Arterioles regulate regional blood flow by adjusting vascular resistance through their smooth muscle in the vessel walls. Their muscular layer allows them to constrict or dilate in response to neural signals, hormones, and local metabolic needs. Because resistance to flow depends very strongly on radius (even small changes in diameter dramatically change resistance), arterioles act as the main control points for distributing blood to specific tissues. Venules and capillaries have little smooth muscle and are more about drainage and exchange, while large arteries are thick-walled conduits that stay wide most of the time and don’t regulate flow to individual regions.

Peristaltic propulsion is produced by the muscularis externa, the GI wall layer that contains smooth muscle arranged in an inner circular layer and an outer longitudinal layer. When these smooth muscles contract in a coordinated, wave-like fashion—driven by the myenteric (Auerbach) plexus—contents are pushed forward along the tract. The mucosa lines the lumen and handles secretion and absorption, not movement. The submucosa provides support and houses vessels and the Meissner plexus that regulates local secretions and blood flow. The serosa is the outer protective covering. So the muscularis is the driver of peristaltic movements.

The main idea here is that peripheral resistance is driven mainly by the diameter of small arteries and arterioles. Because resistance rises dramatically when vessels narrow, small changes in radius have a big effect. Vasoconstriction caused by sympathetic activity and hormones like catecholamines and angiotensin II decreases the radius of arterioles. Since resistance is roughly proportional to 1 divided by the radius to the fourth power, even modest constriction leads to a large increase in peripheral resistance. That’s why this factor most increases PR. Vasodilation from parasympathetic activity would widen vessels and lower PR, not increase it. Decreased blood viscosity would reduce PR, but the effect is smaller than the impact of changing vessel radius. An increased heart rate by itself doesn’t directly raise peripheral resistance, though it can influence flow and pressure indirectly through cardiac output.

When tissues sense low oxygen, local chemical signals cause small arteries and arterioles to dilate. This vasodilation lowers peripheral vascular resistance because there are fewer constricted vessels for the blood to push through. Blood pressure depends on cardiac output times systemic vascular resistance, so if resistance falls and the heart doesn’t immediately increase its output enough to compensate, mean arterial pressure tends to drop. So the healthy, immediate response to hypoxia is decreased resistance and, often, decreased blood pressure.

In the deep venous drainage of the upper limb, the radial and ulnar veins of the forearm join to form a single deep vein that travels with the brachial artery. This vein is the brachial vein. It continues upward and becomes part of the axillary vein as it passes the lower border of the teres major. The other options are either superficial (cephalic and basilic) or a more proximal vessel (subclavian) and do not fit the described pathway.