What keeps blood from leaking through capillaries?

Blood does not leak through capillaries primarily due to the structural integrity of the endothelial cells that line the capillaries. These cells are tightly joined together in most capillaries, forming a selective barrier that regulates the passage of substances. Additionally, the pressure within the capillaries is carefully balanced to prevent excessive leakage while allowing for the exchange of nutrients and waste. In certain capillaries, such as those in the kidneys or liver, specialized structures like fenestrations or sinusoids facilitate selective permeability without compromising overall containment.