Considering its importance in this process, the small intestine is a complex structure, with each part devoted to the absorption of a particular nutrient(s). The lacteals are one such important part, though their function is often poorly understood. What do the lacteals absorb? Let’s find out.
Did You Know?
The surface area of a human small intestine is equal to that of a tennis court.
The digestion of food starts right in the mouth, when enzymes in our saliva begin to break down starch and fats, while chewing. Upon swallowing, it passes into the throat, and enters a tube called the esophagus, or foodpipe. Powerful, involuntary contractions push the food down into the stomach, where the real process of digestion begins. Food is churned amidst strong acids to extract its nutrients.
When it enters the small intestine, food is already in the form of paste, and it is here that the majority of digestion takes place. After digestion in the small intestine, the waste from the food enters the large intestine, where water is further absorbed from it. The feces are then stored until they can be expelled from the body.
The inner lining of the small intestine contains an epithelial layer, called the mucosa. This lining contains millions of small projections called villi (plural). These projections serve the purpose of increasing the surface area of the intestine in contact with the food, so that maximum nutrients can be absorbed from it.
The structure of each villus consists of a network of blood vessels (capillaries and venules) along with a single lymphatic capillary running through its middle, called the lacteal. This tube is closed on its end near the peak of the villus, and is connected to the lymphatic system of the intestine through its other end.
Since the villi have an extensive network of blood vessels in them, and these vessels are separated from the food passing through the intestinal canal (lumen) by a thin epithelial layer, most nutrients are absorbed directly into the bloodstream. However, fat molecules in the digested food are too large to enter the blood capillaries. The lacteals, on the other hand, have a larger diameter than the blood vessels inside the villi, and are more suited to absorb fat from the intestinal canal.
Blood flows at a higher pressure through the intestinal villi, which allows only small molecules like sugars, amino acids, and short-chain fatty acids to diffuse into the capillaries from digested food, and not large, fatty acid molecules. On the other hand, the lacteals carry a fluid called lymph, at a much lower pressure, which enables the absorption of long-chain fatty acids from food into them. The lymph is a fluid that plays a key role in the removal of wastes, and immunity of our body.
The lacteals absorb fats that is already emulsified by intestinal, pancreatic, and liver secretions to form chylomicrons (fat molecules surrounded by transport proteins). This emulsified fat combines with the lymph carried by the lacteals to form a milky-white liquid called ‘chyle’. This is why the lacteals appear white when food is being digested in the small intestine.
The main purpose of the lacteals is to transport fatty acids, vitamins like A, D, E, and K, along with cholesterol from the intestine to the bloodstream. First, fat, in the form of chyle, is transported by a lacteal to larger lymphatic vessels in the mesentery (tissue that supports and nourishes the intestines), from where it travels to the cisterna chyli, a large, sac-like structure behind the abdomen. Lymph and chyle from the cisterna chyli is transported to the thoracic duct.
This is a large tube running from the lower spine to the neck, where it connects to the left subclavian vein, that transports blood to the heart. This allows fat to be available throughout the bloodstream, so that it can be absorbed from anywhere in the body for instant energy.
The presence of the lacteals in the small intestine shows how the lymphatic system plays a key role in the digestion of food, apart from its main role of collecting tissue fluid, filtering cell debris, and protecting the body from infections.