Histology: Blood Vessels & Lymphoid Organs

Histology is the study of tissues under a microscope, and two systems you will examine again and again are blood vessels and lymphoid organs. Blood vessels are layered tubes that carry blood to and from the heart, while lymphoid organs are the tissues where immune cells mature and wait to fight infection. Together they form the body's transport and defence network.

This matters because most major diseases show up in these tissues. Atherosclerosis hardens artery walls, lymphomas begin in lymph nodes, and varicose veins are failed valves. A doctor who can read a vessel wall or a lymph node slide can judge heart-attack risk or cancer spread.

Key Concepts

• Tunica intima: the innermost layer — a single sheet of endothelial cells on a basement membrane, the smooth surface blood touches.
• Tunica media: the middle layer of smooth muscle and elastic fibres — thick in arteries, thin in veins — controlling vessel diameter. The outer adventitia anchors the vessel.
• Capillary: the smallest vessel, ~8 µm wide with a ~0.5 µm wall — one endothelial cell. Types: continuous (brain, muscle), fenestrated (kidney, gut), sinusoidal (liver, bone marrow).
• Lymphoid organs: primary — red bone marrow (B cells) and thymus (T cells); secondary — lymph nodes, spleen, and MALT like tonsils, where immune responses begin.
• Lymph node zones: cortex (B-cell follicles), paracortex (T cells), medulla (sinuses). The spleen has white pulp (lymphoid) and red pulp (blood filter).

How It Works

Follow one heartbeat of blood. It leaves the heart through the aorta, whose elastic media stretches with each pulse and recoils to push blood onward. Muscular arteries adjust diameter to direct flow; arterioles, with one or two muscle layers, are the resistance vessels setting blood pressure. At capillaries the wall thins to a single endothelium so oxygen and nutrients diffuse across within seconds, and veins with one-way valves return the blood to the heart.

Now follow lymph. Tissue fluid drains into lymphatic capillaries and flows into a lymph node, percolating past B-cell follicles in the cortex and T cells in the paracortex, where dendritic cells display captured germs. Filtered lymph leaves through one efferent vessel — which is why infections make the nearest nodes swell.

Worked Example

Compare the thickest and thinnest vessel walls. The aorta's wall is about 2 mm thick, a capillary wall about 0.5 µm. Step 1: convert — 2 mm equals 2000 µm. Step 2: divide — 2000 ÷ 0.5 = 4000. So the aortic wall is roughly 4000 times thicker than a capillary wall, because the aorta must absorb the full force of every heartbeat while a capillary only needs to be thin enough for diffusion.

Common Misconceptions

• Arteries always carry oxygen-rich blood. Wrong — the pulmonary arteries carry oxygen-poor blood to the lungs. The rule is about direction from the heart, not oxygen content.
• Lymph nodes make blood cells. They house mature lymphocytes and filter lymph; blood cells are made in bone marrow.
• Capillaries are just tiny passive pipes. Their endothelium actively controls clotting, inflammation, and which molecules leave the blood in each tissue.

Real-World Applications

• Pathologists grade atherosclerosis by reading plaque changes in the intima of artery walls.
• Lymph node biopsies stage cancers — finding tumour cells in a node changes treatment completely.
• Varicose veins are diagnosed when ultrasound shows venous valves failing to close.
• Lymphedema therapy after lymph-node removal is designed around how lymph reroutes through remaining vessels.

Blood vessels are three-layered tubes, thick at the aorta and a single cell at the capillary. Lymphoid organs filter lymph — B cells in follicles, T cells in the paracortex.