Introduction to organ systems:

Digestive system:

Digestionis the process of breaking down of the complex food stuff with the help of digestive juices into simple constituents, which can be normally absorbed and assimilated in the body. The food after ingestion through oral cavity passes along the various parts of the digestive tract, where it is converted into simple absorbable constituents. The digestive tract, thus in addition to digestion is also concerned with the process of absorption. The undigested and the unabsorbed food along with the other waste products from the body and bacterial flora constitutes the faeces which is excreted out from the terminal part of the tract (anus) by act of defecation.

In the human digestive system, food enters the mouth and mechanical digestion of the food starts by the action of mastication, a form of mechanical digestion, and the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the digestion of starch in the food. After undergoing mastication (chewing) and starch digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action of peristalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly contains hydrochloric acid and pepsin. As these two chemicals may damage the stomach wall, mucus is secreted by the stomach, providing a slimy layer that acts as a shield against the damaging effects of the chemicals. At the same time protein digestion is occurring, mechanical mixing occurs by peristalsis, which is waves of muscular contractions that move along the stomach wall. This allows the mass of food to further mix with the digestive enzymes.

After some time (typically 1-2 hours in humans, 4–6 hours in dogs, 3-4 hours in house cats), the resulting thick liquid is called chyme. When the pyloric sphincter valve opens, chyme enters the duodenum where it mixes with digestive enzymes from the pancreas, and then passes through the small intestine, in which digestion continues. When the chyme is fully digested, it is absorbed into the blood. 95% of absorption of nutrients occurs in the small intestine. Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is slightly acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K produced by bacteria in the colon are also absorbed into the blood in the colon. Waste material is eliminated from the rectum during defecation.

In most vertebrates, digestion is a multistage process in the digestive system, starting from ingestion of raw materials, most often other organisms. Ingestion usually involves some type of mechanical and chemical processing. Digestion is separated into four steps:

1. Ingestion: placing food into the mouth (entry of food in the digestive system),2. Mechanical and chemical breakdown: mastication and the mixing of the resulting

bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures,

3. Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and

4. Egestion (Excretion): Removal of undigested materials from the digestive tract through defecation.

Parts of the digestive tract:

Oral cavity, pharynx, oesophagus, stomach, small intestine (duodenum, jejunum and ileum), large intestine (caecum, ascending colon, transverse colon, descending colon, iliac colon, sigmoid colon, rectum and anus).

The salivary glands, liver and pancreas (secrete digestive juices) are also developmentally associated with alimentary canal.

Oral cavity:

In humans, digestion begins in the mouth where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day) by three pairs of exocrine salivary glands (parotid, submandibular, and sublingual) in the oral cavity, and is mixed with the chewed food by the tongue.

Saliva cleans the oral cavity, lubricates the food thus facilitating the act of mastication and swallowing. It contains digestive enzymes such as salivary amylase, which aids in the chemical breakdown of polysaccharides such as starch into disaccharides such as maltose. It also contains mucus, a glycoprotein that helps soften the food and form it into a bolus. An additional enzyme, lingual lipase, hydrolyzes long-chain triglycerides into partial glycerides and free fatty acids.

Swallowing transports the chewed food into the esophagus, passing through thepharynx.

The pharynx is the part of the neck and throat situated behind the mouth and nasal cavity, and cranial, or superior, to the esophagus. It is part of the digestive system and respiratory system. Because both food and air pass through the pharynx, a flap of connective tissue, the epiglottis closes over the trachea when food is swallowed to prevent choking.

Esophagus:

The esophagus is a narrow muscular tube about 20-30 centimeters long, which starts at the pharynx at the back of the mouth, passes through the thoracic diaphragm, and ends at the cardiac orifice of the stomach. The wall of the esophagus is made up of two layers of smooth muscles, which form a continuous layer from the esophagus to the colon and contract slowly, over long periods of time. The inner layer of muscles is arranged circularly in a series of descending rings, while the outer layer is arranged longitudinally. At the top of the esophagus, is a flap of tissue called the epiglottis that closes during swallowing to prevent food from entering the trachea (windpipe). The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles. It takes only about seven seconds for food to pass through the esophagus and now digestion takes place.

Stomach: is normally a J shaped structure. It communicates above with esophagus through cardiac orifice and below with duodenum through pyloric orifice. Food that enters the stomach

through cardiac orifice goes through further digestion with the help of gastric juice, pepsin and mucous present in the stomach.

The gastric acid is acidic with a pH that ranges between 0.9-1.5. The acidic nature is due to the presence of HCl. The gastric acid also contains KCl and NaCl. The HCl present in the gastric acid activates pepsinogen that are secreted by stomach to pepsin which along with HCl converts proteins into smaller peptides/aminoacids. Lipase, another component of the gastric juice acts on fats and helps in their digestion. The acid itself does not digest the food; rather it provides an optimum pH for the reaction of the enzyme pepsin and kills many microorganisms that are ingested with the food. It can also denature proteins.

Mucous, along with gastric juice plays an important role in lubrication and protection of the mucosal epithelium from excoriation by the highly concentrated hydrochloric acid.

After consumption of food, digestive "tonic" and peristaltic contractions begin, which helps break down the food and move it onward.[15] When the chyme (Food in the stomach is in semi-liquid form, which upon completion is known as chyme.) reaches the opening to the duodenum known as the pylorus, contractions "squirt" the food back into the stomach through a process called retropulsion, which exerts additional force and further grinds down food into smaller particles. Gastric emptying is the release of food from the stomach into the duodenum; the process is tightly controlled with liquids being emptied much more quickly than solids.

Small Intestine:

It has three parts: the Duodenum, Jejunum, and Ileum.

Duodenum does not secrete any digestive juice as such.

After being processed in the stomach, food is passed to the small intestine via the pyloric sphincter. The majority of digestion and absorption occurs here after the milky chyme enters the duodenum. Here it is further mixed with three different liquids:

Bile, which emulsifiesfats to allow absorption, neutralizes the chyme and is used to excrete waste products such as bilin and bile acids. Bile is produced by the liver and then stored in the gallbladder where it will be released to the small intestine via the bile duct.

Pancreatic juice made by the pancreas, which secretes enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen (inactive form of protease).

Trypsinogen is activated by enterokinase into trypsin which inturn activates chymotrypsinogen into chymotrypsin. These converts proteins in to dipeptides.

Amylase converts starch into maltose.

Lipase converts fat into glycerol and Fattyacids.

Intestinal juice secreted by the intestinal glands in the small intestine. It contains enzymes such as

-enteropeptidase, erepsin (peptidases- converts simpler peptides into aminoacids), -

-Nuclease, nucleosidase and nucleotidase (digestion of nucleoproteins)

-trypsin, chymotrypsin,

-maltase (converts into two molecules of glucose),

-lactase(converts lactose to glu and gal),

-arginase (converts arginine into urea) and

-sucrase– converts sucrose to glucose and fructose(all three of which process only sugars).

The small intestine and remainder of the digestive tract undergoes peristalsis to transport food from the stomach to the rectum and allow food to be mixed with the digestive juices and absorbed.

Large intestine

After the food has been passed through the small intestine, the food enters the large intestine. Within it, digestion is retained long enough to allow fermentation due to the action of gut bacteria, which breaks down some of the substances that remain after processing in the small intestine; some of the breakdown products are absorbed. In humans, these include most complex saccharides (at most three disaccharides are digestible in humans)

In general, the large intestine is less vigorous in absorptive activity. It produces sacculation, renews epithelial cells, and provides protective mucus and mucosal immunity. In humans, the large intestine is roughly 1.5 meters long, with three parts: the cecum at the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs water from the chyme and stores feces until it can be egested. Food products that cannot go through the villi, such as cellulose (dietary fiber), are mixed with other waste products from the body and become hard and concentrated feces. The feces is stored in the rectum for a certain period and then the stored feces is eliminated from the body due to the contraction and relaxation through the anus. The exit of this waste material is regulated by the anal sphincter.

Excretory system:

Kidneys are the chief excretory organs, which are concerned with the excretion of most of the waste and other disposable products of the body through urine formation.

Each kidney is a bean shaped structure and lies on either side of the vertebral column behind the peritoneum. On an average, each kidney measures 12 cm x 7 cm x 3 cm and weighs 150 gms. It is embedded in a thick pad of fat and covered by a tough capsule.

The kidney is divided into an outer portion called the cortex and an inner one called the medulla.

Functions of the kidney:

1. The major function of the kidney is to maintain the fluid and electrolyte balance of body fluids.

2. Regulate the blood pH.3. Helps to maintain osmotic pressure 4. Helps to retain important plasma constituents like glucose, aminoacids etc.5. Erythropoietin affects the erythrocytes formation.6. Excrete waste products through urine formation. The main waste products are the end

products of protein metabolism like urea and uric acid.

Structure of kidney:

Each kidney has about one million nephrons. A nephron is a single excreting unit of the kidney. The nephron consists of the following parts.

1.Bowman’s capsule

2. renal glomerulus.

Both of the above together form the malpighian corpuscle.

3. Proximal convoluted tubule

4. Henles loop with descending and ascending loops

5. Distal convoluted tubule

6. Collecting tubule

3-6 constitute the renal tubule.

Urine formation:

The urine formation occurs by glomerular ultrafiltration, selective reabsorption and tubular secretion.

Glomerular filtration:

The glomerular filtering membrane acts as ultrafilter. The glomerular filtrate is the plasma minus its protein in its composition. Average blood flow through kidney is at the rate of 1300 ml/min (700 ml plasma) while the glomerular filtrate is 130 ml/min.

The ratio of glomerular filtration rate to plasma flow rate is called the filtration fraction, ie, FF = 130/700 = 0.18.

On an average 170 liters of glomerular filtrate is formed daily, out of which 168.5 liters is reabsorbed, so that the normal average volume of urine excreted is 1.5 L / day.

Tubular reabsorption:

The GF during passage through the renal tubules gets concentrated by reabsorption of about 99% of its water content. In addition there occurs reabsorption of its solid contents as well. Some of the contents are reabsorbed almost completely e.g., glucose and potassium (100%), water (99%), sodium, chloride and calcium (98-99%). These are high threshold substances. Substances like urea, uric acid and phosphates are reabsorbed to some extent only and are called as low threshold substances., while creatinine and sulphates are not reabsorbed at all and are known as no threshold substances.

Glucose is reabsorbed completel in proximal tubules only. However if glucose concentration exceeds its renal threshold, the excess glucose is excreted in urine and is called a sglucosuria.

Water : about 80- 85 % of water is reabsorbed in proximal tubules passively. It occurs along with active sodium reabsorption. The passive water reabsorption is alled obligatory water reabsorpton. The fluid leaving the proximal tubule is isotonic. The descending limb of henles loop is permeable to water, so that fluid on reaching ascending limb of the loop gets hypertonic, while sodium is reabsorbed here. The system of keeping the fluid hypertonic in descending limbs to multiply the effect of sodium transport in ascending limp is called a system of counter current osmotic multipliers. This results in fluid becoming hypotonic by the time it reaches distal tubules, where water reabsorption occurs with the aid of anti-diuretic hormone.

Potassium is reabsorbed completely in proximal tubules and loops of henle. The potassium found in urine is from the secretory activity of the distal tubules.

Sodium and chloride absorption is interlinked and about 98-99 % of it is reabsorbed.

Urea: on an average, 20 -30 gms of urea is formed everyday in the body, and the amount of urea excretion is dependent on amount filtered and the amount reabsorbed.

Proteins :alost all of 50 gm of protein filtered daily is reabsorbed. During the process of reabsorption, it changes to aminoacids, some of which is excretd in urine.

Uric acid: Normally 90 % of uric acid is reabsorbed but is secreted by tubular cells as well.

Calcium may be excreted upto 200 mgm daily if the blood calcium levels are high. With blood calcium levels below 8 mgm % there is no calcium excretion in urine.

Bicarbonate reabsorption is almost complete so that the alkalinity of body fluids is preserved. CO2 formed in tubular cells is hydrated with the help of carbonic anhydrase to form h2CO3 which ionizes into H+ and HCO3-. The H+ ions diffuse into tubular fluid and combine with HCO3- of Glomerular filtrate to form H2CO3, which form H2O and CO2. Thus the H+ ions secreted by tubular cells formed in exchange of Na+ ions are important in HCO3- absorption. CO2 formed diffuses into tubular cells while Na+ ions enter the blood along with HCO3- formed in the tubular cells.

Secretion:

K+ and H+ are actively secreted by tubular cells.

Nervous system:Nervous system is important for regulation, control and co-ordination of body functions. I enable the individual:

1. To respond in a coordinated manner to environmental changes2. To coordinate and control various movements3. To prolong the life by protecting the body against harmful stimuli

Nervous system consists of innumerable neurons, which are nerve cells with dendrites and axons. There are about 10,000 million neurons in the nervous system.

The nervous system is divided in to

1. Cranial nervous system is the nerves that connect the brain to eyes, mouth, ears and other parts of the head.

2. Central nervous system is the integration and command system for the body. Contains brain and spinal cordTogether with PNS, it has role in the control of behavior.

3. Peripheral nervous system: consists mainly of nerves, which are long fibers that connects the CNS to the limbs and organs, essentially serving as a communication relay going back and forth between the brain and the extremities.

All the nerves and nerve cells outside your central nervous system make up your peripheral nervous system. Its task is to relay information from your brain and spinal cord to the rest of your body and from your body to your brain and spinal cord.

Your peripheral nervous system consists of 12 pairs of cranial nerves, which emerge from your brain and mainly serve your head and neck. It also contains 31 pairs of spinal nerves, which branch off from your spinal cord and supply the rest of your body.

Voluntary and involuntary actions

With the help of your peripheral nerves, you are able to carry out voluntary and involuntary actions.

If you pick up a mug, clap your hands or lift weights in the gym, you are performing voluntary actions. You are conscious of what you're doing. Your brain receives nerve impulses and analyses them before you decide what to do next.

In contrast, your heart beats and your intestines digest without your conscious control. Involuntary actions such as these are regulated by your autonomic nervous system. The autonomic part of your peripheral nervous system ensures that all your internal organs and glands function smoothly.

Your autonomic nervous system has two parts: the sympathetic and the parasympathetic. Both supply essentially the same organs but cause opposite effects. This is because their activating chemicals, or neurotransmitters, are different.

Fight or flight

Often referred to as your 'fight-or-flight' system, your sympathetic nervous system prepares your body for emergencies. It shunts your blood to your muscles and increases your blood pressure, heart rate and breathing rate, enabling you to cope with stressful situations.

Rest and digest

Your parasympathetic nervous system maintains and restores your energy. It directs blood to your digestive tract and makes sure you actively digest food. It also maintains your blood pressure, heart rate and breathing rate at a low level. That's why it is sometimes called your 'rest and digest' system.

Continuation of nervous system – please see nervous control and coordination.