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18. How is the balance of water, and essential ions such as sodium, regulated in the renal system, regardless of conditions such as abundance of water to drink? How do the hormones ADH and aldosterone contribute to this osmoregulation (Review negative feedback mechanisms in slides 49 and 51)? 19. Be able to follow the process of digestion, as discussed in lecture, all the way from mouth to anus. Where does mechanical digestion occur? Where does chemical digestion begin?! Where is chemical digestion most intense and complete? 20. Where are each of our macronutrients (carbohydrates, proteins and fats) broken down into components that our cells can absorb? What are the names of the major enzymes found in each of these locations along the digestive tract? How are the digested macronutrients absorbed into the circulatory system? 21. Know the basic functions of the esophagus, stomach, small intestine and large intestine. 22. Know and recognize the histological layers of the digestive tract, and their functions. What is peristalsis, and how is this movement achieved?

Answer 1

Ans 18) The water, essential ions such as Na+,K+,Ca2+ etc are important for regulating the functions in the body and needs to be selectively reabsorbed in the blood and is done by selective reabsorption process.It can be passive or active (Requires ATP).Osmotic pressure of blood is higher (High solute concentration) than glomerular filtrate.Thus,by osmosis,water moves from the region of low solute concentration to the region of high solute concentration i.e from the glomerular filtrate to the blood.This is how the water and essential ions are reabsorbed and such maintenance of salts or solutes and water balance in the blood for the regulation of osmotic pressure of blood is known as osmoregulation.

ADH (Anti-diuretic hormone plays an essential role in osmoregulation).It is secreted during dehydration or low water concentration in the blood which results in high solute concentration (like ions etc)inthe blood due to dehydration,hypothalamus gets stimulated and releases ADH which results in increase permeability of water by DCT (Distal convoluted tubule),PCT (Proximal conoluted tubule) and CT (collecting tubule) and thus increased water reabsorption from the glomerular filtrate in the blood will occur during dehydration conditions which will replenish the water concentration in the blood.This is how osmoregulation occurs by ADH.

Ans 19) Mechanical digestion deals with the breaking up of food into small particles by chewing,churning and peristaltic movements. In mouth, Food is broken down into smaller particles by teeth and now the food is called as bolus which enters the stomach. In stomach,due to contraction and relaxation of muscles,the food in stomach gets churned and causes further breakdown of food which enters in small intestine. In small intestine,the bolus is mixed with the enzymes for further breakdown.

Chemical digestion is the breakdown of food particles with the help of enzymes. In mouth,the salivary amylase enzyme is present which breaks down starch into maltose and maltase enzyme converts maltose into glucose.

In stomach,pepsin enzyme gets active due to acidic pH i.e 1 and breakdown of proteins into peptides occurs.Fats are also emulsified into fat droplets.The semi-digested acidic food is called chyme. In small intestine, bile and pancreatic juice acts for further chemical digestion. Bile salts neutralize the acidity of food and makes the food alkaline so that the enzymes of small intestine can work on it efficiently. Most of the chemical digestion will occur in the duodenum and gets completed by the action of bile and pancreatic juice.

Ans 20) Carbohydrates are broken down by the salivary amylase in the saliva into Maltose.Another enzyme present in saliva is maltase which converts maltose into Glucose. Glucose is absorbed into blood vessels by diffusion,osmosis and active reabsorption in uriniferous tubules or nephron.

In stomach, pepsin enzyme can break proteins into peptones and proteoses. In small intestine, Trypsin breaks down peptones and proteoses into polypeptides. Carboxypeptidase enzyme breaks down polypeptides into peptides which is further broken down into amino acids.Amino acids can be absorbed directly and readily just like diffusion process etc.

In small intestine, Lipase enzyme breaks down fats into fatty acids and glycerol which are absorbed in the lymph capillaries or lacteals.Lacteals open into blood vessels.

Ans 21) Oesophagus is narrow and long tube and connects pharynx to stomach through which the food is carried to the stomach. It is 25 cm in length.

Stomach has many digestive enzymes which work in the acidic pH i.e 1. In stomach, chemical digestion occur as well as mechanical digestion (churning of food by the muscles of the stomach) and pyloric sphincter opens to pass food from stomach to small intestine.

Small intestine is the place where most of the chemical digestion occurs and also absroption of important ions and other materials will occur with the help ofmicrovilli.

Large intestine is the place of formation and storage of faeces (all the undigested material) in the rectum.

Ans 22) We have already discussed that the muscles of the stomach will contract and relax which will inturn help to propel the food forward for further digestion.This is known as peristaltic movement.

Histological layers are :

1) Serosa (outer thin layer) which protects the covering of stomach.

2) Muscularis externa (function of churning food). It consists of 3 muscle layers : a) Longitudinal muscle

b) circular muscle

c) Oblique muscle

3)Sub-mucosa (Blood vessels are present in this layer). It prevents blood vessels from getting collapsed or damaged.

4) Mucosa is the inner most layer.It consists of 3 layers: a)Muscularis mucosa or interna (has longitudinal and ciruclar muscles)

b)Lamina propria (has blood vessels, nerves etc).

c) Epithelium (Inner most lining layer). It is highly folded and has gastric glands visible.