All About Arteries

Arteries are muscular tubes that carry blood flow away from the heart to the tissues and organs of the body (by contrast, veins are the return path tubes).

The arterial layer that is in direct contect with the flow of blood is the tunica intima, commonly called the intima. This layer is made up of mainly endothelial cells.

Just deep to this layer is the tunica media, known as the media. This “middle layer” is made up of smooth muscle cells and elastic tissue.

The outermost layer (furthest from the flow of blood) is known as the tunica adventitia or the adventitia. This layer is composed of connective tissue.

The arterial system is the higher-pressure portion of the blood system.

Since the heart output is pulsatile, arterial pressure varies between systolic, the peak pressure during heart contraction, and diastolic, the minimum pressure between heart contractions, values with each heart cycle. This pressure and blood volume variation within the artery produces the pulse which is palpable in any artery, reflecting the heart action.

Systemic Arterial Pressures

The systemic arterial pressures, e.g 120/80 mmHg, are generated by the forceful contractions of the heart’s left ventricle. Similarly, the pulmonary arterial pressures, e.g 25/6 mmHg, are generated by the contractions of the heart’s right ventricle.

Healthy resting arterial pressures, compared to many man-engineered system are relatively low, mean systemic pressures typically being under 100 mmHg, about 1.8 lbf/in², above surrounding atmospheric pressure (about 760 mmHg or 14.7 lbf/in² at sea level).

To withstand and adapt to the pressures within, the arteries are surrounded by a varying degree of smooth muscle which has extensive elastic and inelastic connective tissue and also exhibits muscular contraction or relaxation in response to adrenergic, cholinergic, other locally produced peptides, nitrous oxide, etc.

The effects of arterial responsiveness is most dramatic in the arterioles, the smallest end arteries, typically about 20,000 of them in a 150 lb (68 kg) individual.

These arteries, the arterioles, have the greatest collective influence on both local blood flow and, collectively, on overall blood pressure. They are the primary “adjustable nozzles” in the blood system, across which the greatest pressure drop occurs.

The combination of heart output (cardiac output) and total peripheral resistance, which refers to the collective average resistance of all the arterioles, are the principal determinates of the mean arterial blood pressure at any given moment.

The pulse pressure, i.e. Systolic vs. Diastolic difference, is determined primarily by the amount of blood ejected by each heart beat, stroke volume, versus the volume and elasticity of the major arteries.