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.

Peristalsis is a rhythmic wave of muscle contraction along the alimentary canal that mixes its contents and moves them forwards.

• Circular muscle contraction narrows the canal behind the bolus.
• Coordinated contraction and relaxation move a wave along the gut.
• Movement mixes food with digestive juices and brings it into contact with the gut lining.

Keep in mind: Muscle movement propels and mixes food. Chemical digestion requires enzymes to break large molecules into smaller soluble products.

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.

Villi provide a large absorbing surface, a thin barrier and an effective transport supply. Glucose and amino acids enter blood capillaries; absorbed fat is carried mainly through lacteals.

• Many villi and microvilli give a large surface area.
• The villus epithelium is one cell thick, providing a short absorption distance.
• Blood flow carries absorbed nutrients away, helping maintain concentration gradients. Lacteals carry absorbed fat through lymph.

Keep in mind: A large surface helps absorption, but starch must first be digested into small soluble products such as glucose. More area does not remove the need for digestion.

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.

Alcohol can slow responses and reduce self-control. Repeated excessive consumption can damage the liver and brain, with consequences for the person and others.

• Alcohol can increase reaction time and reduce judgement and self-control.
• Long-term excessive consumption can cause liver disease and brain damage.
• Social consequences can follow from impaired decisions, injuries, financial costs and strain on relationships.

Keep in mind: Reaction time is a duration. An increase means the response starts later, which can leave less time to avoid a hazard.