Final answer:
The size of the rectum in babies is small because their entire digestive system, including all internal organs like the liver, is still developing and growing. After birth, the meconium is the first substance to be passed, which later transitions to normal stools as the baby starts feeding.
Step-by-step explanation:
The question you've asked pertains to why the rectum of a baby is small compared to later stages of life. Throughout fetal development and after birth, the digestive system of an infant undergoes significant growth and changes. During the second half of gestation, a baby's intestines accumulate a substance called meconium, which is a sterile, tarry, greenish-black material. This meconium is composed of materials the fetus has ingested during pregnancy, including amniotic fluid and cellular debris.
After birth, the infant's first stools are almost entirely made up of meconium. Over time, as the baby begins to ingest breast milk or formula, their stools transition, and meconium is replaced by digested food, changing the appearance and consistency of their stools. The small size of an infant's rectum is simply because their entire digestive tract is smaller and less developed than that of an older child or adult. The rectum is the final section of the large intestine where feces are stored before elimination, and it grows as the child grows.
It's important to understand that a baby's entire body is proportionately smaller than that of an older child or adult, and this includes all internal organs. The liver, for example, also plays a crucial role in the digestion and processing of nutrients, and just like the digestive tract, it needs to mature and develop as the baby grows. Notably, disorders of the developing fetus can affect the digestive system, as well as the growth and cognitive development of children.