Absorption, liver and disorders

MDCAT Biology · Biology · Digestion. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Most absorption takes place in the small intestine, mainly the jejunum and ileum. Its lining is folded and covered with finger-like villi, and each epithelial cell has microvilli. Together they give a huge surface. A villus has a wall one cell thick, a rich capillary network and a central lymph vessel called a lacteal.

Glucose and amino acids enter the capillaries and travel in the hepatic portal vein to the liver. Fatty acids and glycerol are rebuilt into fats in the epithelial cells and enter the lacteal.

The liver processes absorbed food before it reaches the rest of the body. The pancreas supplies both digestive enzymes and hormones.

Rules to remember

Glucose and amino acids: absorbed by diffusion and active transport (cells have many mitochondria) → blood capillaries → hepatic portal vein
Fatty acids and glycerol → reformed into fats → lacteal → lymph; fat-soluble vitamins A, D, E and K are absorbed with fats and need bile
Liver: makes bile, stores glucose as glycogen, deaminates excess amino acids and makes urea, detoxifies poisons, makes plasma proteins (albumin, fibrinogen, prothrombin), stores iron and vitamins
Pancreas: exocrine part sends pancreatic juice into a duct; endocrine part (islets of Langerhans) sends insulin and glucagon into the blood
Peptic ulcer = a sore where acid and pepsin erode the wall of the stomach or duodenum; often linked with Helicobacter pylori
Dyspepsia = indigestion, with discomfort in the upper abdomen
Food poisoning = illness from food carrying bacteria or their toxins, e.g. Salmonella, Campylobacter

Common mistake

The liver does not secrete insulin or any digestive enzyme. Insulin comes from the islets of Langerhans in the pancreas; the liver only responds to insulin by storing glucose as glycogen.

Worked example

Two hours after a meal rich in starch, blood is sampled from the hepatic portal vein and from the hepatic vein. Which sample has the higher glucose concentration, and why?

Starch is digested to glucose, which is absorbed into the capillaries of the villi.
This blood flows straight to the liver in the hepatic portal vein, so its glucose level is very high.
Under the influence of insulin, the liver cells take up the extra glucose and store it as glycogen.
Blood leaving the liver in the hepatic vein therefore has a lower, near-normal glucose level.

Answer: The hepatic portal vein, because the liver removes excess glucose and stores it as glycogen before the blood leaves.

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