A 68-year-old man with chronic renal disease complains of headaches and dizziness. Blood pressure is 220/130 mm Hg (malignant hypertension). While in the emergency room, the patient suffers a massive stroke and expires. Microscopic examination of the kidney at autopsy shows areas of fibrinoid necrosis. This type of necrosis is typically seen in which of the following anatomic structures or locations?