Kidney Structure and Function Quiz

Renal Function Tests

• Renal function tests are a collection of individual tests used to assess kidney function.
• These tests help determine if the kidneys are performing their tasks adequately.

Kidney Structure and Function

• Each kidney contains one million functional units called nephrons.

• Each nephron can be divided into:

  • The glomerulus (filtration)
  • Tubules (reabsorption and secretion).
    • Proximal tubule
    • Loop of Henle
    • Distal tubule
    • Collecting duct

• The glomerulus filters blood, producing an ultrafiltrate.

• The tubules reabsorb and secrete necessary substances.

Functions of the Kidney

• Excretory function: Removes waste products, like urea, creatinine, and uric acid. It also removes other undesirable waste products.
• Regulatory function (Homeostasis): Maintains a consistent internal chemical balance by reabsorbing and secreting glucose, amino acids, minerals, and vitamins. It also regulates water and electrolyte balance, the body's pH, and blood pressure.
• Endocrine function: The kidneys produce hormones like erythropoietin, renin, and vitamin D3. They're also influenced by other hormones like ADH, aldosterone, and PTH.

Urine Formation

• Urine is a sterile fluid composed primarily of water (95%). It also contains nitrogenous waste and electrolytes.
• Urine formation in the nephron occurs through three processes:
  • Glomerular filtration: Cells and large proteins are filtered from the blood to produce an ultrafiltrate.
  • Tubular reabsorption: The body reabsorbs essential molecules like glucose and amino acids from the filtrate, returning them to the blood.
  • Tubular secretion: Molecules are transported from the blood into the filtrate, which contributes to urine.

Glomerular Filtration

• Glomerular filtration is a passive process.
• Glomeruli act as filters that are permeable to water and smaller molecules but impermeable to larger molecules like proteins.

Glomerular Filtration Rate (GFR)

• GFR measures the blood volume filtered through the glomeruli each minute.
• It varies based on body size and is usually expressed per 1.73 m² of body surface area.
• A normal GFR for an adult is around ~ 120 mL/min/1.73 m². It is approximately 180 liters per day in normal adults.

Tubular Reabsorption and Secretion

• Water and solutes are reabsorbed throughout the entire length of the tubule.
• ~99% of water is reabsorbed back into the body, resulting in 1.5-2 liters of urine per day.

Renal Function Tests (cont.)

• Glomerular Tests:

  • Urine analysis (physical and chemical examination).
  • Serum NPN (BUN, creatinine, uric acid)
  • Creatinine clearance (CrCl)
  • Cystatin C
  • Urine albumin

• Tubular Tests:

  • Osmolality
  • Water deprivation test
  • Urine pH
  • Glycosuria
  • Aminoaciduria

• Glomerular Function Tests (cont.)

• Physical exam (volume, color, odor, specific gravity (osmolality), reaction (pH) and aspect.

• Chemical exam (constituents such as proteins, glucose, ketone bodies, bile salts, bilirubin and blood

• Microscopic exam (cells, casts, and crystals)

• Serum NPN compounds

  • Plasma concentration of NPN (urea, creatinine, uric acid) are inversely related to GFR.
  • Creatinine is a more reliable indicator of GFR.

• Creatinine Clearance (CrCl)

  • Measurement of blood completely cleared of creatinine per minute.
  • Best measurement of GFR.
  • Normal value ~ 120 mL/min

• Cystatin C: Is a reliable indicator of GFR, useful in particular for younger or older patients

• Albumin in urine: Small amounts (< 25 mg/day) are normal. High urinary albumin indicates glomerular damage.

Tubular Function Tests

• Osmolality: Urine osmolality is a marker of tubular function.
  • Normal plasma osmolarity: 280-295 mOsm/kg
  • 24-hr urine osmolality should be 2-3x that of plasma osmolality.
• Water deprivation test: Used to identify diabetes insipidus.
• Urine pH: Normal urine pH is slightly acidic (~5).

Urine Formation: a1 and β2 microglobulins; Glycosuria; Aminoaciduria

• a1 and β2 microglobulins: present in urine in increased concentrations, suggesting renal tubular damage.
• Glycosuria: Presence of glucose in urine when blood glucose is normal suggests renal tubular inability to reabsorb glucose.
• Aminoaciduria: Presence of amino acids in urine in excessive amounts suggests problems with tubular reabsorption or high plasma levels of amino acids.

Acute Renal Failure (AKI)

• Sudden loss of kidney function, often over hours or days, indicated by rising serum urea and creatinine
  • Urine output <400 mL/24 hrs
  • Oliguria or anuria
• Causes: reduced blood supply, kidney stones

Chronic Renal Failure (CRF)

• Progressive loss of kidney function over months or years.
• Causes various kidney diseases leading to loss of nephrons
• If untreated, leads to end-stage renal failure (ESRF).
• GFR drops below 15 mL/min
• Consequences, like disordered water and sodium metabolism, electrolyte imbalances; hyperkalemia, hypocalcemia, etc., and anemia.