Stomach

Stomach

The esophagusThe muscular tube that transports food from the pharynx to the stomach via peristalsis. pierces the diaphragm at the esophageal hiatus and leads to the stomach, which is a J-shaped organ. The stomach functions primarily as a storage compartment. The stomach is expandable and typically holds about half a liter when empty. However, it can expand to hold up to one and a half liters when full.

This is a picture of a stomach resection. The stomach is made smaller to regulate the feedback to the urge of hunger.  If the stomach is full, you are satiated.  If you have a smaller stomach, you feel fuller sooner.

The stomach is really made for mechanical and chemical digestion.  You don’t absorb a lot through the stomach’s lining.  Most nutrient absorption happens in the small intestine. However, substances like caffeine and aspirin can be absorbed in the stomach.

Stomach Vascularization and Innervation

The stomach is a parasympathetic organ involved in resting and digesting.  The parasympathetic neuronsThe functional cells of the nervous system that transmit signals. innervating the stomach branch from cranial nerve X, the vagus nerve.

The stomach receives its blood supply from the celiac trunk. This trunk comes directly off the descending aorta. The veinsBlood vessels that return deoxygenated blood to the heart (except pulmonary veins, which carry oxyge then dump the collected blood into the hepatic portal system.  Remember that a portal system is a direct connection of blood vessels. One example is the hypophyseal portal systemBlood vessel network connecting hypothalamus to anterior pituitary. between the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions. and the anteriorThe front of the body or toward the front when standing in the anatomical position. pituitary glandEndocrine gland at the brain’s base controlling many hormones..  This hepatic portal system collects blood from the digestive organs and transports it to the liverA large organ that produces bile, detoxifies blood, and stores nutrients.. There, it is cleansed before being released into systemic circulation.

Stomach Macroanatomy

The fundusThe dome-shaped upper part of the uterus., the upper part of the stomach, often presses against the diaphragm and can collect gases, leading to burping. The fundus is sensitive and contains chemoreceptors and baroreceptors that signal fullness to the brain. Hiatal hernias occur when a stomach part, usually the fundus, pushes through the esophageal hiatus. This movementA fundamental property of life involving motion of the body or its parts. places it into the thoracic cavityThe body cavity housing the heart and lungs.. This action makes the stomach smaller. It also causes discomfort. This can also lead to acidA substance that releases hydrogen ions (H⁺) in solution. reflux as stomach contents move upwards. While surgery is an option, exercises and other non-invasive methods are often recommended for management.

The pyloricRelating to the pylorus, the region of the stomach that connects to the small intestine. sphincter is a smooth muscle valve that controlsVariables that remain constant to ensure a fair test. the exit of slurry to the small intestine.  When you are hungry, your stomach growls partly because this valve opens. The lack of slurry entering into the small intestine also contributes to the growling. This valve very tightly controls delivery of the slurry into the duodenumThe first section of the small intestine where most digestion occurs; it receives chyme, bile, and p or top part of the small intestine that is pictured here

Stomach Serosa

The stomach performs both mechanical and chemical digestion through churning and the action of enzymesProteins that speed up chemical reactions in the body.. Its anatomyThe study of the structure of the human body. includes the lesser and greater curvatures, to which the lesser and greater omentumA fold of peritoneum that contains fat and blood vessels, providing protection and support to abdomi are attached. These omenta are mesenteric-like sheets of fat containing blood vessels, nerves, and lymph vessels, vascularizing and innervating the stomach.

These wrinkles on the inner lining of this stomach are called rugaeFolds in the stomach lining that allow expansion when food enters.. They are macroscopic features that can be seen with the naked eye.  These folds are called rugae, which disappear as the stomach expands.  These folds are what allow the stomach to be a great storage compartment, holding lots of food.

Stomach Muscularis Externa

The stomach’s muscularis externaA smooth muscle layer of the digestive tract responsible for peristalsis and segmentation. consists of three layers—longitudinal, circular, and oblique—that aid in churning and mechanical digestion. This differs from the two layers found in most other parts of the gastrointestinal tract.   This extra layer of obliquely arranged smooth muscle helps the stomach in pummeling your food.

Segmentation

The stomach with its third oblique layer of smooth muscle does a lot of segmentation. Although segmentation uses those 3 layers of smooth muscle segmentation does not propel food forward.  It pummels the food breaking it down with mechanical digestion. This segmentation leads to that mixing and churning. You can sometimes feel this in your stomach after you’ve eaten a big meal.

Check out this length of GI tract on the left. It has very dark brown boluses. There are also lighter colored beige boluses. Segmentation mixes and churns these boluses together until they are 1 homogeneous mixture. There is no breaking down of food by enzymes in this process, that would be chemical digestion. Although chemical digestion happens in the stomach, the stomach’s contribution to digestion is primarily through segmentation. As we will find out in the next slide, the stomach secretes many enzymes. It also secretes liquid substances, such as hydrochloric acid. These liquid forms mix with the bolusA rounded mass of food mixed with saliva that is ready for swallowing. of compacted food. This process leaves us with a food slurry called chymeA semi-fluid mixture of partially digested food and digestive juices that moves from the stomach int. This makes me think of the Simpson’s episode. In that episode, Lisa works with Burns to clean up the ocean. Burns ends up taking out all the mammals and makes them into a slurry called Little Lisa’s slurry.

Stomach Mucosa

The gastric mucosaThe innermost lining of the digestive tract that contains mucus-secreting cells for protection and a lines the stomach. It is sometimes called the “secretory sheet” because it secretes mucus, hormones, and digestive juices. The lining consists of simple columnar epithelium, which includes goblet (mucus) cellsThe basic structural and functional units of life.. These cells secrete mucus to protect the stomach lining. Gastric pits are located within the mucosa and extend into gastric glands, producing various secretions essential for digestion.  These glands change in cell composition as the stomach transitions from the fundic or cardiac area to the pyloric area. They also become more numerous toward the pyloric section A cut or slice of the body or an organ for study. of the stomach.

Cells of Gastric Glands

Pictured here are 2 of the different types of gastric glands that you can find in the stomach’s mucosa. Sometimes students get confused as to what’s a gastric pit and what’s a gastric gland. They’re kind of the same thing. The pit is the top portion with the opening. The gland is the baseA substance that accepts hydrogen ions (H⁺) or releases hydroxide ions (OH⁻). with many secretory cells.

Parietal cells create two things. They produce intrinsic factorA stomach-secreted protein necessary for vitamin B12 absorption, which helps you absorb B12 and is essential for preventing anemiaA condition characterized by a deficiency of red blood cells or hemoglobin, leading to reduced oxyge. They also create hydrochloric acid. These cells make your stomach contents very acidicA solution with a pH below 7, having a higher concentration of H⁺ ions.. When these cells are overactive, they can create too much hydrochloric acid, leading to issues like heartburn.

Chief cells secrete pepsinogen, an enzyme that breaks down proteinsLarge molecules made of amino acids with various functions in the body.. Protein breakdown begins in the saliva, and once food reaches the stomach, more breakdown occurs.

G cellsGastric cells that secrete gastrin, a hormone that stimulates acid production. secrete gastrinA hormone that stimulates gastric acid secretion and motility., a digestive hormone that signals other digestive processes to start. Gastrin is often called the “go hormone.” Interestingly, G cells are activated even before eating, triggered by stimuliChanges in the environment that are detected by sensory receptors. like the smell of food.

In contrast, D cells Cells in the stomach and pancreas that secrete somatostatin, which inhibits gastric acid secretio secrete somatostatinInhibits GH, insulin, and glucagon secretion., a “stop hormone” that signals digestive processes to halt when the stomach is empty.

 As you move from the fundus to the pyloric region, the glands become more numerous. They become deeper, extending into the submucosa.

Parietal Cells are Important

Parietal cells can become overactive for various reasons. These include excess gastrin, histamine release, possibly due to mild allergies, or overstimulation by the vagus nerve releasing acetylcholinealso know as ACh A neurotransmitter that stimulates muscle contraction.. Treatment options include antihistamines, and anticholinergic drugs.

The first line of defense for heartburn is usually a group of medications called proton pump inhibitors (PPIs). Remember that a proton is a hydrogen cation.  pHA measure of hydrogen ion concentration in a solution. is a measurement of hydrogen cations(H⁺): Protons that influence pH levels in body fluids. and more of them means that a solutionA homogeneous mixture of two or more substances. is acidic.  These drugs block hydrogen cations at the source, preventing these parietal cells Cells in the stomach that secrete hydrochloric acid (HCl) and intrinsic factor, essential for vita from secreting the hydrochloric acid. However, finding the right PPI can take time.