Chapter 1
Anatomy and Physiology: The Digestive System

It is important to have a solid understanding of the digestive system (the gastrointestinal [GI] tract); this will allow healthcare providers to assess a patients' GI health and identify potential issues. Many patients require care related to the digestive system. Monitoring and managing these patients effectively are key to ensuring that they are safe, comfortable, pain-free and receiving the appropriate medications, dietary restrictions or other treatments. Knowledge of the digestive system is essential for providing comprehensive, patient-centred care, thereby contributing to better patient outcomes and overall healthcare quality.

This digestive system is around 10 m long. It traverses the length of the body, begins at the mouth, passes through the thoracic, abdominal and pelvic cavities and ends at the anus (see Figure 1.1). The digestive system has one primary function: to convert food into a form that can be used by the cells of the body, allowing them to perform their specific functions. The food, once absorbed by the body, is converted into energy, this process is known as metabolism.

Figure 1.1 The digestive system

The digestive system is also known as the GI tract, this is a long, tube-like structure that starts at the mouth and ends at the anus. It plays a crucial role in the digestion and absorption of food and nutrients in the human body and the elimination of waste products through the excretion of faeces.

This chapter discusses the structure and function of the digestive system and explains how dietary nutrients are broken down and used by the body for cell metabolism, growth and repair. This chapter provides a comprehensive overview of the structure and function of the digestive system, offering the reader a foundation for their practice. See Figure 1.1 for the digestive system.

The Digestive System

The activity of the digestive system can be classified into five processes:

1. Ingestion: taking food into the digestive system.
2. Propulsion: movement of the food along the length of the digestive system.
3. Digestion: breaking down of food. This can be achieved mechanically as food is chewed or moved through the digestive system or chemically through the action of enzymes that are mixed with the food.
4. Absorption: the products of digestion leave the digestive system and enter the blood or lymph capillaries for distribution to where they are required.
5. Elimination: the waste products of digestion are excreted from the body as faeces.

The digestive system consists of the main digestive system structures and the accessory organs. The main digestive system structures include the mouth, pharynx, oesophagus, stomach, small intestine and large intestine. Accessory organs also contribute to the function of the digestive system. The accessory organs are the salivary glands, the liver, the gallbladder and the pancreas.

The Digestive Organs

These digestive organs work in a coordinated manner as they break down food into its basic components (nutrients) and eliminate waste products from the body. Table 1.1 outlines the various organs and their functions.

Table 1.1 Digestive organs and functions

Digestive organ Function Mouth The digestive process begins in the mouth, where food is broken down mechanically through chewing and mixed with saliva, which contains enzymes that start to break down carbohydrates. Pharynx Plays a role in swallowing, pushing food down the oesophagus. Oesophagus The oesophagus is a muscular tube that transports food from the mouth to the stomach. It does this through a series of coordinated muscle contractions called peristalsis. Stomach The mechanical breakdown of food. The stomach is a muscular organ that continues the digestion of food by mixing it with stomach acid and digestive enzymes. It breaks down proteins and forms a semi-liquid substance called chyme, which commences the digestion of protein. Small intestine: efficient nutrient absorption

The small intestine is where most of the digestion and nutrient absorption occurs. It is divided into three sections:

1. Duodenum
2. Jejunum
3. Ileum

The pancreas and liver release enzymes and bile into the duodenum to further break down carbohydrates, fats and proteins. The small intestine has numerous tiny, finger-like projections called villi and microvilli that increase its surface area for efficient nutrient absorption.

Liver An accessory organ. The liver produces bile, which is stored in the gallbladder and released into the small intestine to help emulsify and digest fats. Pancreas An accessory organ. The pancreas releases digestive enzymes into the small intestine to break down carbohydrates, fats and proteins. It also plays a crucial role in regulating blood glucose levels by producing insulin and glucagon. Large intestine (the colon) The large intestine is responsible for absorbing water and electrolytes, forming and storing faeces, It also houses beneficial bacteria that assist in the fermentation of undigested carbohydrates. Rectum The rectum is the final portion of the large intestine and acts as a temporary storage area for faeces before they are eliminated from the body. Anus The anus is the opening through which faeces are expelled from the body during defaecation.

Source: Adapted from Peate (2022).

The Mouth

The mouth or oral cavity is where the process of digestion begins (Figure 1.2). It has three functions related to digestion:

1. Mastication (the mechanical chewing of food)
2. Salivation (moistening of the food)
3. Deglutition (swallowing of the food)

Figure 1.2 The oral cavity

The lips and cheeks are made of muscle and connective tissue, allowing them to move food mixed with saliva around the mouth and begin mechanical digestion. The teeth also play a role in mechanical digestion by grinding and tearing food. This process of chewing and mixing food with saliva is known as mastication. The oral cavity can be exposed to very hot and cold food as well as rough food particles. It is covered with mucus-secreting, stratified squamous epithelial cells, which provide some protection against abrasion, the effects of heat and continuous wear and tear.

The lips and cheeks are also involved in speech and facial expression.

Tongue

The tongue is a large, voluntary muscular structure that occupies much of the oral cavity. It is attached posteriorly to the hyoid bone and inferiorly by the lingual frenulum (see Figure 1.3).

Figure 1.3 Salivary glands

The tongue is covered in stratified squamous epithelium for protection against wear and tear. This surface also contains many small projections called papillae. The papillae (or taste buds) house the nerve endings responsible for the sense of taste (Tortora and Derrickson 2012). The taste buds contribute to our enjoyment of food. In addition to taste, other functions of the tongue include swallowing (deglutition), holding and moving food around the oral cavity and speech.

Palate

The palate forms the roof of the mouth, consisting of two parts: the hard palate and the soft palate. The hard palate is located anteriorly and is bony. The soft palate lies posteriorly and consists of skeletal muscle and connective tissue (see Figure 1.2). The palate plays a role in swallowing. The palatine tonsils are located laterally and are composed of lymphoid tissue. The uvula is a fold of tissue that hangs down from the centre of the soft palate.

Teeth

The function of the teeth is to chew food. Temporary teeth, also known as deciduous teeth or milk teeth, begin to appear at about six months old. There are 20 temporary teeth, which are replaced by permanent teeth around six years (McErlean 2020). There are 32 permanent teeth in total with 16 located in the maxilla arch (upper) and 16 located in the mandible (lower; see Figure 1.2).

The visible part of the tooth is called the crown and sits above the gum or gingiva. The centre of the tooth is called the pulp cavity where blood and lymph vessels as well as nerves enter and leave the tooth. The tooth receives nutrients and sensations via the pulp. Surrounding this pulp is a calcified matrix, similar to bone, called the dentine and around that is a very hard, protective material called enamel. The neck of the tooth is where the crown meets the root.

Salivary Glands

There are three pairs of salivary glands (see Figure 1.3). The parotid glands are the largest, located anterior to the ears. Saliva from the parotid glands enters the oral cavity close to the level of the second upper molar tooth. The submandibular glands are located below the jaw on each side of the face. Saliva from these glands enters the oral cavity beside the lingual frenulum of the tongue. The sublingual glands are the smallest, located on the floor of the mouth.

While saliva is continuously secreted to keep the oral cavity...