Digestion and Absorption
(i) Process of conversion of complex food substances to simple absorbable forms is called digestion.
(ii) Intracellular: When the process of digestion occurs within the cell in the food vacuole.
Examples:
Protozoa, Porifera, Coelenterata and free living Platyhelminthes
(iii) Extracellular: When the process of digestion occurs outside the cell, Examples: Coelenterates and phylum Platyhelminthes to phylum Chordata.
(iv) Digestion in vertebrates occurs in the digestive tract or alimentary canal. The various parts involved in digestion can be broadly grouped in two groups –
(1) Digestive tract or alimentary canal
(2) Digestive glands
On the basis of the embryonic origin, the alimentary canal of vertebrates can be divided into three parts–
(1) Fore gut / Stomodaeum: Ectodermal. It includes buccal cavity / oral cavity, pharynx, oesophagus, stomach and small part of duodenum.
(2) Mid gut/Mesodaeum: Endodermal. It includes small intestine, and large intestine.
(3) Hind gut/Proctodaeum: Ectodermal. It includes anal canal and anus.
Human Digestive System
The human digestive system is a complex series of organs and glands that processes food. It converts ingested food so that it can be assimilated by the organism. The human digestive system consists of following parts:-
Mouth
Mouth is also known as the oral cavity or buccal cavity. It is the first portion of the alimentary canal. Food and saliva are received by mouth. Mouth has inner lining of mucous membrane epithelium.
Digestive Glands: -
(A) Salivary glands:-
(i) These are the exocrine glands that produce saliva.
(ii) These are the glands with ducts which also secrete amylase.
(iii) Amylase is an enzyme that breaks down starch into maltose.
(iv) Three types of salivary glands are: -
1) Parotid gland
2) Submandibular gland
3) Sublingual gland
(B) Gastric glands:-
(i) The gastric glands (fundic gland) secrete acids and digestive enzymes.
(ii) Secretion of gastric gland is called gastric juice.
(iii) There are approximately 35 million gastric glands present in human stomach.
(C) Intestinal glands:-
(i) Intestinal glands in maamals is a collective name for crypts of Liberkuhn (secretes alkaline enzymatic juice) and Brunner’s glands (secrete mucous).
(ii) Intestinal glands secrete intestinal juice or sucus entericus.
(D) Pancreas:-
(i) Pancreas has two different kind of tissue- exocrine and endocrine.
(ii) Pancreatic secretion is stimulated by cholecystokinin and secret in both.
(iii) Complete digestive juice is pancreatic juice as it contains amylolytic, lipolytic and proteolytic enzymes.
(iv) It produces several important hormones like insulin, glucagon, somatostatin and pancreatic polypeptide.
(E) Liver:-
(i) Liver is the largest digestive gland of the body, weighing about 1.2 to 1.5 Kg in an adult human.
(ii) It is situated in the abdominal cavity, just below the diaphragm and has two lobes (small left and large right lobe).
(iii) The liver has a wide range of functions to perform in the body:
a) It detoxifies various metabolites
b) It helps in protein synthesis.
c) Various biochemical necessary for digestion are produced by liver.
Pharynx: -
It is the opening of oral and nasal cavities. It is classified as: -
1) Nasopharynx
2) Oropharynx
3) Laryngopharynx
Oesophagus: -
Oesophagus connects pharynx with stomach.
Opening of oesophagus is regulated by gastro-oesophageal sphincter.
Stomach: –
It is a J-shaped, muscular, hollow and dilated part of the digestive system. It is located between the oesophagus and the small intestine. It has 1 liter capacity. It secretes protein-digesting enzymes (proteases) and strong acids which aid in food digestion.
The stomach has three parts:
Cardiac: The part of the stomach into which oesophagus opens.
Fundus: It is the air filled portion of stomach.
Pyloric: The portion of the stomach that opens into the small intestine
Breathing and exchange of gases
Pulmonary Volumes and Capacities
There are following respiratory volumes and capacity:
(i) Tidal volume (TV): It is volume of air normally inspired or expired in one breath (i.e. inspiration and expiration) without any extra effort. It is about 500 ml in normal healthy adult. In infants it is 15 ml and in fetus it is 0 ml.
(ii) Inspiratory reserve volume (IRV) : By taking a very deep breath, you can inspire a good deal more than 500 ml. This additional inhaled air, called IRV is about 3000 ml.
(iii) Expiratory reserve volume (ERV) : If you inhale normally & then exhale as forcibly as possible,you should be able to push out 1200 ml of air in addition to 500ml. of T.V. The extra 1200 ml is called ERV.
(iv) Residual volume (RV): Even after expiratory reserve volume is expelled, considerable air remains in the lung, this volume, which cannot be measured by spirometry, and it is called residual volume is about 1200 ml.
(v) Dead space: Portion of tracheobronchial tree where gaseous exchange does not occur is called dead space. It is also called conductive zone. Dead space is 150 ml.
(vi) Functional residual capacity (FRC): It is the amount of air that remains in the lungs after a normal expiration. It is about 2300 ml.
FRC = ERV + RV
= 1100 + 1200 = 2300 ml.
(vii) Vital capacity (VC): This is the maximum amount of air that can be expired forcefully from his lungs after first filling these with a maximum deep inspiration. It is about 4600 ml.
VC = IRV + TV + ERV
= 3000+500+1100 = 4600 ml.
(viii)Total lung capacity (TLC): TLC is the sum of vital capacity (VC) and residual volume (RV). It is about 5800ml.
TLC = VC + RV
= 4600 + 1200 = 5800 ml.
(ix) Inspiratory capacity (IC): It is the total amount of air a person can inspire by maximum distension of his lungs.
I.C. = TV + IRV
= 500 + 3000 = 3500 ml.
Process of Respiration
The process of respiration is completed in 4 steps:
(i) Breathing or ventilation
(ii) Exchange of gases or External respiration
(iii) Transport of gases
(iv) Cellular respiration
(i) Ventilation or breathing:
Breathing is movement of thorax, expansion (inflation) and deflation of lungs and flow of air into the lungs and from the lungs. It is extracellular, energy consuming and physical process. Sum of inspiration and expiration is called respiratory movement. There are two steps of breathing:
(a) Inspiration: Intake of fresh air in lungs from outside. It is an active process. Blood pressure increases during later part of respiration.
(b) Expiration: Out flow of the air from the lungs is called expiration. When expiration occurs,the inspiratory muscles relax. As the external intercostal relax, ribs move inferiorly and as the diaphragm relaxes, its dome moves superiorly owing to its elasticity.
(c) Mechanism of ventilation/breathing:
(ii) Exchange of gases:
(a) Exchange of gases in lungs: It is also called external respiration. In this gaseous exchange oxygen passes from alveoli to pulmonary capillary blood and CO2. Comes to alveoli from pulmonary capillary.
(b) Release of CO2 by the blood: The PCO2 (partial pressure of carbon dioxide) of blood reaching the alveolar capillaries is higher than the PCO2 of alveolar air. Therefore, carbon dioxide diffuses from the blood of alveolar capillaries into the alveolar air.
(c) Exchange of gases in tissues: In the tissues, exchange of gases occurs between the blood and the tissue cells. This exchange occurs via tissue fluid that bathes the tissue cells. The blood reaching the tissue capillaries has PO2 higher than that in the tissue cells and PCO2 lower than that in the tissue cells.
(iii) Transport of gases: Blood carries O2 from respiratory organs to the tissue cells for oxidation and CO2 from tissue cells to respiratory organs for elimination. Blood should be slightly alkaline to help the transport of O2 and CO2 properly.
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