Digestion and Absorption

Digestion is a catabolic process where complex nutrients are broken down to simple and ultimately get absorbed that releases energy.

Human Digestive System consists of:

Mouth:

It has a hollow cavity called the buccal cavity or oral cavity whose roof is made up of bone called the upper palate and base called the lower palate. The interior most portion of upper palate is hard called hard palate and posterior is soft called soft palate. The soft palate hangs freely which prevents the entering of food into the nasal cavity called the uvula.

The Mouth has teeth and human teeth are Diphyodont - having two sets of teeth.

Milk Teeth
Permanent Teeth.

Human teeth are heterodont type means we have different types of teeth.

Incisor,
Canine,
Premolar,
Molar.

Human teeth are Theocodont type which means that it is embedded in the bony socket.

The numerical representation of different types of teeth is called Dental Formula.

Tongue:

Human beings have a tongue which is a muscular organ that remains attached at the posterior by cartilage in the hyoid bone and the lower portion is attached to the lower palate by the lingual frenulum.

All over the tongue, there are small projections called papillae which are of four types.

Circumvallate:
Filiform:
Fungiform:
Foliate:

The tongue also contains taste buds. Such as:

Pharynx:

It is divided into three parts:

Nasopharynx: The part of the pharynx where the nasal cavity terminates into the buccal cavity. It is blocked by the uvula during the swallowing of food that prevents the entering of food into the nasal cavity.
Oropharynx: It is a common passage for both food and air.
Laryngopharynx: It is the region where the food pipe and windpipe get separated. The windpipe has a cap-like structure called epiglottis which blocks the windpipe during the swallowing of food and prevents food from inter into the windpipe.

Esophagus (Food pipe):

It is a hollow 12 to 15 cm long tube-like structure that helps in the passing of food from the pharynx to the stomach. The wall of the food pipe is made up of involuntary muscles. When food enters into the food pipe it undergoes rhythmic contraction and expansion which pushes the food down into the stomach this type of movement is called peristalsis.

Stomach:

It is a hollow "J" shaped organ, where food is stored for 4 to 5 hours where digestion of protein takes place. It is divided into four parts:

Fundus: It is the uppermost part of the stomach where gases are stored produced during digestion.
Cardiac: This part is situated near the heart.
Body: It is the largest part of the stomach where digestion takes place.
Pyrolic: The lowermost part of the stomach that terminus into the small intestine.

In between the food pipe and the stomach it is guarded by a muscular cap-like structure called a cardiac sphincter which prevents the backflow of food into the food pipe. And in between the stomach and small intestine, it is guarded by a Pyrolic sphincter which prevents the backflow of food into the stomach.
The stomach wall has regular foldings called rugae which increases the surface area of digestion.
The stomach secrets gastric juice which converts food to a thick dark liquid called chyme.

Small intestine:

This is the longest part of the digestive system but has the smallest area of cross-section.

It is divided into three parts:

Duodenum: It is the C-shaped region of the small intestine.
Jejunum: It is the largest part of the small intestine.
Ileum: The part of the small intestine which terminates into the large intestine.

The small intestine has numerous finger-like projections called villi which have blood and lymphatic vessels where absorption of food takes place.

Large intestine:

It is divided into 4 parts:

Caecum: The part of the large intestine where is small intestine terminates is called the caecum. It has a small outgrowth called the vermiform appendix which is a vestigial organ other than the digestion of a small amount of cellulose and it also provides immunity.
Colon: it is divided into 4 parts viz. ascending, transverse, descending, and sigmoid. In the whole colon absorption of water takes place.
Rectum: Undigested solid food is stored in the rectum.
Anus: Undigested solid food passes away through the anus as feces or excreta.

Digestive glands:

Glands are modifications of either cubicle or columnar epithelial cells whose function is to secrete. Glands are of two types:

Exocrine
Endocrine

EXOCRINE	ENDOCRINE
It secretes enzymes.	It secretes hormones.
These are called duct glands.	These are called ductless glands.
They are produced at the site of function and flow through the duct.	They are produced away from the site of function and transported through the blood.

Salivary gland:

Situated in the mouth which consist of three glands:

Parotid: This is the largest of all salivary glands situated in the upper jaw.
Sublingual: Situated below the tongue.
Submaxillary or Submandibular: Situated below the lower jaw.

Its secret enzyme salivary amylase or ptyalin, pH 6 to 8, daily output 1000 to 1500 ml that digest starch to maltose.

Gastric glands:

Situated in the stomach which have the following cells:

Oxyntic cell: its secrets hydrochloric acid.
Chief or peptic cell: Its secret enzymes like pepsinogen, prorenin, gastric lipage.
Goblet cells: It secrets mucus that prevents the stomach from being digested by its own enzymes.

The secretion of the gastric gland is called gastric juice, pH 1.5 to 2.5, daily output 2000 to 3000 ml which mainly digest protein.

Liver:

This is the largest gland of our body reddish-brown in color 1.2 to 1.6 kg in weight and is divided into two lobes left and right lobe respectively both the lobes remain joined by the falciform ligament.
Internally liver is divided into a large number of chambers where each chamber is called hepatic lobules which remains surrounded by a membrane called Glisson's capsule.
Each hepatic lobules contain a large number of cells called hepatocytes or hepatic cells which are the structural and functional unit of the liver that secrets bile.
In between hepatic cells, there is some phagocyte cell called kupffer cell that kills microorganisms by a process called phagocytosis.
From the left lobe of the liver left hepatic duct that arises and the right hepatic duct from the right lobe both unite to form a common hepatic duct which carries bile to the gallbladder. From the gallbladder bile duct arises which unites with the pancreatic duct to form a hepatopancreatic ampulla that carries the bile and pancreatic juice to the duodenum of the small intestine.

Pancreas:

This is a heterocrine gland i.e. it secrets both enzymes as well as hormone. The endocrine part is called the Islet of Langerhans which have two types of cells:

Beta-cell: Secrete hormones Insulin which converts excess glucose to glycogen and store in the liver this process is called glycogenesis.
Alpha-cell: It secrets the hormone glucagon that converts excess glycogen to glucose during starvation this process is called glycogenolysis.

The exocrine part is called lobules of acini which secretes enzyme-like chymotrypsinogen, trypsinogen, pancreatic lipase, pancreatic amylase, DNase, RNase. The enzymes are commonly called pancreatic juice, pH 7.8, daily output 500 to 800 ml which through the pancreatic duct and hepatopancreatic ampulla falls into the denim region of the small intestine.

Intestinal gland:

Situated at the Jejenum region of the small intestine. It contains two types of cells:

Crypts of lieberkuhn: It secretes enzymes such as maltase, isomaltase, sucrase, lactase, dextrinase, enterokinase, nucleotidase, and nucleosidase.
Brunner's gland: It is a type of goblet cell that secretes mucus that protects our small intestine from being digested and it helps in the passing of food from the alimentary canal.

The enzyme secreted from it is called intestinal juice or succus entericus, daily output 2000 to 3000 ml, pH 7.8.

Digestion of Carbohydrate

During digestion, carbohydrates are broken down into their simplest form i.e. Glucose, Fructose, Galactose, etc.

Digestion of Carbohydrate takes place as follows: 1. Mouth: In the mouth digestion of starch takes place by the action of salivary amylase or ptyalin. Amylase broke starch into maltose. 2. Food pipe or esophagus: No digestion takes place 3. Stomach: In the stomach very small amount of starch, with the help of gastric amylase is converted to maltose. 4. Small Intestine: With the help of Pancreatic juice, which contains pancreatic amylase starch is broken down to maltose, isomaltose, and alpha-dextrin.

Starch → Maltose + Isomaltose + Alpha-Dextrin

With the help of Intestinal juice, which contains enzyme-like maltase, isomaltase, dextrinase, lactase, and sucrase.

Maltose → Glucose + Glucose (maltase)
Lactose → Glucose + Galactose (lactase)
Isomaltose → Glucose (isomaltase)
Alpha-Dextrin → Glucose (dextrinase)
Sucrose → Glucose + Fructose (sucrase)

Digestion of Protein

Protein is digested into its simplest form i.e. amino acids. Digestion of protein takes place as follows: 1. Mouth: No digestion. 2. Stomach:

In the stomach, activation of inactive enzymes like pepsinogen into pepsin takes place with the help of HCl (hydrochloric acid).
Pepsin converts other inactive pepsinogens into pepsin, this process in which an enzyme converts its inactive form to active is called Auto-catalytic reaction.
Prorenin by the help of HCl is converted into renin.
Protein → Peptones (HCl)
Casein (milk protein) → paracasein (Renin)
Paracasein + Ca → Calcium paracaseinate
Calcium paracaseinate → peptones (Pepsin)

3. Small Intestine: With the help of pancreatic juice: Enzyme Chymotrypsinogen and trypsinogen by the help of enterokinase is converted into chymotrypsin and trypsin which digest protein as follows:-

Chymotrypsinogen⟶ Chymotrypsin (enterokinase)
Trypsinogen ⟶Trypsin (enterokinase)
Protein ⟶Peptones (chymotrypsin)
Protein ⟶Peptones (trypsin)
Peptones ⟶ Large peptide (carboxypeptidase)

With the help of Intestinal juice: It contains the enzyme aminopeptidase and dipeptidase which digest protein as follows:

Large peptide ⟶ Dipeptidase (aminopeptidase)
Dipeptidase ⟶ Amino acids (dipeptidase)

Digestion of Fat:

Fats are broken down into their simplest form i.e. fatty acid and glycerol. Digestion of fats takes place as follows: 1. Mouth: No digestion 2. Stomach: No digestion 3. Small Intestine: Bile converts fats into small droplets this process is called emulsification of fat and the fats are called emulsified fats. Emulsified fat with the help of pancreatic lipase is broken down to fatty acid and diglycerides; then fatty acid and monoglyceride; and then to fatty acid and glycerol.

Emulsified fats ⟶ Fatty acid + Diglyceride (pancreatic lipase)
Fatty acid + Diglyceride ⟶ Fatty acid + monoglyceride
Fatty acid + monoglyceride ⟶ Fatty acid + glycerol

Digestion of Nucleic acid:

Nucleic acid is digested into its simplest form pentose sugar and Nitrogenous bases. Digestion of nucleic acid takes place as follows: 1. Mouth: No Digestion 2. Stomach: No Digestion 3. Small Intestine: With the help of pancreatic juice: Pancreatic juice contains enzymes, like DNase and RNase that digest nucleic acid to the nucleotide.

DNA ⟶ Nucleotide (DNase)
RNA ⟶ Nucleotide (RNase)

With the help of intestinal juice: Intestinal juice contains enzyme nucleotidase and nucleosidase that digest nucleic acids as follows:

Nucleotide ⟶ Nucleoside + P (Nucleotidase)
Nucleoside ⟶ Pentose sugar + N2 base (Nucleosidase)

Absorption:

The uptake of digested nutrients by the body is called absorption.
Maximum absorption takes place in the small intestine that has numerous villi that increase the surface area of absorption and it takes place either in blood vessels or lymphatic vessels.
Absorption takes place by diffusion either by active, simple, or facilitated.

Absorption of Carbohydrate:

Glucose and galactose are absorbed by the process of active diffusion and fructose by the process of facilitated diffusion in the blood vessels of villi.

Absorption of Protein:

Amino acids are absorbed in the blood vessels of villi by active diffusion.

Absorption of fats:

Fats are broken down to fatty acid and glycerol which by the help of bile converted into a water-soluble form called micelles; which are uptaken in the villi by simple diffusion. Micelles in the endoplasmic reticulum of the intestinal wall are converted into small fats droplet called chylomicrons which are absorbed by the lymphatic vessels. The lymphatic vessels where absorption of fats takes place are called lacteal.