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for the maintenance of homeostasis and consists of two parts: (i) the GI tract, which starts with the mouth and ends with the anus; and (ii) the GI organs and glands, which are the liver, pancreas, and gall bladder.

      GI system function is impaired with age, while the anatomy changes little. As a result, chewing problems due to impaired tooth integrity, swallowing problems, constipation due to slow peristalsis, dyspeptic complaints, decreased appetite, hiatal and inguinal hernias, gallstones, and diverticulosis all become more prevalent with age.

      Age‐related changes in the GI tract

      The GI tract starts with the mouth. Mouth health is essential for digestion. Dental problems tend to occur more often in the elderly and can result in tooth loss. The salivary glands produce less saliva, and mucosal erosions due to physical trauma heal more slowly. All of these factors contribute to chewing problems and can cause an elderly person to eat less. This situation may result in inadequate nutritional intake or dyspeptic symptoms because of impaired mechanical digestion with chewing. A decreased ability to taste or smell causes loss of appetite as well.^45‐46

Swallowing is essential for transferring a bolus from the mouth to the oesophagus. The swallowing function requires oropharyngeal muscle integrity and coordination. Swallowing is under the control of cranial nerves V, VII, IX, X, and XI, which coordinate the tongue and pharyngeal muscles. The oropharyngeal muscles tend to get weak, and muscle coordination tends to become impaired with age. Comorbidities such as cerebrovascular disease and dementia may contribute to impaired swallowing function and dysphagia. Impaired swallowing may result in another frequent complication: food aspiration into the trachea.⁴⁷

      Once the bolus is transferred into the oesophagus, smooth muscle contractions of the oesophageal wall cause peristalsis toward the stomach. Peristalsis of the oesophagus slows with advanced age. Until age 80, oesophagus motility remains intact. After 80, the transfer time of the bolus from the mouth to the stomach becomes longer. Thus the risk of esophagitis increases if the bolus includes irritating component such as pills. In addition, hiatal hernia prevalence increases by age 60, which causes reflux symptoms.⁴⁸

      A feeling of early satiety occurs with age because gastric compliance and the emptying rate decrease with age. Gastric acid secretion decreases, and atrophic gastritis occurs more frequently after 60, which is responsible for reduced produced of gastric acid. Acidic pH is required for protein digestion and vitamin B12 absorption. Dyspeptic complaints such as heartburn and fullness may increase with age. Accompanying comorbidities such as dementia, chronic obstructive pulmonary disease, and heart failure may also contribute to loss of appetite.^49‐50

      Slower peristalsis and slightly decreased enzymatic activity occur in the small intestine with age. Although enzymes linked to the brush border of the mucosa decrease, carbohydrate, fat, and protein digestion and absorption remain normal. Absorption of fat‐soluble vitamins A and D changes with age, with an increase in vitamin A absorption and a decrease in vitamin D absorption. Water‐soluble vitamin absorption remains intact unless hypochlorhydria is present, leading to decreased absorption of B12. Folate absorption slightly decreases with age. These changes rarely cause malabsorption unless a pathological condition coexists.^51‐52

      Slower peristalsis occurs with age in the colon, and anal sphincter tone decreases, contributing to constipation. The rectoanal angle and perineal descent can be impaired in older women following labour traumas, and thus elderly women tend to have more rectal evacuation problems than elderly men.⁵³ The colonic microbiota also changes with age. While the Bacteroides spp. population increases, the Firmicutes spp. population decreases in the colon mucosa. These changes are components of the inflammageing process,⁵⁴ which is discussed separately in the immune system section. Bacterial population changes were found to be correlated with obesity, carcinogenesis, and metabolic age in previous studies. Maintaining the microbiota from a younger age remains a key factor for preventing multiple geriatric syndromes, such as frailty and dementia.

      Age‐related changes in GI organs and glands

      Liver size and blood flow are reduced with age. The mechanism underlying these changes is yet to be discovered, but increased fibrosis and inflammation may play a role. Enzymatic activity, necessary for oxidation and glucuronidation in hepatocytes, decreases with age, resulting in susceptibility to drug overdose. Drug clearance from the blood decreases with age; this is very important in managing pharmacotherapy for the elderly, as is the risk of polypharmacy.⁵⁵ Bile acid production is managed by hepatocytes and has a balanced amount of cholesterol and bile acids. Increased cholesterol level in the bile and decreased muscle tone of the gallbladder contribute to an increased risk of stone formation.⁵⁶

      Pancreatic enzyme secretion decreases, but no significant effects were shown in fat, carbohydrate, and protein digestion in the elderly.⁵⁷

      The prevalence of carcinogenesis increases along the entire GI tract with age. Mucosal irritation, inflammation, impaired antioxidant mechanisms, and genetically damaged epithelium in the GI tract play a role in excess carcinogenesis. GI tract–related cancers may cause symptoms such as constipation, early satiety, and bleeding in the early stages. If caught and treated early, they may be cured. Blood analysis of the stool is a rapid and easy screening test for colorectal cancers, and the sensitivity of this test is 82%. If positive, additional diagnostic testing is recommended regardless of age. Endoscopic screening of the colon is recommended every decade over the age of 50 by American Cancer Society guidelines. Most of the guidelines also add that screening can be stopped after the age of 80.⁵⁸

The genitourinary tract

      The urinary tract is vital for excreting toxic end products of metabolism and balancing blood pressure and electrolyte levels. Age‐related changes in the urinary tract are mostly inevitable. Decreasing renal function is a challenging factor for the appropriate dosing of medications in the setting of multiple comorbidities. Physiologic changes in the urinary tract are discussed in this section.

      Age‐related changes in the kidneys

Kidney size and the thickness of the renal cortex decrease slightly with age, while the structure of the renal medulla is not affected much. Microscopically, the nephron number is reduced and interstitial, and glomerular fibrosis occurs.⁵⁹ While some of the glomeruli become sclerotic and shrink, the remaining glomeruli become hypertrophic in an attempt to compensate for an age‐related reduction in the glomerular filtration rate (GFR). GFR declines by 0.8–1 ml/min per 1.73 m² each year in approximately one‐third of older adults after age 40.⁶⁰ Why this reduction is not seen in all older adults has not been explained, and the existence of older adults with normal GFR demonstrates that renal function decline is not an inevitable destiny. GFR is mostly calculated by using equations related to the serum creatinine level. However, an age‐related decrease in muscle mass (as the primary source of creatinine) may result in false estimations of GFR that use the creatinine level. Measuring cystatin C is recommended in such circumstances.^61‐62

      Renal blood flow decreases with age, too. Perfusion of the renal medulla remains nearly intact compared with the cortex. Atherosclerotic processes and glomerulosclerosis contribute to the reduction of renal blood flow. Glomerular blood flow is regulated by angiotensin 2. The vasoconstrictive response to angiotensin 2 in the afferent arteriole is accelerated, while the compliance of renal arterioles decreases in older adults.⁶³

      Tubular functions are slightly impaired in older adults, and water, sodium, and potassium resorption and excretion decrease. The aldosterone response of Henle is also reduced. Thus, older adults need longer to reset their electrolyte balance after it becomes unbalanced.⁶⁴

      Age‐related changes in the lower urinary tract

      In the bladder, the detrusor muscle collagen content and nerve endings increase with age. Both striated and smooth muscle density in the bladder wall decrease.⁶⁵

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