The kidneys are a pair of bean-shaped organs that are situated within the retroperitoneal space, high in the abdominal cavity and, relative to the spinal column, occupy a position between the twelfth thoracic and fourth lumbar vertebrae. The right kidney is slightly lower than the left because of the position of the liver.
Figure 1 – The position of the kidneys
Adult kidneys measure approximately 10 to 12cm in length and 5 to 7 cm in width, and weigh approximately 150g.
The lateral surface of the kidney is convex, and the medial surface is concave with an indentation in the middle of the organ called the renal hilum. This is where the ureter, blood vessels, lymphatics and nerves enter and exit the kidney.
Figure 2 – Diagram of the Kidney
The kidneys have three layers of supportive tissue; a renal capsule, which is a smooth, fibrous layer, that acts as a barrier against infection and direct trauma. Next is the perinephric fat, this holds the kidney in place and cushions it from trauma. The third (outer) layer is the renal fascia; this is a dense fibrous layer of connective tissue that anchors the kidney to its surrounding structures and the posterior abdominal wall.
The kidneys serve the body as a natural filter of the blood. The kidneys receive blood via the paired renal arteries and as the blood passes through each kidney it is “cleaned” (i.e. filtered), and the body’s waste products and excess fluid are removed. These waste products are then sent through the ureters, to the bladder to be passed out of the body as urine. Many of the kidneys’ functions are accomplished by relatively simple mechanisms of filtration, reabsorption, and secretion, which take place in the nephron.
Nephron comes from the Greek “nephros” meaning “kidney”. Each kidney consists of approximately one million nephrons. The nephrons are the blood processing, urine-producing working centres of the kidneys. They carry out nearly all of the functions of the kidney necessary to maintaining the chemical balance that is vital for life. The chief role of the nephrons is to filter the blood, reabsorb what is needed and excrete the rest as urine. The key functional areas of the nephrons can be divided into the following stages: filtration, mass salvage of essential substances, water balance, fine control of water, sodium and hydrogen, and elective water reabsorption under hormonal control.
If you were to cut a kidney in half you would see that it is divided into a dark outer area called the renal cortex, and an inner lighter-coloured area called the renal medulla (medulla means “inner portion”). Within the medulla there are between 10 and 18 triangular structures known as renal pyramids. The renal pyramids consist of renal tubules that transport the urine produced to the renal calyces, which are the initial part of the collecting system. The base of each pyramid adjoins the kidney’s cortex; the apex terminates at a renal calyx. The renal cortex and pyramids make up therenal parenchyma, the functional substance of the kidney, and it is here that the one million nephrons can be found.
Urine formation occurs in the nephrons. Fluid filtered out of the blood by the nephrons flows into the tubules of the renal pyramids, which drain in the renal calyces. The renal calyces are funnel-like extensions where urine is collected before it passes through the ureter to the bladder. Minor calyces merge to form major calyces, which merge to form the renal pelvis, the upper portion of the ureter. The calyces are made of smooth muscle and lined with transitional cell epithelium; this layer of tissue also lines the renal pelvis, the ureter and bladder. Contractions of the smooth muscle in the walls of the calyces and renal pelvis propel the urine down toward the ureter.
The blood supply to the kidneys is via the renal arteries which branch directly from the abdominal aorta. The right renal artery is longer than the left as the aorta lies to the left of the midline. As the artery enters the hilum it divides into five segmental arteries, these then subdivide several times to ultimately divide into arterioles, which perfuse the nephrons where filtration takes place. Blood flows from the kidney through the renal vein after waste products have been removed. The venous drainage from the kidneys is roughly parallel to the arterial supply. The renal veins empty directly into the inferior vena cava, and the left renal vein is slightly longer than the right because the inferior vena cava lies to the right of the midline.
The kidneys have several other functions and you can find more about this from a good urology or renal book (such as Urological Nursing by Fillingham and Douglas 2004). These functions include blood pressure regulation, acid-base balance, toxin removal, red blood cell production, activation of vitamin D, water balance and electrolyte balance.