Absorption and Excretion - Anatomy and Physiology

INTRODUCTION TO THE DIGESTIVE AND EXCRETORY SYSTEM

The digestive and excretory system are essential components of the human body.

Digestive System: a body system made up of the GI tract and other organs to break down food and absorb the nutrients from it

Excretory System: a body system that removes unneeded materials and wastes from the bloodstream and other fluids of the body

They are responsible for the vital processes of breaking down food for nourishment and eliminating waste products. These interconnected systems are very important to maintain homeostasis and overall health and well-being.

The main functions of the digestive system are…

Metabolism

Breaks down food in nutrients absorbed by the bloodstream. These nutrients serve as the energy source for cellular metabolism. Metabolism, in turn, generates waste products, which the excretory system must eliminate to prevent harmful accumulation.

Nutrient Absorption

Nutrients absorbed by the digestive tract are critical for cellular functions and energy production. The excretory system, particularly the kidneys, ensures that these nutrients are conserved during filtration and that only waste products, excess minerals, and toxins are excreted.

Water Balance

The digestive system absorbs water from food and drinks. Meanwhile, the kidneys of the excretory system regulate the volume and composition of bodily fluids by reabsorbing water and electrolytes (minerals in your blood or other bodily fluids that can carry an electrical charge) while excreting excesses. This balance ensures that cells and tissues function optimally and prevents dehydration or salt imbalances.

Toxin Removal

Part of the digestive system neutralizes and excretes toxins from the liver into the intestines, where they eliminated in feces. The excretory system filters toxins and metabolic byproducts from the bloodstream and excretes them as urine. Waste is also removed from the body through the lungs when we respire and through the skin when we sweat.

DIGESTIVE SYSTEM ORGANS

The primary organs of the digestive system are the gastrointestinal (GI) tract, liver, pancreas, and gallbladder. The GI tract is a long, twisting tube that includes the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus.

Mouth: Digestion begins with the mechanical breakdown of food through chewing and the chemical breakdown of food through an enzyme in saliva called amylase.

Esophagus: A muscular tube transporting food from the mouth to the stomach.

Liver: Produces bile that breaks down fats to acid in their digestion and absorption. It also processes nutrients and detoxifies harmful substances.

Stomach: Is responsible for further mechanical and chemical digestion of food, primarily proteins, using stomach acid and enzymes like pepsin.

Gallbladder: stores and releases bile produced by the liver when needed for fat digestion

Pancreas: stores digestive enzymes, such as amylase, lipase, and proteases that are used in the small intestine to break down carbohydrates, fats, and proteins

Large Intestine: absorbs water and electrolytes and forms feces from undigested material, it also houses beneficial gut bacteria that aid in the fermentation if certain indigestible carbohydrates

Small Intestine: composed of the duodenum, jejunum, and ileum, its the primary site for nutrient absorption and digestion, with the help of various enzymes from the pancreas and bile from the liver and gallbladder

Appendix: small finger-like pouch attached to the cecum of the large intestine that releases chemicals to keep the digestive system in balance

Anus: where digestion ends, and feces leaves the body

EXCRETORY SYSTEM ORGANS

The urinary system is crucial to maintaining the body’s internal balance and removing waste products and combined the main organs remove metabolic waste products, excess water, and electrolytes.

Fluids are filtered and regulated in the kidneys with the help of a hormone called the antidiuretic hormone. The main function of ADH is to regulate the amount of water excreted by the kidneys. High levels of ADH result in less urine production, while low levels of ADH result in greater urine production. Health conditions such as stroke, infection, trauma, and mental illness can cause abnormalities in ADH and excretory homeostasis.

Kidney: acts as filters, and remove excess water and toxic substances from the bloodstream, including urea, creatine and excess salts. They also help regulate the body’s fluid and electrolyte balance, blood pressure, and the concentration of various ions

Ureter: two narrow tubes that connect kidneys to the bladder. The ureters transport urine, a concentrated solution of waste and excess substances from the kidneys to the bladder for temporary stage

Bladder: a muscular sac that stores urine until it’s convenient to expel it from the body. The bladder can expand as it fills with urine and contract when its time to eliminate urine as waste

Urethra: the tube that carries urine from the bladder to the outside of the body

Formation of Urine

Filtration: Blood enters the millions of nephrons in the kidney. Each nephron has a glomerulus (a cluster of small blood vessels in nephrons where wastes are filtered from blood). The high pressure in the tiny vessels of this structure forces water and urea (a compound excreted in urine) out of the blood.

Reabsorption: Filtrate enters the renal tubules where essential substances like glucose and water are reabsorbed into the bloodstream

Secretion: hydrogen ions and certain substances such as medication are transported from the blood into the renal tubules to be secreted in urine. Hydrogen ions also help regulate pH, which promotes homeostasis

Concentration: The remaining filtrate in the renal tubules becomes more concentrated as water continues to be reabsorbed

Collection: The concentrated urine moves through the ureters to the bladder, where it is stored until it leaves the body

Other parts of the body ALSO help remove waste products. These organs eliminate waste and maintain balance.

Skin: the bodies largest organ and is involved in waste removal through sweat. Sweat contains water, salt, and small amounts of metabolic waste products, helping to cool the body and excrete some waste substances, such as urea

Respiratory System: plays a role in waste elimination by expelling CO2 as a byproduct of cellular respiration. When you exhale, you release CO2 which is a waste product of metabolism

Blood: helps remove waste from the body by transporting it to filtering organs like the kidneys and liver. These organs process and eliminate waste products from the bloodstream through urine and bile

Feces: The digestive system excretes undigested solids in the form of feces or stool. This digestive process filters and absorbs nutrients the body needs, and excretes the rest. This process occurs mainly in the large intestines.

Cells Removal of Waste

The proteasome, cellular garbage disposal breaks down proteins into reusable components. Lysosomes act like cellular stomachs in a cell to digest old organelles and proteins through autophagy, preventing lysosomal storage diseases. They also combat infections and may offer therapeutic potential. Additionally cells can aggregate waste, spit it out through exocytosis, or pump out toxins

autophagy: the digesting of damaged cell organelles or tissues to components that can be repurposed or reused by the body in other ways

exocytosis: a process that occurs when a cell vacuole releases wastes to the outside of a cell by fusing the vacuole membrane with the cell membrane

TISSUES OF THE EXCRETORY SYSTEM

Kidneys: removes nitrogen-containing wastes from the blood, and controls the concentrations of the substances and body fluids, they also regulate blood volume and pH levels. The kidneys remove waste from the blood and processes them into urine (a sterile liquid containing salts, urea, and water). The fundamental filtering units in the kidneys are nephrons (each kidneys contains millions of nephrons).

ex: salts, water, minerals, and vitamins

Urine passes out of the kidneys to the bladder (for temporary storage) through small tubes called ureters, and from time to time urine is emptied from the bladder through the urethra and exits the body. The bladder can be under conscious and unconscious control, but as it fills, the urge to urinate cannot be held back. Urine is composed of about 95% water and 5% waste products, this drinking water promotes the overall health of the urinary system. Not drinking enough water can lead to kidney stones or kidney disease (gradual loss of kidney function). A urinalysis (test completed on a sample of urine that indicates the health of body systems and organs) helps a medical professional monitor the function of excretory and other organ systems of a patient.

What can you Learn from Urinalysis Results?

water - less water in urine can indicate that a person is dehydrated

urea - waste product generated from the breakdown of proteins in the body. Elevated levels of urea in urine can indicate kidney malfunction

creatinine - waste product produced by muscle metabolism. High levels may indicate kidney problems

glucose - elevated levels of glucose can be a sign of uncontrolled diabetes

proteins - elevated proteins in the urine could indicate kidney disease or other health issues

blood - blood in urine can be a sign of a urinary tract infection, kidney stones, or more severe problems like kidney disease or cancer

bilirubin - waste product from the breakdown of red blood cells. Elevated levels can suggest liver or gallbladder issues

leukocytes - white blood cells in urine can signal an infection or inflammation in the urinary tract

STRUCTURES OF EXCRETORY ORGANS

Urine Creation

Each kidney is made up of many filtering units called…

Nephrons

Proximal Convoluted Tubule: a tube responsible for the reabsorption of ions, organic molecules, vitamins, and water

Bowman’s Capsule: a cup-like sac of the nephron that surrounds the glomerulus

Afferent Arteriole: feeds blood into the glomerulus

Collecting Ducts: microscopic passages that connect multiple nephrons to one another

Efferent Arteriole: carries blood away from the glomerulus

Glomerulus and Glomerular Capillaries: the glomerulus filters our blood and produces filtrate; a specialized bundle of capillaries in the glomerulus allow small proteins to pass but prevent larger proteins from leaving the blood

Urine Transport

Peritoneum: a membrane lining the abdominal cavity that provides structure

Ureters: two narrow tubes that connect the kidneys to the bladder

Detrusor Muscle: a muscle in the wall of the bladder that contracts during urination to move urine through the urethra

Trigone: a small triangular region of the bladder that, when stretched, helps signal the brain when it is time to empty the bladder

Bladder Neck: a group of muscles that contract to hold in urine and relax to release urine

External Urethral Orifice: external opening of the urethra

HISTOLOGY OF EXCRETORY ORGANS

The excretory organs contain various tissues, for example, stratified epithelium helps protect the bladder from the acidity of urine. Connective tissues support the structure of the bladder, and smooth muscle and transitional epithelium in the bladder wall allow it to contract and expand, aiding in urine storage and release through the excretory system

Connective Tissue…

Blood Vessels: arteries carry oxygenated blood and nutrients to the excretory organs. Veins carry deoxygenated blood and waste away from the excretory organs

Fatty Tissues: found outside the ureters, bladder, and urethra; it helps provide protection and separates them from other organs

Smooth Muscle Tissue…

Smooth Muscle: involuntary muscle that allows the excretory organs to expand and contract during the process of excretion. Smooth muscle is dominant in the ureters making them involuntary. The bladder and urethra contain less smooth muscle, which makes them more voluntary, as a result any build-up will be absorbed by the organs’ surrounding tissues

Epithelium Tissue…

Epithelium Tissue: This tissue line and protects the inside of the ureters, bladder, and urethra

Transitional Epithelium: Tissue that lines and protects the inside of the excretory organs. Any damage to this tissue will prevent the smooth muscle from functioning correctly, preventing the organs from widening to release urine

EXCRETORY SYSTEM DISORDERS
Urinary Tract Infection (cystitis): They can cause the bladder, ureters, and urethra to become inflamed. In some cases, kidney infections, called pyelonephritis, can cause pain, discomfort, and fever, leading to more severe complications like kidney damage is left untreated, UTIs are a commonly observed medical condition, affecting millions of individuals each year, women more than men

Kidney Disease: a progressive disorder where the kidneys cannot effectively filter waste and excess fluids. Over time, this disease can become chronic. With the body unable to filter blood properly, there is a buildup of waste products. This can result in high blood pressure, anemia, fluid retention, and electrolyte imbalances. These things can affect various organs and systems in the body, resulting in symptoms like fatigue and swelling of the body

Polycystic Kidney Disease: is a genetic disorder characterized by the growth of fluid-filled cysts in the kidneys. Over time, these cysts can enlarge, causing kidney enlargement and impairment of their function. Over time this can lead to a decline in kidney function, resulting in chronic kidney disease and in severe cases kidney failure, urinary tract infections, and other health issues. Cysts disrupts the natural filtering ability of the kidney, casing effects throughout the body systems.

MACROMOLECULE BASICS

The 4 major macromolecules are nucleic acids, carbohydrates, proteins and lipids.

monosaccharide: simple molecule/monomer that can join together with other monomers to form a larger molecule - if you put two monosaccharides together and you get s disaccharide like lactose - if you put three or more monosaccharides together you get a polysaccharide like glycogen

glycogen: a complex carb with several branches for long-term energy

monomers: a small molecule that can be joined to other monomers to form more complex molecules

Lipids dont uses monomers like carbohydrates do, instead lipids have long chains of carbon and hydrogen arranged in many different ways. Fatty acids have energy storage (important sources of fuel). Phospholipids build protective barriers and steroids take ultimate control of all types of body functions. However, lipids one weakness is water. Carbon and hydrogen chains are hydrophobic.

Proteins are large, powerful biological macromolecules made up of smaller molecules called amino acids. There are 20 different amino acids that can combine in various ways in order to make a protein, and a difference in one amino acid in a chain can make a whole different function for the protein.

Nucleic acids are chains of nucleotides bonded together, each nucleotide is made up of a phosphate and sugar backbone with a nitrogenous base and makes up DNA and RNA.

SUMMARY

Carbohydrates: provides energy - made-up of simple monomers that can be combined to make simple sugars, like glucose or more complex carbohydrates, like glycogen

Lipids: diverse group of organic molecules that are insoluble in water - serves essential bodily function: energy storage, cellular structure - are the key components of cell membranes because they are hydrophobic and provides a natural barrier that protects the inside of the cell - some lipids for different types of hormones for the endocrine system

Nucleic Acids: contains the genetic information of living things - there are two different types of nucleic acids: DNA and RNA, both are composed of nucleotides - they provide instructions for the growth and development of organisms, and regulate protein synthesis and cellular processes

Proteins are made-up of nitrogen-based monomers called amino acids that are linked together in different combination to make all types of proteins the body needs to function - plays a critical role in structure and function of cells, tissues, and organs - acts as enzymes in various biological processes

hydrophobic: a substance or molecule that is water repelling

RNA (ribonucleic acid): a nucleic acid in all living things that is similar to DNA and is often single-stranded

nucleotides: the basic molecular structure of nucleic acids

amino acids: organic molecules that contain carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur

enzymes: a protein that lowers the activation energy of a chemical reaction

STRUCTURES OF NUCLEIC ACIDS, CARBOHYDRATES, AND LIPIDS

Carbohydrates

Made of carbon, hydrogen and oxygen that form a ring. These rings can be combined to made various structures. The bonds between rings allow the molecule to be broken apart and used for INSTANT energy or stored and broken apart later.

1. CH2OH alcohol group

2. carbon ring with altering H and OH

3. Bond between rings

4. n represents the number of monomers in the chain which can be very large or small

Monosaccharides: made up of ONE simple sugar molecule and are the simplest form of carbohydrates (this form cannot be broken down to make smaller molecules)

ex: glucose or fructose

Disaccharides: two monosaccharides bonded with a covalent bond, they are fairly easy for the body to break apart into easy-to-use monosaccharides

ex: sucrose, lactose and maltose

Polysaccharides: long chains or three or more monosaccharides and acts as a long-term energy source as the body must break apart many bonds to be able to use them

ex: starch, cellulose or glycogen

Lipids

Unlike other macromolecules, lipids are not made-up of monomers, they are made from glycerol molecule attached to a fatty acid composed of atoms C,H, and O. Saturated fatty acids are made of single bonds and surrounded by hydrogen. They are tightly bonded together, so it takes more energy to break them apart, making them useful for energy storage. Unsaturated fatty acids are made from single and double bonds and have fewer hydrogens than saturated fats. The double bonds of unsaturated fatty acids are easier to break apart than the single bonds, so energy from unsaturated fats is more readily available to the cells.

1. Three carbon chain of glycerol

2. Chain of all single bonds is a saturated fatty acid-many hydrogens

3. Branching chains of carbon and hydrogen

4. Chain with double bonds is an unsaturated fatty acid - fewer hydrogens

Cholesterol and Steroids

Made of C,H, and O that build ring-like structures. These rings are called cholesterol, a natural molecule found in your body that serves as the building block for steroids. The body modifies cholesterol into different types of steroids, such as hormones like testosterone and cortisol.

1. Hydroxyl group

2. Hydrocarbon rings

3. Hydrocarbon tail

Nucleic Acids

Made up of C,H, O and P (phosphorus). There is a sugar-phosphate backbone and nitrogenous bases that creates nucleotides. The phosphate backbone is easy to recognize due to the P and the double bonds of O. The sugar can be one of two types and has a characteristic ring shape. The nitrogenous bases have N within them and can be one of 5 structures.

1. Phosphate backbone

2. Sugar of either ribose or deoxyribose

3. Nitrogenous base

4. All four molecules together makes a nucleotide that connect together to make either RNA or DNA

DNA is one of the two nucleic acids in the body. It serves as the genetic blueprint of an organism, carrying instructions in its sequence of nucleotide bases. RNA acts as a messenger that translates the genetic information from DNA to give to a ribosome, which use this information to assemble amino acids in the correct order, producing proteins according to the instructions encoded in DNA.

PROTEINS AND ENZYMES
Amino Acids

Proteins are made up of one or more chains of amino acids, which are made up of the elements C,H,O, N (nitrogen), or sometimes S (sulfur). The characteristics of the amino acids include the amino acid group at one end and the carboxyl group at the other. A peptide bond connects the N of one amino acid to the C of another.

1. Amine group with nitrogen

2. R group, which is any group that a C or HH is attached to

3. Carboxyl group with a double-bonded O and an OH that was removed. There is another one at the end of a protein with the OH group

4. Peptide bond between the nitrogen and carbon of two amine groups

Protein Structures

Proteins fold into unique three-dimensional shapes that perform specific biological functions. The sequence and number of amino acids dictate how the protein will fold and what shape it will create, which are broken into four different categories:

Primary Structure: The sequence of amino acids that make up a protein is determined by the genetic code encoded in DNA. This sequence is considered the proteins primary structure

Secondary and Tertiary Structures: The amino acid chain starts to fold into a three-dimensional structure due to various chemical interactions, including hydrogen bonds and disulfide bridges. This folding creates secondary and tertiary structures of a protein.

hydrogen bonds: a weak force between a hydrogen atom and an electronegative atom

disulfide bridges: covalent bonds between two sulfur atoms

Quaternary Structure: multiple protein subunits may come together to form a more complex quaternary structure

Enzymes

Enzymes are special proteins in our bodies that act as biological catalysts. Enzymes speed up chemical reactions that occur in our body and help these reactions happen more efficiently. An enzyme has a substrate that it fits perfectly with. If an enzyme and substrate fit together, chemical and biological changes occur. As this process occurs, both the substrate and enzyme can slightly change shape. The result is a catalysis reactions, the enzyme functioned as it should to speed up the reaction. If the shape of the protein structure is not EXACTLY correct, the substrate will not fit and a reaction cannot occur. Enzymes can be involved in chemical reactions that make a product (synthesis) or reactions that break apart molecules (degradation).

substrate: a molecule that fits within the active site of an enzyme and causes a reaction

catalysts: substances, often enzymes that speed up chemical reactions that occur in the body and help these reactions happen more efficiently

catalysis: the acceleration of a chemical reaction using enzymes as a catalyst

Enzymes have characteristics that make them essential for digesting food, synthesizing molecules and repairing damaged cells

Specificity: enzymes are highly specific, each designed for a specific reaction or substrate, they have an active site, a region that fits and interacts with specific molecules, ensuring that enzymes only work with their designated substrates

Reusability: Enzymes can be used repeatedly in reaction without being consumed in the process. After catalyzing one reaction, they can move on to catalyze another making them very efficient in conserving resources

Regulation: various mechanisms can control enzyme activity. Feedback inhibition is when the process stops when enough of something is made. This helps maintain balance and prevent excessive or unnecessary reactions

There are two categories of enzymes…

Digestive - proteins that help breakdown food into smaller molecules such as sugars, amino acids, and fatty acids. These essential molecules are absorbed and used by our bodies for energy and other essential functions.

ex: amylase (breaks down complex carbohydrates into simpler sugars), pepsin (digests proteins by breaking peptide bonds), lipase (breaks down complex fats into fatty acids and glycerol)

Metabolic: controls the chemical reactions necessary for our bodies metabolism, including energy production and molecule synthesis required for growth, repair, and maintenance

ex: hexokinase (jump-starts glucose metabolism by catalyzing the first step of glycolysis), citrate synthase (used in the citric acid cycle to from citrate), ATP synthase (makes ATP from ADP and inorganic phosphate during cellular respiration)

ENZYMATIC ACTIVITY

Activation energy is amount of energy required to start a chemical reaction, break the initial chemical bonds and allow new bonds to form. Enzymes lower the activation energy of chemical reactions, making it easier for the reaction to start and proceed. This reduction in activation energy allows reactions to happen at a rate that sustains life.

Enzymes and Activation Energy

Potential energy: represents the energy stored within a system of particles, molecules, or atoms and it is plotted along the vertical axis of the diagram to show how the energy changes as the system goes through a chemical reaction

Initial state: represents the reactants (substrates) before the enzyme-catalyzed reaction begins. At this stage, the reactants have a certain amount of energy associated with their chemical bonds

Transition state: a high-energy, unstable intermediate that occurs during the reaction. It is the point at which the reactants have undergone the necessary chemical changes but have not yet formed the final products. The transition state is the peak that needs to be overcome for the reaction to proceed. It represents the highest potential energy point in the reaction pathway

Activation energy: energy difference between the initial state and the transition state. The energy required to reach the transition state allows the reaction to occur, Enzymes lower this activation energy

Final State: represents the products of the enzymatic reaction. The chemical bonds rearrange to form the final products after the reactants pass through the transition state and the activation energy is overcome

Enzymes work best when the environmental conditions are just right. If the environment is not optimal for the enzyme, then the reactions happen more slowly or not at all. The concentration of enzymes/substrate, temp, and pH levels affects enzyme activity.

Reactant Substrate Amount: The more substrate molecules available to bind with enzymes, the faster the reaction will go. But when available enzymes become filled with substrates, adding more substrate does not speed up the reaction. Increasing substrate concentration beyond this point won’t lead to faster reaction rates

pH: Enzymes have an optimal pH at which they work most efficiently. Extreme pH levels, either too acidic or alkaline, can denature enzymes, altering their shape and rendering them ineffective.

Temperature: at lower temp, reactions are slower because the molecules more more sluggishly and collide less often. When the temp is higher the reaction rate increases as the molecules move faster and collide more frequently, but if the temp is too high an enzymes shape can change, or become denatured, preventing it from bonding with the substrate

Enzymes and Health

When there are problems or changes in enzymatic factors, health issues or poor nutrition, difficulty with muscle coordination, and gastrointestinal conditions can occur

Uncontrolled Blood Sugar - in individuals with diabetes, there may be insufficient production of the enzyme insulin. Without enough insulin, blood glucose levels cannot be regulated. High blood sugar can cause damage to the kidneys, heart, nerves, and blood vessels

Digestion - the enzyme pepsin helps with digestion in the stomachs acidic environment. When people take antacids to reduce stomach acidity, it can raise the pH of the stomach, impairing pepsins ability to digest dietary proteins effectively

Toxin Build-up - the enzyme catalase breaks down toxic hydrogen peroxide in the body. If the body experiences a fever due to infection or illness catalase can denature. This can cause a build-up of hydrogen peroxide in the blood.

DIGESTIVE SYSTEM BASICS

Food moves through the alimentary canal (gastrointestinal tract) during digestion. The alimentary canal includes the mouth pharynx, esophagus, stomach, small intestine, large intestine, rectum and anus. This tract is responsible for nutrient absorption, digestion, and excretion. Accessory organs help with digestion by releasing secretions, such as digestive fluids and enzymes into the mouth, stomach, and intestines, examples include liver, pancreas, gallbladder, appendix, and glands.

Path of Digestion

1. Food enters the mouth and is broken down by the mechanical process of chewing. Enzymes in the mouth help break down the food before it is swallowed

2. Muscles of the esophagus move the partially broken-down food into the stomach

3. Food enters the stomach as a mixture. It is broken down further by hydrochloric acid and pepsin.

4. Food leaves the stomach and enters the small intestine as a thick, acidic chyme. Proteins have started to break down within the chyme, and fats have been emulsified (to become a smooth mixture).

5. Nutrients are absorbed by the villi and microvilli of the small intestines. Villi and the even smaller microvilli are small, finger-like projections that contain blood vessels that absorb nutrients into the bloodstream from the chyme. Lacteals (lymphatic capillary) in the villi absorb dietary fats in the chyme.

6. Villi increase the total surface area exposed to the chyme as it moves through the intestines

7. Indigestible fiber and other substances continue into the large intestines, where water is absorbed, and the remaining waste forms feces

TYPES OF DIGESTION

mechanical digestion: involves the physical breakdown of food into smaller pieces without changing its chemical composition

• this process begins in the mouth where teeth chew and grind food, breaking it into smaller bits to increase its surface area for further digestion

• in the stomach, muscular contractions churn and mix the food with gastric fluids

• in the small intestine, contractions and peristalsis continue to mechanically mix and move chyme, facilitating contact with digestive enzymes and absorption of nutrients

peristalsis: the involuntary constriction and relaxation of the muscle of the intestine or another canal creates wave-like movements that push the canals contents forward

chemical digestion: involves the breakdown of food into simpler chemical substances through the action of digestive enzymes

• in the mouth saliva contains enzymes like amylase, which starts breaking down carbohydrates into simpler sugars

• in the stomach, hydriodic acid and pepsin begin the digestion of proteins

• the pancreas secretes digestive enzymes (lipase, protease, amylase) into the small intestine to break down proteins

• bile produced by the liver and stored in the gallbladder helps emulsify fats, making them easier for enzymes to break down

cardiac sphincter: a valve like structure regulates the passage of food into the stomach

The stomach is a thick-walled sac made up of smooth muscle and can hold up to two liters of food and drink. Muscular contraction of the stomach churn and mix food. Food is chemically digested in the stomach when it mixes with hydrochloric acid, pepsin, and mucus secreted by glands in the walls of the stomach. Liquids pass through the stomach rather quickly but solids are reduced slowly to a thin soupy liquid called chyme. After 2-6 hours, chyme passes through a valve, called the pyloric sphincter into the small intestine.

The small intestine is a hollow tube and is only 2 ½ centimeters in diameter. The chemical digestion of carbohydrates and proteins is completed within the small intestine. Absorption of fats, vitamins, minerals, and amino acids into the bloodstream also occurs in the small intestine. Small fingerlike projections called villi, line the inner walls of the small intestine are packed with capillaries and lymph vessel called lacteals. Capillaries absorb carbohydrates and proteins while lacteals absorb fats and fatty acids.

In the small intestine chyme is mixed with bile from the liver, pancreatic juice from the pancreas and intestinal juice from glands in the intestinal wall. These secretions contain enzymes and other substances necessary for digestion. Undigested and unabsorbed materials then pass from the small intestine into the large intestine (colon). Water and water-soluble vitamins are absorbed, and when most of the water is removed, what is left is solid waste called feces, and is removed by exiting through the anus.

Digestive Enzymes

Enzymes break down carbohydrates into sugar (amylase, maltase lactase), proteins (pepsin, trypsin, protease) into amino acids, and fats (lipase) into fatty acids and glycerol.

ACCESSORY ORGANS

Salivary Glands…

• lubricates food with saliva

• contains enzymes that begin digestion

Health conditions can impact saliva production, impairing breakdown of carbohydrates in the mouth and making it hard to swallow or eat

Stomach (gastric) Glands…

• produces digestive enzymes and fluids

ex: hydrohalic acid, pepsin, and mucus

If the production of these fluids is impacted it can cause indigestion, bloating, and poor nutrient absorption

Liver…

• secretes bile required for lipid digestion

Fatty liver disease and cirrhosis can impact bile production or secretion and hinder the breakdown of fats, leading to issues like bloating, diarrhea and malabsorption of fat-soluble vitamins

fatty liver disease: when fat inside your liver can affect liver function and cause liver injury

cirrhosis: scarring of the liver that interferes with functioning

Gallbladder…

• stores bile for liver for release into the small intestine

If the gallbladder has gallstones or is not functioning correctly, it may not release enough bile leading to difficulty digesting fats, causing symptoms such as bloating, gas, and diarrhea after eating fatty foods

gallstones: a hard piece of material, usually made of cholesterol or bilirubin, that develops in your gallbladder

Pancreas…

• secretes digestive enzymes into the small intestine

If the pancreas doesn’t produce enough digestive enzyme due to conditions like pancreatitis, it can lead to malabsorption of nutrients, causing weight loss and nutritional deficiencies

pancreatitis: inflammation or swelling of the pancreas

Appendix…

• stores good bacteria that assists with digestion

Is attached to the first part of the colon and it is inflammed or affected (appendicitis) it can lead to severe abdominal pain and discomfort, affecting a person’s appetite and eating habits

System Connections

Nerves are connected to digestive system organs and accessory organs that send messages to the brain and spine. The autonomic nervous system, helps regulate and coordinate the digestive processes based on the body’s needs. Interconnections between the nervous and digestive systems are called the enteric nervous system.

Sensory receptors in the digestive tracts communicate information about the type and quantity of food present. When food is ingested, the receptors can pass signals to the brain to alert the body that the stomach is full or that the body is dehydrated. This information sends signals to regulate the release of digestive enzymes and hormones that maintain digestive processes.

REGULATORS OF DIGESTION

Food in the digestive tract triggers nerves in the esophagus, stomach, and intestine to release substances that assist the movement of food and the production of liquids by the digestive organs. There are also nerve impulses from the central nervous system that signal the release of the neurotransmitter acetylcholine, which allows the digestive organs muscles to contract and push food and chyme through the gastrointestinal tract and the stomach/pancreas to produce more digestive juices. Adrenaline relaxes the muscles of the stomach and intestines and decreases the flow of blood to these organs during a fight-or-flight situation.

Digestion is also regulated through the endocrine system which can regulate cell growth (in the gastrointestinal tract) and signal the release of bile, enzyme and digestive liquids, which are produced by mucosa cells of the stomach and small intestine.

Three main hormones secreted during digestion are…

Gastrin - causes the stomach to produce an acid for dissolving and digesting food, it promotes growth of the lining of the stomach, small intestine and colon.

Secretin - causes the pancreas to release bicarbonate, which reduces the acidity of the chyme, stimulates the stomach to produce pepsin, and also stimulates the liver to produce bile

Cholecystokinin - signals the gallbladder to release its bile and causes the pancreas to grow and produce the enzymes in pancreatic juices

Digestive Disorders

IBS

A chronic disorder that affects the large intestine and can cause symptoms like abdominal pain, bloating, diarrhea and constipation - thought to be related to irregular muscle contractions in the colon and abnormal nervous system signals that control digestive processes

Gastroesophageal Reflux Disease (GERD)

Occurs when the stomach acid frequently flows back into the esophagus, causing heartburn, irritation in the upper GI tract, and nausea/vomiting - the lower esophageal sphincter doesn’t function properly in GERD

Peptic Ulcers

Are open sores that develop on the inner lining of the stomach or the upper part of the small intestine - can be caused by bacterial infection or by excessive production of stomach acid, which can be influenced by hormones like gastrin

LAYERS OF THE GI TRACT

The organs of the alimentary canal have walls of four distinct tissue layers:

serosa

muscle layer

submucosa

mucosa

These layers of tissues within the GI tract ensure that food is being properly processed, digested, and absorbed. They also protect tissues from the abrasive nature of digestive enzymes and mechanical digestive movements.

serosa: outermost layer of the GI tract - covers, protects and controls movement of digestive organs within the abdominal cavity - secretes fluids so organs can easily slide against one another - is flexible to adapt to changes in size and shape of the contents flowing through this structure

muscle layer: contains a circular muscle layer and a longitudinal muscle layer. When these muscles contract and relax they move the chyme through the intestinal tract

submucosa: contains blood vessels and nerves that supply nutrients and oxygen to the epithelial cells in the mucosa. This layer also provides structural support to the mucosa

mucosa” the innermost layer. It consists of a thin layer of specialized epithelial cells lying on top of a base of connective tissue and smooth muscle. It contains folds called plicae circulares. These folds have tiny projections called villi that increase the surface area for nutrient absorption

TISSUES OF THE GASTROINTESTINAL TRACT

Tissues within the tract work together to perform ingestion, digestion, absorption, and elimination.

Epithelial Tissues:

• stratified squamous epithelial tissue lines the tongue and esophagus

• simple columnar epithelial tissue is found in the stomach and intestines

• specialized cells and glands within the epithelial tissues release saliva in the mouth. Mucous and digestive enzymes are released in the stomach and intestines

Smooth Muscle

• found in the esophagus, stomach, and intestines

• smooth muscle in the stomach is responsible for contractions

• smooth muscle of the intestines propels food through the digestive tract

• a mixture of smooth muscle fibers and striated muscle fibers (large reddish circles in the image)

Connective Tissue

• support and hold digestive organs in place

• blood transfers nutrients to the bloodstream from the intestines

• collagen fibers, connective tissue cells and several blood vessels can be seen

Nervous Tissue

• regulates muscle contractions in the esophagus, stomach, and intestines

• controls voluntary muscle movement in the mouth

• signals the secretion of digestive enzymes

• sends signals of hunger and fullness from the stomach

STRUCTURES OF THE MUCOSA

The mucosa membrane of the stomach and small intestine contains glands that secrete gastric or intestinal fluids for digestion, below this area are fibroblasts (a connective tissue that supports the structure of the mucosa). Once digested nutrients move into the body’s cells using the blood capillaries and lactels within the small intestines villi. The small intestines villi provide a large surface area for nutrient absorption.

ACCESSORY ORGAN STRUCTURES

Specialized cells are responsible for making digestive enzymes and secretions. They also have ducts to release their secretions into the G1 tract and most accessory organs make hormones that regualte digestions and other body functions…

Salivary glands: lubricates food; contain enzymes that begin digestion