The kidneys maintain the pH, ion, and fluid balance of the blood. Humans usually have two of these bean-shaped organs in the abdominal cavity. To achieve their function, the kidneys are made of basic filtering units, called nephrons, intertwined with a web of blood vessels and funnel-like drainage canals.
Basic Function - Homeostasis
To achieve fluid balance, the kidney filters waste, excess ions and molecules, and excess water from the blood. This filtered material becomes urine and drains into the bladder. The filtering and re-absorption of ions is regulated by endocrine hormones, including the renin-angiotensin system and some secreted by the adrenal glands, which are found on top of the kidneys. This balance of water and salt ions also regulates blood pressure.
Balance is achieved by filtering. The basic functional unit of the kidney is the nephron. Parts of the nephron are closely situated to blood vessels, including the glomerulus, where a very high pressure capillary extension is fed by the renal circulation. This high pressure pushes water and other molecules, such as sugars and ions, into the interstitium. This extra-tissue space is protected by the Bowman's capsule. The filtered fluid then proceeds through the structures of the nephron to the collecting ducts, where some additional filtering may occur.
Parts of the renal tubules, the structures downstream from glomerulus filtering, further filter the waste fluid and reabsorb some solutes depending on what the body needs. The proximal tubule reabsorbs water, salts, glucose, and amino acids. The loop of Henle concentrates salts in the interstitium. The distal convoluted tubule has cells specialized in active transport and is involved in maintaining the pH of the urine and blood. The collecting ducts become permeable or impermeable to water depending on whether the urine needs to be concentrated or diluted, and lower portions of the ducts are also permeable to urea, the nitrogen-containing component of urine.
The filtered material, now called urine, drains from the ducts to the renal calyx and into the renal pelvis via peristalsis, which are consistent muscular contractions. The renal pelvis acts as a funnel for the urine to leave the kidneys. Each kidney drains into its own ureter, a roughly 3 mm diameter muscular tube, or duct, connecting the kidney to the bladder. Urine backflow into the kidney is prevented by valves in the ureter and bladder pressure. Healthy urine is sterile, but backflow into the kidney can cause kidney disorders and sometimes infection depending on the health of the bladder.
Blood Pressure Regulation
Though other hormones and organs, including the heart and blood vessels, are involved in blood pressure regulation, the kidney also plays a major role. Physical trauma or defects of the kidney and a weakening of the renal system can result in high blood pressure (hypertension) because of aberrant filtering of water and metabolic products from the blood. As a person ages, the kidneys may not work as well as they once did, leading to an increased incidence of hypertension as a population ages.