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Basic Renal Functions and Structures
An understanding of the basic functions of the kidney is necessary to understand interferences with these functions in disease states. The basic functional unit of the kidney is the nephron.
The nephron is an exquisite example of a highly complex, minute tissue unit. It is adapted in fine detail to its vital function maintaining an internal fluid environment compatible with life. These vital units of the kidney are the master chemists of our bodies. We have the kind of body fluids and tissues that we have not merely because of what the mouth takes in but because of what the kidneys keep. Only because they work in the way they do has it become possible for us to have specific tissues of specific natures to do specific tasks.
Each kidney contains some one million nephrons. As the body fluid flows through these finely structured units, the nephrons perform four significant functions to support life:
1. Filtration of most constituents from the entering blood except red cells and proteins
2. Reabsorption of needed substances as the filtrate continues along the winding tubules
3. Secretion of additional ions to maintain acid-base balance
4. Excretion of unneeded materials in concentrated urine.
Specific nephron structures perform unique metabolic tasks to maintain body balance.
Glomerulus. At the head of the nephron the blood enters in a single capillary and then branches into a group of collateral capillaries. This tuft of collateral capillaries is held closely applied in a cup-shaped membrane. This cup-shaped capsule is named Bowman's capsule, which establishes the basis of plasma filtration and consequent urine secretion on this intimate relationship of blood-filled glomeruli and enveloping membrane. The filtrate formed here is cell free and virtually protein free. Otherwise it carries the same constituents as does the entering blood.
Tubules Continuous with the base of Bowman's capsule the nephron tubule winds in a series of convolutions toward its terminal in the kidney pelvis. Specific reabsorption functions are performed by the four sections of the tubule: the proximal tubule the loop of the Henle the distal tubule, and the collecting tubule.
PROXIMAL TUBULE In the first section nearest the glomerulus major nutrient reabsorption occurs, essentially 100% of the glucose and amino acids and 80% to 85% of the water sodium, potassium. Chloride and most other substances are reabsorbed. Only 15% to 30% of the filtrate remains to enter the next section the loop of Henle.
LOOP OF HENLE This is the midsection of the renal tubule. Here the tubule narrow's, and its thin loop dips into the central renal medulla. Through a balanced system of water and sodium exchange in the limbs of the loop important fluid density is created surrounding the loop. This area of increased density is important to concentrate the urine by osmotic. Pressure as it later passes through this same area of the kidney in the collecting tubule.
DISTAL TUBULE The latter portion of the tubule functions primarily in Kid-base balance through secretion of ionized hydrogen. It also conserves sodium by reabsorbing it under the influence of aldosterone.
COLLECTlNG TUBULE. In the final section of the tubule water is absorbed under the influence of the hormone ADH and the osmotic pressure of the more dense surrounding fluid. The resulting volume of urine now concentrated and excreted is only 0.5% to 1% of the original filtered water.