Oral Cavity
The oral cavity is the starting point of the digestive tract and is responsible for functions such as sucking, swallowing, chewing, digestion, taste, sensation, and speech. Full-term newborns already possess relatively well-developed sucking and swallowing abilities at birth. The oral mucosa of newborns and infants is delicate, richly vascularized, and easily prone to injury or localized infections because the salivary glands are underdeveloped. Saliva production starts increasing around 3 to 4 months of age. Infants have a shallow mouth floor and cannot swallow all the saliva they produce in a timely manner, which often leads to drooling.
Esophagus
The length of the esophagus is approximately 8–10 cm in newborns, 12 cm at 1 year of age, 16 cm at 5 years old, 20–25 cm in school-aged children, and 25–30 cm in adults. The total length corresponds to the distance from the pharynx to the xiphoid process. When inserting a gastric tube, the correct insertion length is determined by measuring from the tip of the nose to the earlobe and then to the xiphoid process. The esophageal diameter is approximately equal to the diameter of the base of the child's thumb. In infants, it ranges from 0.6–0.8 cm, in toddlers about 1 cm, and in school-aged children 1.2–1.5 cm. The esophageal pH typically ranges from 5.0 to 6.8. In newborns and infants, the esophagus has a funnel shape, thin mucosa, underdeveloped glands, poorly developed elastic tissue and muscle layers, and immature lower esophageal sphincter function. This immaturity often leads to gastric reflux.
Stomach
The stomach capacity of a newborn is approximately 30–60 ml, increasing to 90–150 ml at 1–3 months, 250–300 ml at 1 year, 700–850 ml at 5 years, and about 2,000 ml in adults. After feeding, the pylorus immediately opens, allowing gastric contents to enter the duodenum gradually, so the actual feeding volume may exceed the previously listed capacities. The infant's stomach lies more horizontally, transitioning to a vertical position as the child begins walking. Gastric secretions, including hydrochloric acid and various enzymes, are produced in smaller quantities and with lower activity levels compared to adults, resulting in weaker digestive functions. The smooth muscles of the stomach are not fully developed, making it prone to distension after being filled with liquid food. Gastric emptying time varies depending on the type of food consumed: water empties in about 1.5–2 hours, human milk in 2–3 hours, and cow's milk in 3–4 hours. Premature infants have slower gastric emptying times and are more susceptible to gastric retention.
Intestines
The intestinal tract in children is relatively longer compared to adults, typically 5 to 7 times the body length (compared to about 4 times in adults), or roughly 10 times the sitting height. The primary functions of the small intestine include movement (peristalsis, oscillatory, and segmental movements), digestion, absorption, and immunity. The large intestine primarily stores food residues, reabsorbs water, and forms feces. In infants and young children, the intestinal mucosa and muscle layers are underdeveloped, and the mesentery is soft and long. The colon lacks clearly developed taeniae coli and epiploic appendices, and the ascending colon has poor fixation to the posterior abdominal wall, making it prone to intestinal volvulus and intussusception. The intestinal wall is thin, resulting in high permeability and poor barrier function. Thus, endotoxins and partially digested substances can function as antigens that enter the body through the intestinal mucosa. Additionally, the mechanisms for oral immune tolerance are not yet fully developed, which can lead to systemic infections and allergic diseases. Due to the underdeveloped cerebral cortex in infants, eating often triggers a gastrocolic reflex, resulting in frequent bowel movements compared to older children.
Liver
The liver is relatively larger in younger children. In infants, connective tissue in the liver is poorly developed, but hepatocytes have strong regenerative ability, making it less likely for liver cirrhosis to develop. However, the liver can be easily affected by adverse factors such as hypoxia, infection, drugs, or congenital metabolic abnormalities, which may lead to hepatocellular swelling, fatty infiltration, degeneration, necrosis, fibrosis, and enlargement, thereby impairing liver function. Bile secretion in infancy is relatively low, resulting in weaker fat digestion and absorption.
Pancreas
The pancreas develops rapidly between the ages of 3 to 4 months, with increased pancreatic juice secretion. By the end of the first year of life, the growth of the exocrine pancreas is about three times that at birth. Pancreatic juice secretion continues to increase with age. The appearance of pancreatic enzymes follows a specific sequence: trypsin, chymotrypsin, carboxypeptidase, lipase, and amylase. In newborns, pancreatic lipase activity is low and does not approach adult levels until the child is 2–3 years old. During infancy and early childhood, pancreatic juice and its digestive enzymes are highly susceptible to suppression by hot weather or various illnesses, leading to digestive difficulties. Recurrent pancreatitis during childhood may indicate congenital abnormalities in the pancreaticobiliary ducts or hereditary pancreatitis.
Intestinal Bacteria and Feces
In the womb, the fetal intestine is sterile, but within hours after birth, bacteria begin to colonize the intestinal tract, primarily in the colon and rectum. The composition of the intestinal microbiota is influenced by factors such as the mode of delivery, the timing of complementary food introduction, and the dietary composition. In exclusively breastfed infants, bifidobacteria dominate the microbiota. In formula-fed or mixed-fed infants, the proportions of Escherichia coli, Lactobacillus acidophilus, Bifidobacterium, and Enterococcus are nearly equal. The normal intestinal microbiota not only play a role in resisting pathogenic bacteria that invade the gut but also contribute, along with their metabolic products, to the development and maturation of physiological functions in children, such as immunity, metabolism, nutrition, digestion, and absorption. The normal intestinal microbiota in infants is fragile and can be easily disrupted by various internal and external factors, leading to dysbiosis and impaired digestive function.
The time it takes for food to pass through the digestive tract and be excreted as feces varies by age. For breastfed infants, the average transit time is 13 hours, while for formula-fed infants it is approximately 15 hours. In adults, the transit time ranges from 18 to 24 hours. The average time from ingestion of barium to its excretion is 8 hours for neonates and infants but approximately 24 hours for adults. Fecal output is about 5–10 grams per kilogram per day in infants, while in adults it is approximately 200 grams per day.
Meconium
Meconium is the thick, olive-green, odorless substance excreted by newborns during the first three days postpartum. It consists of shed intestinal epithelial cells, concentrated digestive secretions, and ingested amniotic fluid. Within 2–3 days, meconium transitions into regular infant feces.
Feces in Breastfed Infants
Feces in breastfed infants are typically yellow or golden-yellow, often uniform and pasty, though they may sometimes contain small yellow fecal particles or appear slightly loose. The feces are greenish, odorless, and acidic (pH 4.7–5.1). Breastfed infants usually defecate 2–4 times per day, with the frequency decreasing after the introduction of complementary foods.
Feces in Formula-Fed Infants
Feces in formula-fed infants are generally pale yellow or grayish-yellow, drier and thicker than breastfed infants, and display a neutral or alkaline reaction (pH 6–8). Earlier formulations of cow's milk and its derivatives, which contained higher levels of casein, produced feces with a strong odor due to protein decomposition products and, occasionally, the presence of white casein curds. However, with advancements in infant formula production over the past two decades, these phenomena have significantly decreased. Formula-fed infants typically defecate 1–2 times per day.
Feces in Mixed-Fed Infants
The feces of mixed-fed infants resemble those of formula-fed infants but tend to be softer and yellowish. The addition of starchy foods can increase fecal output, reduce fecal consistency slightly, and result in a darker brown color with a stronger odor. Mixed-fed infants typically defecate 1–3 times per day. When vegetables, fruits, and other complementary foods are introduced, the appearance of feces becomes similar to that of adults. During the initial introduction of vegetable purees, small amounts of greenish stool may occur, sometimes containing undigested food particles.