86 Structure and functions
Learning Objectives
After reading this section you should be able to-
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Describe the major functions of the digestive system.
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Explain the differences between the gastrointestinal (GI) tract (alimentary canal) and the accessory digestive organs.
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Compare and contrast mechanical digestion and chemical digestion, including where they occur in the digestive system.
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Define peristalsis.
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Trace the pathway of ingested substances through the gastrointestinal (GI) tract.
The function of the digestive system is to break down the foods you eat, release their nutrients, and absorb those nutrients into the body. Although the small intestine is the workhorse of the system, where the majority of digestion occurs, and where most of the released nutrients are absorbed into the blood or lymph, each of the digestive system organs makes a vital contribution to this process (Figure X.1).
Digestive System Organs
The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of the digestive system.
Alimentary Canal Organs
Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = “to nourish”) is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body by digesting food and absorbing released nutrients. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body’s “inner space.”
Accessory Structures
Accessory structures within the digestive system play vital roles in facilitating the breakdown of food and the absorption of nutrients. These structures include the following:
- Teeth and Tongue: Primarily involved in mechanical digestion, teeth chew and grind food into smaller pieces, while the tongue helps manipulate food for swallowing.
- Salivary Glands: These glands secrete saliva containing enzymes like amylase, which initiate the breakdown of carbohydrates in the mouth.
- Gallbladder: Serving as a storage reservoir for bile produced by the liver, the gallbladder releases bile into the small intestine to aid in the emulsification and digestion of fats.
- Liver: Producing bile, the liver plays a crucial role in fat digestion and assists in the metabolism of nutrients absorbed by the digestive system.
- Pancreas: The pancreas secretes digestive enzymes (such as lipase, protease, and amylase) into the small intestine to further break down fats, proteins, and carbohydrates.
These accessory organs significantly contribute to the digestive process, enhancing the efficiency of nutrient breakdown and absorption.
Nerve Supply
Nerve innervation is integral to the functioning of the digestive system, ensuring the coordination of various digestive processes. Key aspects of nerve supply include:
- Cranial Nerves: Upon food entry into the mouth, receptors detect stimuli and send impulses along sensory neurons of cranial nerves. These impulses enable taste sensation, tactile feedback, and motor control necessary for chewing and swallowing. Cranial nerves play a vital role in initiating and regulating the digestive process.
- Enteric Nervous System: Spanning from the esophagus to the anus, the enteric nervous system comprises approximately 100 million neurons, forming complex neural networks within the gastrointestinal tract. Two main plexuses, the myenteric plexus (Auerbach) and submucosal plexus (Meissner), regulate motility, secretion, and sensory functions of the digestive system. The myenteric plexus coordinates muscle contractions, facilitating the rhythmic movement of food along the digestive tract, while the submucosal plexus regulates glandular secretions and responds to luminal stimuli.
- Extrinsic Innervation: The autonomic nervous system, consisting of sympathetic and parasympathetic divisions, provides extrinsic innervation to the digestive system. Sympathetic activation, associated with the fight-or-flight response, inhibits digestive activity, reducing secretion and motility. In contrast, parasympathetic activation, linked to the rest-and-digest response, enhances digestive functions by stimulating enteric neurons, promoting secretion and motility.
This intricate network of nerve supply ensures the precise regulation of digestive processes, allowing for efficient digestion, absorption, and nutrient assimilation. Synergistic interactions between cranial nerves, the enteric nervous system, and autonomic innervation coordinate the intricate choreography of gastrointestinal function, supporting overall digestive health and homeostasis.
Blood Supply
Adequate blood supply is essential for sustaining the metabolic demands of the digestive system and facilitating nutrient absorption. Key aspects of blood supply include:
- Arterial Circulation: Blood vessels branching from the aortic arch and thoracic aorta supply the anterior parts of the alimentary canal, while arteries branching from the abdominal aorta provide blood to the remaining gastrointestinal organs. The celiac trunk services the liver, stomach, and duodenum, while the superior and inferior mesenteric arteries supply blood to the small and large intestines, respectively.
- Venous Drainage: Veins collect nutrient-rich blood from the small intestine and transport it to the hepatic portal system. From there, blood enters the liver, where nutrients are processed or stored for later use. Veins draining blood from the alimentary canal viscera eventually return it to the heart for recirculation. The hepatic portal system plays a crucial role in regulating nutrient metabolism and detoxification processes within the liver.
- Function: Blood vessels serving the digestive system not only transport absorbed nutrients but also supply organs with oxygen and nutrients required for cellular processes. Additionally, the venous drainage of the small intestine via the hepatic portal system allows for efficient processing of absorbed nutrients by the liver before their distribution to the rest of the body.
This intricate network of blood vessels ensures the delivery of oxygen, nutrients, and metabolic substrates to support the metabolic activities of the digestive organs. The hepatic portal system, in particular, plays a critical role in regulating nutrient metabolism and maintaining homeostasis within the body.
Adapted from Anatomy & Physiology by Lindsay M. Biga et al, shared under a Creative Commons Attribution-ShareAlike 4.0 International License, chapter 23.
