Pyramid Science

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Thursday, January 01, 2009


The kidneys produce urine. Under normal resting conditions the kidneys, which comprise less than 0.5% of the body weight, receive 25% of the cardiac output. Each minute some 1300ml of blood enter the kidneys through the renal arteries, and approximately 1298-1299ml leave via renal veins, with the difference (1-2ml), leaving as urine via the ureter.

The Importance of Urine

The composition of the urine is not impressive: water, salt, small amounts of acid, and a variety of waste products like urea. What's impressive is how urine composition and volume change to compensate for any fluctuation in volume or composition of body fluids. The composition of the body fluids is apparently determined not by what the mouth takes in, but by what the kidneys keep. The design of the gastrointestinal tract appears to maximize absorption indiscriminately without regard for quantities. The kidneys are the guardians of the internal environment and rework the body fluids fifteen times a day. When the body is dehydrated, the volume of water excreted decreases and when body fluids become more acid, the kidneys excrete more acid. If the K+ content of body fluids rises the kidneys excrete more K+.

The kidneys are about the size of a clenched fist and lie against the back abdominal wall, just above the waistline. The outer covering of the kidney, called the capsule is thin but tough and fibrous. When it is cut open, two regions appear: an outer zone (cortex) and an inner region (medulla). The microscopic view reveals the unit of kidney function: the nephron. Each kidney has about 1 million nephrons, which are tubular structures about 45mm to 65mm long and about 0.05mm wide. Their walls are made of a single layer of epithelial cells. A funnel like structure about 0.2mm in diameter called Bowman's capsule comprises the top end of the nephron. These capsules are always found in the cortex. Fluid flows through the lumen of the tubule from the Bowman's capsule into the next section, the proximal tubule, which has a "curly" or convoluted section and then a straight portion that dips into the medulla. This section, about 15mm long, is called the proximal tubule because it is near the origin of the nephron (Bowman's capsule).

Fluid then flows into a long, thin tube that plummets straight toward the depths of the medulla. This is the descending limb of the loop of Henle. At its lowest point, the loop makes a hairpin turn and begins to ascend out of the medulla back toward the cortex, becoming considerabiy thicker toward the later portions of its ascent in the cortex. The ascending limb of the loop becomes continuous with the distal tubule. Finally, the distal tubule empties into the collecting duct, a tube that gathers fluid from several nephrons.

There are two major classes of nephrons. The majority, called cortical nephrons, originate in the outer portions of the cortex and are characterized by short loops of Henle that reach only the outer regions of the medulla. The remaining nephrons, which comprise only about 15% of the total, originate closer to the medulla and are known as juxtamedullary nephrons. These have very long loops of Henle that reach deep into the medulla. They are important for water conservation in the body. Individual collecting ducts coalesce into larger tubular structures, and this pattern repeats until several of the larger tubes empty into a still larger funnel structure, the renal pelvis. Fluid in the renal pelvis is identical to urine. The renal pelvis itself is continuous with the ureter, which leaves each kidney to convey urine to the bladder, where it is stored until eliminated via the urethra. The blood supply to the nephrons is special because it consists of two capillary beds in series. Each Bowman's capsule has its own capillary bed called a glomerulus. The combined structure of the Bowman's capsule and glomerulus is sometimes referred to as just the glomerulus. The vessel bringing blood to the glomerulus is called the afferent arteriole. Blood leaving the glomerulus does not enter a venule, but rather it enters another arteriole, the efferent arteriole, which serves as a conduit to the second capillary bed, called peritubular capillaries. These capillaries are so interconnected that it is difficult to tell which capillary came from which efferent arteriole: the tubules of any one nephron probably receive blood from several efferent arterioles.

Efferent arterioles from juxtamedullary nephrons also form peritubular capillaries in much the same way, but, in addition, they send off branches which are straight tubes that follow the descending limbs of loops of Henle deep into the medulla, turn at the bend of the loop, and ascend back toward the cortex. The design of these hairpin loops of blood vessels important for water conservation. By the time the fluid in the nephron has passed through the collecting ducts to reach the pelvis, it has become urine. From here, it flows along the lumen of the nephron and is modified by the epithelial cells of the tubules and the collecting ducts until it finally becomes urine.


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