Arteries are blood vessels that supply blood to the tissues of body. They carry the blood away from heart and because the blood inside them is oxygenated, they appear red in color.
Characteristic Features of Arteries:
- Arteries are thick-walled, being uniformly thicker than the accompanying veins, except for the arteries within the cranium and vertebral canal where these are thin.
- Their lumen is smaller than that of the accompanying veins,
- Arteries have no valves.
- An artery is usually accompanied by veins and nerves, and the three of them together form the neurovascular bundle which is surrounded and supported by a fibro-areolar sheath.
Types and Structure of Arteries:
- Large arteries of elastic type, e.g. aorta and its main branches (brachiocephalic, common carotid, subclavian and common iliac) and the pulmonary arteries.
- Medium and small arteries of muscular type, e.g. temporal, occipital, radial, popliteal, etc.
- Smallest arteries of muscular type are called arterioles. They measure 50-100 micron in diameter. Arterioles divide into terminal arterioles with a diameter of 15-20 micron, and having one or two layers of smooth muscle in their walls. The side branches from terminal arterioles are called metarterioles which measure 10-15 micron at their origin and about 5 micron at their termination.
The terminal narrow end of metarteriole is surrounded by a precapillary sphincter which regulates blood flow into the capillary bed. It is important to know that the muscular arterioles arc responsible for generating peripheral resistance, and thereby for regulating the diastolic blood pressure. Microscopically, all arteries are made up of three coats.
- The inner coat is called tunica intima.
- The middle coat is called tunica media.
- The outer coat is called tunica adventitia. It is strongest of all coats and merges with the perivascular sheath.
The relative thickness of the coats and the relative proportion of the muscuiar, clastic and fibrous tissues vary in different types of arteries.
Blood Supply of Arteries:
The large arteries (of more than I mm diameter) are supplied with blood vessels. The nutrient vessels, called vasa vasorum, form a dense capillary network in the tunica adventitia, and supply the adventitia and the outer part of tunica media. The rest of the vessel wall (intima + inner part of media) is nourished directly by diffusion from the luminal blood. Minute veins accompanying the arteries drain the blood from the outer part of arterial wall. Lymphatics are also present in the adventitia.
Some arteries can be palpated through the skin. These are: common carotid, facial, brachial, radial, abdominal aorta, femoral, posterior tibial and dorsalis pedis.
Nerve Supply of Arteries:
The nerves supplying an artery are called nervi vascularis. The nerves arc mostly nonmyelinated sympathetic fibres which are vasoconstrictor in function. A few fibers are myelinated, and arc believed to be sensory to the outer and inner coats of the arteries. Vasodilator innervation is restricted to the following sites.
- The skeletal muscle vessels are dilated by cholinergic sympathetic nerves.
- The exocrine gland vessels are dilated on parasympathetic stimulation.
- The cutaneous vessels arc dilated locally to produce the flare (redness) after an injury. The vasodilatation is produced by the afferent impulses in the cutaneous nerves which pass antidromically in their collaterals to the blood vessels (axon reflex).