4.1 Renal Anatomy and Physiology Quiz

Renal Anatomy and Physiology

• Kidneys are paired organs located retroperitoneally.
• A renal artery enters and a renal vein exits each kidney at the hilum.
• Approximately 20% of cardiac output goes to the kidneys.
• Kidneys filter blood, remove waste (urea, nitrogen), and regulate electrolytes and intravascular volume.

Nephron Structure and Function

• Nephrons are the basic structural and functional units.
• Each nephron consists of a glomerulus (tuft of capillaries), Bowman's capsule, and renal tubule.
• Approximately 1 million nephrons per kidney.
• Glomerulus capillaries are fenestrated but have negatively charged glycoproteins that exclude plasma proteins like albumin.
• Bowman's capsule surrounds the glomerulus.
• Renal tubule reclaims salts and water.
• Foot processes, basement membrane, and epithelial cells (podocytes) are crucial components of the glomerulus.

Glomerular Filtration

• 100-120 ml/minute of filtrate is produced in a healthy adult.
• Filtered substances are reabsorbed along the renal tubule network.
• Reabsorption varies by substance and location.
  • 60-70% of sodium, almost all potassium and glucose are reabsorbed.
  • Water is reabsorbed passively, following the osmotic gradient of sodium reabsorption.

Tubular Secretion and Urine Formation

• Substances like creatinine, histamine, drugs, and toxins are secreted into the tubular fluid.
• 30 ml/minute of isotonic filtrate enters the Loop of Henle.
• Countercurrent multiplier mechanism concentrates urine.
  • Sodium secretion in the thick ascending limb establishes a hypotonic gradient to absorb water from the descending limb.
• 5-10 ml/minute of urine reaches the collecting ducts.
• Vasopressin (ADH) controls water absorption in collecting ducts.
• Sodium is reabsorbed in exchange for potassium and hydrogen excretion.
• Collecting ducts regulate urine volume, and water, sodium, acid-base, and potassium balance.

Blood Pressure Regulation

• Kidneys regulate blood pressure via sodium and water balance.
• Macula densa in the juxtaglomerular complex senses sodium concentration and blood pressure.
• Low sodium or low perfusion pressure triggers renin release from juxtaglomerular cells.
• Renin converts angiotensinogen to angiotensin I, then angiotensin II by ACE.
• Angiotensin II constricts blood vessels and stimulates aldosterone release (sodium and water retention).
• Intravascular volume depletion stimulates vasopressin release, increasing water reabsorption.
• This creates a negative feedback loop to maintain blood pressure.
• Maladaptive renin-angiotensin-aldosterone system activity can contribute to disease (e.g., hypertension).
• Low effective circulating volume (fluid out of vascular space) can trigger the system even without low total volume. Heart failure, decreased oncotic pressure (e.g., nephrotic syndrome or cirrhosis), or vascular disease can contribute.

Acid-Base Homeostasis

• Kidneys play a crucial role in acid-base balance.
• Decreased pH (increased hydrogen) triggers a carbonic anhydrase-bicarbonate buffer system response.
• Kidneys secrete additional hydrogen, reversing reaction, restoring bicarbonate.
• Pulmonary system compensates quickly; kidneys take hours/days.
• Hydrogen secretion in the distal collecting duct (combining with ammonia to form ammonium).

Metabolic Acidosis

• Causes include overwhelmed kidney hydrogen excretion, ingestion of exogenous acids, or bicarbonate loss.
• Advanced kidney failure, methanol/ethylene glycol ingestion.
• Bicarbonate-depleting solution administration can cause dilutional acidosis.

Calcium and Phosphate Homeostasis

• Kidneys convert vitamin D (from liver) to its active form (1,25-dihydroxyvitamin D), increasing calcium absorption.
• Kidneys respond to parathyroid hormone by retaining calcium and excreting phosphate.

Erythropoietin Production

• Kidneys produce erythropoietin, stimulating red blood cell production.
• Low blood oxygen triggers erythropoietin release.
• Anemia is common in renal disease.

Glomerular Filtration Rate (GFR) Regulation

• Tubular glomerular feedback: Kidneys sense distal tubular sodium concentration and adjust afferent arteriole vasoconstriction to regulate GFR.
• Decreased flow allows for more time to reabsorb sodium.
• Several vasoactive substances influence regulation (vasodilators like adenosine, prostaglandins, and nitric oxide).

Medullary Blood Flow

• Sufficient cortical blood flow to maintain GFR is necessary, but medullary flow must be carefully maintained to not disrupt the countercurrent system. Medullary blood flow insufficiency can cause anoxic injury.

Potassium Regulation

• Hyperkalemia (high potassium) can result from intracellular potassium shifts, extra-renal losses (diarrhea), renal losses (diuretics), or hyperaldosteronism.
• Diuretics and hyperaldosteronism are often causes.
• Hyperaldosteronism can be caused by tumors or secondary conditions.