renal medulla
Summary
The renal medulla is the inner region of the kidney, consisting of pyramids that contain the loops of Henle, vasa recta, and collecting ducts. It is critical for establishing the medullary osmotic gradient, which enables urine concentration via the countercurrent multiplier and exchanger systems. It is more susceptible to ischemia and papillary necrosis due to relatively low blood flow.
Detail
The renal medulla is organized into renal pyramids, each with a base at the corticomedullary junction and an apex forming a renal papilla that empties into a minor calyx. Structurally, it is divided into outer (further split into outer and inner stripes) and inner medulla, with the inner medulla containing the thin loops of Henle and terminal collecting ducts.
Key functional structures: - Loops of Henle (especially thin descending and thick ascending limbs of juxtamedullary nephrons) generate the countercurrent multiplier, establishing a hypertonic interstitium (up to ~1200 mOsm/kg at the papillary tip). - Vasa recta act as countercurrent exchangers, preserving the medullary gradient by passive exchange of water and solutes, minimizing washout. - Collecting ducts, under ADH (vasopressin) influence via aquaporin-2 channels, reabsorb water driven by this osmotic gradient, allowing urine concentration. - The medulla also houses the vasopressin-sensitive urea transporters (UT-A1, UT-A3) contributing to urea recycling, important for maximal urine concentration.
Clinical significance: - The medulla has a relatively low oxygen tension due to low blood flow (only ~1-2% of renal blood flow) despite high metabolic demand (active reabsorption), making it prone to hypoxic injury, especially in the thick ascending limb (S3 segment vulnerability in acute tubular necrosis, particularly ischemic ATN). - NSAIDs, sickle cell disease, diabetes mellitus, and analgesic abuse can cause renal papillary necrosis due to compromised medullary blood flow. - Medullary sponge kidney and medullary cystic disease are congenital/genetic conditions affecting this region. - Juxtamedullary nephrons (with long loops of Henle) are essential for maximal urine concentrating ability, contrasted with cortical nephrons (short loops).
Understanding medulla physiology is essential for grasping renal concentration/dilution mechanisms, diuretic mechanisms (loop diuretics act on thick ascending limb within the medulla), and the pathophysiology of conditions like diabetes insipidus and SIADH.
Sources
- Guyton and Hall Textbook of Medical Physiology
- BRS Physiology
- First Aid for the USMLE Step 1
- Costanzo Physiology
- Robbins Basic Pathology
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