Kidneys

KIDNEYS

Basic Functions

The kidneys are essential for homeostasis (maintaining a constant internal environment) of the body's extracellular fluids. Their basic functions include:

1. Regulation of extracellular fluid volume. The kidneys work to ensure an adequate quantity of plasma to keep blood flowing to vital organs.

2. Regulation of osmolarity. The kidneys help keep extracellular fluid from becoming too dilute or concentrated with respect to the solutes carried in the fluid.

3. Regulation of ion concentrations. The kidneys are responsible for maintaining relatively constant levels of key ions including sodium, potassium and calcium.

4. Regulation of pH. The kidneys prevent blood plasma from becoming too acidic or basic by regulating ions.

5. Excretion of wastes and toxins. The kidneys filter out a variety of water-soluble waste products and environmental toxins into the urine for excretion.

6. Production of hormones. The kidneys produce erthryopoietin, which stimulates red blood cell synthesis, and renin, which helps control salt and water balance and blood pressure. They are also involved in regulating plasma calcium and glucose levels.

ANATOMY

The two kidneys are located to the rear of the abdominal cavity on either side of the spine. They normally weigh about 5 onces each, but receive about 20% of the blood flow coming from the heart. The urine produced by each kidney drains through a separate ureter into the urinary bladder, located in the pelvic region. The bladder is emptied in turn by a single urethra, which exits the body.

FUNCTIONAL STRUCTURE OF THE KIDNEYS

Each kidney has about 1 million nephrons, the functional units of the kidney. Each nephron is composed of a tubule that begins in the outer layer of the kidney and eventually joins other tubules to empty into the ureter. The tubule has a number of functional segments:

1. The tubule begins with a hollow enlargement called Bowman's capsule, which is where water and solutes initially enter the tubule from the bloodstream. This process is known as filtration. The structure comprised of Bowman's capsule and associated capillaries is called the renal corpuscle.

2. From Bowman's capsule the tubular fluid flows towards the proximal tubule, which remains in the outer layer (cortex) of the kidney. The proximal tubule is the major site of reabsorption of water and solutes in equal proportions from the filtered tubular fluid.

3. Then the tubule dips into the hairpin loop of Henle, which descends toward the center of the kidney (medulla) and then rises back to the cortex. The loop of Henle is also a major site of reabsorption, but unlike the proximal tubule, proportionately more solute than water is reabsorbed, so the tubular fluid is dilute relative to plasma by the end of this segment.

4. The next segment is the distal tubule, which like the proximal tubule remains in the cortex. Both reabsorption and secretion take place in this segment, which is where sodium and potassium concentrations (and other electrolytes) and the pH of the tubular fluid are adjusted to ensure homeostasis.

5. The final segment of the nephron is the collecting duct, where multiple tubules join and descend toward the center of the kidney, where the ureter collects the remaining tubular fluid as urine. The collecting duct is a major site of regulation of water balance, where additional water may be reabsorbed from the tubular fluid depending on the body's hydration status.

Surrounding each tubule is a complex system of blood vessels that exchange water and solutes with the tubule. This system is special in that blood must pass through two capillary beds.
1. An afferent arteriole takes blood to the renal corpuscle, where the blood passes through the first capillary bed, a ball-shape tuft known as the glomerulus.

2. An efferent arteriole takes blood away from the glomerulus.

3. From there the blood passes into a set of peritubular capillaries, which follow the remainder of the tubule and are the site of further exchange of water and solutes between plasma and tubular fluid.