Chemicals pass between cells and blood in the capillaries, the smallest blood vessels in your body. These microscopic vessels, barely wide enough for a single red blood cell to squeeze through, form vast networks in nearly every tissue. Their walls are just one cell thick, creating a thin barrier where oxygen, nutrients, hormones, and waste products move back and forth between your bloodstream and the fluid surrounding your cells.
How the Capillary Wall Works
The capillary wall is made of a single layer of flat cells called endothelial cells. Chemicals cross this barrier through several distinct routes, depending on their size. Small molecules like oxygen, carbon dioxide, and water slip through gaps between neighboring endothelial cells, known as intercellular clefts. These gaps act as tiny channels that allow anything smaller than about 3 nanometers to pass freely by simple diffusion. Molecules larger than 3 nm need help from specialized transport proteins embedded in the cell membrane.
For much larger molecules like insulin, antibodies, and the protein albumin (which makes up a big share of blood plasma), the body uses a more elaborate system. The endothelial cell essentially swallows the molecule on one side, packages it into a small bubble called a vesicle, shuttles it across the cell interior, and releases it on the other side. This three-step process of intake, trafficking, and release moves cargo that would otherwise be far too large to fit through any gap in the capillary wall. The vesicles involved have an opening of roughly 25 nanometers, making them the “large pore” system of the capillary.
Three Types of Capillaries
Not all capillaries are built the same. Your body uses three structurally different designs, each matched to the job a particular organ needs done.
- Continuous capillaries are the most common type, found in skin, lungs, muscles, and the heart. Their endothelial cells sit tightly together with no holes in the membrane. The gaps between cells allow pores of roughly 5 nm in most tissues, letting water, ions, and small molecules through while holding back larger proteins.
- Fenestrated capillaries have small windows (fenestrations) punched through the endothelial cells themselves. These pores range from 6 to 12 nm in glands and intestinal lining, and up to about 15 nm in the kidney’s filtering units. This design lets organs that need to absorb or secrete large volumes, like the intestines and hormone-producing glands, exchange chemicals much faster.
- Sinusoidal (discontinuous) capillaries are the leakiest of all. Found in the liver, spleen, and bone marrow, they have large, irregular gaps and an incomplete basement membrane. This allows even whole cells and large proteins to pass through, which is why the liver can so efficiently filter and process blood.
What Pushes Chemicals Across
Two main forces drive fluid and dissolved chemicals out of or into capillaries. The first is hydrostatic pressure, which is essentially the physical push of blood against the capillary wall. The second is osmotic pressure created by proteins dissolved in the blood plasma. These proteins are too large to leave the capillary easily, so they pull water back toward themselves.
At the arterial end of a capillary (where blood first enters from a small artery), hydrostatic pressure is high, around 35 mmHg. This force pushes fluid carrying dissolved nutrients and oxygen out through the capillary wall and into the surrounding tissue. As blood travels along the capillary, that pressure drops to roughly 18 mmHg by the venous end. Meanwhile, the inward pull from plasma proteins stays relatively constant at about 25 mmHg. So by the venous end, the protein-driven pull exceeds the outward push, and fluid flows back into the capillary, bringing dissolved waste products like carbon dioxide with it.
About 90% of the fluid that filters out at the arterial end gets reabsorbed at the venous end. The remaining 10% drains into the lymphatic system.
The Space Between Blood and Cells
Chemicals don’t jump directly from capillary blood into your body’s cells. They first enter the interstitial fluid, a thin layer of liquid that bathes every cell in your tissues. This fluid has a protein concentration of roughly 20.6 grams per liter, much lower than blood plasma, and contains dissolved nutrients, electrolytes, and gases that cells need.
Interstitial fluid acts as the go-between. Oxygen and glucose diffuse from the capillary into this fluid, then from the fluid into nearby cells. Carbon dioxide and other metabolic waste travel the reverse path, moving from cells into interstitial fluid and then into the capillary for removal. The constant flow of this fluid keeps cells supplied and clean.
How the Lymphatic System Picks Up the Rest
Not everything that leaves the capillary makes it back in at the venous end. Larger waste products, stray plasma proteins, and excess fluid that can’t easily diffuse back into the blood get collected by lymphatic capillaries instead. These are blind-ended vessels scattered throughout your tissues with a clever design: their endothelial cells overlap to form microscopic one-way flaps called button junctions. Fluid can push in, but it can’t flow back out.
This lymphatic drainage serves two purposes. It prevents fluid from building up in tissues (which would cause swelling), and it delivers foreign particles and potential threats to lymph nodes for immune screening. The collected lymph eventually empties back into the bloodstream near the heart, completing the loop.
The Blood-Brain Barrier Is a Special Case
Capillaries in the brain play by different rules. Their endothelial cells are sealed together with extremely tight junctions that shrink the effective pore size to less than 1 nanometer, compared to 5 nm in muscle capillaries. They also lack fenestrations entirely and do very little of the vesicle-based transport that capillaries elsewhere rely on for large molecules.
This means most water-soluble chemicals in the blood simply cannot reach brain tissue. Instead, the brain’s capillaries use dedicated transport proteins to selectively import only what neurons need: glucose, essential amino acids, and a handful of other molecules. They also contain pumps that actively eject foreign substances back into the bloodstream, and enzymes inside the endothelial cells that break down unwanted compounds before they ever reach the brain side. This triple-layered defense, a physical barrier, an active pump system, and a metabolic filter, is why so many drugs that work elsewhere in the body can’t easily treat brain conditions.