0610 · 7.5Absorption

Where absorption happens

0:00Here is a question. You eat a sandwich at lunchtime, and by the afternoon your muscles are using the glucose that came from its bread. But the sandwich went into a tube running through your body, and your muscles are nowhere near that tube. So how does the food get out of the gut and into the rest of you? The answer is absorption. By the end of this video you will be able to state where nutrients and water are absorbed, explain how villi and microvilli make absorption fast, describe the structure of a villus, and explain what the capillaries and the lacteal inside each villus do.

0:30You need three ideas from earlier videos. First, absorption is the movement of nutrients from the intestines into the blood. Second, chemical digestion breaks large food molecules into small, soluble ones. Starch becomes glucose, proteins become amino acids, and fats become fatty acids and glycerol. Third, small molecules can cross cell membranes by diffusion, down a concentration gradient, and by active transport, against a concentration gradient, using energy from respiration.

0:58Let us start with where it happens. The small intestine is the region where nutrients are absorbed. In the small intestine, digestion is completed. Food is broken down into small, soluble molecules, and these are small enough to pass through the wall of the small intestine and be absorbed. Water is absorbed too. Most of the water is absorbed from the small intestine. Some more water is absorbed from the colon, which is the main part of the large intestine.

1:21That is why the waste that leaves the colon is fairly solid. This is a very common mistake. Many students say that the colon absorbs most of the water. No, it does not. The small intestine absorbs most of it. The colon only absorbs some of what is left.

Villi and microvilli surface area

1:37For the extended syllabus, you need to explain why the small intestine is so good at absorption. The key idea is surface area. Molecules can only be absorbed where they touch the lining of the gut. The more lining there is, the more molecules can be absorbed at the same time, so the faster absorption happens. If the small intestine were a smooth tube, its inner surface would be quite small.

1:59Instead, its lining is covered in millions of tiny finger-shaped projections called villi. One of them is called a villus. Each villus sticks out into the space in the middle of the gut, where the digested food is. Think of a towel. A smooth sheet of cloth has a small surface. A towel is covered in little loops, so it has a much larger surface, and that is why it soaks up water so well.

2:22Villi do the same job for the small intestine. And there is a second level. The cells on the surface of each villus have their own, even smaller folds on their outer edge. These are called microvilli. So the lining is covered in villi, and the cells on the villi have their membranes folded into microvilli. Together, they increase the internal surface area of the small intestine many times over.

2:42A larger surface area means a faster rate of absorption, so very little digested food passes through without being absorbed.

Villus structure and nutrient routes

2:49Now let us look inside one villus. The syllabus wants you to describe its structure, so let us build it up from the outside in. On the outside is the epithelium, a wall that is only one cell thick. Each of these epithelial cells has microvilli on its outer edge. Because the wall is so thin, the distance that molecules have to travel from the gut into the blood is very short, and a short diffusion distance means faster diffusion.

3:14Just under the epithelium is a dense network of capillaries. Blood arrives at the base of the villus in a small artery called an arteriole, flows through the capillaries, and leaves in a small vein called a venule. So every villus has a very good blood supply. And up the centre of each villus runs one more tube. This is the lacteal. The lacteal is not a blood vessel. It is part of the lymphatic system, a separate network of tubes that carries a fluid called lymph around the body.

3:40For the extended syllabus, you need to describe what the capillaries and the lacteal do. Different nutrients take different routes. The capillaries absorb glucose and amino acids, as well as water and mineral ions. These molecules pass through the epithelial cells and into the blood. The blood then carries them away from the small intestine, in a vessel called the hepatic portal vein, to the liver. Here is why the moving blood matters.

4:04Glucose diffuses from where it is more concentrated, in the gut, to where it is less concentrated, in the blood. If the blood stood still, the glucose would build up in it, and diffusion would slow down. But the blood is always flowing, so it keeps carrying the glucose away. This keeps the concentration in the blood low, and so it maintains a steep concentration gradient between the gut and the blood.

4:25You met another part of this story in the video on active transport. When the concentration of glucose in the gut falls lower than in the blood, the epithelial cells use active transport to keep absorbing it, using energy from respiration. So diffusion and active transport work together, and almost none of the glucose is wasted. The lacteal deals with fats. Fatty acids and glycerol are absorbed into the lacteal, and they travel away in the lymph.

4:49The lymph does not stay separate forever. Later, it drains into the blood near the heart, so the fats reach the rest of the body in the end.

Common mistakes and exam answer

4:56Let us collect the common mistakes. The first is saying that the lacteal carries blood. No, it does not. It is part of the lymphatic system, and it carries lymph. The second is mixing up the two routes. Glucose and amino acids go into the capillaries. Fatty acids and glycerol go into the lacteal. The third is mixing up villi and microvilli. Villi are finger-like projections of the gut lining, each made of many cells.

5:20Microvilli are tiny folds of the cell membrane of a single epithelial cell. The fourth is just writing that villi increase the surface area, and stopping there. In an exam, always finish the idea. A larger surface area gives a faster rate of absorption. Let us try an exam-style question. Explain how the structure of a villus is adapted for absorption. This could be worth four or five marks, so you need several separate features.

5:44For each one, say the feature, and then say why it helps. Here is a model answer. Villi and microvilli give a large surface area, so absorption is faster. The wall of the villus is only one cell thick, so there is a short diffusion distance. There is a dense network of capillaries, which gives a good blood supply. The capillaries absorb glucose and amino acids, and the flowing blood carries them away, which maintains a steep concentration gradient.

6:09There is a lacteal in the centre, which absorbs fatty acids and glycerol into the lymphatic system. Notice the pattern. Every feature is linked to a reason. Writing only the features, as a list, is a common way to lose marks. You can also gain credit for saying that the epithelial cells have many mitochondria, which release energy for active transport. Let us recap. Nutrients are absorbed in the small intestine.

6:33Most water is absorbed in the small intestine as well, and some more is absorbed in the colon. Villi and microvilli greatly increase the internal surface area of the small intestine, which makes absorption faster. Each villus has a wall that is one cell thick, a dense network of capillaries, and a single lacteal up its centre. The capillaries absorb glucose and amino acids, and the flowing blood carries them to the liver, keeping the concentration gradient steep.

6:59The lacteal absorbs fatty acids and glycerol into the lymph. And that is how a sandwich in your gut becomes fuel for your muscles.