Today's topic for biology class 12 is The Human Digestive System.Human body is made up of very small units known as cells which are also known as the building blocks of the body. Cells join to form tissues and tissues join to form organs. Different organs perform different functions and a group of organs form an organ system such as organs like nose,wind pipe and lungs form the Respiratory System which helps in breathing.
There are eight organ systems in the human body. These are-
1.The Digestive System
2.The Circulatory System
3.The Respiratory System
4.The Nervous System
5.The Skeletal System
6.The Muscular System
7.The Reproductive System
8.The Excretory System
The Human Digestive System
The human digestive system helps our body in the process of digestion i. e to digest or break down the food.The main organs of this system are mouth, oesophagus or food pipe, stomach, liver, pancreas, small intestine, large intestine and anus . These all organs are a part of the alimentary canal which is a long, muscular tube that extends through the whole length of the body. Digestive glands also form an important part of the human digestive system.
The alimentary canal consists of -
1.Mouth- It is a slit like opening through which food is injested . This slit like opening is also known as mouth orifice. It has two parts-
(a) Vestibule - It is found just inside the opening and is externally surrounded by lips and internally by the teeth and gums.
(b) Buccal Cavity - It is also known as the oral Cavity and it lies just side the teeth and gums.Palate is present on the upper side or the roof of the buccal cavity. Throat is present on its lower side which is joined with the tongue and the teeth.
2.Pharynx - It is a 12 cm long tube which extends behind the buccal cavity. Its lower part leads to the larynx (upper part of wind pipe or trachea) in the front side. The opening between larynx and pharynx is called glottis. Glottis is closed by a flap called epiglottis to stop the food to enter the wind pipe during swallowing.
3.Oesophagus - It is a narrow, long tube which joins the pharynx and the stomach.The opening of oesophagus is controlled by oesophageal sphincter which is ring shaped and made up of muscles. Oesophagus bends upwards before entering the stomach. This stops the gastric contents to flow back into the oesophagus.
4.Stomach - It is present below the diaphragm in the upper portion of abdominal cavity on the left side. Its length is 30 cm and width is 15 cm. It has a lesser curvature towards its upper side and greater curvature towards its lower side.
It has four parts -
(a) Cardiac Part - Cardiac part is that part of the stomach in which the oesophagus opens. This opening is called Cardia. There is a valve present on this opening which stops the food to move back (regurgitation). This valve is known as cardiac sphincter.
(b) Fundus - It is present above the cardia and is dome shaped.
(c) Body - It is the main central part of the stomach.
(d) Pyloric part - It is the lower part of the stomach which is narrower. It opens into the duodenum (upper part of small intestine). This opening is called pyloric aperture and is regulated by a valve known as pyloric sphincter. It controls the movement of food from stomach to small intestine.
5.Small Intestine - Its length is 6 metres long and its width is 2-3 cm. It is the longest part in the alimentary canal . It has three main parts-
(a) Duodenum- It is the upper part of small intestine in which the pyloric part of the stomach opens. Its length is 25 cm.It has leaf like projections called villi.Villi bear numerous microvilli on their surface.Villi contain blood and lymph vessels and both viIli and microvilli help in the absorption of food by increasing the surface area. Duodenum is followed by jejunum.
(b) Jejunum- It occupies the middle portion of the small intestine. Its length is 2.4 metres. It has a thick and vascular wall. The villi are tongue shaped in jejunum.It is followed by ileum.
(c) Ileum - It occupies the lower portion of the small intestine. Its length is 12 ft.The villi are finger like in ileum.It has a less thinner and less vascular wall than that of jejunum.
6.Large Intestine - Its length is 5 feet and is coiled up around the small intestine. It has three parts. These are-
(a) Caecum - Its width is 6 cm.It doesn't have any role and is just a vestigial organ. The region where it is joined with ileum is known as ileocaecal junction which is guarded by a valve called ileocaecal valve. It is followed by colon.
(b) Colon - It has 4 parts - Ascending colon is found on the right side of the abdominal cavity and moves upwards. Transverse colon appears below the below the pancreas and runs horizontally above the abdominal cavity. Descending colon is present on the left side of the abdominal cavity and moves downwards. Sigmoid colon turns in the right direction in the pelvis. It is followed by rectum.
(c) Rectum - Its width is 13 cm. It stores faeces.
7.Anus - It is the external opening of the alimentary canal and has two sphincters- internal sphincter made up of smooth muscle fibres and external sphincter made up striated muscle fibres.
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