The kidney is a bean shaped organ in the abdominal cavity. It is made of intricate webs of blood vessels and drainage canals for filtering wastes from the blood and maintaining the fluid and electrolyte balance of the body. Humans normally have two kidneys but can function with only one in the absence of disease.
Waste filtered out by the kidney drains into the bladder as urine, making the kidney one of the main structures of the urinary tract. The basic structural and functional unit of the kidney is the nephron. Nephrons filter waste from the blood and maintain the fluid and electrolyte balance via a series of tubules and ducts weaving around the blood vessels. Their function, which ions are filtered or resorbed, is regulated by endocrine hormones.
The kidneys are surrounded by fatty tissue called perinephric fat, which also protects the attached adrenal gland. The fibrous renal capsule is connective tissue that creates a tough fibrous layer around the kidney and is covered the perinephric fat. These layers cushion and protect the kidney from damage. The renal artery enters the medulla through the hilus of the kidney. The renal tubules connect the medulla and cortex, the two sections of the kidney.
The renal cortex is the outer portion of the kidney. Often smooth on the outside, the cortex has projections known as cortical columns between the renal pyramids. The cortex contains the corpuscles and tubules, but not the loops of Henle, which enter the medulla. The cortex is also where the blood vessels and cortical collecting ducts are found and where ultrafiltration occurs. The renal medulla is the core of the kidney. The human renal medulla contains 10-20 renal pyramids.
Basic units and filtration
Nephrons combine together to make up the structures known as the renal pyramids. The pyramids are sections of cone shaped tissue. The base is in the cortex and the apex points inward to the medulla.
The renal corpuscle is the initial filtering component of the nephron and is made up of two structures - the glomerulus and the Bowman's capsule. The glomerulus is a capillary extension that receives blood from the renal circulation via an arteriole. The high pressure in the glomerulus pushes water and solutes into a space outside the blood vessel. The space is protected by the Bowman's capsule. The fluid then proceeds to the renal tubule.
The renal tubule carries filtered fluid from the glomerulus to the collecting ducts. In the process, parts of the tubule act on the fluid to further filter the waste and maintain balance by reabsorbing water, salts, glucose, and amino acids. The parts of the tubule are the proximal tubule, the loop of Henle, and the distal convoluted tubule.
The proximal tubule reabsorbs water, salts, glucose, and amino acids from the fluid filtered out of the blood by the renal corpuscle. The loop of Henle turns to pass back into the cortex from the medulla. This portion of the tubule concentrates salts from the urine in the interstitium, the tissue space around the loop, and is also known as the nephron loop. The distal convoluted tubule is the distal portion of the renal tubule and has cells specialized in active transport. It is involved in maintaining the pH of the urine and blood. The distal tubule is heavily regulated by the endocrine system.
The collecting ducts are the final portion of the nephron. Fluid that has been filtered and concentrated by the corpuscle and tubule enters the collecting tubule. The system extends through both the renal cortex and medulla. Depending on the body's needs, the ducts become permeable or impermeable to water in order to either concentrate or dilute the urine. Lower portions of the collecting ducts are also permeable to urea, the nitrogen-containing component of urine.
The renal calyces are structures where urine drains from the nephron. Consistent muscular contractions, called peristalsis, force the fluid through the calyx into the renal pelvis. The renal pelvis is a funnel-like structure. Urine is propelled out of the calyces into the pelvis where it drains into the ureter, which transports urine to the bladder where it is stored for urination.