Further Physiology

• Neurosecretory cells in the hypothalamus synthesise and transport ADH to the posterior pituitary gland where it is stored
• Osmoreceptors in the hypothalamus detect an increase in blood solute concentration when levels of water in the blood drop
• In response to this nervous activation, ADH is released from the posterior pituitary and travels in the blood to the kidneys
• ADH makes the epithelium of the collecting ducts more permeable to water
• This increases the amount of water reabsorption by the kidneys, causing blood solute concentration to drop
• With a drop in blood solute concentration, ADH secretion is inhibited (negative feedback)
• If blood solute concentration is low (due to excessive water intake), the collecting duct remains impermeable to water and urine is dilute

• Exocrine glands have ducts through which they secrete their product 
• The ducts / ductules arise from a cluster of cells called an acinus
• Acini are lined by a single layer of secretory cells which release the exocrine product into the lumen of the duct via secretory vesicles
• Secretory cells are held together by tight junctions, and possess a highly developed RER and golgi network for material secretion

Similarities:

• Contain water (universal solvent)
• Contain mucus (as a lubricant or an intestinal lining)
• Contain salts and ions (calcium, phosphate, etc.)

Pre-Ingestion

• The sight and smell of food triggers a reflex response in which gastric juice is secreted from gastric pits in the stomach wall
• This ensures that gastric juice is in the stomach by the time the food is consumed

Post-Ingestion

• Food entering the stomach causes distension, which is detected by stretch receptors in the stomach lining
• Impulses are sent to the brain, which triggers the secretion of gastrin from the pits lining the stomach wall
• Gastrin causes the sustained release of gastric juice, particularly its acid component
• When the pH drops too low, gastrin secretion is inhibited by hormones 

Some digestive enzymes are immobilised on the plasma membrane of the epithelial cells of the small intestine, serving two main benefits:

• The enzyme is fixed in place and does not pass through the digestive system, meaning it can be reused

• The enzyme can be linked to secondary functions (e.g. membrane transport)

Example:

• Maltase is immobilised on the epithelial lining with its active site facing towards the intestinal lumen
• Maltase digests the disaccharide maltose into two glucose monomers, which are then absorbed by localised transporters

Glucose can exist in one of two isomeric forms: a-glucose or ß-glucose

While humans can digest polymers of a-glucose (e.g. starch, glycogen), they cannot digest the polymer of ß-glucose (cellulose).

• This is because they do not produce the necessary enzyme (cellulase) and lack bacteria in their gut capable of digesting cellulose
• Cellulose is a component of plant cell walls and the main source of dietary roughage (undigested, it creates bulk which stimulates peristalsis)

• Both pepsin and trypsin are protease enzymes (specifically endopeptidases) which hydrolyse peptide bonds to digest proteins
• As proteins are ubiquitous and essential components of cells, these enzymes could digest the cells that secrete them (auto digestion)
• Instead, they are synthesised as inactive forms (zymogens) and subsequently activated in the digestive tract (lined with mucus to protect cells)

• Stomach ulcers are inflammed and damaged areas in the stomach wall
• Heliobacter pylori:
  • can survive the acid conditions of the stomac because it secretes urease, which neutralizes the gastric acid to lower the acidity of the stomach for further colonisation
  • it also secretes proteases to degrade the mucos lining of the wall, allowing it to burrow in it
• Gastric acid:
  • The degeneration of the protective lining allows for damage to the stomach, causing ulcers
  • The prolonged presence of stomach ulcers may lead to the formation of stomach cancers

• Lipids are hydrophobic and hence insoluble within the aqueous environment of the body
• They will group together to form large droplets of fat (fat globules)
• The enzyme responsible for lipid digestion (lipase) is water soluble, and therefore unable to access the interior of hte fat globule

• Bile is a watery fluid that contains bile salts and pigments produced by liver stored in the gall bladder 
• Bile salt molecules have both a hydrophobic and hydrophilic end
• The hydrophobic end attaches to the lipid while the hydrophilic end interacts with water dividing the globule into smaller droplets (emulsification), increasing the surface area available for enzyme activity

• Lipids are hydrophobic and hence insoluble within the aqueous environment of the body
• They will group together to form large droplets of fat (fat globules)
• The enzyme responsible for lipid digestion (lipase) is water soluble, and therefore unable to access the interior of hte fat globule

• Bile is a watery fluid that contains bile salts and pigments produced by liver stored in the gall bladder 
• Bile salt molecules have both a hydrophobic and hydrophilic end
• The hydrophobic end attaches to the lipid while the hydrophilic end interacts with water dividing the globule into smaller droplets (emulsification), increasing the surface area available for enzyme activity

Microvilli:  increase surface area of plasma membrane, allowing for more absorption of digested food materials. The membrane is embedded with digestive enzymes and channels to assist in uptake.

Mitochondria: Prrovide energy (ATP) for active transport processes.

Pinocytotic vesicles: Contain fluid and dissolved food materials from the lumen of the ileum. 

Tight junctions: Occluding associations between the membrane of two adjacent cells, creating an impermeable barrier to keep digestive fluids separate from tissue fluids and ensure a one way flow

Diffusion: Lipids are absorbed by simple diffusion (can pass freely through hydrophobic core of the plasma membrane).

Facilitated Diffusion: Channel proteins help hydrophilic food molecules pass through the hydrophobic portion of the phospholipid bilayer. Water-soluble molecules (e.g. fructose), minerals and vitamins are absorbed by facilitated diffusion.

Active Transport: Protein pumps in the plasma membrane hydrolyse ATP to translocate molecules against the concentration gradient. Glucose, amino acids and mineral ions are all absorbed by active transport.

Endocytosis: Endocytosis involves the invagination of the plasma membrane to create an internal vesicle containing extracellular materials. Each pinocytotic vesicle contains a small droplet of fluid from the lumen of the ileum. The vesicles contain channels and pumps from the plasma membrane and so digested food can be absorbed from the vesicle into the cytoplasm.