Nutrition in humans, at a glance

Food travels along the alimentary canal. Digestive glands supply substances into it; small nutrients are absorbed and then used by body cells.

• Ingestion: taking food in. Digestion: breaking large insoluble food molecules into small soluble molecules.
• Absorption: nutrients cross into blood or lymph. Assimilation: absorbed nutrients are used or incorporated by cells.
• Egestion removes undigested material through the anus; it is different from excretion of metabolic waste.

Keep in mind: These organs send secretions into the canal. The swallowed food itself remains in the canal until nutrients are absorbed or undigested material is egested.

Enzymes chemically digest food molecules. Chewing and bile emulsification increase surface area so enzymes can act more effectively.

• Amylase: starch to maltose. Maltase: maltose to glucose.
• Proteases: proteins are broken down through smaller peptides to amino acids.
• Lipase: fats to fatty acids and glycerol. Bile emulsifies fat and contains no digestive enzyme.

Keep in mind: Bile breaks up droplets containing many fat molecules. The molecules remain chemically unchanged during emulsification. Lipase produces the smaller digestion products.

The hepatic portal vein carries nutrient-rich blood from the small intestine to the liver. The liver processes nutrients before blood carries them onwards.

• The hepatic portal vein carries absorbed glucose and amino acids from the intestine to the liver.
• The liver converts glucose to glycogen for storage and glycogen back to glucose when needed.
• Assimilation includes making body substances and using absorbed nutrients in cells.

Keep in mind: It carries blood from the intestine to the liver. Bile travels towards the intestine through bile ducts, which are not blood vessels.

The liver stores and processes nutrients, produces bile, converts excess amino acid nitrogen into urea, and breaks down alcohol and hormones.

• Glucose can be stored as glycogen; glycogen can be converted back to glucose.
• Excess amino acids are deaminated in the liver; urea formed there travels in blood to the kidneys for excretion.
• The liver produces bile and breaks down alcohol and hormones. Breakdown capacity does not make excessive alcohol harmless.

Keep in mind: The liver forms urea after amino acid deamination. The kidneys remove the urea from blood and excrete it.

A hormone is a chemical substance produced by a gland, carried in the blood and affecting specific target organs.

• Gland produces hormone -> blood carries it -> specific target changes activity.
• The pancreas releases insulin and glucagon into blood.
• Hormones are signals; digestive enzymes catalyse digestion in the gut.

Keep in mind: A hormone affects suitable target cells with the appropriate receptors. Circulation and response are different things.

Insulin lowers blood glucose by promoting uptake and storage; glucagon raises it by promoting glucose release from the liver.

• Insulin: blood glucose falls; glucose uptake and glycogen formation increase.
• Glucagon: liver glycogen breaks down; blood glucose rises.
• Glucose is a sugar; glycogen is its storage polymer.

Keep in mind: Both are hormones that signal changes in target-cell activity. Glucose and glycogen are the substances interconverted during storage and release.

Type 2 diabetes involves persistently high blood glucose because cells respond less effectively to insulin, insulin supply is insufficient, or both.

• Insulin resistance is reduced responsiveness of target cells.
• Persistent high blood glucose differs from a brief normal rise.
• Risk factors affect likelihood; management depends on the individual.

Keep in mind: Insulin resistance and/or insufficient insulin are involved. Insulin may be present, and the balance of these problems varies between individuals.