Renal System & Kidney Anatomy
The kidneys regulate the volume and concentration of fluids in the body by producing urine. Urine is produced in a process called glomerular filtration, which is the removal of waste products, minerals, and water from the blood. The kidneys maintain the volume and concentration of urine by filtering waste products and reabsorbing useful substances and water from the blood.
The kidneys also perform the following functions:
- Detoxify harmful substances (e.g., free radicals, drugs)
- Increase the absorption of calcium by producing calcitriol (form of vitamin D)
- Produce erythropoietin (hormone that stimulates red blood cell production in the bone marrow)
- Secrete renin (hormone that regulates blood pressure and electrolyte balance)
The kidneys are a pair of bean-shaped organs located below the ribs near the middle of the back. They are protected by three layers of connective tissue: the renal fascia (fibrous membrane) surrounds the kidney and binds the organ to the abdominal wall; the adipose capsule (layer of fat) cushions the kidney; and the renal capsule (fibrous sac) surrounds the kidney and protects it from trauma and infection.
The renal artery enters the kidney and the renal vein emerges from the kidney at an indentation in the middle of the organ called the hilum. The renal artery supplies oxygen and blood to the kidney. Blood flows from the kidney through the renal vein after waste products have been removed.
Formation and Elimination of Urine
The formation of urine occurs in the basic units of the kidney, called nephrons. Each human kidney contains over 1 million nephrons. Nephrons consist of a network of capillaries (called a glomerulus), a renal tubule, and a membrane that surrounds the glomerulus and functions as a filter (called Bowman's capsule). The glomeruli are where urine production begins. Urine formation occurs in the renal tubules, which travel from the outer tissue of the kidney (called the cortex), to the inner tissue (called the medulla), and return to the cortex.
Extensions of the cortex project into the medulla and divide the tissue into renal pyramids. The renal pyramids extend into funnel-like extensions (called calyces), where the collection of urine occurs. Minor calyces merge to form major calyces and major calyces merge to form the renal pelvis, the upper portion of the ureter.
Each section of the renal tubule performs a different function. As the tube leads away from Bowman's capsule into the cortex, it forms the proximal convoluted (highly coiled) tubule. In this section, waste products and toxic substances (e.g., ammonia, nicotine) are forced out of the blood through a permeable membrane and useful substances (e.g., glucose, amino acids, vitamins, minerals) are reabsorbed.
Urine then travels through the loop of Henle, a long U-shaped extension of the proximal convoluted tubule. It consists of a descending limb and an ascending limb. Some sections of the loop are permeable to water and impermeable to substances in the urine (e.g., salt, ammonia), and some sections are impermeable to water and permeable to other substances.
The next section is the distal convoluted tubule. Normally, this section is water permeable. Substances that remain in the urine are reabsorbed, increasing the concentration of the urine. After passing through the distal convoluted tubule, the urine consists almost entirely of waste products. Most of the water and other useful substances have been reabsorbed.
Next, urine enters the collecting tubule. Urine from several nephrons empties into each collecting tubule. These tubules form the calyces, and the calyces form the renal pelvis (upper portion of the ureter). Urine travels from the kidneys through the ureters to the bladder, where it is stored until it is eliminated from the body through the urethra.
The hypothalamus in the brain detects the level of substances in the blood and controls the secretion of hormones. Antidiuretic hormone, aldosterone, and atrial natriuretic factor are hormones that change the permeability of the distal convoluted tubule and the collecting tubule, regulating urine volume and helping to maintain blood pressure.
For example, when water content in the blood is low (called dehydration), the secretion of antidiuretic hormone (ADH) increases and the kidneys reabsorb more water. This increases the concentration of the urine and decreases urine output. When water content in the blood is high, ADH production ceases and the kidneys reabsorb less water. This decreases the concentration of the urine and increases urine output.
BUN and Creatinine
The concentration in the blood (blood level) of blood urea nitrogen (BUN), known as urea, and creatinine (Cr) can be measured by routine laboratory tests. BUN and creatinine levels indicate the general function of the kidneys. BUN is a metabolic by-product of protein-rich food such as meat, poultry, and certain vegetables. BUN is filtered out of the blood by the kidneys and excreted in the urine. Creatinine is continuously generated by normal cell metabolism within the muscles. Creatinine is also filtered out of the blood by the kidneys and excreted in the urine.
The amounts of BUN and creatinine in the blood are equal to the amount excreted by the kidneys. The blood levels of BUN and Cr remain unchanged unless there is sudden deterioration of renal (i.e., kidney) function. If the kidneys are suddenly unable to function, BUN and Cr increase daily. This condition is known as acute renal failure. Chronic renal failure is a condition distinguished by a gradual increase in BUN and Cr over a long period of time.
When renal function decreases, blood levels of Cr and BUN increase because the kidneys are unable to clean the blood effectively. Factors not related to the kidneys also impact BUN and Cr levels. Creatinine, in particular, is affected by age, sex, weight, and muscle mass.