U01.04.005 Ischemia

Ischemia refers to an inadequate blood supply that fails to meet the metabolic demands of tissues, leading to tissue damage or infarction.

Mechanisms of Ischemia

Reduced Arterial Perfusion: Often caused by conditions like atherosclerosis, where narrowed arteries limit blood flow.
Impaired Venous Drainage: Seen in conditions like testicular torsion and Budd-Chiari syndrome, where blood cannot properly exit the tissue.
Shock: A systemic condition leading to decreased blood flow to organs and tissues.

Regions Most Vulnerable to Hypoxia/Ischemia

Organ	Region Most Vulnerable
Brain	ACA/MCA/PCA boundary areas (watershed zones)
Heart	Subendocardium (left ventricle)
Kidney	Straight segment of the proximal tubule (medulla), Thick ascending limb (medulla)
Liver	Area around the central vein (zone III)
Colon	The area around the central vein (zone III)

Regions Most Vulnerable to Hypoxia/Ischemia

Points to Remember

Watershed Areas: These are regions at the border zones of two arterial supplies (e.g., ACA/MCA in the brain), where blood flow is the most distal and collateral circulation is limited, making them highly susceptible to ischemia during hypoperfusion.
Neurons Susceptible to Hypoxic-Ischemic Injury:
- Purkinje cells in the cerebellum.
- Pyramidal cells in the hippocampus and neocortex (zones 3, 5, 6).

Clinical Relevance

Understanding the regions most susceptible to ischemia is crucial for diagnosing and managing conditions like stroke, myocardial infarction, acute kidney injury, and ischemic colitis. These regions are often the first to be damaged during ischemic events, and timely intervention can mitigate long-term effects.

USMLE Step 1 Review Series

Curriculum

U01.04.005 Ischemia

Mechanisms of Ischemia

Regions Most Vulnerable to Hypoxia/Ischemia

Points to Remember

Clinical Relevance

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