Human Digestive System

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Introduction to human digestive system: The human digestive system is a complex network of organs that play a vital role in breaking down food and extracting essential nutrients for the body’s growth, energy, and overall well-being. Understanding the intricacies of the human digestive system is crucial for maintaining a healthy lifestyle. From the moment food enters the mouth, the human digestive system begins its remarkable journey. The process starts with chewing and saliva production, preparing the food for further breakdown in the stomach. The stomach, a significant component of the human digestive system, secretes gastric juices to break down food particles into a semi-liquid mixture called chyme. As chyme moves into the small intestine, the most critical section of the human digestive system, nutrients are absorbed into the bloodstream to nourish the body. Finally, the remaining waste material passes through the large intestine and is eliminated as feces. With a comprehensive understanding of the human digestive system, individuals can make informed choices to support their digestive health and overall wellness.

Human digestive system: The food we eat consists mostly of large and complex molecules (starch, proteins, and lipids). These molecules cannot cross the cell membrane. So cells cannot use them for energy or growth. Therefore, the food molecules must be broken down into smaller molecules that can pass through the cell membrane.

Intake of food or ingestion is followed by digestion. Digestion is the biological process by which food is broken down into smaller components through mechanical and chemical means, allowing the body to absorb and utilize nutrients for energy, growth, and maintenance.ā€

Once the food is digested, it is absorbed. The uptake of diffusible food from the digestive tract into the blood is called absorption. The blood transports the absorbed food to the body tissues, where it diffuses into the cells. Cells utilize the food molecules according to their specialties and needs. Food molecules can be oxidized to get energy or changed to make new materials for growth. This process is known as assimilation. The digestion process does not encompass the complete breakdown of all the food we consume. A considerable amount of food is not digested and it ends in a waste material that the body must get rid of. This elimination or removal of undigested food (feces) from the body is called egestion.

The human digestive system consists of a long tube called the alimentary canal and associated glands.

Alimentary canal: The human alimentary canal or the digestive tract is differentiated into the following parts:

  1. Oral Activity
  2. Pharynx
  3. Oesophagus
  4. Stomach
  5. Small intestine
  6. Large intestine

a). Oral Activity

The oral cavity, being the initial segment of the alimentary canal, is where the food is received. In the oral cavity, there are three important organs:

  1. Teeth
  2. Tongue
  3. Salivary glands

Mammals possess specialized teeth that serve specific functions. Among the permanent teeth, the incisor teeth are adapted for cutting or biting. Canine teeth are designed for tearing, whereas premolars and molars are adapted for grinding and chewing food. This is known as the mechanical digestion of food.

The tongue contains taste buds which can help us to sense the taste of food. The tongue also helps during the grinding of food by keeping it between the teeth. It also helps in the swallowing process.

Within the oral cavity, there exist three pairs of salivary glands. These glands secrete saliva. Saliva is composed of water, mucus, salts, and a digestive enzyme, forming a combination. Mucous lubricates the food. The presence of sodium bicarbonate in saliva contributes to the elimination of germs and bacteria while also alkalizing the food. The enzyme called salivary amylase begins the chemical digestion of starch and glycogen. It breaks the starch and glycogen into maltose. After the mechanical and 0artial chemical digestion, the food turns into the form of a ball called a bolus.

Parts of Buccal Cavity

b). Pharynx and Swallowing

The oral cavity opens into the pharynx. The pharynx is located at the rear of the oral cavity. Food in the form of bolus is pushed back by the action of the tongue. The process of food moving downward from the buccal cavity is referred to as swallowing. The beginning of the swallowing is voluntary but once the food reaches the back of the oral cavity, swallowing becomes an automatic or reflex action Peristalsis propels the food into the esophagus and downwards through its muscular contractions.

c). Oesophagus

The esophagus, also known as the food pipe, is a muscular tube that stretches from the pharynx to the stomach. It runs through the neck and thorax between the trachea and vertebral column. Food is passed rapidly through the oesophagus by peristalsis.

Peristalsis: Peristalsis is a wave-like contraction and relaxation in the muscular walls of the alimentary canal by which the food is moved along the alimentary canal.

Peristalsis in oesophagus

d). Stomach

The stomach is a sac-like organ and is located between the esophagus and the intestines. Located below the diaphragm in the abdominal cavity, the stomach is a muscular, elastic organ with a pear-like shape At its broadest point, it measures approximately 6 inches in width and extends about 12 inches in length. The capacity of the stomach is about 1 liter.

Food enters the stomach from the oesophagus. At the junction of the oesophagus and the stomach, there are muscles called cardiac sphincter. The cardiac sphincter serves to prevent the backward flow of food into the esophagus.

The stomach possesses thick and muscular walls. Once the food enters the stomach, it’s walls contract to churn (break down) the food into smaller particles. This is the mechanical digestion of food. The churning action also produces heat which helps to melt the lipids in the food.

The inner walls of the stomach have many gastric glands. They secrete gastric juice into the stomach. The gastric juice contains mucus, pepsinogen enzyme, and hydrochloric acid. The acid converts the inactive pepsinogen into shorter chains of inactive enzymes called peptides. Hydrochloric acid also kills bacteria present in food. After these actions on food, it is converted into a thick soup-like fluid called chime.

The present in gastric juice forms a coating in the inner walls of the stomach. It protects the inner lining of the stomach from HCI and enzymatic action of pepsin. In this way, the walls are protected from breakdown. The other end of the stomach called the pyloric end empties into the first section of the small intestine called the duodenum. The pyloric sphincter acts as a boundary between the stomach and the duodenum.

Structure of the Stomach

e). Small intestine

The stomach opens into the small intestine. The small intestine is an elongated narrow and coiled tube. It is the longest and most important part of the human digestive system. It is divided into three parts :

(i) duodenum

(ii) jejunum

(iii) ileum

(i). Duodenum

It is the first part of the small intestine, about 12 inches long and curved like ā€˜Cā€™. It receives chyme from the stomach. In the duodenum, the chyme meets the bile from the liver and pancreatic juice from the pancreas. Both are alkaline secretions.

Liver as digestive gland: liver is a large reddish brown gland. It consists of two main lobes; each is further divided into smaller lobes. The liver produces bile which is duct. Bile is a green alkaline liquid without any enzyme. The bile contains water, sodium bicarbonate, excess calcium, and breakdown products of emulsification of fats. Bile also neutralizes the acidity of the chyme.

Pancreas as digestive gland: Pancreas is a yellowish organ present beneath the stomach. It is connected to the duodenum. It secretes pancreatic juice into the pancreatic duct, which joins the common bile duct before entering the duodenum. Pancreatic juice contains sodium bicarbonate and enzymes. Sodium bicarbonate neutralizes the acidity of chyme. The enzymes present in pancreatic juice are trypsinogen (inactive enzyme), pancreatic amylase, and lipase. The inactive trypsin by the duodenal enzyme. Trypsin acts on proteins, which have escaped the action of pepsin and converts them into amino acids. The pancreatic amylase acts on the starch, which has escaped the action of salivary amylase, and converts them into simple sugars. Lipase functions by acting upon emulsified fats, transforming them into glycerol and fatty acids.

Digestive Glands

(ii). jejunum: Jejunum comes after duodenum and is about 2.4 meters long.

(iii). Ileum

The ileum is about 2.6 meters long. The glands of the ileum secrete intestinal. It contains enzymes that break down the leftover undigested proteins, carbohydrates, and fats. Through this process, the digestion of food reaches its completion within the small intestine.

Absorption of food: After the complete digestion of food, the process of absorption begins in the small intestine. The inner lining of the small intestine is adorned with abundant finger-like protrusions known as villi, which are further adorned with smaller projections called microvilli. These structures greatly increase the surface area available for absorption.

Nutrients, such as glucose, amino acids, and fatty acids, are absorbed into the bloodstream through the thin walls of the villi. The capillaries within the villi absorb water-soluble nutrients like glucose and amino acids, while the lacteals, specialized lymphatic vessels, absorb dietary fats in the form of fatty acids and glycerol.

The absorbed nutrients are then transported by the bloodstream to various tissues and organs throughout the body, where they are utilized for energy production, growth, and repair. Glucose is an important source of energy for cells, amino acids are used for protein synthesis, and fatty acids are involved in energy production and the synthesis of cell membranes and hormones.

Water and some electrolytes, such as sodium and potassium, are also absorbed in the small intestine. These substances are crucial for maintaining fluid balance and proper cellular function.

Any remaining undigested material, such as dietary fiber and waste products, continues through the large intestine, where water and electrolytes are further absorbed. The waste is eventually eliminated from the body as feces.

Structure of Villus

f). Large Intestine

Large intestine is part of the digestive system where undigested food is collected and converted into feces. The large intestine spans approximately 1.5 meters in length and comprises three main parts: the caecum, colon, and rectum. The caecum, serving as the initial segment of the large intestine, is responsible for the first stage of digestion.

Large Intestine

Caecum: The caecum is positioned as the initial segment of the large intestine. It receives the mixture from the small intestine and moves it towards the colon. The mixture in caecum contains undigested food, water, some vitamins, and some salts.

Colon: The colon, which constitutes the lengthiest section of the large intestine, extends for a significant distance. The colon has four sections i.e. ascending, transverse, descending, and sigmoid colon. As the material travels through the colon, the lining of the colon absorbs most of the water and some vitamins and minerals. The mixture of undigested material, water, dead cells of the alimentary canal, bile pigments, etc., mixes with mucus and takes the form of feces. Through muscular movements of the colon, feces is pushed into the rectum.

Rectum: The rectum serves as the concluding segment of the large intestine. It is where stool is stored before being passed out from the anus. It is called egestion.

Other roles of the liver

  1. The liver produces bile which emulsifies fats and breaks into droplets. Besides its digestive function, the liver also performs many other bodily functions. For example;
  2. It stores glucose as glycogen and provides glucose when required.
  3. It also stores fat-soluble vitamins.
  4. The liver converts amino acids to other organic compounds. In this process, ammonia is produced. The liver converts this harmful compound to less toxic urea, which is then excreted from the body through urine.
  5. It makes many of the body’s proteins.
  6. Toxic substances e.g. alcohol are broken down in the liver.
  7. It also breaks the red blood cells which have completed their life spans.
  8. The liver makes vitamin A from carotene.
  9. In cold temperatures, the liver carries out basic metabolism at faster rates and so produces heat.

Added by: Sadiq Ameen member of

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