The kidneys are bean shaped organs in the abdominal cavity that are part of the urinary tract. The organs are held in place by connective tissue, the fibrous renal capsule, and cushioned by fatty tissue, the perinephric fat. Humans normally have two kidneys, but they can survive with one if healthy. The kidneys maintain the fluid and ion balance in the body, filtering out wastes in the process.
Basic Kidney (Renal) Structures
The renal medulla is the inner portion of the kidney; the renal cortex is the smooth outer portion with projections called cortical columns. The medulla and cortex are connected by the renal tubules, drainage canals that carry wastes removed from the blood, a process regulated by endocrine hormones that target the kidneys. The kidney receives blood from the renal artery, which enters the renal medulla through the hilus, the concave portion of the kidney. Within the kidney, the artery branches into smaller blood vessels.
The basic building blocks of the kidney are the nephrons, which consist of intricate webs of blood vessels and drainage canals. The nephrons form units called renal pyramids, sections of cone-shaped tissue. The apices of the pyramids point inward to the medulla, which contains 10-20 pyramids.
Briefly, the other structures, which are discussed in detail in terms of physiology below, include:
The renal tubules, which are made of the proximal tubule, loop of Henle (the portion of the tubules in the medulla), and distal convoluted tubule.
Within the cortex – the cortical collecting ducts and corpuscles, the initial filtering component of the nephron. Corpuscles are made up of the glomerulus and Bowman’s capsule.
The renal calyces, which are structures through which urine (the waste fluid) drains from the nephron into the ureters, drainage canals leading to the bladder.
As mentioned above, the initial filtering component of the nephron is the corpuscle in the cortex. Ultrafiltration also occurs in the cortex in the cortical collecting ducts of the final portion of the nephron. These two mechanisms occur in the renal cortex because of the presence of the blood vessels in this region of the kidney: The glomerulus is a very high pressure capillary extension fed by the renal circulation. The high pressure pushes water and other material (such as sugars and salt) into the space outside the blood vessel, the interstitium, which is protected by the Bowman’s capsule. The filtered fluid then proceeds to the renal tubule, which carries it to the collecting ducts.
Parts of the tubule further filter the fluid and maintain the body’s ion and fluid balance by reabsorbing some solutes. The proximal tubule reabsorbs water, salts, glucose, and amino acids. The loop of Henle, also known as the nephron loop, turns to pass back into the cortex from the medulla and concentrates salts in the interstitium. The distal convoluted tubule is the distal portion of the renal tubule; it has cells specialized in active transport and is involved in maintaining the pH of the urine and blood.
Fluid filtered and concentrated by the corpuscle and tubule enters the collecting tubule. The ducts become permeable or impermeable to water in order to either concentrate or dilute the urine depending on the body’s needs. Lower portions of the collecting ducts are also permeable to urea, the nitrogen-containing component of urine.
Consistent muscular contractions, called peristalsis, force the fluid through the renal calyx into the funnel-like renal pelvis. It is here that urine leaves the kidneys, having started as an assortment of water and solutes in the blood.