Nephron Function and Urine Concentration

The Loop of Henle

• The descending part of the Loop of Henle reabsorbs additional water into the bloodstream, but not solutes, resulting in a highly concentrated urine.
• The ascending part of the Loop of Henle reabsorbs solutes, but not water, resulting in a less concentrated urine.

Nephrons and Distal Convoluted Tubule

• From the Loop of Henle, the fluid goes to the distal convoluted tubule.
• In the distal convoluted tubule, some Na+ and Ca+ are reabsorbed into the bloodstream.
• The calcium is reabsorbed by the effects of parathyroid hormone, which enhances the synthesis of transport proteins in the distal convoluted tubule.
• Depending on the pH of the blood, the distal convoluted tubule may:
  • Excrete hydrogen ions and reabsorb bicarbonate into the blood.
  • Excrete bicarbonate and reabsorb hydrogen ions into the blood.

Collecting Duct

• From the distal convoluted tubule, the fluid goes to the collecting duct.
• Collecting ducts merge from several different nephrons to form cortical collecting ducts.
• The cortical collecting ducts are sensitive to antidiuretic hormone (vasopressin) and aldosterone, allowing for:
  • Water and salt reabsorption into the bloodstream if these hormones are present.
  • Excretion of sodium, potassium, hydrogen, and bicarbonate into the collecting duct if these hormones are not present.

Renal Blood Flow (RBF)

• The kidneys are highly vascular, receiving 20-25% of the cardiac output.
• Approximately 20% of the plasma that flows through the kidneys is filtered by the glomerulus.

Erythropoietin

• Function: Stimulates the bone marrow to produce RBCs to prevent or correct hypoxia.
• Decreased oxygen delivery to the kidneys stimulates the secretion of erythropoietin from the peritubular cells of the kidneys, which then stimulates the bone marrow to produce RBCs.

Tests of Renal Function

• Renal clearance tests determine how much of a substance can be cleared from the blood by the kidneys during a given unit of time.
• Tests are useful for:
  • Evaluating kidney function for diagnostic/prognostic purposes.
  • Dosing certain medications like antibiotics.
  • Determining if a patient can have a test or procedure.

Renal Stones

• Can form in the urinary tract due to changes in temperature and pH.
• Types of stones:
  • Calcium phosphate stones (alkaline).
  • Uric acid stones (acidotic).

Lower Urinary Tract Obstruction

• Can be caused by:
  • Neurogenic bladder dysfunction.
  • Anatomic obstructions (e.g., urethral stricture, prostate enlargement, pelvic organ prolapse).
• Manifestations:
  • Frequent daytime voiding.
  • Nocturia.
  • Urgency.
  • Dysuria.
  • Poor force of stream.
  • Feelings of incomplete bladder emptying.

Tumors

• Renal adenomas: encapsulated benign tumors usually in the renal cortex.
• Renal cell carcinoma (RCC):
  • Most common renal cancer (85%).
  • Develops from the proximal tubule epithelial cells.
  • Risk factors: tobacco use, obesity, long-term analgesic use.
  • Manifestations:
    • Asymptomatic early on.
    • Weight loss.
    • Fatigue.
    • Classic triad: hematuria, dull and aching flank pain, and palpable flank mass in thinner individuals.

Urinary Tract Infections

• Definition: Inflammation of the urinary epithelium following invasion and colonization of a pathogen.
• Most common cause: bacteria.
• Classification:
  • Uncomplicated UTI: occurs in a healthy individual with a normal urinary tract.
  • Complicated UTI: occurs in those with defects in the urinary system or other health problems that compromise their ability to prevent pathogen colonization or fight organisms.
• Potential complications:
  • Cystitis: inflammation of the bladder.
  • Pyelonephritis: inflammation of the upper urinary tracts.
• Manifestations:
  • Asymptomatic.
  • Urinary frequency.
  • Dysuria.
  • Urgency.
  • Flank and/or suprapubic pain.
  • Cloudy urine.
  • Foul-smelling urine.
  • Fever/chills.

Acute Glomerulonephritis

• Definition: Inflammation of the glomerulus, most often related to group A poststreptococcal infection.
• Mechanism: Antibodies produced against the organism cross-react with the glomerular endothelial cells.
• Manifestations:
  • Hematuria with red blood cell casts.
  • Proteinuria exceeding 3–5 g/day.
  • Hypoalbuminemia.
  • Edema.
  • Decreased GFR.

Chronic Glomerulonephritis

• Definition: Glomerular diseases with a progressive course, usually related to a chronic disease, leading to renal failure.
• Manifestations:
  • Proteinuria.
  • Hypercholesterolemia.
• Diagnosis: H&P, urinalysis, serum creatinine, creatinine clearance.

Nephrotic Syndrome

• Definition: Excretion of 3.5 g or more of protein in urine.
• Mechanism: Protein excretion as a result of glomerular injury.
• Etiologies: Genetic, rare primary diseases, diabetes mellitus, lupus, amyloidosis, medication-related, HIV, leukemia, lymphoma, vascular disorders.
• Manifestations:
  • Hypoalbuminemia.
  • Edema.
  • Oliguria.
  • Hypertension.
• Diagnosis: H&P, serum albumin level, 24-hour urine protein test.

Classification of Kidney Dysfunction

• Acute or chronic; reversible or irreversible.
• Renal insufficiency: decline of renal function to approximately 25% of normal.
• Renal failure: significant loss of renal function.
• End-stage renal failure: less than 15% of renal function remains.

Alterations of Renal and Urinary Tract Function in Children

• The kidneys and urologic structures are continuing to develop in children, and dysfunction can look very different to that seen in adults.
• Some disorders seen in children are congenital structural issues.
• As with adults, renal and urologic conditions can have significant impacts on other systems.
• Fluid and electrolyte imbalance:
  • Immaturity and smaller tubule surface area diminish the water reabsorption response to antidiuretic hormone (ADH).
  • Immature tubular transport capacity.
  • Diarrhea, infection, fasting, and poor feeding can rapidly lead to severe acidosis and fluid imbalance.
• Congenital abnormalities:
  • Hypospadias.
  • Epispadias.
  • Exstrophy of the bladder.
  • Ureteropelvic junction obstruction.
  • Bladder outlet obstruction.
  • Renal aplasia.
  • Hypoplastic kidneys.
  • Renal dysplasia.
  • Renal agenesis.