Structure and Function of Blood Vessels

Histological Organization: The Tunics

Most blood vessels (excluding capillaries) are organized into three distinct layers known as tunics.

1. Tunica Intima (Inner Layer): This layer consists of the endothelium, a simple squamous epithelium that provides a smooth surface for blood flow.
2. Tunica Media (Middle Layer): This layer is composed of smooth muscle and elastic connective tissue. The thickness of this layer varies; for example, muscular arteries have significant smooth muscle to regulate vessel diameter.
3. Tunica Externa (Outer Layer): This is a fibrous layer made of collagen that provides structural support and anchors the vessel to surrounding tissues.

The central space through which blood actually flows is called the lumen.

Blood Flow Regulation

The body must constantly adjust blood distribution to meet the metabolic demands of different tissues. This is achieved through blood flow regulation.

Vasodilation and Vasoconstriction

The diameter of the lumen in arterioles and small arteries can be altered by the contraction or relaxation of the smooth muscle in the tunica media:

• Vasoconstriction: The narrowing of the vessel lumen, which increases resistance and can raise blood pressure.
• Vasodilation: The widening of the vessel lumen, which decreases resistance and increases blood flow to specific areas.

This process is primarily controlled by the autonomic nervous system. By modulating vessel diameter, the nervous system can effectively redirect blood volume to where it is most needed (e.g., to skeletal muscles during exercise).

Microcirculation and Precapillary Sphincters

While large vessels undergo vasodilation and vasoconstriction, capillaries themselves do not possess smooth muscle and therefore cannot change their diameter. Instead, blood flow into specific capillary beds is regulated by precapillary sphincters. These are rings of smooth muscle located at the junction where arterioles transition into capillaries. By contracting or relaxing, these sphincters determine whether blood enters a true capillary bed or bypasses it through a metabolic shunt.

Exam Tip: If a question asks why capillaries cannot undergo vasoconstriction, the answer lies in their histology: they lack the smooth muscle layer (tunica media) required for such a movement.

Clinical Relevance: Blood Pressure

Blood pressure is the force exerted by blood against the vessel walls. It is highest in the arteries leaving the heart and decreases as blood moves through the systemic circuit.

During the cardiac cycle, pressure fluctuates between two values:

1. Systolic Pressure: The maximum pressure during ventricular contraction.
2. Diastolic Pressure: The minimum pressure during ventricular relaxation.

In humans, a normal reading is approximately $90\text{--}120\text{ mmHg}$ systolic and $60\text{--}80\text{ mmHg}$ diastolic. Dysregulation of vessel diameter (via the autonomic nervous system) and changes in cardiac output are the primary drivers of blood pressure fluctuations.