The kidneys are bean-shaped organs that sit in the back of the lower abdominal cavity on either side of the spinal cord. The organs are cushioned by perinephric fat, fatty tissue that surrounds the kidneys and adrenal glands to prevent damage and movement in the abdominal cavity. Under normal conditions, humans are born with two kidneys, but it is possible to survive with only one as long as disease or other factors do not strain the organ.
Kidney Function
The main function of the kidney is to maintain the fluid, electrolyte, and pH balance of the body by filtering ions, macromolecules, water, and nitrogenous wastes from the blood based on the body’s condition. Wastes filtered out of the blood drains from canals in the kidney into the bladder as urine. Blockage of the drainage system can cause the kidney to become congested, stretched, and potentially scarred. Functioning kidneys are necessary to maintain life, and one measure of their function is the glomerular filtration rate. A loss of kidney function results in the need for dialysis, which is an artificial method of removing wastes from the blood by running the blood from the body, through an artificial kidney, and then back into the body.
Renal Anatomy
The kidneys are situated in the perinephric fat but are also surrounded by a tough fibrous layer of connective tissue called the fibrous renal capsule. The hilus of the kidney is the central indent that gives the bean shape to the organ, and it is where the artery carrying oxygenated blood enters, and the ureter carrying urine exits, the kidney. Within the kidney, the renal artery branches into smaller blood vessels.
The basic unit of the renal tissue is the nephron. Each kidney contains approximately 1 million nephrons making up 10 to 20 renal pyramids. Each nephron consists of a renal corpuscle and renal tubule that interweave with small blood vessels called capillaries. Nephron function and the determination of which ions are filtered or resorbed are regulated by endocrine hormones.
Fluid filtered out of the blood by the nephrons flows into the collecting tubules, which drain in the renal calyces. Peristalsis, the name for consistent muscular contractions, forces the fluid, which is now urine, through a calyx into the funnel-like renal pelvis. Urine drains from the pelvis to the ureter, a vessel that transports urine to the bladder where it is stored for urination.
Renal Medulla and Cortex
The inner portion, or core, of the kidney is the renal medulla. The outer portion of the kidney is the renal cortex, which is smooth and contains projections called cortical columns. The renal tubules of the nephrons connect the medulla and cortex, with the loop of Henle being found in the medulla.
The base of each renal pyramid is in the cortex and the apex of each pyramid points inward to the medulla, giving the round shape to the outer kidney. The cortical columns are placed between the renal pyramids. The cortex contains the corpuscles and is where the blood vessels and cortical collecting ducts are found.
In general, filtration occurs in the cortex and the calyces and pelvis are found in the medulla.
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