How Are Nutrients Transported into the Blood?

November 17, 2025 •

3 min read

The small intestine is responsible for absorbing approximately 90% of all nutrients we consume. Understanding precisely how are nutrients transported into the blood from this vital organ is key to grasping how our body fuels its functions, repairs tissues, and grows.

Quick Summary

Nutrient absorption from the small intestine follows two main pathways based on solubility: water-soluble compounds enter the blood via the hepatic portal vein, while fats use the lymphatic system.

Key Points

Small Intestine is Key: The vast majority of nutrient absorption occurs in the small intestine due to its large surface area created by villi and microvilli.
Two Transport Pathways: Water-soluble nutrients (like sugars and amino acids) and fat-soluble nutrients (like fatty acids and certain vitamins) use different pathways to enter the bloodstream.
Water-Soluble Path: Nutrients are absorbed into capillaries in the villi and travel via the hepatic portal vein directly to the liver for initial processing.
Fat-Soluble Path: Lipids and fat-soluble vitamins are absorbed into lacteals, entering the lymphatic system before eventually joining the blood near the heart.
Four Absorption Mechanisms: The body uses passive diffusion, facilitated diffusion, active transport, and endocytosis to move nutrients across the intestinal cell membranes.
Liver as a Filter: The hepatic portal system ensures the liver can regulate, store, and detoxify nutrients absorbed from the gut before they reach the rest of the body.

The Hub of Absorption: The Small Intestine

Before any nutrient can enter the blood, it must be broken down into its simplest components during digestion. Proteins become amino acids, carbohydrates become monosaccharides (simple sugars), and fats are broken into fatty acids and monoglycerides. This critical process of absorption primarily occurs within the small intestine, a remarkable organ designed for maximum efficiency.

Villi and Microvilli: The Surface Area Solution

The inner surface of the small intestine is covered in millions of tiny, finger-like projections called villi, which in turn are covered with even smaller, microscopic hair-like projections known as microvilli. This creates a massive surface area—roughly the size of a tennis court—for efficient nutrient transfer through the thin, one-cell-thick walls of the villi.

The Two Main Transport Pathways

The method of transport for a nutrient depends on its solubility. Water-soluble nutrients, such as simple sugars and amino acids, follow one path, while fat-soluble nutrients and lipids take another.

Pathway 1: The Water-Soluble Route

This pathway is utilized by most nutrients, including glucose, galactose, amino acids, minerals, and water-soluble vitamins like B and C. These nutrients are absorbed through the epithelial cells of the villi into the blood capillaries within each villus, which then merge into the hepatic portal vein. The hepatic portal vein carries this nutrient-rich blood directly to the liver for processing, filtering, and storage before it enters the general circulatory system for distribution.

Pathway 2: The Fat-Soluble Route

Fats (lipids) and fat-soluble vitamins (A, D, E, K) require a different route as they cannot dissolve directly into the watery blood. Digested fats form micelles in the small intestine, which are then absorbed into intestinal cells. Inside the cells, they are reassembled into triglycerides and packaged into chylomicrons. These chylomicrons are too large for blood capillaries and instead enter specialized lymphatic vessels within the villi called lacteals. The lacteals transport the chylomicrons through the lymphatic system, eventually emptying into the bloodstream near the heart.

Cellular Mechanisms of Absorption

Nutrients cross the membranes of the intestinal cells using several distinct transport mechanisms. These include passive diffusion, facilitated diffusion, active transport, and endocytosis.

Comparison of Nutrient Transport Pathways


Aspect	Water-Soluble Nutrients	Fat-Soluble Nutrients
Primary Nutrients	Monosaccharides, Amino Acids, Water-Soluble Vitamins (B/C), Minerals	Fatty Acids, Monoglycerides, Fat-Soluble Vitamins (A, D, E, K)
Absorption Site	Capillaries within the intestinal villi	Lacteals (lymphatic vessels) within the intestinal villi
Transport System	Circulatory System (via Hepatic Portal Vein)	Lymphatic System (via Lacteals)
First Pass Organ	Liver	Heart (Systemic Circulation)
Processing	Processed, stored, or detoxified by the liver	Circulates systemically before processing or storage
Risk of Toxicity	Lower; excess typically excreted	Higher; excess can accumulate
Energy Required	Often requires active transport	Required for chylomicron formation

Conclusion

In summary, the absorption of nutrients from the small intestine into the bloodstream is a sophisticated process utilizing extensive surface area from villi and microvilli. For more detailed information on nutrient absorption and transport, you can refer to {Link: Pearson https://www.pearson.com/channels/nutrition/learn/bruce-bryan/3-digestive-system/nutrient-absorption-and-transport} or {Link: NCBI website https://www.ncbi.nlm.nih.gov/books/NBK537158/}.

Frequently Asked Questions

The small intestine is the primary site for nutrient absorption. Its inner surface contains millions of tiny finger-like projections called villi, which dramatically increase the surface area available for nutrient uptake.

Water-soluble nutrients, such as sugars and amino acids, are absorbed into the capillaries located within the villi of the small intestine. They then travel through the hepatic portal vein directly to the liver before entering the general bloodstream.

Fat-soluble nutrients (fats and vitamins A, D, E, K) cannot dissolve in the watery blood. Instead, they are absorbed into specialized lymphatic vessels called lacteals and enter the lymphatic system, bypassing the liver initially.

The hepatic portal system is a network of veins that transports nutrient-rich, but oxygen-poor, blood from the digestive organs (stomach, intestines, pancreas, spleen) to the liver for filtering and processing before it is sent to the rest of the body.

Villi and microvilli are finger-like projections that line the small intestine and significantly increase its surface area. This larger surface allows for more efficient absorption of nutrients as food moves through the digestive tract.

Nutrients are absorbed via four main cellular mechanisms: passive diffusion (high to low concentration, no energy), facilitated diffusion (with a carrier protein, no energy), active transport (against a concentration gradient, requires energy), and endocytosis (cell engulfs molecule, requires energy).

Once absorbed, nutrients are carried by the bloodstream to the body's cells, where they are used for energy, growth, and repair. Excess nutrients may be stored in organs like the liver or in fat tissues for later use.