Question

A muscular wall is absent in (a) Venule (b) Vein (c) Capillary (d) Arteriole

Short answer

The capillary is the blood vessel that does not have a muscular wall.

Step-by-step solution

Understanding different blood vessels

Start by understanding the physical structure of the different types of blood vessels. Venules are small vessels that collect blood from the capillaries and transport it to the veins. Veins carry blood towards the heart. Arterioles are small vessels that extend and branch out from an artery and carry blood to the capillaries. Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation, where exchange of water, oxygen, carbon dioxide, and many other nutrients and wastes occurs.

Identifying the blood vessel without a muscular wall

Next, identify which of these vessels lacks a muscular wall. Venules, veins, and arterioles all have a muscular wall in their structure. However, capillaries do not have a muscular wall. The walls of capillaries are only one cell thick to allow exchange of materials between the blood and body's cells.

Key concepts

Structure of Blood Vessels

The human body has a vast network of blood vessels intricately woven to form the circulatory system, serving as highways for blood to transport essential substances. Each type of blood vessel plays a critical role in this process, and they vary significantly in structure to accommodate their functions.

Venules are small vessels that bridge the gap between capillaries and the larger veins. They gather the blood that has passed through capillary beds and usher it into the larger veins, which then transport it back to the heart. Veins act as return conduits for deoxygenated blood, except for the pulmonary vein which carries oxygenated blood from the lungs to the heart. These vessels have thinner walls compared to arteries, equipped with valves to prevent backflow of blood as it moves towards the heart against gravity.

Meanwhile, arteries and their smaller counterparts, arterioles, deliver oxygenated blood from the heart to the rest of the body. These vessels have a distinctively thick, muscular wall to handle high-pressure blood flow. The muscle layer, or tunica media, enables arteries and arterioles to control the blood flow by contracting and dilating — a process known as vasoconstriction and vasodilation respectively.

Capillaries and Microcirculation

Nestled within the microcirculation network, capillaries are the body's smallest blood vessels and serve as the primary location for the exchange of gases, nutrients, and wastes between the blood and tissues. This exchange is made possible by the incredibly thin capillary wall, which is only one cell layer thick, known as the endothelium.

The capillary beds weave through tissues and organs, providing a vast surface area for efficient exchange. The thin walls allow oxygen and nutrients to diffuse from the blood into the tissues, while carbon dioxide and other waste products move from the tissues back into the bloodstream. Due to their role, capillaries lack a muscular wall which is found in larger vessels, thus differing fundamentally in both structure and function from venules, veins, and arterioles.

Blood Vessel Muscular Wall

A predominant feature of blood vessels, especially arteries and arterioles, is their muscular wall, which plays a pivotal role in regulating blood flow and pressure in the circulatory system. This muscular wall is composed of smooth muscle cells organized in a layer called the tunica media.

In veins and venules, the muscular wall is less pronounced but still present. It assists with blood flow back to the heart and can constrict or dilate to accommodate varying volumes of blood. However, capillaries do not have this muscular layer as their primary function is the exchange of materials with tissues, requiring a significantly thinner wall for permeability. Hence, capillaries can afford to sacrifice the muscular wall for a single-celled lining that facilitates efficient transfer of substances.