What is the physiological process of absorption?

October 17, 2025 •

3 min read

Over 90% of all nutrient absorption occurs in the small intestine, a marvel of biological engineering. The physiological process of absorption is the essential final step of digestion, where simple, usable nutrients are transported from the gut into the circulatory or lymphatic systems to be distributed throughout the body.

Quick Summary

This article explores the journey of digested nutrients, explaining the key mechanisms of transport and the specialized structures within the small intestine that make absorption highly efficient. It covers the fate of carbohydrates, proteins, and fats, including the special pathway for lipids via the lymphatic system.

Key Points

Small Intestine as the Hub: The vast majority of nutrient absorption occurs in the small intestine due to its massive surface area created by circular folds, villi, and microvilli.
Multiple Transport Methods: Absorption utilizes several mechanisms, including simple diffusion, facilitated diffusion, and active transport, which move molecules into the bloodstream or lymphatic system.
Fat Absorption via Lymph: Unlike other macronutrients, fats are processed into chylomicrons inside intestinal cells and transported into the body via the lymphatic system before entering the bloodstream.
Active and Passive Processes: Energy-dependent active transport is used for nutrients like glucose and most amino acids, while passive methods like diffusion handle other substances, such as some fatty acids.
Water Absorption in the Large Intestine: After the small intestine absorbs most nutrients, the large intestine takes on the crucial role of absorbing water and electrolytes, as well as vitamins produced by gut bacteria.
Distinction from Digestion: Absorption is the process of moving digested nutrients across the intestinal wall into circulation, a distinct stage that follows the enzymatic breakdown of food.

The Journey from Lumen to Bloodstream

Following the mechanical and chemical breakdown of food during digestion, the resulting small, soluble molecules—such as monosaccharides, amino acids, and fatty acids—are ready for absorption. This process is not a single event but a complex series of transport mechanisms occurring primarily across the epithelial lining of the small intestine. The journey can be categorized into four main mechanisms of transport:

Simple Diffusion: Movement of substances from an area of higher concentration to an area of lower concentration, directly through the cell membrane. This passive process requires no energy input from the body.
Facilitated Diffusion: Similar to simple diffusion, but relies on a carrier protein to move substances down a concentration gradient. While it doesn't require energy, it is faster and more specific than simple diffusion.
Active Transport: The movement of molecules against their concentration gradient, from a lower concentration to a higher one. This process requires energy, typically in the form of adenosine triphosphate (ATP), to pump substances across the membrane using specific carrier proteins.
Endocytosis: A process where the cell membrane engulfs larger molecules, forming a vesicle to transport them across the cell.

The Small Intestine: A Specialized Absorptive Surface

The structure of the small intestine is a key physiological adaptation for maximizing absorption. Its immense surface area is achieved through several features:

Plicae Circulares (Circular Folds): Large, permanent ridges on the inner lining of the small intestine that slow the passage of food, providing more time for absorption.
Villi: Millions of tiny, finger-like projections covering the circular folds. Each villus is richly supplied with a capillary network and a central lymphatic vessel called a lacteal.
Microvilli: Microscopic projections on the surface of the absorptive epithelial cells (enterocytes) that line the villi. Together, they form a "brush border" that further multiplies the surface area.

This multi-layered structure can increase the absorptive surface area to that of a tennis court, making the uptake of nutrients incredibly efficient.

Absorption Pathways for Different Nutrients

Different classes of nutrients follow specific physiological pathways for absorption:

Carbohydrates

Carbohydrates are broken down into monosaccharides like glucose, galactose, and fructose. Glucose and galactose utilize active transport, while fructose primarily uses facilitated diffusion to enter intestinal cells.

Proteins

Proteins are digested into amino acids, dipeptides, and tripeptides. Amino acids are mostly absorbed via active transport, often with sodium. Di- and tripeptides are absorbed and then converted to amino acids within the cells before entering the blood.

Lipids

Lipid absorption is complex due to their insolubility. Bile salts emulsify fats, and enzymes break them into fatty acids and monoglycerides. These form micelles, which transport them to intestinal cells. Inside, they reform into triglycerides and are packaged into chylomicrons, entering the lymphatic system via lacteals before reaching the bloodstream.

Comparison of Absorption Mechanisms


Feature	Simple Diffusion	Facilitated Diffusion	Active Transport
Energy Required	No (passive)	No (passive)	Yes (active)
Concentration Gradient	Moves down gradient	Moves down gradient	Moves against gradient
Carrier Protein	No	Yes	Yes
Specificity	Non-specific	Highly specific	Highly specific
Example	Small fatty acids, glycerol	Fructose	Glucose, amino acids, sodium ions

The Role of the Large Intestine

The large intestine absorbs remaining water and electrolytes, and vitamins produced by gut bacteria. This maintains fluid balance and prepares waste for elimination.

Conclusion: The Final Crucial Step

The physiological process of absorption is a vital mechanism completing digestion by delivering nutrients from the gut to body cells via specialized structures and transport methods. This complex process is fundamental for energy, growth, and overall health.

Learn More About Digestion

For more detailed information on digestive physiology and related topics, consult a reliable medical or scientific source, such as the comprehensive resources available from the National Center for Biotechnology Information (NCBI) at the National Institutes of Health. Drug Absorption - StatPearls - NCBI Bookshelf

Frequently Asked Questions

The primary site for nutrient absorption is the small intestine, specifically within its three sections: the duodenum, jejunum, and ileum.

Carbohydrates are absorbed as monosaccharides (simple sugars). Glucose and galactose are absorbed via active transport with sodium ions, while fructose is absorbed through facilitated diffusion.

Villi and microvilli are finger-like projections and microscopic extensions, respectively, that line the small intestine. Their primary role is to drastically increase the surface area available for the absorption of nutrients.

Fat absorption is unique because fats are insoluble in water. They are first emulsified by bile salts, then absorbed into intestinal cells as micelles, re-formed into chylomicrons, and transported via the lymphatic system instead of directly into the bloodstream.

After entering the bloodstream, nutrients are transported to the liver via the hepatic portal vein. The liver processes these nutrients, which are then distributed to the body's cells for energy, growth, and repair.

Absorption is the process of moving digested nutrients from the intestinal lumen into the blood or lymph. Assimilation is the subsequent process where these absorbed nutrients are taken up by body cells and tissues for use in metabolic activities.

The stomach does not play a major role in nutrient absorption, though some minor absorption of substances like alcohol and certain drugs can occur there.