Is it the sphincter that flexes when a human pushes a bowel movement?

Is it the sphincter that flexes when a human pushes a bowel movement?

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Of course the sphincter muscle is at the exit point. To use a toothpaste tube analogy, if I want to squeeze out some toothpaste, it does me little to no good to jostle the nozzle; I need to squeeze the tube (which is analogous to the colon) to get the paste (payload) to come out.

So when a human is sitting on the toilet squeezing, is that squeezing flexing of the sphincter, or squeezing muscles along the sides of the colon pushing on it as one's fingers push on the tube to get toothpaste to come out.

If it's the sphincter that's getting flexed, how is that helping get a big log out?

The pressure that you apply when you push during a bowel movement derives from an increase in the pressure of the abdomino-pelvic cavity. You generate this pressure by closing the glottis (the opening to the lungs) and contracting the anteriolateral abdominal muscles (i.e., the external oblique, internal oblique, and transversus abdominus). This reduces the volume of the (now sealed) abdominopelvic cavity and increases pressure helping to drive feces from the rectum. This is mechanism is also used to expel urine and during childbirth.

If you do this with the glottis open, then you forcibly exhale because the increased pressure of the abdominopelvic cavity is transferred across the diaphragm and into the thoracic cavity compressing the lungs.

Journey Through the Digestive Tract

Your favorite foods take a wild ride after you take a bite. Picture a go-to snack and consider its journey through your digestive system. Take a ripe, juicy apple, for example. The fruit is full of nutrients for your body to use.

But how does that fruit become usable in your body? After all, your blood isn’t pumping microscopic apples through your arteries and veins. Your body is utilizing the chemical compounds that make apples crunchy and sweet.

Those compounds are extracted from your food through the digestive process. It’s the method by which your diet’s fats, sugars, proteins, fiber, and essential vitamins and minerals—as well as other important nutrients—find their way out of the food you eat to power your body. Digestion also removes waste. And this process is constantly going on in your body.

From dinner plate to elimination, the food you eat takes a long trip through your digestive system. Take a look at the path your food will follow as it is digested:

Mouth >> Esophagus >> Stomach >> Small Intestine >> Large Intestine

At each step along the digestive journey, food is modified and broken down into usable pieces. By modeling this system step-by-step, you can gain a better understanding of the fate of your food after it enters your body.

What is a defecation reflex?

The defecation reflex is an involuntary response of the lower bowels to various stimuli thereby promoting or even inhibiting a bowel movement. These reflexes are under the control of the autonomic system and play an integral role in the defecation process along with the somatic system that is responsible for voluntary control of defecation. The two main defecation reflexes are known as the intrinsic myenteric defecation reflex and parasympathetic defecation reflex.

Intrinsic Myenteric Defecation Reflex

The entry of feces into the rectum causes the distention of the rectal wall. This stretching triggers signals to the descending and sigmoid colon via the myenteric plexus to increase peristalsis. The myenteric plexus is part of the enteric nervous system which is the gut’s own internal neural network as discussed under stomach nerves.

The peristaltic waves extend all the way to the rectum an anus. In this manner, fecal matter is moved closer to the anus. When the wave reaches the anus, it causes the internal anal sphincter, which is always constricted, to relax. This is achieved by inhibitory signals via the myenteric plexus to reduce sphincter constriction.

Defecation may occur at this point if the external anal sphincter also relaxes. However, without the parasympathetic defecation reflex, defecation solely dependent on the intrinsic reflex would be weak.

Parasympathetic Defecation Reflex

The parasympathetic defecation reflex works in essentially the same way as the intrinsic myenteric defecation reflex but involves parasympathetic nerve fibers in the pelvic nerves. It triggers peristaltic waves in the descending and sigmoid colon as well as the rectum. It also causes relaxation of the external anal sphincter. The difference is that the parasympathetic defecation reflex enhances this process and makes the intrinsic reflex much more powerful. If sufficiently stimulated, it may even cause the sigmoid colon to completely empty all of its contents in the rectum rapidly.

The force triggered by the parasympathetic defecation reflex can be powerful enough to result in defecation, despite conscious efforts to keep the external anal sphincter constricted.

Other Defecation Reflexes

Apart from the two main defecation reflexes mentioned above, other reflexes can also influence the defecation process.

  • Gastrocolic reflex – distention of the stomach while eating or immediately after a meal triggers mass movements in the colon.
  • Gastroileal reflex – distention of the stomach while eating or immediately after eating triggers the relaxation of the ileocecal sphincter and speeds up peristalsis in the ileum (end portion of the small intestine). This causes the contents of the ileum to rapidly empty into the colon.
  • Enterogastric reflex – distention and/or acidic chyme in the duodenum slows stomach emptying and reduces peristalsis.
  • Duodenocolic reflex – distention of the duodenum a short while after eating triggers mass movements in the colon.

Irritation within the stomach or duodenum can stimulate or even inhibit the defecation reflexes. In addition to these gastrointestinal reflexes, there are other reflexes involving the peritoneum, kidney and bladder that can affect the defecation process. This includes the :

  • Peritoneointestinal reflex involving the peritoneum and intestines.
  • Renointestinal reflex involving the kidney and intestines.
  • Vesicointestinal reflex involving the bladder and intestines.

When these organs are irritated and the reflexes are triggered, it inhibits intestinal activity.

How the Digestive System Works

Let's say you've picked up a ham and cheese sandwich for lunch. Before you even take a bite, your nose smells it and signals the brain, which sends word to the nerves controlling your mouth's salivary (spit) glands. Once the glands have their cue, they get busy secreting juices, making your mouth water. When you bite into the sandwich, the salivary glands get even more excited and secrete more saliva, making the food moister and easier to swallow.

Before the sandwich even leaves your mouth, an enzyme in your saliva called amylase begins to break down the carbohydrates in the bread. When you swallow, the sandwich pieces slide down your pharynx, also known as the throat. They then come to a fork in the road: One pathway is the esophagus, which leads to the stomach, and the other follows the trachea, which leads to the lungs. Of course, the correct path is through the esophagus, but sometimes food can take a careless detour. So when we say something went down the "wrong pipe," it means it went through the trachea, usually because you were breathing or laughing when you swallowed. Not to worry, this rarely happens -- the act of swallowing closes the epiglottis, a flexible flap over the trachea. So the sandwich pieces will normally slide into the esophagus through the upper esophageal sphincter, a ring-shaped muscle that opens only when food is swallowed.

Once the sandwich is in the esophagus, involuntary muscle contractions -- or peristalses -- push it toward the stomach. At the end of the esophagus, the lower esophageal sphincter lets the food into the stomach. It opens and then quickly closes to keep the food from escaping back into the esophagus. Ever had heartburn? This occurs when this sphincter isn't working properly and stomach acid manages to splash into the esophagus. If this happens chronically, you might have Gastroesophageal Reflux Disease, or GERD.

About Artificial Urinary Sphincters

An AUS is a device that works like your natural urinary sphincter (see Figure 2). An AUS is made up of 3 parts:

Figure 2. Artificial urinary sphincter (AUS)

  • Urethral cuff: This wraps around your urethra to control the flow of urine. When the cuff is closed (filled with fluid), no urine can pass through. When it’s open (not filled with fluid), urine can pass through.
  • Pump: This moves fluid into or away from the urethral cuff. It’s placed in your scrotum.
  • Balloon: This holds the same fluid as the cuff. This is where the fluid is moved to when the urethral cuff is open or deflated. It’s placed under your abdominal (belly) muscles.

To urinate with an AUS, you must squeeze the pump in your scrotum. The pump moves fluid from the urethral cuff into the balloon, allowing the cuff to open or deflate and your urethra to open. Your cuff stays open for about 3 minutes so that you can urinate. The cuff will close automatically.

An AUS is placed during a surgery.

Risks associated with having an AUS

Although not common, the following problems can occur:

  • Pain
  • Bleeding
  • Infection, which can require that the AUS be taken out
  • Problems with any part of the AUS, which can require surgery to take it out or replace it
  • Injury to your urethra
  • Ongoing leakage of urine
  • Long-term or permanent problems urinating
    • You may have to insert a catheter into your bladder to drain your urine.
    • You may also need to have another surgery to fix the problem.


    Peristalsis is generally directed caudad, that is, towards the anus. This sense of direction might be attributable to the polarisation of the myenteric plexus. Because of the reliance of the peristaltic reflex on the myenteric plexus it is also referred to as the myenteric reflex. [4]

    Mechanism of the peristaltic reflex Edit

    The food bolus causes a stretch of the gut smooth muscle that causes serotonin to be secreted to sensory neurons, which then get activated. These sensory neurons in turn activate neurons of the myenteric plexus, which then proceed to split into two cholinergic pathways: a retrograde and an anterograde. Activated neurons of the retrograde pathway release substance P and acetylcholine to contract the smooth muscle behind the bolus. The activated neurons of the anterograde pathway instead release nitric oxide and vasoactive intestinal polypeptide to relax the smooth muscle caudad to the bolus. This allows the food bolus to effectively be pushed forward along the digestive tract. [5]

    Esophagus Edit

    After food is chewed into a bolus, it is swallowed and moved through the esophagus. Smooth muscles contract behind the bolus to prevent it from being squeezed back into the mouth. Then rhythmic, unidirectional waves of contractions work to rapidly force the food into the stomach. The migrating motor complex (MMC) helps trigger peristaltic waves. This process works in one direction only and its sole esophageal function is to move food from the mouth into the stomach (the MMC also functions to clear out remaining food in the stomach to the small bowel, and remaining particles in the small bowel into the colon). [6]

    In the oesophagus, two types of peristalsis occur:

    • First, there is a primary peristaltic wave, which occurs when the bolus enters the esophagus during swallowing. The primary peristaltic wave forces the bolus down the esophagus and into the stomach in a wave lasting about 8–9 seconds. The wave travels down to the stomach even if the bolus of food descends at a greater rate than the wave itself, and continues even if for some reason the bolus gets stuck further up the esophagus.
    • In the event that the bolus gets stuck or moves slower than the primary peristaltic wave (as can happen when it is poorly lubricated), stretch receptors in the esophageal lining are stimulated and a local reflex response causes a secondary peristaltic wave around the bolus, forcing it further down the esophagus, and these secondary waves continue indefinitely until the bolus enters the stomach. The process of peristalsis is controlled by the medulla oblongata. Esophageal peristalsis is typically assessed by performing an esophageal motility study.
    • A third type of peristalsis, tertiary peristalsis, is dysfunctional and involves irregular, diffuse, simultaneous contractions. These contractions are suspect in esophageal dysmotility and present on a barium swallow as a "corkscrew esophagus". [7]

    During vomiting, the propulsion of food up the esophagus and out the mouth comes from contraction of the abdominal muscles peristalsis does not reverse in the esophagus.

    Stomach Edit

    When a peristaltic wave reaches at the end of the oesophagus, the cardiac sphincter (gastroesophageal sphincter) opens allowing the passage of bolus into the stomach. Gastroesophageal sphincter normally remains closed and does not allow food contents of the stomach to move back. The churning movements of stomach's thick muscular wall mixes the food thoroughly with the acidic gastric juice and is called the chyme. The muscularis layer of the stomach is thickest and maximum peristalsis occurs here. After short intervals, the pyloric sphincter keeps on opening and closing so the chyme is fed into the intestine in installments.

    Small intestine Edit

    Once processed and digested by the stomach, the semifluid chyme is passed through the pyloric sphincter into the small intestine. Once past the stomach, a typical peristaltic wave lasts only a few seconds, travelling at only a few centimeters per second. Its primary purpose is to mix the chyme in the intestine rather than to move it forward in the intestine. Through this process of mixing and continued digestion and absorption of nutrients, the chyme gradually works its way through the small intestine to the large intestine. [6]

    In contrast to peristalsis, segmentation contractions result in that churning and mixing without pushing materials further down the digestive tract.

    Large intestine Edit

    Although the large intestine has peristalsis of the type that the small intestine uses, it is not the primary propulsion. Instead, general contractions called mass action contractions occur one to three times per day in the large intestine, propelling the chyme (now feces) toward the rectum. Mass movements often tend to be triggered by meals, as the presence of chyme in the stomach and duodenum prompts them (gastrocolic reflex). Minimum peristalsis is found in the rectum part of large intestine as a result of thinnest muscularis layer.

    Lymph Edit

    The human lymphatic system has no central pump. Instead, lymph circulates through peristalsis in the lymph capillaries, as well as valves in the capillaries, compression during contraction of adjacent skeletal muscle, and arterial pulsation.

    Sperm Edit

    During ejaculation, the smooth muscle in the walls of the vas deferens contracts reflexively in peristalsis, propelling sperm from the testicles to the urethra. [8]

    The earthworm is a limbless annelid worm with a hydrostatic skeleton that moves by peristalsis. Its hydrostatic skeleton consists of a fluid-filled body cavity surrounded by an extensible body wall. The worm moves by radially constricting the anterior portion of its body, resulting in an increase in length via hydrostatic pressure. This constricted region propagates posteriorly along the worm's body. As a result, each segment is extended forward, then relaxes and re-contacts the substrate, with hair-like setae preventing backward slipping. [9] Various other invertebrates, such as caterpillars and millipedes, also move by peristalsis.

    A peristaltic pump is a positive-displacement pump in which a motor pinches advancing portions of a flexible tube to propel a fluid within the tube. The pump isolates the fluid from the machinery, which is important if the fluid is abrasive or must remain sterile.

    Robots have been designed that use peristalsis to achieve locomotion, as the earthworm uses it. [10] [11]

    • Ureter (x2)
    • Peritoneum
    • Detrusor muscle
    • Sub-mucosa
    • Mucosa
    • Rugae
    • Ureter orifices
    • Trigone
    • Neck of bladder
    • Urethral sphincter
    • Urogenital diaphragm
    • Internal urethral sphincter
    • Urethra
    • Prostatic Urethra
    • Membranous Urethra
    • Spongy Urethra
    • External Urethral sphincter

    Labeled components that are not part of the urinary system:

    • Cowper's Gland
      (Bulbourethral Gland)
    • Bulb
    • Crus
    • Penis
    • Corona
    • Glans Penis

    These are described in the section about the reproductive system.

    Quick Re-cap from previous pages:

    • The urinary bladder stores urine prior to its elimination from the body (functions of the urinary system).
      At micturation / urination, the bladder expels urine into the urethra, leading to the exterior of the body. The bladder is a musculomembranous sac located on the floor of the pelvic cavity, anterior to the uterus and upper vagina (in females).
    • Outer surfaces of the Bladder: The upper and side surfaces of the bladder are covered by peritoneum (also called "serosa"). This serous membrane of the abdominal cavity consists of mesthelium and elastic fibrous connective tissue. "Visceral peritoneum" covers the bladder and other abdominal organs, while "parietal peritoneum" lines the abdomen walls.
    • Ureters: The ureters deliver urine to the bladder from the kidneys (one ureter from each kidney - see components of human urinary system). The ureters pass through the posterior surface of the bladder at the ureter orifices (shown above). Urine drains through the ureters directly into the bladder there are no sphincter muscles or valves at the ureter orifices.
    • Structure of Bladder (Detail): The bladder itself consists of 4 layers:-
      1. Serous - this outer layer being a partial layer derived from the peritoneum
      2. Muscular - the detrusor muscle of the urinary bladder wall, which consists of 3 layers incl. both longitudinal and circularly arranged muscle fibres
      3. Sub-mucous - a thin layer of areolar tissue loosely connecting the muscular layer with the mucous layer
      4. Mucous - the innermost layer of the wall of the urinary bladder loosely attached to the (strong and substantial) muscular layer. The mucosa falls into many folds known as rugae when the bladder is empty or near empty.
    • The features observable on the inside of the bladder are the ureter orifices, the trigone, and the internal orifice of the urethra.
    • The trigone is a smooth triangular region between the openings of the two ureters and the urethra and never presents any rugae even when the bladder is empty because this area is more tightly bound to its outer layer of bladder tissue.
    • Exit from Bladder: When urine is released from the bladder is flows out via the neck of the bladder (in the trigone region).
      The internal urethral sphincter is a sphincter (circular) muscle located at the neck of the bladder and formed from a thickening of the detrusor muscle. It closes the urethra when the bladder has emptied.
    • More detail about the above is included on the page about the bladder.

    The male urethra

    At about 8-9 inches (200 mm) long, the adult male urethra is longer than the adult female urethra (of approx. 1.5 inches 35 mm). It has three portions (the prostatic urethra, the membranous urethra, and the spongy urethra - see below) and extends from the neck of the bladder (shown in the diagram above) to the meatus urinarius at the end of the male penis / urethra.

    The general shape of the urethra (the tube itself) varies along it's length: The prostatic section of the male urethra is somewhat arched. Except during passage of urine or semen through the urethra, it is a transverse slit along it's length, the upper and lower surfaces of the transverse slit being in contact when fluid is not passing along the urethra. However, at exit from the body at the meatus urinarius (external orifice of urethra) the slit is vertical.

    The three regions / sections of the male urethra are:

    1. Prostatic Urethra
      The prostatic urethra begins at the neck of the bladder and includes all of the section that passes through the prostrate gland. It is the widest and most dilatable part of the male urethral canal.
    2. Membranous Urethra
      The membranous urethra is the shortest and narrowest part of the male urethra. This section measures approx. 0.5 - 0.75 inches (12 - 19 mm) in length and is the section of the urethra that passes through the male urogenital diaphragm.
      The external urethral sphincter (muscle) is located in the urogenital diaphragm (as for the female urethra). This muscle is referred to as the "compressor urethrae muscle" in some older textbooks.
      The passage of urine along the urethra through the urogenital diaphragm is controlled by the external urethral sphincter, which is a circular muscle under voluntary control (that is, it is innervated by the somatic nervous system, SNS).
      See the page about micturation for more about control of these structures by the nervous system.
    3. Spongy Urethra
      The spongy urethra is the longest of the three sections. It is approx. 6 inches (150 mm) in length and is contained in the corpus spongiosum that extends from the end of the membranous portion, passes through the penis, and terminates at the external orifice of the urethra, the location at which the urine leaves the body.

    Structure of the Male Urethra

    The structure of the urethra (tube) itself is a continuous mucous membrane supported by submucous tissue connecting it to the other structures through which it passes.

    • The mucous coat is continuous with the mucous membrane of the bladder, ureters and kidney. In the membranous and spongy sections (2. and 3. above), the mucous membrane is arranged in longitudinal folds when the tube is empty.
    • The submucous tissue consists of a vascular (i.e. containing many blood vessels) erectile layer surrounded by a layer of smooth (involuntary) muscle fibres. These muscle fibres are arranged in a circular configuration that separates the mucous membrane and submucous tissue from the surrounding structure, the tissue of the corpus spongiosum (labeled 'penis' in the diagram above).

    Unlike the female urethra, the male urethra has a reproductive function in addition to it's urinary function. It conveys semen out of the body at ejaculation. For further information about this function red the section about the male reproductive system.

    For further details about the structures described here see Gray's Anatomy.

    Gross Anatomy

    The intestines are located inferior to the stomach in the abdominal body cavity. They are connected to the posterior wall of the abdomen by the mesentery, a thin vascular membrane. Blood vessels of the mesentery carry oxygenated blood to support the tissues of the intestines and carry nutrient-rich venous blood away from the intestines to feed the tissues of the body.

    The small intestine is about 1 inch in diameter and about 10 feet long in a living body. It extends from the stomach to the large intestine and consists of 3 major regions: the duodenum, jejunum, and ileum.

    • The duodenum receives partially digested food from the stomach, bile from the liver and gallbladder, and pancreatic juice from the pancreas. These substances mix in the duodenum to further digest food into its most basic units. The duodenum begins to absorb nutrients from the food passing through its lumen.
    • Food next passes into the jejunum, a longer region of intestine where the bulk of the absorption of nutrients takes place.
    • Finally, food passes into the ileum, the longest region of the small intestine. Any last nutrients that were not absorbed in the jejunum are absorbed in the ileum before the food passes into the large intestine.

    The large intestine is about 2.5 inches in diameter and about 5 feet long in a living body. It receives fecal matter from the small intestine through the ileocecal sphincter. The smooth walls of the large intestine absorb water from fecal matter. These intestinal walls also absorb vitamins released from the fermentation of feces by bacteria living in the large intestine.

    Our large intestine consists of 4 major regions: the cecum, colon, rectum, and anal canal.

    • The cecum is a pouch-like dead-end passage that branches inferiorly from the end of the ileum. Fecal matter entering the large intestine from the ileum passes into the cecum before being pushed superiorly into the ascending colon. The appendix is attached to the inferior end of the cecum and is believed to store beneficial bacteria that help break down undigested food.
    • Fecal matter passes from the cecum into the colon, the largest region of the large intestine. The ascending colon carries feces superiorly from the cecum to the transverse colon. The transverse colon then carries feces transversely from the right side of the abdomen to the left side, where it enters the descending colon. Next, the descending colon carries the feces inferiorly to the S-shaped sigmoid colon and rectum.
    • The rectum stores feces until they are ready to be defecated (eliminated from the body).
    • During defecation, the anal sphincter muscles of the anal canal relax to allow feces to exit the body.

    Thursday, 17 July 2014

    Homework - Why are Birds and Reptiles with abundant yolk sac polyspermic?

    I was given an explanation that birds and reptiles are polyspermic because they have an abundant yolk sac. But how does it explain the thing?

    Chicken as an adult is not using in my opinion yolk as an energy source.
    Yolk is used during embryogenesis as the primary energy source with blastula and gastrula -stages and during organogenesis, since the embryo needs proteins and energy somewhere.

    How does abundant yolk sac make birds and reptiles polyspermic?


    The rectum (Latin for "straight") is an 8-inch chamber that connects the colon to the anus. It is the rectum's job to receive stool from the colon, to let you know there is stool to be evacuated, and to hold the stool until evacuation happens. When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters (muscles) relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates, so that the sensation temporarily goes away.

    The anus is the last part of the digestive tract. It consists of the pelvic floor muscles and the two anal sphincters (internal and external muscles). The lining of the upper anus is specialized to detect rectal contents. It lets us know whether the contents are liquid, gas, or solid. The pelvic floor muscle creates an angle between the rectum and the anus that stops stool from coming out when it is not supposed to. The anal sphincters provide fine control of stool. The internal sphincter keeps us from going to the bathroom when we are asleep, or otherwise unaware of the presence of stool. When we get an urge to go to the bathroom, we rely on our external sphincter to keep the stool in until we can get to the toilet.

    Watch the video: I Quit Drinking Alcohol For 30 Days.. Heres What Happened (October 2022).