Digestive System Notes PDF

**Digestive System I** When we are hungry, we may grab something to eat. Imagine running over to Macca's to get a McChicken burger -- because you're starving! What happens to this food, so it can be of use to us? The burger doesn't travel through us like an egg goes through a snake. When you are c...

**Digestive System I** When we are hungry, we may grab something to eat. Imagine running over to Macca's to get a McChicken burger -- because you're starving! What happens to this food, so it can be of use to us? The burger doesn't travel through us like an egg goes through a snake. When you are cut or injured, you don't see bits of bread, lettuce, or chicken floating around in the blood -- do you? On this journey through the digestive system, we will look at how food gets into your blood, so it can be of use to the body. Let's start by reviewing the different nutrients in our burger: **Digestion** is the process of breaking down the food you eat to dissolve it basically, so it can become part of the body. Your body may add chemicals to the food in order to do this. Most of our nutrients as you may recall are complicated chemicals and need to be dissolved to be of use to us. Our food is made soluble by two methods: 1. [Mechanical/Physical digestion:] 2. [Chemical digestion:] [Where is food digested?] The main organs are the mouth, oesophagus, stomach and small intestine. To get rid of waste, the main organs are the large intestine, rectum and anus. The name we give to all these parts is the **alimentary canal** or the gastrointestinal (GI) tract. Basically, this is one long tube from your mouth to the anus. Different parts become specialised (e.g. the stomach acts as a temporary storage organ so has developed a pouch-like structure.  [The walls of the alimentary canal have the same four basic layers (tunics):] - **Mucosa**: moist epithelium that lines the surface and secretes mucus containing digestive enzymes and hormones, and also absorbs nutrients into the blood. This mucus also helps protect us from disease. Lying under the epithelium is some loose areolar connective tissue, that may have little lymph nodes (part of the MALT system) that defend us against pathogens. A layer of smooth muscle allows the epithelium to move, and also help increase the surface area in the small intestine, by creating lots of little folds. - **Submucosa**: richly vascularised areolar tissue, whose elastic fibres allow structures to maintain their shape after food passes through. - **Muscularis externa**: this is responsible for moving food through the alimentary canal. There are two layers of smooth muscle -- the inner layer is circular, and the outer layer is longitudinal. When the muscle contract, the cause 'wave-like' contractions to occur which squeeze the food along (a bit like getting a tennis ball into a sock). - **Serosa**: the outermost layer is made up of areolar tissue covered with a single layer of squamous cells (although this may be replaced by fibrous connective tissue in places like the oesophagus to help the organ remain in place by attaching to other structures. This fibrous tissue is called adventitia). Lets start our journey in the mouth by taking a bite out of the burger -- lets assume we have managed to get all the nutrients into that one bite. This is called **ingestion**. [In the mouth:]  So, inside the mouth the teeth, tongue and skeletal muscles break the food up into smaller pieces as you chew it around. This is called mastication (not to be confused with anything else!). your incisor teeth cut and the canine teeth tear up the food, whereas the molars and premolars grind the food up. Your tongue turns the food over, and as it does it mixes in the saliva, forming a ball called a bolus. All the food taken in during that bite is being mashed up -- the chicken, the bread and the salad. Saliva comes from three pairs of salivary glands which empty into your mouth. The salivary amylase starts to break down complex carbohydrates (polysaccharides) such as starch into sugar called maltose (oligosaccharide). The bread from our burger is being chemically changed by the salivary amylase and if we savour the burger, it may even taste sweet (if you pop a plain cracker on your tongue and let the saliva soak into it, and then swish it around inside your mouth, it ought to taste slightly sweet as the salivary amylase breaks the starch into smaller molecules -- try this at home!). As your tongue pushes the bolus towards the back of the mouth, the tongue 'kicks off' swallowing. This also goes by the fancier name of **deglutition**. The bolus must go down the oesophagus after passing the pharynx. As you swallow, the **soft palate** lifts back and closes the entrance to your nose. The **epiglottis** covers your trachea, so food won't go down into the lungs. [In the oesophagus:] The oesophagus is basically a muscular tube from the throat to the stomach, joining at the cardiac orifice and puncturing the diaphragm on its way down. Your food is propelled along, by the muscles in an involuntary process called peristalsis. Once you have swallowed your food, you have lost any voluntary control over it. The muscles contract behind and in front of the bolus to move it in a constrained way -- don't want it whizzing along too quickly! (Even if you stood on your head to eat this burger, it would still be propelled towards your stomach... although, peristalsis in reverse is what causes you to vomit!). When you are not eating our oesophagus is crumpled inwards. The mucosa contains non-keratinised squamous epithelium as the layers provide protection from abrasive foods!  [In the stomach:] After your food has gone through the oesophagus, it ends up in the stomach, which is like a bag that stretches to hold onto food. Chemical digestion also occurs in the stomach. Gastric juice comes out of gastric pits in the walls of the stomach. Gastric juice contains **pepsin** and **rennin**. The chief cells make **pepsinogen** and when this mixes with the **hydrochloric acid** (HCl) made by the **parietal cells**, the pepsinogen changes to pepsin. This enzyme starts to chemically work on the proteins we have ingested, such as the chicken in our burger (remember, up until this point, no chemical digestion has happened to our chicken). The HCl is very corrosive and strong and can even make holes in the lining of the stomach. The acid is needed to make pepsin work, as well as stopping the salivary amylase from working. The acid is also useful for killing bacteria (remember, in order for enzymes to work, the pH has to be suitable). *Find out how the gastric mucosa protects itself from the corrosive effects of HCl. What is a gastric ulcer? Find out why people with gastric ulcers should avoid alcohol and aspirin.* Another enzyme called **rennin** helps to thicken milk into a custard/yoghurt thickness so that the pepsin can then work on it. This is particularly important in infants and babies. After churning around and adding gastric juice, your food is a messy paste called **chyme**. A small ring of muscle lets the chyme out into the duodenum, a wee bit at a time. So, this means from our burger, that all the major nutrients of carbs, lipids and protein from the chicken are being pummelled up. [In the duodenum:]  **Digestive System II** [In the duodenum continued:] By the time the chyme enters the small intestine, the food (well, it was once food) has been prepared for the next part of the journey. The small intestine is about 6 metres long, but it is narrow. The small intestine runs from the **pyloric sphincter** to the **ileocecal valve**, and is divided into three parts -- the **duodenum**, **jejunum** and **ileum** where the nutrients such as fatty acids are absorbed (remember so far only our carbohydrates and proteins have been partially chemically digested). The chyme leaves the stomach through the pyloric sphincter and enters the duodenum. The duodenum is the\ C-shaped bit at the beginning of the small intestine. Here, a couple of juices are added: 1. **Bile** comes from the liver and **gall bladder**. 2. **Pancreatic** **juice** comes from the **pancreas**. [Bile] Bile is a greeny-yellow liquid made in the liver and enters the duodenum via the bile duct. Any extra bile is stored and concentrated in the gall bladder and is used when needed. The gall bladder is a thin walled, muscular sac on the front side of the liver. Bile helps to **emulsify** fats. This means that it breaks large blobs of fat into smaller drops, so that fat can be dissolved and used -- no fat digestion has taken place yet (Egg is an example of an emulsifier used in cooking as it stops the oil separating out of mayonnaise). The smaller droplets can then be worked on by **lipase** (a fat digesting enzyme), to help fat and cholesterol absorption. Bile contains bile salts, cholesterol, phospholipids, and electrolytes. This makes bile alkaline so that it helps to neutralise the HCl acid that was added in the stomach. The greeny-yellow colour comes from a pigment called **bilirubin**, which is a waste product of haeme (in RBCs). [Pancreatic juice] The pancreas is considered to not only be an accessory organ in digestion, but also an **endocrine gland** as it releases **insulin** and **glucagon** to maintain glucose levels in the blood. The pancreas sits a wee bit behind the stomach and liver and extends across from the duodenum to the spleen. **Acinar cells** in the pancreas secrete pancreatic juice which contains enzymes to break down all the major nutrients. Pancreatic juice flows through the pancreatic duct into the common bile duct opening at the **ampulla** in the duodenum. The **hepatopancreatic sphincter** controls the entry of both bile and pancreatic juice. Pancreatic juice is also alkaline as it contains bicarbonate to help neutralise stomach acid. Some of the enzymes that are released are inactive but become active in the duodenum. We have lipase, pancreatic amylase, and trypsin. - **Pancreatic amylase**: this continues to change or digest our carbohydrates such as the bread that made up our burger, into maltose as most of the starch still needs to be broken down. - **Lipase**: this enzyme works on fatty droplets. - **Trypsinogen**: continues our digestion of proteins such as the chicken into smaller polypeptides and is changed to **trypsin** in the duodenum by substances in the walls of the duodenum. *This is an example of protease -- all enzymes that break up proteins are proteases; likewise, carbohydrases break up carbohydrates.* The small intestine itself secretes its own enzymes to finish off chemical digestion of the major nutrients. We say the small intestine is enzyme poor as the enzymes that finish off this chemical digestion are made by the plasma membrane of the **microvilli** -- we call this the brush border. [Structure of the villi:]  Many finger-like projections increase the surface area of the small intestine and allow for absorption to be more efficient. The villi have their own villi, and these are called microvilli (a villi is about the same size as a comma -- ","). The **lacteals** are a part of the **lymphatic system** and join the lymphatic vessels. Fats, steroids, fat soluble vitamins go through the lymph system before returning to the normal circulatory system via the thoracic duct. The epidermis of the villi is only one cell thick for absorption. Most nutrients pass through the villi and into the capillaries where they join to the hepatic portal vein which takes them to the liver for processing, or reallocation, or storage. [The liver] The liver cells are called **hepatocytes** and each of these can help the liver perform the following functions: - Make bile - Store fat soluble vitamins such as D, E, K & A. - Detoxify the blood -- any substances such as drugs, coffee, alcohol need to be made less harmful and undergo a process to make them less poisonous. - Deamination -- the nitrogen from the digested amino acids needs to be removed, by converting it to urea, which the kidneys then excrete out as urine. - Stores excess glucose as glycogen. - Stores minerals like iron. - Helps to regulate body temperature -- as all the blood passes through the liver, which is the largest internal organ, and is rather warm as it is so busy doing all this other stuff, the blood can be warmed up. [The colon] By the time the chyme has moved to the end of the small intestine, all the good stuff has been absorbed (e.g. amino acids, glycerol, fatty acids, vitamins, minerals, and simple sugars). Mostly all that is left is the fibre and water, and dead cells that are scraped off your insides as the material is pushed along. This material needs to be eliminated from the body. It enters the **colon** or **large intestine** through the **ileocecal valve**, which stops the waste going back the way it came from. The main job of the large intestine is to reabsorb water into the surrounding capillaries, so that the waste faecal matter can be compacted and egested. The waste matter is stored in the **rectum** until it can be eliminated via the **anus** by opening the **anal sphincters**. [Follow up work to do:] - Valsalva manoeuvre in the rectum. - Epiploic appendages of the colon. - Plicae circulares of the small intestine. - The blood supply to the stomach and liver, small intestine and large intestine. - The blood vessels that drain the stomach, liver, small intestine and large intestine. - The nerves that control the digestive system. - The peritoneum and the peritoneal cavity including the omentum, mesentery, parietal and visceral peritoneum, and peritoneal fluid. [A helpful pneumonic...] +-------------+-------------+-------------+-------------+-------------+ | | **Enzyme/ch | **Function* | **Comes | **Released | | | emical** | * | from:** | into:** | +=============+=============+=============+=============+=============+ | *Shoving* | Salivary | Starts carb | Salivary | Mouth | | | amylase | digestion | glands | | +-------------+-------------+-------------+-------------+-------------+ | *Prams* | Pepsin | Starts | Stomach | Stomach | | | | protein | | | | *Round* | Rennin | digestion | | | | | | | | | | | | Thickens | | | | | | milk | | | +-------------+-------------+-------------+-------------+-------------+ | *Blinking* | Bile | Emulsifies | Gall | Duodenum | | | | fats | bladder | | | *Papatoetoe | Pancreatic | | | | | * | amylase | Continues | Pancreas | | | | | carb | | | | *Leads* | Lipase | digestion | | | | | | | | | | *To* | Trypsin | Finishes | | | | | | lipid | | | | | | digestion | | | | | | | | | | | | Continues | | | | | | protein | | | | | | digestion | | | +-------------+-------------+-------------+-------------+-------------+ | *Exhausted* | Erepsin | Finishes | Small | Small | | | | protein | intestine | intestine | | *Stressed* | Sucrase | digestion | | | | | | | | | | *Lovable* | Lactase | Finishes | | | | | | carb | | | | *Mothers* | Maltase | digestion | | | +-------------+-------------+-------------+-------------+-------------+ **Digestive System III: Movement through the digestive system** [Deglutition (swallowing)] Tip of the tongue reaches for the roof of the mouth and the muscles in the tongue contract forcing the bolus backwards into the **oropharynx**. This is a voluntary process. Once the bolus has gone into the pharynx it all basically becomes an involuntary reflex. Receptors send information to the swallowing centre in the brain (medulla/pons) and breathing is restricted and only one pathway to the digestive system remains open (soft palate closes of entry to the nasal cavity; the epiglottis covers the trachea; the sphincter muscle at the top of the oesophagus relaxes). This phase is called the **pharyngeal-oesophageal phase**. Peristalsis then drives food down the oesophagus into the stomach -- this journey can take between 1 and 8 seconds depending on the content of the food. Another sphincter at the end of the oesophagus relaxes to allow food to enter the stomach, and then closes to stop the food coming back up! [Gastric Emptying:] How much you eat can affect emptying - having lots of liquid food can speed up emptying. Carbohydrates remain in the stomach for maybe 3 hours, and protein about four. Simpler fruits like apples may last about 20 minutes, yet a more complex fruit like a banana maybe 40 minutes. As we know the pyloric sphincter relaxes and let's a little chyme out a bit at a time into the duodenum. So, if there is any "food" in the duodenum it may limit the amount of food moving from the stomach. Fatty foods of course need to be emulsified by the bile from the gall bladder -- this can take a bit of time and cause a bit of a back log as food moves out of the stomach even slower. [Movement through the small intestine:] Besides waves of peristaltic contractions, we also have a process called **segmentation** occurring in the small intestine. Marieb describes this as like giving the food a "massage". The chyme is sort of moved back and forward, back and forward to ensure the chyme is thoroughly mixed up with the intestinal juice, pancreatic juice and the bile. This segmentation is controlled by **pacemaker cells** (think about the heart's pacemaker) and moves food bit by bit towards the ileocecal valve just in time to make sure that digestion and absorption has been completed. **Parasympathetic nerve activity** aids this process -- in other words when you are relaxed you digest your food better. [Movement through the large intestine (Colon)] Pretty much the large intestine is inactive. Once the ileocecal valve opens and allows chyme into the colon, the valve closes to prevent the chyme moving back in. Once the undigested food reaches the colon the colon starts to move -- this is a very lethargic or slothful movement -- like it's a bit lazy. Segmentation is occurring and is referred to as **haustral contractions** -- what were the haustra? These contractions occur maybe every half an hour and only last for about a minute. Long, lazy but forceful contractions move large areas of the colon about three or four times a day (peristalsis) to get the undigested food towards the rectum. When food enters the stomach it starts what we call the **gastrocolic reflex** (and the gastroileal reflex in the small intestine) to move the substances in them towards the rectum -- its like what is there has to be shunted along to fit what's coming in next! Fibre in the diet helps bulk up food and make the undigested food move better. [Defaecation] Peristalsis causes faecal matter to enter the rectum. As the rectum fills, the walls stretch and start the **defaecation reflex**. Parasympathetic nerves coming from the spinal cord make the walls of the **sigmoid colon** and rectum contract, and those internal anal sphincter muscles to relax (involuntary control). As the faeces pass into the anal canal, messages are sent to the brain allowing us to decide whether or not now is a good time to let those eternal anal sphincters to relax! If we decide now is not a good time- the rectal walls relax until the next peristaltic wave hits and we have to decide again. We can aid defaecation by engaging the diaphragm and abdominal muscles to increase abdominal pressure and force the stuff out. (what happens in your throat at the same time? The glottis closes!!) We also use our **levator ani** muscle which lifts the anal canal, so it is above the faeces and outside of the body! [Now, let's go back and look at how our glandular secretions are controlled ] - **Salivary glands**:\ Salivary glands secrete enough saliva all the time just to keep your mouth moist. When food is ingested receptors in the mouth detect this and send messages to the brain (pons and medulla) which sends a message back along the parasympathetic nerves to the glands to secrete enzyme rich salvia (when you produce saliva in the lab for your experiments you had to dribble remember? Hoicking or spitting doesn't produce enzyme rich saliva). When you are scared or frightened you may experience a dry mouth as the sympathetic nerve pathways restrict blood flow to digestive organs including those going to your salivary glands! - **Gastric secretions**:\ The smell, sight or even taste of food can cause gastric juice to be released into the stomach. This is called the Cephalic (head) phase (for example your nose detects food smells and sends messages to the hypothalamus -- this stimulates the medulla oblongata which sends messages along the parasympathetic nerves to the glands in the stomach). During the Gastric phase, gastric juice is released once food has reached the stomach. The hormone responsible for this is GASTRIN. Semi digested food dilutes the acid levels in the stomach (causing pH to rise) activates the G-Cells to release Gastrin, which in turn causes enzymes to be released. Once the Chyme starts to enter the duodenum the mucosal cells in the intestine release a hormone called intestine gastrin, which encourages the gastric glands to keep secreting their stuff. - **What about the bile and pancreatic juice**?\ When cells of the duodenum are exposed to fatty substances, they release a hormone called Secretin, which in turn causes the liver to release bile. When we are not eating or digesting, the sphincter that controls the exit of material from the cystic duct (where bile and pancreatic juice enter) is closed off and bile that is made backs up and gets stored in the gall bladder as overflow. To get the bile out of the gall bladder and being used, the walls of the gall bladder need to contract. This comes about when the duodenum detects fat in the chyme and releases CCK (cholecystokinin). This causes the pancreas to release pancreatic juice and relaxes the hepatopancreatic sphincter so that both bile and pancreatic juice can enter the duodenum. Again, it is the parasympathetic nerves that cause the gall bladder walls to contract. *[References:]* *Marieb Textbook, Chapter 23.*