluted tubule can lead to glucose in the urine. Another possible cause would be extremely high glucose levels in the filtrate, such that the proximal convoluted tubule is incapable of properly removing all of the solute. This condition is a trademark of diabetes.

The loop of Henle, the distal convoluted tubule, and, under special circumstances, the collection duct are responsible for the reabsorption of other nutrients (such as water and various ions). 

The glomerulus, proximal tubule, distal tubule are all located in the cortex (outer portion) of the kidney, where the osmolarity of the interstitial fluid is relatively low. Both limbs of the loop of Henle and the collecting duct are located in the medulla (central portion) of the kidney, where the osmolarity of the interstitial fluid is much greater.

Filtrate movement through the nephron into different surrounding osmolarities is what allows water and sodium to be retained, if necessary, while other waste products are concentrated in the urine. The evolution of the loop of Henle is specifically designed to create the countercurrent multiplier system that allows for urine concentration and water retention in land animals. Water-based animals generally have shorter loops of Henle that may not intersect with the renal medulla, since water retention is less important.

Together, the glomerulus and Bowman's capsule form the structure known as the renal corpuscle. Blood in the capillaries of the glomerulus is forced against the walls of the vessels, where specialized epithelium and cell junctions allows fluids and small solutes to diffuse across the walls of the glomerulus and into Bowman's capsule. This process is highly dependent on pressure differentials; higher hydrostatic pressure in the glomerulus and greater solute concentration in Bowman's capsule will work to remove water and fluids from the capillary. Blood cells and large proteins are unable to pass through filtration and remain in circulation. Ions, small sugars, amino acids, and nitrogenous wastes pass through filtration, and are either reabsorbed back into the blood or excreted in the filtrate.

The proximal and distal tubules are regions of the nephron that are located in the renal cortex and specialize in ion reabsorption. The renal pelvis is located in the renal medulla and serves as the final collecting point for filtrate from multiple collecting ducts before transferring it to the ureter.

The function of the kidney is to filter wastes out of the blood and concentrate them into a filtrate that can be excreted from the body. Nephrons are the functional unit of the excretory system, meaning that each nephron is capable of concentrating wastes into filtrate. Each nephron is made of a single long tubule, with different regions modified to transport different ions and wastes into or out of the filtrate. The proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule are the principle regions of the nephron.

The glomerulus and Bowman's capsule form the renal corpuscle, the site of blood filtration. While the nephron serves to concentrate filtrate, the renal corpuscle separates the filtrate from the blood. The glomerulus is a system of capillaries, and carries blood rather than filtrate. It is kept separate from the nephron by the barriers in the glomerulus walls and Bowman's capsule.

Filtration of blood occurs in the renal corpuscle, which is composed of the glomerulus and Bowman's capsule. The glomerulus is a complex net of capillaries that carry blood adjacent to Bowman's capsule. Bowman's capsule collects fluid filtrate from the blood and transfers to the rest of the nephron. The glomerulus is the only portion of the nephron to contain blood rather than filtrate.

After entering Bowman's capsule, filtrate is passed to the proximal convoluted tubule, loop of Henle, distal convoluted tubule, and finally the collecting duct.

The nephrons are the smallest functional unit in the kidney. They are responsible for the filtration and concentration of urine.

Neurons are the smallest functional unit of the nervous system. The renal medulla is the region on the interior of the kidney, as opposed to the renal cortex. Renal corpuscles are used to filter blood into the nephron, and consist of the glomerulus and Bowman's capsule.

The kidneys are responsible for the excretion of urine, while the skin secretes sweat from the body. The pancreas secretes digestive enzymes into glands and ducts. Exocrine function differs from endocrine function, in that exocrine excretions involve ducts that generally lead to the body's exterior. Endocrine functions, in contrast, will enter the blood stream and be distributed throughout the body.

The overall organ systems that perform excretory functions are highly variable between animal groups. All animals, however, share some common features. Water and ion balances must be tightly regulated by reabsorption mechanisms, in order to maintain organismal homeostasis and functions. To assist in this process, the vessels for filtrate transport must have a large surface area. Proteins, pumps, and channels along this surface serve to moderate water and ion transport.