Chapter 33 BOWEL ELIMINATION
The digestive process enables consumed food and fluids to be broken down into nutrients and electrolytes that can be absorbed by the body for cell energy and function. This process produces waste products that must be eliminated regularly. Healthy bowel function is maintained by routine elimination habits, a nutritional diet with the recommended amount of fibre and fluid intake, and daily mild exercise to stimulate colonic motility. The variety of factors that can affect normal bowel elimination are explored in this chapter. Observation of the individual’s ability to eliminate faeces, together with observation of the faeces, provides the nurse with an objective assessment of the client’s bowel elimination status. As a result, appropriate nursing actions may be planned and implemented to assist the individual to meet their bowel elimination needs.
I was raised by my mother, who gave me laxatives — chemical laxatives are harsh and irritating to the bowel and should never be used. They provide only temporary relief and add to the problem. I have learnt that diet and exercise and drinking adequate water prevents constipation.
Every cell in the body requires energy to carry out its normal functions. Cellular energy is produced when nutrients in food are broken down and absorbed. Solid wastes that accumulate during the digestive process must be eliminated. The digestive system is the means by which food is ingested, digested and eliminated. In the digestive tract, food is digested to its elemental components — nutrients, fluid and electrolytes. The digested elements are absorbed into the bloodstream for transport to all body cells, and the solid wastes that accumulate during digestion are excreted from the body.
The digestive tract is a muscular tube about 9–10 metres in length, which extends from the mouth to the anus (Figure 33.1). The structure of the alimentary canal is similar for most of its length and consists of an outer covering, middle layers of involuntary muscle and connective tissue, and an inner mucous membrane lining (Figure 33.2).
The outer covering of the alimentary canal consists of fibrous tissue (the serosa) or, in the abdomen, peritoneum. The peritoneum is a double-layer serous membrane that secretes serous fluid to prevent friction between the abdominal organs. The inner layer is called the visceral serous membrane and the outer layer the parietal serous membrane. The two layers of the peritoneum are kept proximate and separated by peritoneal fluid. The peritoneum forms a lining for the abdominal cavity (parietal layer) and a covering for most of the abdominal organs (visceral layer). The mesentery, which is formed by the peritoneum, covers the intestines and attaches them to the posterior abdominal wall. Ligaments are formed by folds of peritoneum to attach some organs to each other or to the abdominal wall. The greater omentum is attached to the lower border of the stomach and hangs down like an apron and loops up to be attached to the transverse colon. The lesser omentum extends from the lower border of the liver to the lesser curvature of the stomach.
The innermost layer of the digestive tract is composed of connective tissue (the submucosa), which contains many large blood and lymph vessels, and an inner lining of mucous membrane (the mucosa), which secretes mucus into the digestive tract. The mucosa also contains a small amount of loose connective tissue and a thin layer of smooth muscle.
The mouth, or oral cavity, has boundaries of muscle and bone and is lined by mucous membrane. The lips protect its anterior opening, the cheeks form the lateral walls, the hard palate forms its anterior roof, and the soft palate forms its posterior roof. The uvula is a finger-like muscular projection hanging down from the midline of the soft palate. It helps prevent the entry of food and fluids into the nasal cavities.
The mouth contains the tongue and the teeth. The tongue is a muscular organ that occupies the floor of the mouth. It is attached to the hyoid bone and also to the floor of the mouth by folds of mucous membrane called the frenulum. The tongue consists of a mass of voluntary muscle and is covered by squamous epithelium. On the upper surface of the tongue there are many small projections called papillae, which contain tastebuds, the sensory endings of the nerve that perceives taste. The tongue is a very mobile organ which is important in the chewing (mastication) of food, assists in swallowing and is essential for speech.
The teeth are embedded in the maxillae and the mandible. All teeth have the same basic structural organisation but differ in shape and size. Each tooth has one or more roots embedded in the maxilla or mandible, a portion (the crown) above the gum, and a neck, which joins the root and the crown and which is surrounded by the gum. Each tooth is made up of an ivory-like substance called dentine; a central pulp cavity containing blood and lymphatic vessels, nerves and connective tissue; and a thin layer of enamel covering the crown. In children there are 20 deciduous (milk) teeth, consisting of 10 in each jaw; in adults there are 16 permanent teeth in each jaw. The teeth are named according to their shape and function. In each jaw there are four incisors, used for biting; two canines, used for tearing; four premolars, used for crushing; and six molars, used for grinding. A wisdom tooth is the third molar tooth and it is the last tooth to erupt. Wisdom teeth usually erupt from age 18 to 25.
The oropharynx is the muscular canal forming the passage between the oral cavity and the major parts of the alimentary canal. Lined by mucous membrane, it is a tube about 13 cm long and is continuous with the nasopharynx above and the oesophagus below.
The oesophagus is a muscular tube about 20–25 cm long, extending from the pharynx above to the stomach below. It lies behind the larynx and trachea, in the midline through the neck and thorax, and passes through the diaphragm to join the stomach. The oesophagus has an outer layer of fibrous tissue, a layer of involuntary muscle, a layer of connective tissue, and a lining of mucous membrane. The function of the oesophagus is to carry food to the stomach by means of peristaltic action.
Peristalsis is a wave-like progression of alternate contraction and relaxation of the muscle fibres of the oesophagus or intestines, by which contents are propelled along the alimentary canal. At rest, the opening between the oesophagus and pharynx (oesophageal sphincter) is closed. During swallowing the muscles contract and cause the sphincter to open, thereby allowing the bolus of food to pass down into the oesophagus. A wave of contraction in the circular muscle layer then propels the bolus down to the stomach. The bottom end of the oesophagus acts as a functional sphincter, which is normally in a state of tonic contraction. As the peristaltic wave approaches the sphincter, the muscle relaxes and allows food to enter the stomach. The sphincter then closes again and prevents regurgitation of gastric contents back into the oesophagus.
The stomach is a hollow muscular organ that lies primarily in the upper left quadrant of the abdomen, beneath the diaphragm (Figure 33.3). It is commonly described as being ‘J shaped’, but the size and shape of the stomach varies according to its contents. The stomach is divided into four areas: the fundus (the upper portion); the cardia, where the oesophagus joins the stomach; the body, or main part of the stomach; and the pylorus (the narrowed lower portion).
The opening of the oesophagus into the stomach is called the cardiac orifice, and is surrounded by a functional sphincter called the cardiac sphincter. The pyloric orifice is the opening between the stomach and the small intestine and is surrounded by the pyloric sphincter, which consists of a thickened layer of circular muscle and is normally partly open. Peristaltic waves in the stomach push some of the gastric contents through the orifice and into the duodenum. The orifice then closes.
The stomach acts as a temporary reservoir for food, allowing the digestive enzymes time to act, breaks up the food into a liquid state by rhythmic muscular contraction of its walls, and produces gastric juice, which contains:
The small intestine is a coiled muscular tube about 5–6 metres in length, extending from the pyloric end of the stomach to the large intestine. The small intestine is divided into several anatomically recognisable areas: the duodenum — the proximal section, which is the widest section and is about 20 cm long and curved; the jejunum, which is the middle section and is about 2.5 metres long; and the ileum, which is the distal section and is about 4 metres in length.
Covering the mucosa are very fine projections called villi (Figure 33.4). Intestinal glands in the mucous membrane secrete an intestinal juice containing enzymes to complete the digestion of food. Solitary and aggregated patches of lymphatic tissue (Peyer’s patches) are also found in the mucosa of the intestinal wall, especially in the lower ileum. An opening in the duodenum (ampulla of Vater) allows for the entry of the common bile duct, carrying bile from the liver, and the pancreatic duct, carrying pancreatic juice. At the junction of the ileum and the caecum of the large intestine is the ileo-caecal valve, which prevents a backward flow of contents from the large to the small intestine.
The large intestine is a muscular tube about 1.5 m in length and 6 cm in diameter, and extends from the end of the ileum to the anus (Figure 33.5). Lying in the abdominal and pelvic cavities, the large intestine may be divided into regions that are distinguished by their anatomical structure and position:
The accessory organs secrete enzymes into the alimentary canal, secretions (enzymes) that are actively involved in the process of digestion. An enzyme is a substance, usually protein in nature, that initiates and accelerates a chemical reaction. The accessory organs are the salivary glands, the pancreas, the liver and the biliary tract.
There are three pairs of salivary glands that secrete saliva into the mouth. The anatomical position of each pair is described above. Saliva is a watery fluid containing ions, mucin and the digestive enzyme salivary amylase. Salivation is largely initiated by sensory stimulation, including the presence of food in the mouth, and by taste and smell. Salivation may also be induced by the presence of irritating substances in the stomach or small intestine. Saliva has the following functions:
The pancreas is a soft gland, lying across the abdominal cavity behind the stomach. It is divided into a head, which fits into the curve of the duodenum; a central portion, or body; and a tail, which extends out to the spleen. The pancreatic duct runs centrally through the length of the pancreas, while smaller ducts carry the pancreatic juice secreted by the pancreas into the central duct. The pancreatic duct joins the common bile duct from the liver, to enter the duodenum (Figure 33.6).
The bulk of the tissue in the pancreas is composed of exocrine cells, which produce pancreatic juice. The pancreas secretes about 1200 mL of pancreatic juice daily. Pancreatic juice is a watery alkaline fluid rich in digestive enzymes. The enzymes and their actions are summarised in Table 33.1. The overall function of pancreatic juice is the digestion of nutrients. Scattered among the exocrine tissue are groups of hormone-secreting cells, the islets of Langerhans. The function of the islets of Langerhans is described in Chapter 41 on endocrine health, as these cells belong to the endocrine system.
The liver is an organ situated in the upper part of the abdominal cavity, immediately beneath the diaphragm. The greater part of the liver lies in the right upper abdomen but the organ extends across to the left upper abdomen (Figure 33.7). The liver is divided into two parts, a large right lobe and a much smaller left lobe. Like the alimentary canal, the liver is almost entirely covered by a layer of peritoneum. Beneath this is a fibrous capsule, which is continuous with areolar connective tissue situated within the liver. The areolar tissue forms a tree-like structure, which carries branches of the hepatic artery, hepatic portal vein, bile ducts and lymphatic vessels. These vessels enter and leave the liver through the porta hepatis, a short transverse fissure on the inferior surface of the liver.
The hepatic artery carries oxygenated blood to the liver. The portal vein carries deoxygenated blood, rich in nutrients from the small intestine, to the liver. Three hepatic veins carry deoxygenated blood from the liver to the inferior vena cava. The right and left hepatic ducts carry bile, secreted by the liver, to the common hepatic duct. The latter combines with the cystic duct from the gall bladder to form the common bile duct, which drains into the duodenum. The biliary tract, which transports bile from the liver to the duodenum, consists of the left and right hepatic ducts, the common hepatic duct, the cystic duct, the gall bladder and the common bile duct (Figure 33.8).
During the process of digestion, food is reduced to its simplest chemical form so that it can be absorbed into the bloodstream and used by the tissues. Digestion occurs through both mechanical and chemical actions. Mechanical action involves the physical process of liquefying the food, mixing it with digestive juices and moving it through the alimentary canal. Chemical action occurs when the digestive juices mix with the food, resulting in complex chemical substances being split into simple substances.
In the mouth food is broken down physically by the process of chewing (mastication), and mixed with saliva to bring about the formation of a moist ball, or bolus. Chewing softens the food so that it passes more easily through the alimentary canal. The presence of food in the mouth, together with its taste and smell, stimulates the secretion of saliva, gastric and pancreatic juices and bile (by means of parasympathetic pathways). The enzyme salivary beta-amylase in saliva begins to digest starches.
After the food has been formed into a bolus it is passed through the pharynx and down the oesophagus into the stomach by the act of swallowing. Swallowing is a complex reflex regulated by a ‘swallowing centre’ in the medulla oblongata of the brain. Swallowing is initiated when the tongue muscles push the bolus upwards and backwards into the oropharynx. The soft palate is elevated and comes into contact with the posterior wall of the pharynx, thereby closing off the nasopharynx. The larynx is pulled upwards and forwards, and the bolus pushes the epiglottis back over the glottis to prevent food from entering the respiratory tract. The oesophageal sphincter opens and the bolus enters the oesophagus.
Peristaltic waves carry the bolus through the oesophagus, and the cardiac sphincter relaxes to allow food and fluids to enter the stomach. The cardiac sphincter contracts to close the cardiac orifice at the end of each wave of contraction of the oesophagus, then relaxes to open the orifice when the next wave of contraction begins.
The stomach stores the food and later releases it at a rate that is optimal for digestion. Food is mixed with gastric juice, thereby changing its consistency so that it will be more easily transported along the alimentary canal. The food is exposed to enzymes (pepsins) which begin the digestion of proteins, and the gastric juice converts ferric iron (Fe3+) to ferrous iron (Fe2+). When the stomach muscles are stretched by swallowed food, peristaltic contractions are stimulated, which results in a churning movement. When the food is mixed with gastric juice it develops a pasty consistency and becomes known as chyme. The rate of emptying of the stomach depends on the:
The average time for the stomach to empty after a meal is 4–6 hours. When the food has been well mixed in the stomach, peristalsis begins in the lower half of the stomach and forces the chyme through the pyloric sphincter. Because this sphincter is only partially opened, only small amounts of chyme enter it at one time. When the duodenum is filled with chyme a nervous reflex (the enterogastric reflex) occurs, which inhibits the vagus nerves from stimulating the stomach muscles, and slows the emptying of the stomach. This mechanism ensures that food does not enter the small intestine too rapidly, to enable its digestion.
After it leaves the stomach, chyme is mixed with intestinal secretions as well as with bile and pancreatic juice. Digestion is completed and the products are absorbed through the villi of the intestinal wall. The wall of the small intestine is capable of several different types of movement:
Figure 33.9 Movements of the small intestineA: Peristalsis: food moves along the digestive tract, as neighbouring segments of the intestine contract and relax in turn. B: Segmentation: single segments contract and relax alternately. Because there are inactive segments between the active ones, the food mixes but does not move along the tract
Absorption is the passage of the end-products of digestion through the villi into the bloodstream. Although absorption mainly occurs in the villi in the small intestine, some absorption takes place in the stomach and large intestine. Neighbouring segments of the intestine alternately contract and relax, moving food along the digestive tract. Small, lipid-soluble substances such as alcohol, water, glucose and drugs such as salicylic acid (aspirin) are able to diffuse through cell membranes and can therefore be absorbed in the stomach. Absorption of water and digested nutrients occurs all along the length of the small intestine.
Most substances, for example, amino acids and monosaccharides, are absorbed through the villi walls by the process of active transport, and enter the capillaries in the villi to be transported in the blood to the liver via the portal vein. The exception is lipids (fats), which are absorbed passively by the process of diffusion. Lipids enter central lacteals in the villi to be transported in the lymph. Lymphatic vessels empty their contents into the thoracic duct, which then drains the lymph into the blood at the subclavian vein. The proximal colon is the main site for the absorption of certain substances from chyme, including mineral salts and water. Vitamins B and K are synthesised by bacteria in the colon and absorbed into the bloodstream.
Chyme enters the caecum through the ileo-caecal valve, which is normally closed but opens briefly to allow a small amount of chyme through with each peristaltic wave. Movement of chyme through the large intestine is a slow process. Various types of movement occur in the colon, including peristalsis, segmentation, and mass movements. Mass movements are brought about as a result of distension of the stomach or duodenum by ingested food. This sudden movement of colonic contents can push large amounts of faeces into the rectum and initiate the desire to defecate.
The faeces consists of a semi-solid brown mass. Even though absorption of water takes place in the large intestine, water still remains and makes up 60–70% of the weight of faeces. The remainder consists of fibre, indigestible cellular plant and animal matter, dead and live microbes, digestive enzymes, epithelial cells from the walls of the gastrointestinal tract together with the mucus they secrete, fatty acids and bile pigments.
Most of the time the rectum is empty of faeces; however, when a mass movement forces faeces into the rectum the desire to defecate is initiated. Faeces collect in the sigmoid colon before entering the rectum. As the faecal mass enters the rectum the defecation reflex and the desire to defecate are initiated. Impulses from sensory neurons in the rectal wall travel to the spinal cord, and peristalsis is stimulated in the descending colon, rectum and the anal canal. When the peristaltic wave reaches the internal anal sphincter it relaxes. Voluntary relaxation of the external anal sphincter enables the faeces to be excreted. Voluntary contraction of the abdominal muscles and deep inhalation raise the intra-abdominal pressure and assist evacuation. If it is not convenient, defecation can be delayed temporarily, as within a few seconds the reflex contractions cease and the rectal walls relax.