Digestive System

Unit 18 Synopsis

Functions

Conversion of consumed food into the energy and raw materials that are needs to build, repair, and fuel our bodies.

Macromolecules get mechanically and chemically broken down into simplified forms, which can then be absorbed into our blood and transported

Anatomy

The main structures make up the alimentary canal (gastrointestinal tract) is the hollow muscular tube from the mouth to the anus that digests and absorbs our food.

Structures include:

mouth
pharynx
esophagus
stomach
small intestine
large intestine

Accessory structures include:

teeth
tongue
salivary glands
gallbladder
liver
pancreas

Digestive Process

1. Ingestion

Eating, occurs in the mouth

Teeth begin mechanical breakdown

Chemical digestion begins with enzymes released from the salivary glands

The ball of chewed food we swallow is referred to as a bolus

2. Propulsion

Movement of food throughout the GI tract

Starts with the voluntary swallowing of the bolus

Continues with involuntary peristalsis

The contraction and relaxation of smooth muscles in the esophagus, stomach, and intestines to move food through the canal

3. Mechanical breakdown

Physical breakdown of food to increase surface area, which helps with absorption that can then be mixed with enzymes

4. Chemical digestion

Enzyme secretion to chemically break down into monomers for absorption

5. Absorption

The transport of the end result of our digested food through the walls of the GI tract and into the blood or lymph to be transported

These monomers, vitamins, minerals can then be used for cellular respiration, in cells, or whatever other process that is required.

6. Defecation

Elimination of indigestible substances through the anus as feces

Histology

Four main layers of the GI tract (from deepest to most superficial)

1. Mucosa (innermost):

Secretes mucus, enzymes, and hormones

Absorbs nutrients into the blood

Protects against infectious disease

2. Submucosa

Middle layer of loose areolar connective tissue that provides elasticity to the tube

3. Muscularis externa

Middle layer of smooth muscle that is essential for peristalsis

4. Serosa

Outermost layer comprised of loose connective tissue that houses the nerves, blood vessels, and lymphatic vessels for each structure

Note: the layers above refer to the actual layers of the organs that are surrounded by the peritoneum (serous membrane that line the abdominopelvic cavity)

Epithelial Tissue has key roles in creating a selectively permeable lining

Stratified squamous epithelial tissue is found in the mouth, esophagus, and anus to protect against abrasive food

Simple columnar epithelial cells line the stomach and intestines for absorption and secretion

Enzymes

Amylase

Location: saliva

Digests: Starches (complex carbs)

Product formed: maltose (disaccharide)

Pepsin and HCl

Location: Gastric juice

Digests: Proteins

Product formed: partially digested proteins

Proteases, lipases, amylase

Location: pancreatic juice

Digests: proteins, fats (in bile), and starches

Product formed: Peptides and AA, fatty acids, glycerol, maltose

Peptidases, sucrase, lactase, maltase

Location: intestinal enzymes

Digests: Peptides (short chains of AA), sucrose, lactose, maltose

Product formed: AA, glucose, fructose, galactose

Bile salts

Location: Liver bile

Digests: fats

Product formed: fat droplets

Amylase

Regulation

The enteric nervous system is the in-house nerve supply for the GI tract for regulating activities of the digestive system

It is part of the autonomic nervous system

Receptors respond to external stimuli (seeing, smelling, tasting, thinking about food) or internal stimuli

Internal stimuli can be mechanical (stretching) or chemical (pH changes or presence of certain end products)

Even if the Vagus nerve is severed, the enteric nervous system can still continue to function independently.

The enteric NS communicates with the CNS via the Vagus nerve

Has both sensory and motor functions

Connects the brain to the GI tract

Hormones play a significant role

G cells produce gastrin

Enterendocrine cells release hormones like serotonin and histamine

These hormones increase/activate the release of HCl and stimulate muscle contractions in the stomach and intestines

Gastric fluid secretion is controlled in three phases:

1. Cephalic phase (reflex phase in the brain)

External stimuli (like seeing or smelling food) --> hypothalamus and medulla oblongata in the brain --> triggers parasympathetic fibers in Vagus nerve --> prep stomach for food

2. Gastric phase occurs in the stomach

Mechanical stimuli activate stretch receptors in stomach --> secrete more gastric fluid

Chemical stimuli activate chemoreceptors in the stomach --> G cells release gastrin --> increase in HCl secretion

3. Intestinal phase occurs in the intestines

Receptors in the duodenum receive chyme, in response intestinal cells secrete more gastrin

These stages are represented in the cascade signaling mechanism, it also works in an inhibitory fashion

Nutrition

Nutrients are substances in food that the body uses to promote normal growth, repair, and maintenance

There are 40 essential nutrients

Essential: the human body cannot synthesize, therefore must obtain through diet

Kilocalorie: unit of measurement of energy stored in the chemical bonds of our food

Six essential nutrients:

Water

Macronutrients

Carbohydrates - 4 cal/g

Lipids - 9 cal/g

Proteins - 4 cal/g

Remember that although you will consume nucleic acids, they are not broken down for energy (they can be broken down for the pentose, phosphate, and nitrogenous base to build other molecules)

Micronutrients

Vitamins

Minerals

Carbohydrates

Dietary sources

Plants - simple sugars in fruits and starches in grains and vegetables

exception of lactose in milk

Break down: starches to oligosaccharide and disaccharides to monosaccharides (glucose, fructose, galactose) by salivary amylase, pancreatic amylase, lactase and others in small intestine in the mouth and small intestine. They will then be transported via facilitated diffusion into capillary blood, then head to the liver via the hepatic portal vein

Functions: glucose fuels body cells with ATP

ATP is unstable so we can't store it as is, therefore we need glucose or glycogen

Approximately 45-65% of calories should come from carbs

Lipids

Dietary sources include meat and dairy foods, some plants such as avocado and coconut, and oils. We cannot synthesize omega 6 and 3 fatty acids. Lipids are used in protective cushioning, insulation, energy storage, phospholipids (cell membrane), cholesterol serves as a precursor to bile salts and steroid hormones, triglycerides serve as energy for skeletal muscles, helps absorb fat-soluble vitamins, and prostaglandins act as regulatory molecules.

The breakdown is triglycerides to monoglycerides and fatty acids. Emulsification by the liver's bile salts and pancreatic lipases. This occurs in the stomach and small intestine.

Some lipids diffuse, some use exocytosis, while short fatty acid chains are transported via diffusion into capillary blood, then head to the liver via the hepatic portal vein.

Out of daily calories, 20-30% should be lipids.

Proteins

Dietary sources include eggs, milk, fish, meat, and soybeans, all of which are considered complete proteins because they contain all the AA our body needs.

There are 9 essential AA for humans: Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Lysine, Leucine

Proteins function in everything that the body does (enzymes, hormones, protection, communication, regulation, energy, transport, etc.). Proteins are broken down into large polypeptides, into smaller peptides and lastly into amino acidsby pepsin (stomach), pancreatic enzymes and many others. Proteins can be absorbed alongside sodium and hydrogen ions, which can then be transported via facilitated diffusion into capillary blood, then head to the liver via the hepatic portal vein.

General guidelines for proteins: 0.8g of protein per kg of weight

Vitamins

Organic compounds that help the body get the nutrients its needs

Most function as coenzymes, acting alongside a particular enzymes

e.g. vitamin B acts as a coenzyme in the oxidation of glucose for energy

Not used for energy or building other compounds, but do play a critical role in the utilization of compounds

No single food contains all our required vitamins, which is why a balanced diet is required

Are classified as either lipid or water soluble

Water-soluble include B complex and C vitamins, these do not get stored, so if they aren't taken into a cell within an hour of ingestion, they get excreted. An exception is B12 that must bind to an intrinsic factor stomach secretion in order to be absorbed.

Fat-soluble include A,D,E, and K are absorbed with other lipids in the small intestine. They can be stored in the body and therefore supplements must be closely regulated as it is possible to cause an overdose (e.g. humans cannot eat polar bear liver due to the high levels of A)

Minerals

Inorganic chemical compounds found in nature, salts

Do not provide fuel

Make structures in our body stronger, and most bind to organic compounds or get ionized in body fluids

e.g. Iron is a key component of hemoglobin; sodium and chloride ions are key electrolytes in our blood

200 mg are required of calcium, potassium, phosphorous, sulfur, chloride, sodium, magnesium (note: be careful not to confuse this manganese which is a trace mineral)

All other vitamins that are not needed in such high concentrations are trace minerals (such as iodine, fluoride, iron, manganese, molybdenum, selenium, zinc, copper, chromium)

Metabolism

The sum of all biochemical reactions in the body (metabolism= all anabolic +catabolic rxns)

Anabolic: building up of molecules to make larger ones

requires a net input of energy

Catabolic: breaking down of molecules into simpler ones

Is a net release of energy

Energy output results in heat loss, which helps maintain homeostatic body temperature, which is mainly regulated by the hypothalamus

Digestion is a catabolic reaction accomplished through enzymes that are secreted into the alimentary canal

Fast or slow metabolism?

refers to an individual's basal metabolic rate (BMR), this is a reflection of the energy our body needs to perform its most essential functions (like breathing and organ functioning)

Factors that influence BMR:

-Age, younger people have a higher BMR

-Sex, males have a higher BMR than females

Generally because males have greater muscle mass and muscles are very metabolically active even at rest

-Stress can increase BMR by mobilizing our sympathetic nervous system

-The more thyroxine produced, the higher the BMR (because it has a direct effect on most somatic cells (excluding brain) Is to increase oxygen consumption and heat production, which increases the use of ATP, increasing cellular respiration rates to increase ATP production

-Hyperthyroidism (too much thyroxine)

-Hypothyroidism (too little)

Steps of processing food for energy

1. Digestion and absorption

Occurs in the GI tract and is then transported by the blood into our tissue cells

2. Nutrients can be built into other macromolecules (anabolic) or start being processed to be used for energy (catabolic) by going through glycolysis in the cytoplasm of our cells

3. Citric Acid Cycle (Kreb's) and Electron Transport Chain (Oxidative phosphorylation) in the mitochondria

Nutritional States

Absorptive state: fed state that lasts about four hours after someone starts eating

Nutrients are moving from GI tract into blood

Anabolism exceeds catabolism which results in storing nutrients

Amino acids --> proteins

Glucose --> Glycogen

Glycerol and fatty acids --> triglycerides

Major fuel is glucose

Excess glucose gets stored as fat or glycogen

All is controlled or directed by insulin

The body is very efficient so it likes to keep levels of nutrients and energy sources stable in our blood, thus it alternates between nutritional states

Postabsorptive state: fasting state, when the GI tract is empty

Catabolism exceeds anabolism --> stops the synthesis of fats, glycogen, and proteins, and starts breaking them down for the body's needs

Major energy fuel are body reserves so that glucose is available for the brain

Glucose comes from several sources:

1. breaking down glycogen in the liver

2. breaking down glycogen in skeletal muscles

3. converting glycerol from adipose tissue in the liver

4. breaking down proteins from tissues (last resort)

Controlled/directed by glucagon and the sympathetic NS

Main goal is to maintain blood glucose concentration because the brain relies on blood glucose to function as an energy source. Because of this, most of the events of this of this state are focused on getting glucose in the blood and/or switching organs (like skeletal muscles) that can rely on other energy sources (like fats) to those alternative sources so that glucose can be saved for the brain.

The body draws from its nutrient pools (current stocks of amino acids, carbs, and fats) to meet whatever its present needs are – and these pools are interconnected because the body can draw from one to use for another (such as draw from the amino acid pool to make keto acids to be converted into carbohydrates and used as energy)

Liver

Functions: processing the majority of our nutrients and regulating plasma cholesterol levels, made of 4 lobes

Cholesterol is a steroid made by the liver that is found in many body tissues and is a major compoentn of the plasma membrane

It is the structural basis for bile salts, steroid hormones, and vitamind D

Makes lipoproteins for transporting lipids in the blood and regulating entry and exit from specific target cells

The liver is so important that hepatocytes have over 500 metabolic functions

15% of our cholesterol comes from our diet and the other 85% is made by our liver from acetyl-CoA

Urinary System

Major functions: blood filtration and regulation

Sends toxins and waste out of the urine and retains any nutrients our body needs

Urination (micturition) is the act of emptying the bladder and removes toxins from the body and maintains homeostasis of our water volume (and thus our blood pressure)

Key structures: kidneys, bladder, ureters, urethra

At any given moment, approximately 20% of our blood is in our kidneys

Urine composition: 95% water and 5% slightly acidic solutes. our urine can tell a lot of our health

Kidneys: the most important organ in the urinary system

Functions:

*water volume in the body and osmolarity (solute concentration)

an osmotic gradient regulates urine concentration and volume

*ion concentrations

*acid-base (pH) balance

*excretion of foreign substances from the body

*erythrocyte production

*blood pressure

*conversion of leftover AA to carbs or lipids for storage

Nitrogen from AA gets converted to ammonia (NH3) and then urea that can be urinated out

Located in the dorsal body wall (basically on either side of your spine), in the superior lumbar cavity so they are somewhat protected by the lower part of the rib cage. The liver kind of gets in the way of the right kidney, so it is a bit lower than the left

Three major regions:

*Renal cortex:

Surrounded by the outermost layer of the fibrous capsule that protects the kidneys from infection

*Renal Medulla:

Formed of mostly parallel bundles of urine collecting tubules and capillaries (gives them a striped look)

Renal columns separate the medulla

*Renal pelvis:

Funnel shaped tube with smooth muscle walls to propel urine via peristalsis into the ureters and on to the bladder

Blood and Nerve Supply

*Renal Artery

delivers oxygenated blood to the kidney from the heart through the aorta

Is subdivided into smaller arteries and arterioles (segmental, interlobar, arcuate, and cortical radiate arteries)

*Renal Vein

Delivers deoxygenated blood out of the kidneys to the inferior vena cava

Is subdivided into venules and small veins (cortical radiate, arcuate, and interlobar)

*Renal plexus

Network of autonomic nerve gibers and ganglia that innervates the kidneys

NEPHRONS

The structural and functional units of the kidneys

Each kidney has about 1 million nephrons working together to make urine

Microscopic filtering units where blood is processed

Two parts:

*renal corpuscle is located in the renal cortex

*renal tubules that start in the renal cortex and pass into the renal medulla

Nephron Functions

1. Glomerular Filtration

Creates a cell- and protein-free filtrate from blood in the glomerulus of the renal corpuscle

Filtrate: everything in blood plasma except proteins

Urine: the unneeded substances (e.g. excess minerals, metabolic wastes, and toxins) that get filtered out so they can leave the body

*Passive transport

*Glomeruli act as mechanical filters between blood and glomerular capsule

*Hydrostatic pressure pushes fluids (e.g. water) and solutes (e.g. glucose, AA, nitrogenous wastes) through the membrane

2. Tubular reabsorption

Reabsorbs any needed substances from filtrate back into the blood in the renal tubules and collecting ducts

*Most of the filtrate's contents (e.g. sodium ions and water) get reabsorbed back into the blood

*Can be passive (diffusion, facilitated diffusion, or osmosis) or can be active depending on the substances reabsorbed

3. Tubular secretion

In the renal tubules and collecting ducts, any unneeded substances are removed from the capillaries into the filtrate, then moved to the renal pelvis to be excreted as urine

*Moves substances (like hydrogen, potassium, ammonium, and creatinine) from capillaries into the filtrate

-secreted urine will contain both filtered and secreted substances

*Important for maintaining blood pH

Regulation

Glomerular filtration rate: how much blood can pass through the glomeruli every minute

*Controlled intrinsically (renal autoregulation)

*Myogenic mechanism: if blood pressure is too high and pushing too much blood into kidneys, muscles in glomeruli shrink so they won't take as much in

*Tubuloglomerular feedback loop: amount of salt in renal tubules signals glomeruli

*Controlled extrinsically

*Via nervous and endocrine systems

ADH, secreted by the neurohypophysis to help the body retain water and stay hydrated by transporting water into cells via aquaporins to maintain blood pressure

inhibited by caffeine and alcohol

Higher blood pressure pushed more blood into kidneys to be filtered, but the kidneys can only handle so much at a time.

Other Urinary Structures

Ureters:
- Paired tubes that transport urine from the kidneys to the bladder via peristalsis
Bladder:
- Hollow, smooth, and collapsible muscular sac that serves as a temporary storage reservois for urine
- Can comfortably hold about 500 mL of urine, but can stretch to hold up to one liter
- Stretch receptors signal the brain, which is how you know when you need to pee
Urethra
- Thin-walled muscular tube that carries urine from the bladder through the involuntary internal urethral sphincter out of the body through the voluntary external urethral sphincter
  - The internal urethral sphincter opens involuntarily, and our autonomic nervous system helps to keep it closed to prevent urine leakage.
  - The external urethral sphincter is made of skeletal muscle, which is how we can control it voluntarily (and hopefully pee when we want to and not pee when we don’t want to!)
- Much longer in males (travels the entire length of the penis, therefore, they are less susceptible to getting UTIs