The body’s vascular structures are a network of hollow tubes, or vessels, that transport fluids. This network is composed of two primary systems. The first is the blood vascular system, commonly known as the circulatory system, which circulates blood. The second is the lymphatic system, which handles a clear fluid called lymph. Together, these systems ensure all parts of the body receive necessary substances and have waste products removed, maintaining internal balance and health.
The Blood Vascular System
The blood vascular system is a closed network, meaning blood is always contained within a vessel. It consists of three main types of blood vessels: arteries, veins, and capillaries.
Arteries carry blood away from the heart. They are built to handle high-pressure blood flow from the heart’s contractions, so they have thick, muscular, and elastic walls. This elasticity allows them to expand and recoil with each heartbeat, pushing the blood forward. The largest artery, the aorta, receives blood from the heart and branches into smaller arteries to distribute it throughout the body.
Veins transport blood from the body’s tissues back toward the heart. Because the blood in veins is under lower pressure, their walls are thinner and less muscular than arteries. Veins, particularly in the limbs, contain one-way valves. These small flaps of tissue prevent the backflow of blood, helping it return to the heart against gravity.
Capillaries are the smallest blood vessels, connecting the arterial and venous systems. They form vast networks called capillary beds that permeate every tissue. Their walls are incredibly thin, often only a single cell thick. This structure facilitates the exchange of substances between the blood and body cells. Oxygen and nutrients pass from the blood into the tissues, while carbon dioxide and waste products move from the tissues into the blood.
Systemic and Pulmonary Circulation
The blood vascular system operates through two interconnected circuits: the pulmonary circulation and the systemic circulation. These two pathways create a figure-eight loop, with the heart acting as the central pump connecting them.
The pulmonary circulation is the smaller loop between the heart and the lungs for gas exchange. Deoxygenated blood is pumped from the right side of the heart through the pulmonary arteries to the lungs. In the lungs, carbon dioxide is released, and oxygen is absorbed. The now oxygen-rich blood returns to the left side of the heart through the pulmonary veins.
Once oxygenated blood reaches the left side of the heart, it enters the systemic circulation. This larger circuit transports blood to and from the rest of the body. The left side of the heart pumps this oxygen-rich blood into the aorta to supply all organs and tissues. As blood flows through the capillaries, it delivers oxygen and nutrients and picks up waste products. This deoxygenated blood then returns to the right side of the heart through veins, ready to re-enter the pulmonary circuit.
The Lymphatic System
Operating alongside the blood vascular network is the lymphatic system, a second, parallel system of vessels. Instead of blood, these vessels carry a clear fluid known as lymph. Lymph originates from the plasma that leaks out of blood capillaries to bathe the body’s cells. A primary role of the lymphatic system is to collect this excess fluid, called interstitial fluid, and return it to the bloodstream, preventing fluid buildup in the tissues.
The lymphatic system is also a component of the body’s immune defense. As lymph flows through the vessels, it passes through small structures called lymph nodes. These nodes are populated with immune cells that filter the lymph, trapping and destroying pathogens like bacteria and viruses before the fluid is returned to the blood.
Structurally, the lymphatic system differs from the blood circulatory system. It is a one-way drainage system that flows only toward the heart, not a continuous loop. It also lacks a central pump; instead, lymph is moved by the contraction of surrounding skeletal muscles and pressure changes from breathing.
Conditions of the Vascular Structures
When the structure or function of vascular systems is compromised, various health conditions can arise.
Atherosclerosis is a common condition affecting the arteries. It involves the buildup of plaque—a substance made of fats, cholesterol, and other materials—on the inner walls of the arteries. This buildup causes the arteries to harden and narrow, restricting blood flow and potentially leading to blockages. The reduced blood flow can deprive tissues of oxygen.
Varicose veins are a condition of the veins, most commonly occurring in the legs. They develop when the one-way valves within the veins weaken or fail. This failure allows blood to flow backward and pool in the veins, causing them to become enlarged, twisted, and visible under the skin.
Lymphedema is a condition that results from a blockage or damage to the lymphatic system. When lymphatic vessels are unable to properly drain lymph from the tissues, the fluid accumulates, causing swelling, typically in the arms or legs. This condition can be caused by infection, injury, or as a side effect of some medical treatments.