Nutrient Transport in Animals
Describe how nutrients and water are transported within animals, including the role of the circulatory system in delivering nutrients from digestion
What a learner can do afterwards
- Describe how many nutrients, including simple sugars, pass through the wall of the small intestine into the blood
- Explain that blood transports dissolved nutrients and water to all parts of the body
- Connect the digestive system and circulatory system as working together to deliver nourishment
1 · Read
You have learned where food travels and how the heart pumps blood. Digestion breaks large food molecules into smaller molecules the body can absorb. Chewing alone does not finish this job. In the small intestine, many nutrients, including simple sugars, cross the intestine wall into nearby blood vessels. Whole food lumps cannot cross this way.
Blood carries dissolved nutrients and water in its liquid part. It also contains blood cells. The heart pumps blood through vessels to organs such as the brain and muscles. Near body cells, nutrients and water can leave tiny vessels called capillaries. They enter the fluid around cells, from which cells take what they need.
After a rice meal, digestion breaks starch down into simple sugars. This breakdown finishes in the small intestine. A sugar can then cross the small-intestine wall into blood. Blood carries it onward, and muscle cells can use it for energy. For this sugar, release by digestion comes before absorption, which comes before delivery to muscles.
Water from food and drinks crosses the gut wall into the body without being digested into smaller food molecules. Most water absorption happens in the small intestine. The large intestine absorbs some water too. Blood transports absorbed water around the body.
The digestive system breaks food down and absorbs nutrients. The circulatory system carries absorbed nutrients and water around the body. Some absorbed fats first travel in lymph, a fluid in another set of vessels, before entering blood. Digestion and circulation work together: breaking food down and delivering nutrients to distant cells are different jobs.
Digestion releases nutrients. Many enter blood through the small-intestine wall. Blood carries nutrients and water toward cells throughout the body.
These five readings were independently written for this dataset. Supplied OpenStax records informed fact checks and teaching approaches; their prose is not reproduced.
More to read
Different animals move nutrients differently
Your heart pumps blood through vessels, providing a route from the gut to distant body parts. This human transport system is not a rule for all animals. In insects, circulating fluid also flows around organs rather than staying entirely inside vessels. Some thin-bodied animals have no circulatory system. Their cells exchange substances over short distances. Studying humans gives one example of nutrient transport, while other animals use different arrangements.
Chewing does not finish digestion
A chewed grain of rice contains smaller food pieces, but it still contains large starch molecules. Chemical digestion breaks starch down into simple sugars. Those sugars can cross the small-intestine wall into blood. Making food pieces smaller and making food molecules smaller are different changes. Absorption is another process: it moves suitable substances across the gut wall into the body. Whole rice pieces do not enter blood.
Where nutrients leave blood
Blood flows inside vessels during normal circulation. Near muscle cells, the vessels include tiny capillaries. Nutrients can leave blood through capillary walls and enter fluid surrounding the cells. Cells take up substances from that fluid. Blood delivery therefore does not mean blood flows inside each muscle cell. The heart keeps the blood moving, while exchange near cells allows nutrients to reach the places where they are used.
Water can be absorbed without digestion
Water in food and drinks does not need to be broken into smaller food molecules. It crosses the gut wall into the body. Most water absorption happens in the small intestine, and the large intestine absorbs some too. Blood moves absorbed water around the body. Water does not supply food energy as sugar does, but cells still need it. It also provides the liquid in which many nutrients travel.
An extra transport step for fats
Simple sugars absorbed in the small intestine enter nearby blood vessels. Many fats take an extra route. After absorption, cells in the intestine lining package them for transport. These packages enter vessels carrying lymph, a fluid in a separate transport system. Lymph later drains into the blood circulation. Blood then carries the fats onward. Not every absorbed nutrient passes straight from the intestine into blood.
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