Oxygenated blood enters the arm by leaving the aortic arch and flowing into one of two subclavian arteries. These travel under the collarbones of the shoulders and down the arms. From here, blood flows through the brachial artery—the major artery of the upper arm—and into the radial or ulnar arteries, branches of the brachial. Each of these travels down the forearm; the ulnar is on the pinkie side, and the radial is on the thumb side. 

The radial and ulnar arteries continue down the arm and through the wrist before connecting into the deep palmar arch. This arch is a series of arteries formed at the junction of the ulnar and radial arteries in the palm of the hand. This semicircular artery branches into the fingers, where its divisions are known as palmar digital branches.

On the top of the hand, the dorsal venous network is a web of veins that spreads across the back of the hand to return blood to the heart so it can be pumped to the lungs and become reoxygenated.

To get from the hand to the heart, the deoxygenated blood flows up through either the basilic or cephalic veins, which transport it through the shoulder area and back to the heart.

The veins across the back of the hand are also called superficial veins because they are close to the skin’s surface. These veins are only present in the body’s extremities and are easily detected in the back of the hand and the forearm in most people.

Nerves also weave through the hand. They collect sensory information and send it to the brain, which sends appropriate responses. Hand nerves include:

• Ulnar nerve: This nerve begins at the spinal column, through the arm, and branches in the hand. It travels just under the skin at the elbow’s “funny bone,” a knob on the humerus bone. It serves the ring and pinkie fingers.
• Radial nerve: Supplying the triceps and wrist extensors, this nerve’s branches cover most of the back of the hand beginning at the wrist.
• Median nerve: This nerve originates in the upper arm. It branches into the hand to serves the thumb, index, and middle fingers. It is also the only nerve that passes through the carpal tunnel. Pressure on the median nerve can lead to carpal tunnel syndrome and other problems.