The gut wall is the layered tissue forming the intestine — the inner surface across which digested nutrients pass into the blood. Its lining is folded into millions of finger-like villi, each fringed with microvilli, creating a vast absorptive surface area roughly the size of a tennis court. Nutrients cross this barrier by specialised transporters: glucose and amino acids pumped actively, fatty acids diffusing into cells for repackaging. The wall is also a selective fortress — tight junctions between cells, a mucus coat, immune tissue, and the microbiota decide what may pass and what must not. When the barrier fails, as in coeliac disease or severe infection, malabsorption and systemic inflammation follow.