Amino Acid Absorption
Complete protein absorption is only possible after their preliminary hydrolysis into amino acids. This process begins in the stomach, where hydrochloric acid (HCl) induces protein denaturation, and pepsin enzymes initiate their cleavage.
Intestinal epithelial cells (enterocytes) exhibit pronounced polarity, meaning transport mechanisms differ at the opposite poles of the cell:
- Apical membrane (facing the intestinal lumen): Amino acid uptake into the enterocyte occurs via active transport. This process requires specific carrier molecules (5 distinct types are known) and ATP energy consumption.
- Basal membrane (facing the blood vessels): The exit of amino acids from the cell into the blood occurs via facilitated diffusion.
The rate of this process is dynamic. It is higher in younger organisms and directly depends on the body's current protein metabolism status, free amino acid concentrations in the circulating blood, as well as neural and humoral signals.
Carbohydrate Absorption
The intestinal epithelium permits carbohydrate absorption exclusively in the form of monosaccharides. However, their absorption rates vary: hexoses (glucose and galactose) cross the membrane barrier significantly faster than pentoses.
For glucose and galactose, a secondary active symport mechanism with Na⁺ ions operates at the apical membrane. This coupled transport allows glucose to actively accumulate inside the enterocyte. It exits the cell across the basolateral membranes via passive transport, moving down its concentration gradient into the interstitial fluid and blood.
Regulation of Glucose Absorption:
- Upregulated by: Parasympathetic nervous system, certain amino acids, and hormones (insulin, thyroxine, glucocorticoids).
- Downregulated by: Histamine (minimal effect). Significant and strong inhibition is caused by somatostatin, sympathetic nervous system activation, and cellular respiration inhibitors.
Physiology of Fat Absorption: Luminal and Cellular Phases
On average, a human consumes 60–100 g of dietary fat per day. The main challenge in their assimilation is that lipids are hydrophobic, whereas all gastrointestinal metabolic processes occur in an aqueous environment. The process is divided into three sequential phases.
Luminal Phase (Intestinal Lumen) Dietary fats (triglycerides) are broken down by lipase enzymes into free fatty acids (FFAs) and monoglycerides (MGs). To prevent these hydrophobic products from coalescing, bile salts emulsify them into micelles. A micelle is a transport particle with an outer hydrophilic shell of phospholipids and an inner core containing FFAs, MGs, cholesterol derivatives, and fat-soluble vitamins. Micelles deliver lipids across the unstirred water layer directly to the brush border of enterocytes.
Intracellular Phase (Within the Enterocyte) Upon contact with the apical microvillar membrane, the micelle disintegrates. Lipid components cross into the cell via diffusion. Bile acids remain in the lumen and are later reabsorbed in the ileum via active transport. Inside the enterocyte, within the smooth endoplasmic reticulum, resynthesis of triglycerides occurs ($MG \rightarrow DG \rightarrow TG$). Phospholipids, cholesterol, and lipoproteins are then added to form microscopic protein-coated lipid droplets known as chylomicrons.
Lipid Transport into Circulation
Formed chylomicrons are packaged into vesicles and secreted across the basolateral membrane via exocytosis. Because of their large size, chylomicrons cannot cross the basement membrane of blood capillaries. Instead, they enter the central lacteal ( lymphatic capillary) of the intestinal villus (a process mechanically aided by villus contractions).
The bulk of exogenous lipids is transported via the lymph. Soon after a meal, the lymph takes on a milky appearance, hence termed "chyle". Via the thoracic duct, chylomicrons enter the systemic circulation, bypassing the liver. Alternative Pathway: Short-chain fatty acids can be absorbed directly into blood capillaries, bypassing chylomicron packaging. Bound to albumin, they travel via the portal vein directly to the liver.
Regulation: The parasympathetic system, secretin, cholecystokinin-pancreozymin, along with pituitary, thyroid, and adrenal cortex hormones, stimulate fat absorption. Conversely, the sympathetic nervous system slows it down.