The Excretory System and the Nephron

The excretory system is crucial for the removal of wastes and excess substances from the body, maintaining homeostasis. It plays a vital role in ensuring that the body's internal environment remains stable despite changes in external conditions. Focus is placed on the kidneys—two bean-shaped organs that filter blood and produce urine—as well as their functional units: the nephrons.

The Urinary System

• Composed of the kidneys, ureters, bladder, and urethra.

• Functions:

  • Waste removal via urine: The kidneys filter unwanted substances from the blood, which are then excreted as urine.

  • Regulation of blood volume and pressure: The kidneys adjust the volume of fluids excreted to control blood pressure, ensuring proper circulation.

  • Regulation of electrolyte balance: They help maintain the appropriate levels of electrolytes (such as sodium, potassium, and calcium) in the body.

  • Control of blood pH: The kidneys facilitate the regulation of acid-base balance by excreting hydrogen ions and retaining bicarbonate as needed.

Kidney and Urinary Function

The kidneys function through a series of steps to filter blood and produce urine:

1. Filtration

   • The process begins in the glomerulus, where blood is filtered under pressure. The glomerulus allows water, ions, and small molecules to pass through into the Bowman's capsule while retaining larger molecules like proteins and blood cells, which remain in the bloodstream.

2. Reabsorption

   • After filtration, the filtrate enters the proximal tubule, where approximately 65-70% of filtered water, along with glucose, amino acids, and essential ions (sodium, potassium, and bicarbonate), is reabsorbed back into the blood. This reabsorption is highly selective, ensuring that valuable substances are retained.

   • Next, the filtrate moves into the Loop of Henle, which has two limbs: the descending limb and the ascending limb.

     • In the descending limb, water is reabsorbed due to osmotic pressure. This makes the filtrate more concentrated.

     • In the ascending limb, sodium and chloride ions are actively transported out while the segment is impermeable to water, resulting in dilution of the filtrate.

3. Secretion

   • The final processing occurs in the distal tubule and collecting duct. Here, additional waste products and excess ions are secreted into the filtrate. This step is crucial for the regulation of electrolyte balance, pH levels, and waste removal.

4. Excretion

   • The processed filtrate, now referred to as urine, is collected in the collecting duct and transported to the renal pelvis, from where it flows down the ureters to the bladder for storage. Once the bladder is full, urine is expelled from the body through the urethra during urination.

Kidney Failure: Signs and Symptoms

Symptoms can manifest as:

• Fatigue and weakness due to the buildup of toxins in the body.

• Swelling in ankles, feet, or around the eyes (edema), often as a result of fluid retention.

• Changes in urination patterns (frequency and volume) indicating altered kidney function.

• Nausea and vomiting can occur as toxins accumulate, affecting the gastrointestinal system.

• Shortness of breath due to fluid accumulation in the lungs.

Dialysis Options

Dialysis serves as a treatment for kidney failure, filtering and purifying the blood using a machine. The two primary types are:

• Hemodialysis:

  • Blood is circulated outside the body through a machine that filters it before returning it to the body. This method typically requires patients to visit a dialysis center three times a week for about four hours each session.

  • Potential complications: Low blood pressure, muscle cramps, and infections at the access site.

• Peritoneal Dialysis:

  • This treatment takes place inside the body and uses the lining of the abdomen (peritoneum) to filter blood. Patients can perform this type of dialysis at home, allowing for greater flexibility and independence.

  • The process involves filling the abdominal cavity with a dialysis solution, which absorbs waste products and excess fluids. After a dwell time, the solution is drained and replaced with fresh solution.

  • Key benefit: It can be done overnight while sleeping or throughout the day, accommodating patients' daily routines.