What is the Digestive System? 

The human body consists of several systems that help it to perform several essential life processes. The digestive system and the excretory system are some of the many systems that make the human body fit to its environment. 

Food works like fuel to any organism. The digestive system helps in the digestion of food. During digestion, food is broken down into simpler components that are easy to absorb and transport. The human digestive system is complex and divided into two parts:  

1. The alimentary canal  
2. Associative glands 

The digestive tract consists of the mouth, food pipe, stomach, small intestine, and large intestine- together, these organs are known as digestive organs. In addition, the liver and pancreas are two associative glands that secret their digestive juices to aid in digestion. After ingestion (the intake process), the digestion process starts when food enters the mouth. 

Parts of the Digestive System 

1. Mouth: The teeth in the mouth help chewing the food and converting it into small pieces; the process is termed mastication. The saliva converts masticated food particles into a bolus. It contains electrolytes, salivary amylase (digestive enzyme), and lysozyme. The salivary amylase converts starch into maltose, indicating that the digestion process starts in the mouth. The tongue is a muscular and glandular part. It facilitates the movement of food while chewing and helps in swallowing the bolus. 

2. Food pipe: After the mouth, the bolus is pushed down into the food pipe. This process is known as swallowing and deglutition. Further, the bolus is passed by the successive waves generated by the contraction of the muscular wall of the food pipe, a process called peristalsis. 

3. Stomach: The next step of digestion takes place in the stomach. The stomach holds up the food for four-five hours. In the stomach, food is mixed with gastric juices by the churning movement of the stomach wall to form chyme. The hydrochloric acid (HCl) in the stomach provides an acidic medium that activates the inactive digestive enzyme pepsinogen into the active enzyme pepsin. Pepsin converts proteins into proteases and peptones. Thus, food is partially digested inside the stomach. 

4. Small intestine: The partially digested food from the stomach moves into a long-coiled tube called the small intestine. The wall of the small intestine produces a juice called intestinal juice. The intestinal juice with bile juice and pancreatic juice acts on the partially digested food and converts most of it into liquid type digested food. In addition, the small intestine contains various finger-like projections known as villi. These villi facilities maximum surface area to absorb nutrients from the digested food into the blood capillaries. 

5. Liver: It is the largest gland of the body. It secretes bile juice. The hepatic duct in the liver transports bile into the gallbladder. From the gallbladder, the bile duct empties the bile into the small intestine. 

6. Pancreas: The accessory pancreatic ducts include two ducts; one is a non-functional duct of Santorini that directly opens into the small intestine. The other is a functional duct known as the duct of Wirsung. The duct of Wirsung and bile duct together form the hepatopancreatic duct that opens into the duodenum region of the small intestine. 

The food is digested in the small intestine, but the wasteful food gets transported into the large intestine. 

7. Large intestine: The undigested part of food is transported to the large intestine. The large intestinal wall absorbs water and minerals from the undigested part of the food. The undigested food or waste products of digestion are thrown outside the body through the anus. The large intestine helps solidify the undigested food by absorbing the water. 

The Link Between Digestion and Excretion 

The digestive system and excretory system both work together to process the nutrition present in the food. In the digestive system, nutrition is absorbed by the blood to release energy in the presence of oxygen, a process referred to as assimilation. During assimilation, many harmful products get formed. These harmful products are then removed from the body with the help of excretion. 

Other than the urinary system, the excretory system is composed of the liver, skin, and lungs. 

Urinary system: It involves a pair of kidneys, two ureters, a urinary bladder, and a urethra. 

The kidney is the main organ of the excretory system that looks like reddish-brown beans. Humans have a pair of kidneys on both sides of the body. 

Kidneys contain small filtration units known as nephrons. They help remove the unwanted waste material from the blood as urine and return the useful substance to the blood. Urine is made up of urea, water, inorganic salt, and creatinine. Thus, the kidney purifies the blood by forming urine. 

From the kidney, urine is transferred to the urinary bladder through the ureters. During urination, the urine is removed from the body through the urethra. 

• Liver: Blood contains many harmful substances. The liver breaks some harmful substances carried by the blood. One such product is ammonia which is produced as the first waste product from protein metabolism. In the small intestine, protein is converted into amino acids, which are further converted into urea by the liver. 
• Lungs: The removal of carbon dioxide from the blood is considered the main function of the lungs. The carbon dioxide is produced from the digested food that is burned for energy in the presence of oxygen. 
• Skin: The sebaceous gland and sweat gland in the skin secrete sebum and sweat, respectively. Sweat is a watery fluid that contains sodium chloride, small amounts of urea, amino acids, and glucose. Sebum is a waxy protective secretion that contains sterols. 

It can be concluded that the parts of the excretory system remove the harmful substances produced during digestion. 

Disorders of the Digestive Tract and Excretory System  

• Uremia: It is the condition in which blood has a high amount of urea. Uremia can cause gastrointestinal problems like poor appetite, nausea, bad taste in the mouth, peptic ulcer, diarrhea, and vomiting.  
• Chronic kidney disease(CKD): It is commonly known as kidney failure. The kidneys play an important role in the excretory system. A healthy kidney filters and purifies the blood, but the kidney cannot filter the blood in CKD. CKD also affects the absorption function of the small intestine that results in gastrointestinal problems. It also causes peptic ulcers. 
• Small intestinal bacterial overgrowth (SIBO): In this disorder, the gut (digestive tract) bacteria overgrow, especially in the small intestine. The small intestine contains fewer bacteria than the large intestine. The bacteria produce toxin in the small intestine that causes a low absorption of nutrition. SIBO also leads to uremia. 

Hence, the digestive system disorders affect the excretory system, and the excretory system disorders affect the digestive system.  

Context and Applications 

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for 

• Bachelors in Biology 
• Bachelors in Microbiology 
• Bachelors in Neurology 
• Masters in Microbiology 
• Masters in Microbial Biotechnology 
• Masters in Molecular Genetics 

Related Topics 

1. Circulatory system 
2. Respiratory system 
3. Endocrine system 
4. Muscular system 
5. Colon cancer