Absorption and Assimilation

Key Takeaway

Table of Contents

Absorption moves digested nutrients into transport systems; assimilation puts them to use

Digestion prepares small, soluble molecules. Absorption then moves them across the wall of the small intestine, while assimilation happens when body cells use or incorporate those absorbed molecules.

The ileum is specialised for absorption by having many villi and microvilli, a one-cell-thick epithelium and an efficient transport system.
Glucose and amino acids enter blood capillaries; products of fat digestion enter the lacteal.
The hepatic portal vein carries blood rich in absorbed nutrients from the small intestine to the liver.
Assimilation includes using glucose in respiration, amino acids to make proteins, and lipids to form cell structures or energy stores.
The liver converts glucose ⇌ glycogen, produces bile, metabolises amino acids and forms urea, and breaks down alcohol and hormones.

Finishing digestion does not place nutrients inside your body cells. The products are still in the lumen of the small intestine, separated from the blood and lymph by the intestinal lining. They must first cross that lining. This movement is absorption.

Absorption is only the handover. Once the transported nutrients reach tissues, cells may release energy from them, build new cell material or store them for later. That next stage is assimilation. The villi, blood vessels, lymphatic vessels and liver connect these two stages into one continuous journey from the intestine to the cells.

Diagnostic Quiz

What Do You Already Know About Absorption?

Try these five questions before reading. They test the villus, nutrient routes, the hepatic portal vein and the meaning of assimilation.

5 diagnostic questions
Core absorption and assimilation ideas
Instant explanation for every answer option

Question 1 of 5

Which statement best describes absorption?

Large food molecules are broken into small, soluble molecules

That is digestion, which prepares molecules for absorption.

Digested nutrients move through the wall of the small intestine into blood or lymph

Absorption transfers digested nutrients from the intestinal lumen into the body's transport systems.

Body cells use nutrients to release energy or build cell material

That is assimilation, which occurs after nutrients have been absorbed and transported.

Undigested material leaves the body through the anus

That is egestion, not absorption.

Absorption is the movement of digested nutrients across the intestinal wall into blood or lymph.

Which feature of a villus gives absorbed molecules a short distance to travel?

Its outer surface is covered by thick muscle layers

A thick outer barrier would lengthen rather than shorten the absorption distance.

It contains stored glycogen

Glycogen storage does not explain the short route across the villus surface.

Its epithelium is one cell thick

A one-cell-thick epithelium minimises the distance between the intestinal lumen and the transport vessels.

It produces hydrochloric acid

Hydrochloric acid is produced in the stomach and is not a villus adaptation.

The thin epithelium shortens the pathway for diffusion and other transport processes.

After absorption, where do glucose and amino acids enter inside a villus?

The blood capillaries

Glucose and amino acids enter the capillary network and are carried away in the blood.

The lacteal only

The lacteal is the main route for absorbed products of fat digestion.

The bile duct

The bile duct delivers bile to the duodenum; it does not collect absorbed glucose and amino acids.

The lumen of the ileum

The nutrients begin in the lumen and must leave it during absorption.

Water-soluble digestion products such as glucose and amino acids enter blood capillaries in the villi.

What is the function of the hepatic portal vein?

It carries bile from the liver to the gall bladder

Bile travels through bile ducts, not through a vein.

It carries oxygenated blood from the heart to the liver

The hepatic artery supplies oxygenated blood to the liver.

It carries blood from the liver towards the heart

That is the direction of flow in the hepatic vein.

It carries blood rich in absorbed nutrients from the small intestine to the liver

This direct route allows the liver to process absorbed water-soluble nutrients before the blood enters the general circulation.

Remember the direction: digestive organs → hepatic portal vein → liver.

Which event is an example of assimilation?

Amylase changes starch into maltose in the digestive system

This is chemical digestion.

A muscle cell uses absorbed amino acids to make proteins

The cell is incorporating absorbed nutrients into new cell material, so this is assimilation.

Glucose crosses the epithelium of a villus into a capillary

This transfer across the intestinal wall is absorption.

Undigested fibre moves into the colon

Movement of undigested material is not assimilation.

Assimilation occurs when cells use absorbed nutrients, for example in respiration, growth, repair or storage.

Absorption and assimilation describe different stages

Digestion, absorption and assimilation happen in sequence, but each word answers a different question. Digestion asks how food molecules are made small and soluble. Absorption asks how those products enter the body's transport systems. Assimilation asks what cells do with the products after they arrive.

Process	What happens	Example
Digestion	Large, insoluble food molecules are broken down into small, soluble molecules.	Maltase changes maltose into glucose.
Absorption	Digested nutrients cross the intestinal wall into blood or lymph.	Glucose crosses the villus epithelium and enters a blood capillary.
Assimilation	Cells take up and use or incorporate absorbed nutrients.	Amino acids are joined to make proteins for growth and repair.

A useful boundary: crossing from the intestinal lumen into blood or lymph is absorption. Using the nutrient after it has been delivered to a cell is assimilation.

The ileum provides a very large absorbing surface

Most absorption of digested nutrients occurs in the ileum, the long final part of the small intestine. Its inner surface is not smooth. It has many folds covered with millions of finger-like projections called villi. The epithelial cells covering each villus also have microscopic projections called microvilli. These levels of folding create a very large surface area in a limited space.

A large surface area allows many nutrient molecules to cross at the same time. However, surface area alone is not enough. Each villus also provides a short route across the epithelium and vessels that rapidly carry absorbed substances away.

What is inside a villus?

Blood and lymphatic transport inside intestinal villi. Every villus contains both a blood capillary network and a lacteal. The illustration separates the two transport systems across neighbouring villi so that each can be seen clearly. Glucose and amino acids enter the blood capillaries, while absorbed products of fat digestion enter the lacteal.
Adapted from intestinal-villi by Servier Medical Art, licensed under CC BY 3.0 Unported.

How each adaptation improves absorption

Feature	How it helps absorption
Many villi and microvilli	They greatly increase the surface area available for substances to cross.
Epithelium one cell thick	It gives molecules a short distance to travel from the lumen to the transport vessels.
Dense capillary network	It rapidly carries glucose and amino acids away, helping to maintain concentration gradients.
Lacteal inside each villus	It provides a route for absorbed products of fat digestion to enter the lymphatic system.
Epithelial cells with many mitochondria	Respiration supplies energy for active transport, including glucose uptake when it moves against a concentration gradient.

Do not write only “the villus has a large surface area”.

A complete explanation links each structure to its effect: more surface for simultaneous uptake, a shorter transport distance, and rapid removal of absorbed substances.

Different nutrients leave the villus by different routes

The capillaries and lacteal are not interchangeable. Water-soluble nutrients enter the blood capillaries. Products of fat digestion take the lymphatic route through the lacteal. Remembering the destination is more reliable than memorising a list without understanding the vessels.

Absorbed substance	First transport vessel inside the villus	Route after absorption
Glucose and amino acids	Blood capillaries	Blood from the small intestine travels to the liver through the hepatic portal vein.
Products of fat digestion	Lacteal	They enter lymph, which later drains into the bloodstream.
Water, mineral ions and water-soluble vitamins	Blood capillaries	They are transported in the blood plasma.

Fat absorption has an extra cellular step. Fatty acids and glycerol enter the epithelial cells, where much of the material is reassembled into lipids before entering the lacteal. For an O-Level route question, the essential contrast is still clear: glucose and amino acids → capillaries; products of fat digestion → lacteal.

Diffusion and active transport can both contribute

When a substance is at a higher concentration in the intestinal lumen than in the blood, it can move down its concentration gradient by diffusion. Continuous blood flow removes absorbed molecules and helps keep this gradient steep.

Absorption does not stop simply because the concentration gradient becomes unfavourable. Glucose can also be taken up by active transport, moving against a concentration gradient. This requires energy released by respiration in the epithelial cells. Describing active transport merely as “fast diffusion” is therefore incorrect.

The hepatic portal vein takes absorbed nutrients to the liver

Blood leaving the capillaries of the small intestine is rich in recently absorbed, water-soluble nutrients, especially after a meal. Instead of joining the general circulation immediately, it travels directly to the liver through the hepatic portal vein.

Exam-ready function: the hepatic portal vein transports blood rich in absorbed nutrients from the small intestine to the liver.

The word portal matters. The hepatic portal vein connects one capillary bed in the digestive organs to another in the liver. It is different from the hepatic vein, which carries blood away from the liver towards the heart, and from the hepatic artery, which supplies the liver with oxygenated blood.

Assimilation is the use of absorbed nutrients by cells

Once nutrients have been transported to tissues, cells take them up and use them. This is assimilation. It is not limited to the liver: muscle cells, adipose cells and cells throughout the body all assimilate nutrients. The liver is especially important because it processes, stores and redistributes many substances arriving from the intestine.

Nutrient	Examples of assimilation
Glucose	Used in respiration to release energy; joined to form glycogen for storage; converted into fat when intake greatly exceeds immediate needs.
Amino acids	Joined to form proteins needed for growth, repair, enzymes and other cell structures.
Lipids	Used to form cell membranes, used as an energy source, or stored in adipose tissue as a long-term energy reserve and for insulation.

The body does not keep a storage form of excess amino acids. If amino acids are not needed for protein synthesis, the liver metabolises them. This is one reason the hepatic portal route is so important after a protein-containing meal.

The liver links absorption, assimilation and excretion

The liver is not simply a storage organ. It adjusts the chemical composition of blood, produces a substance needed for fat digestion, makes urea from nitrogen-containing waste, and removes substances whose effects should not continue indefinitely.

1. Conversion of glucose to glycogen and vice versa

After a carbohydrate-rich meal, the liver can remove glucose from the blood and join glucose molecules to form glycogen for storage. When more glucose is needed, liver glycogen can be broken down to glucose and released into the blood. These opposite conversions help regulate blood glucose concentration.

Glucose → glycogen: storage after blood glucose rises.

Glycogen → glucose: release when blood glucose needs to be raised.

2. Production of bile for fat digestion

The liver produces bile. Bile is stored and concentrated in the gall bladder before being released into the duodenum. It emulsifies large fat globules into smaller droplets, increasing the total surface area on which lipase can act. Bile assists fat digestion, but it is not an enzyme and does not produce fatty acids and glycerol itself.

3. Metabolism of amino acids and formation of urea

Excess amino acids cannot be stored. In the liver, their amino groups are removed in a process called deamination. This produces ammonia, which is toxic. The liver converts the ammonia into the less toxic compound urea. Urea travels in the blood to the kidneys and is excreted in urine. The remaining carbon-containing part of the amino acid can be used in respiration or converted into other substances.

Formation is not the same as excretion.

Urea is formed in the liver, transported in the blood and excreted by the kidneys. Keeping those organs separate prevents a very common error.

4. Breakdown of alcohol

The liver breaks down alcohol, but it can do so only at a limited rate. Alcohol affects the brain before it is fully removed, so reaction time increases and self-control may decrease. Long-term excessive consumption can cause liver disease and brain damage. It can also contribute to accidents, unsafe decisions and problems affecting families, work and society.

5. Breakdown of hormones

Hormones alter the activity of target organs, but their effects should not continue forever. The liver helps break down hormones after they have acted, allowing their concentration and effects to decrease as conditions change.

Common Mistakes

Common mistake	Why it is wrong	Better approach
Using digestion and absorption as synonyms	Digestion changes food molecules; absorption transfers the products across the intestinal wall.	Identify whether the question concerns molecular breakdown or movement into blood or lymph.
Saying assimilation occurs when nutrients enter a villus	Entry through the villus epithelium is absorption.	Use assimilation for the use or incorporation of nutrients by cells.
Saying one villus is called a villi	Villus is singular; villi is plural.	Write “one villus” and “many villi”.
Saying the whole villus is one cell thick	A villus contains many cells, capillaries and a lacteal.	State that the epithelium covering the villus is one cell thick.
Sending every nutrient into the blood capillaries	Products of fat digestion enter the lacteal and lymphatic system.	Contrast glucose and amino acids → capillaries with fat products → lacteal.
Calling active transport a type of diffusion	Active transport can move a substance against its concentration gradient and requires energy.	Mention the direction relative to the gradient and the energy released by respiration.
Confusing the hepatic portal vein with the hepatic vein	They carry blood in different directions.	Hepatic portal vein: small intestine → liver; hepatic vein: liver → heart.
Saying the liver stores glucose	The named storage carbohydrate is glycogen.	State that the liver converts glucose into glycogen for storage.
Saying bile is produced by the gall bladder	Bile is produced by the liver.	The gall bladder stores, concentrates and releases bile.
Saying the kidneys form urea	Urea is formed in the liver after amino acid metabolism.	The liver forms urea; the kidneys remove it from the blood and excrete it.
Saying the liver can immediately neutralise any amount of alcohol	Alcohol is broken down at a limited rate and affects the brain while present.	Connect excessive intake with increased reaction time, reduced self-control and long-term damage.

What you should be able to do

Distinguish the three stages

Separate digestion, absorption and assimilation using an example of each.

Identify the main site

State that most absorption of digested nutrients occurs in the ileum of the small intestine.

Explain villus adaptations

Link surface area, epithelial thickness, blood flow, capillaries and the lacteal to absorption.

Trace different nutrient routes

Send glucose and amino acids into capillaries and products of fat digestion into the lacteal.

Apply membrane transport ideas

Explain when diffusion is possible and why active transport requires energy.

State the portal-vein function

Describe transport of blood rich in absorbed nutrients from the small intestine to the liver.

Explain assimilation

Describe how cells use glucose, amino acids and lipids for energy, growth, repair, structures or storage.

Recall all five liver roles

Cover glucose ⇌ glycogen, fat digestion, amino acid metabolism and urea, alcohol, and hormones.

Follow nitrogenous waste

Connect deamination in the liver, urea formation, blood transport and excretion by the kidneys.

Explain effects of alcohol

Relate alcohol to the brain, reaction time, self-control, long-term damage and social consequences.

Mastery Quiz

Can You Apply Absorption and Assimilation?

These ten questions test whether you can use villus adaptations, concentration gradients, nutrient routes and liver functions in unfamiliar situations.

10 mastery questions
Application and biological reasoning
Instant explanation for every answer option

Question 1 of 10

A disease damages the villi so that the inner surface of the ileum becomes much smoother. What is the most direct consequence?

A greater surface area is available for absorption

Flattening the villi reduces rather than increases the absorbing surface.

Fewer digested nutrient molecules can be absorbed at the same time

A smaller surface provides fewer sites across which nutrient molecules can move simultaneously.

Digestion in the mouth immediately stops

Damage to ileum villi does not directly stop salivary amylase or chewing in the mouth.

The hepatic portal vein begins carrying lymph

The hepatic portal vein remains a blood vessel and does not become part of the lymphatic system.

The many villi and microvilli provide a large surface area, so flattening them reduces the capacity for absorption.

The epithelium covering a villus becomes several cells thick instead of one cell thick. Which change is most likely?

Absorption becomes faster because molecules cross more membranes

More barriers do not speed transfer; they increase the distance molecules must travel.

Absorption is unchanged because thickness never affects transport

Transport rate depends partly on the distance across the exchange surface.

Absorption becomes slower because the transport distance is greater

A thicker epithelial barrier increases the distance between the lumen and the vessels.

The villus starts producing bile

Bile is produced by the liver, not by villus epithelial cells.

A one-cell-thick epithelium is effective because it creates a short absorption distance.

Blood flow through the capillary network of a villus increases after a meal. How can this increase the rate of glucose absorption by diffusion?

Glucose is carried away more rapidly, helping to maintain a steep concentration gradient

Removing absorbed glucose keeps its concentration lower near the blood side of the exchange surface.

Blood flow chemically digests glucose into glycogen inside the villus

Glucose does not need digestion, and glycogen formation is not caused by blood flow in the villus.

Blood flow makes the epithelium many cells thick

Increasing epithelial thickness would lengthen the transport distance.

Blood flow converts the lacteal into a capillary

The lacteal remains a lymphatic vessel with a different transport role.

Rapid removal of an absorbed substance helps preserve the concentration difference that drives diffusion.

Glucose continues to enter villus epithelial cells even when its concentration is lower in the intestinal lumen than inside the cells. Which process explains this?

Osmosis, because glucose is water

Osmosis is the net movement of water molecules, not glucose molecules.

Simple diffusion, because glucose is moving down its concentration gradient

The stated movement is from lower to higher glucose concentration, so it is against the gradient.

Egestion, because glucose is leaving the alimentary canal

Egestion removes undigested material from the body; it does not absorb glucose.

Active transport, using energy released by respiration

Active transport can move glucose against its concentration gradient and requires energy.

The direction relative to the concentration gradient distinguishes active transport from diffusion.

A blockage prevents blood from the small intestine from flowing through the hepatic portal vein. Which immediate delivery is most directly reduced?

Delivery of bile from the gall bladder to the duodenum

Bile travels through the bile duct, not through the hepatic portal vein.

Delivery of recently absorbed glucose and amino acids to the liver

These nutrients enter intestinal capillaries and normally reach the liver through the hepatic portal vein.

Delivery of oxygen from the lungs directly to the ileum

The hepatic portal vein is not the vessel carrying oxygen from the lungs to the intestine.

Delivery of urea from the kidney to the bladder

Urine travels from kidney to bladder through the ureter, not the hepatic portal vein.

The hepatic portal vein is the direct blood route from the digestive organs to the liver.

The liver stops producing bile, but lipase is still released into the duodenum. What is the most likely effect after a fatty meal?

Fat digestion becomes faster because lipase has fewer droplets to contact

Fewer, larger globules provide less total surface area for lipase.

Fat digestion stops completely because bile is the enzyme that hydrolyses fat

Bile is not an enzyme; lipase is still present, although it works less efficiently without emulsification.

Fat digestion becomes slower because large fat globules provide less total surface area

Without bile, emulsification is reduced, so less fat surface is available for lipase at one time.

Protein digestion stops because bile normally activates every protease

Bile's main stated role here is to assist fat digestion, not to activate every protease.

The liver contributes to fat digestion by producing bile, which increases the surface area available to lipase.

A person absorbs more amino acids than are needed for protein synthesis. Which sequence correctly describes what happens to the excess?

Deamination in the liver → ammonia formed → urea formed → urea carried to the kidneys

The liver removes amino groups and converts toxic ammonia into urea, which the kidneys later excrete.

Storage as amino acids in the gall bladder → release into the duodenum

The body does not store excess amino acids in the gall bladder.

Conversion into urea in the kidneys → storage in the liver

Urea is formed in the liver and excreted by the kidneys, not the reverse.

Absorption into the lacteal → immediate conversion into bile

Amino acids enter blood capillaries and are not directly converted into bile.

This pathway links nutrient metabolism in the liver to nitrogenous-waste excretion by the kidneys.

Soon after a carbohydrate-rich meal, the concentration of glucose in blood reaching the liver rises. Which liver response stores part of this absorbed nutrient?

Conversion of glycogen into glucose

That conversion releases glucose rather than storing the excess.

Conversion of glucose into urea

Urea is formed from nitrogen-containing waste associated with amino acid metabolism, not from glucose storage.

Release of glucose into the bile duct

Bile is not the route used to store or release blood glucose.

Conversion of glucose into glycogen

The liver joins glucose units to form glycogen, the named storage carbohydrate.

High blood glucose favours storage as glycogen; the reverse conversion can supply glucose when needed.

Which observation shows assimilation rather than absorption?

Fatty acids cross from the intestinal lumen into villus epithelial cells

Crossing the intestinal lining is part of absorption.

Amino acids delivered to a muscle are joined to form new proteins

The muscle cells are using absorbed nutrients to build cell material.

Glucose enters a blood capillary inside a villus

Entry into the capillary is absorption.

Products of fat digestion enter a lacteal

Entry into the lacteal is still part of absorption.

Assimilation begins when delivered nutrients are used or incorporated by cells, not merely when they cross the intestinal wall.

A driver has consumed alcohol. Which combination is consistent with alcohol's effects and the liver's role?

Reaction time decreases, self-control improves, and the kidneys immediately remove all alcohol

Alcohol tends to increase reaction time and reduce self-control, and it is not instantly removed.

Reaction time is unchanged because alcohol affects only the digestive system

Alcohol affects the brain and can impair responses and judgement.

Reaction time increases, self-control may decrease, and the liver breaks alcohol down at a limited rate

This explains why drinking can impair driving before the liver has cleared the alcohol.

Reaction time increases only after the liver has converted alcohol into glycogen

Alcohol is not stored as glycogen, and its effects occur while it is present in the body.

Alcohol affects brain function, while liver breakdown is limited in rate; this creates serious safety and social consequences.

Frequently asked questions

Absorption and assimilation: questions students often ask

The ideas become much easier once the boundaries are clear: intestine versus blood, capillary versus lacteal, and absorption versus use by a cell. These questions connect the details so that each biological term has one precise job.

What is the biological definition of absorption?

Absorption is the movement of digested nutrients through the wall of the small intestine into the blood or lymph. The nutrients must already be small enough to cross the intestinal epithelium.

What is the difference between absorption and assimilation?

Absorption transfers nutrients from the intestinal lumen into blood or lymph. Assimilation occurs later, when cells take up and use those transported nutrients. Glucose entering a villus capillary is absorption; a muscle cell using glucose in respiration is assimilation.

Why is the ileum the main site of absorption?

The ileum is long and its folded inner surface is covered with many villi. The epithelial cells also bear microvilli. Together these features provide a very large surface area, while the thin epithelium and internal transport vessels allow absorbed substances to move away efficiently.

What is the difference between a villus and microvilli?

A villus is a finger-like projection of the lining of the small intestine and contains blood capillaries and a lacteal. Microvilli are much smaller projections of the cell membrane on individual epithelial cells. Both increase surface area, but they exist at different scales.

Is a villus really one cell thick?

The entire villus is not one cell thick. A villus contains many cells, a capillary network, a lacteal and supporting tissue. It is the epithelium covering the villus that is one cell thick, giving a short route from the lumen to the vessels inside.

Why does a good blood supply increase absorption?

Blood flow carries absorbed glucose and amino acids away from the villus. This helps prevent them from accumulating on the blood side and maintains a concentration gradient for diffusion from the intestinal lumen. It also transports the absorbed nutrients rapidly to the liver and other tissues.

Which nutrients enter the capillaries and which enter the lacteal?

Glucose and amino acids enter the blood capillaries. Products of fat digestion enter the lacteal and are carried in lymph before eventually joining the bloodstream. This capillary-versus-lacteal distinction is a common examination point.

Do fatty acids and glycerol enter the lacteal exactly as they left the lumen?

Fatty acids and glycerol first enter the villus epithelial cells. Much of this material is reassembled into lipids before it passes into the lacteal. At O-Level, the essential route is that products of fat digestion use the lacteal and lymphatic pathway rather than entering the blood capillaries directly like glucose and amino acids.

Can absorption occur by both diffusion and active transport?

Yes. A nutrient can diffuse when it moves down its concentration gradient. Glucose can also be absorbed by active transport against a concentration gradient. Active transport requires energy released by respiration, so it is not simply a faster form of diffusion.

What exactly does the hepatic portal vein carry?

It carries blood rich in absorbed nutrients from the small intestine to the liver. The blood is especially rich in water-soluble nutrients such as glucose and amino acids after a meal. Products of fat digestion first travel through the lymphatic route.

How is the hepatic portal vein different from the hepatic vein?

The hepatic portal vein carries blood from the digestive organs to the liver. The hepatic vein carries blood away from the liver towards the heart. Writing the start and end points is the safest way to distinguish them.

Does assimilation happen only in the liver?

No. The liver performs many important processing and storage functions, but cells throughout the body assimilate nutrients. For example, muscle cells use glucose in respiration and amino acids for protein synthesis, while adipose cells store lipids.

Does the liver store glucose or glycogen?

The liver stores carbohydrate mainly as glycogen. When blood glucose is high, glucose can be converted into glycogen. When more glucose is needed, glycogen can be converted back into glucose. Saying simply that the liver “stores glucose” misses the named storage form.

What happens to excess amino acids?

Excess amino acids cannot be stored. The liver removes their amino groups by deamination, producing ammonia. Because ammonia is toxic, the liver converts it into urea. Urea is then carried in the blood to the kidneys for excretion.

What is the liver's role in fat digestion?

The liver produces bile. Bile emulsifies large fat globules into smaller droplets, increasing the total surface area available to lipase. The gall bladder stores and concentrates bile, but it does not produce it, and bile itself is not a digestive enzyme.

Why does the liver break down hormones?

Hormones change the activity of target organs, so their concentrations must be able to fall after the required response. By breaking down hormones, the liver helps stop signals from continuing when they are no longer needed.

Why can alcohol still affect the brain if the liver breaks it down?

The liver removes alcohol only at a limited rate. Alcohol can therefore remain in the blood and reach the brain faster than the liver can clear it. Reaction time may increase and self-control may decrease; repeated excessive consumption can also damage the liver and brain.

The journey of a nutrient does not end when digestion is complete. Villi provide the large, thin and well-supplied surface that moves digested products into capillaries or lacteals. The hepatic portal vein then takes absorbed water-soluble nutrients to the liver, while cells throughout the body assimilate what they need. Seen this way, absorption is the transfer, assimilation is the use, and the liver is the processing centre that helps keep the whole system under control.