Kidney Tutorial

One of the easiest ways to learn about Bartter’s and Gitelman’s syndrome is to learn how the normal kidney functions. This page is to teach you more information about the kidneys and how they function. Take your time and read the information a few times. Remember when we went to college for nursing it took a while to learn all of this stuff. So don’t expect it to make sense the first time you read it. The more you read the more you will learn. Learning this will help you understand your disease, why it happens and what affect it has on you and your kidneys.

Understanding The Renal System

Together with the urinary system, the renal system serves as the body’s water treatment plant. These systems work together to collect the body’s waste products and expel them as urine.

The kidneys are located on each side of the abdomen near the lower back. These compact organs contain an amazingly efficient filtration system. The by product of this filtration is urine, which contains water and waste products.

Once produced by the kidneys, urine passes through the urinary system and is expelled from the body. The other structures of the system, extending downward from the kidneys include the:
Ureters - 16″ to 18″ (41- 46cm) muscular tubes that contract rhythmically to transport urine from each kidney to the bladder.
Urinary Bladder- A sac with muscular walls that collects and holds urine that is expelled from the bladder.
Uretha - A narrow tube leading out of the body through which urine is expelled from the bladder. (The place where UTI’s occur)

Regulating, Balancing & Detoxifying

The kidneys perform the following vital functions:

1. Regulating the electrolyte concentration, acid-base balance and amount of body fluids.
2. Detoxifying the blood and eliminating wastes.
3. Regulating Blood pressure
4. Aiding red blood cell (RBC) formation (erythropoiesis)

Maintaining Fluid and Acid-Base Balance

Two important kidney functions are maintaining fluid and acid-base balance.

Fluid Balance

The kidneys maintain fluid balance in the body by regulating the amount and makeup of the fluids inside and around cells. Kidneys maintain the volume and composition of extracellular fluid and, to a lesser extent, intracellular fluid. They do this by continuously exchanging water and solutes, such as hydrogen, sodium, potassium, chloride, bicarbonate, sulfate, and phosphate ions, across their cell membranes.

Acid-Base Balance

To regulate acid-base balance, the kidneys:
Secrete hydrogen ions
Reabsorb sodium and bicarbonate ions
Acidify phosphate salts
Produce ammonia

All of these regulating activities keep the blood at its normal pH of 7.37 to 7.43. Acidosis occurs when the pH falls below 7.35, and alkalosis occurs when the pH is greater than 7.45.

Thanks To Two Hormones

Hormones partially control the kidney’s role in fluid balance. This control depends on the response of specialized sensory nerve endings (osmoreceptors) to changes in osmolality (the ionic concentration of a solution). The two hormones involved are:
Antidiuretic Hormone (ADH), produced by the pituitary gland.
Aldosterone, produced by the adrenal cortex.

All About ADH

ADH alters the collecting tubule’s permeability to water. When ADH concentration in plasma is high, the tubules are most permeable to water. This creates a highly concentrated but small volume of urine. If ADH concentration is low, the tubules are less permeable to water. This creates a larger volume of less concentrated urine.

All that is known about Aldosterone

Aldosterone regulates water absorption by the distal tubules and changes urine concentration by increasing sodium reabsorption. A high plasma aldosterone concentration increases sodium and water reabsorption by the tubules and decreases sodium and water excretion in the urine. A low plasma aldosterone concentration promotes sodium and water excretion.
Aldosterone helps control the secretion of potassium by the distal tubules. A high aldosterone concentration increases the excretion of potassium. Other factors that effect potassium secretion include:
The amount of potassium ingested
The number of hydrogen ions secreted
Potassium levels in the cell
The amount of sodium in the distal tubule
The glomular filtration rate (GFR), the rate at which plasma is filtered as it flows through the glomerular capillary filtration membrane
Problems in hormone concentration may cause fluctuations in sodium and potassium concentrations that, in turn, may lead to hypertension.

Collection

The kidneys collect and eliminate wastes from the body in a three step process:
By Glomerular filtration – filtering the blood that flows through the kidney’s blood vessels, or glomeruli.
By tubular absorption-reabsorbing filtered blood through the minute canals (tubules) that make up the kidney.
By Tubular secretion-release of the filtered substance by the tubules.

Clear The Way

Clearance is the complete removal of a substance from the blood. Clearance is commonly described as the amount of blood that can be cleared in a specific amount of time. For example, creatinine clearance is the volume of blood in milliliters that the kidneys can clear of creatinine in 1 minute.
Some substances are filtered out of the blood by the glomeruli. Dissolved substances that remain in the fluid may or may not be reabsorbed by the renal tubular cells.

Nephrons can only do so much

In patients whose kidneys have shrunk from disease, healthy nephrons (the filtering units of the kidney) enlarge to compensate. But as the nephron damage progresses, the enlargement can no longer adequately compensate, and the GFR slows.

Don’t Saturate the System

The amount of a substance that’s reabsorbed or secreted depends on the substance’s maximum tubular transport capacity – the maximum amount of a substance that can be reabsorbed or secreted in a minute without saturating the renal system.
For example, in diabetes mellitus, excess glucose in the blood overwhelms the renal tubules and causes glucose to appear in the urine (glycosuria). In other cases, when glomeruli are damaged, protein appears in the urine (proteinuria) because the large protein molecules pass into the urine instead of being reabsorbed.

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