PHYS SHORT FINAL SG

How does Gastrin regulate digestion?

• Secreted by G-cells (antrum of stomach)

• Stimulated by peptides, amino acids, vagal input (GRP)

• Functions:

  • ↑ HCl secretion (indirectly via ECL histamine release)

  • ↑ gastric motility

  • ↑ mucosal growth

What is Motilin’s role?

• Secreted by small intestine during fasting

• Initiates Migrating Motor Complex (MMC)

• Clears residual food/bacteria between meals

What does CCK do?

• Released by I-cells in duodenum in response to fats/proteins

• Functions:

  • Stimulates pancreatic enzyme secretion

  • Causes gallbladder contraction → bile release

  • Relaxes sphincter of Oddi

  • Slows gastric emptying

What does Secretin do?

• Released from S-cells in response to acidic chyme

• Functions:

  • Stimulates pancreatic bicarbonate secretion

  • Inhibits gastric acid secretion

  • Protects duodenum from low pH

What is Somatostatin?

• Secreted by D-cells

• Universal inhibitor:

  • ↓ gastrin

  • ↓ HCl

  • ↓ pancreatic secretion

  • ↓ GI motility

What are ECL cells?

• Release histamine

• Histamine binds H₂ receptors on parietal cells → ↑ HCl secretion

What are Chief cells?

• Secrete pepsinogen

• Activated by HCl → pepsin (protease)

What are Parietal cells?

• Secrete:

  • HCl (acidifies stomach, activates enzymes, kills microbes)

  • Intrinsic factor (required for vitamin B₁₂ absorption in ileum)

What are Enteroendocrine cells?

• Hormone-secreting cells distributed throughout GI tract

• Release gastrin, CCK, secretin, etc.

What are Paneth cells?

• Located in intestinal crypts

• Secrete defensins, lysozyme

• Regulate microbiota and innate immunity

What are Goblet cells?

• Secrete mucus (mucin)

• Protect epithelium and facilitate lubrication

What are Tuft cells?

• Chemosensory epithelial cells

• Detect parasites and activate immune responses (type 2 immunity)

What are D-cells?

• Secrete somatostatin

• Inhibit neighboring cells (paracrine regulation)

What are G-cells?

Secrete gastrin

What are ECF (enterochromaffin) cells?

• Release serotonin (5-HT)

• Regulate gut motility and secretion

How are fats digested and absorbed?

• Bile salts emulsify lipids → increase surface area

• Pancreatic lipase breaks triglycerides → fatty acids + monoglycerides

• Products form micelles with bile salts

• Micelles deliver lipids to enterocytes

• Lipids are re-esterified → triglycerides

• Packaged into chylomicrons

• Enter lacteals (lymphatic vessels) → lymph → bloodstream

How do parietal cells produce HCl? (mechanism)

1. Carbon dioxide and water react to form carbonic acid through the action of carbonic anhydrase.

2. Carbonic acid dissociates into hydrogen ions and bicarbonate ions.

3. Hydrogen ions are secreted into the lumen by the hydrogen-potassium ATPase, also known as the proton pump.

4. Chloride ions enter the lumen through specific channels.

5. The hydrogen ions combine with chloride ions to produce hydrochloric acid.

What is the alkaline tide?

bicarbonate transported into blood → temporary increase in blood pH after meals

How do pancreatic duct cells secrete bicarbonate?

1. Carbon dioxide and water react to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions.

2. Bicarbonate is secreted into the duct through a chloride-bicarbonate exchanger.

3. The CFTR channel facilitates the secretion of chloride into the lumen.

4. Chloride ions are recycled to maintain the exchange process.

5. Hydrogen ions are transported into the bloodstream.

6. Stimulated by secretin

Function of bicarbonate?

• Neutralizes acidic chyme in duodenum

• Creates optimal pH for pancreatic enzymes

What is the role of epithelial and immune cells?

• Paneth cells → antimicrobial peptides

• Dendritic cells → antigen presentation

• Tight junctions → barrier integrity

What happens in the PCT?

• Bulk reabsorption

• Reabsorbs glucose, amino acids, bicarbonate, and water

• Secretes hydrogen and drugs

Descending Loop of Henle

• Permeable to water only

• Water exits → filtrate becomes concentrated

Ascending Loop of Henle

• Impermeable to water

• Reabsorbs Na⁺, K⁺, Cl⁻ (NKCC transporter in thick segment)

• Creates medullary osmotic gradient (countercurrent multiplier)

What happens in the DCT?

• Fine-tuning of ions

• reabsorbs sodium and calcium

• secretes hydrogen and potassium

How does the Juxtaglomerular Apparatus (JGA) function?

• Regulates GFR and blood pressure

• Releases renin

Loop diuretics (e.g., furosemide)?

• Inhibit NKCC transporter (Loop of Henle)

• ↓ medullary gradient → ↓ water reabsorption

Thiazide diuretics?

• Inhibit NCC transporter (DCT)

• ↑ Na⁺ excretion

Potassium-sparing diuretics?

• Block ENaC channels or aldosterone receptors

• Reduce K⁺ loss

Osmotic diuretics?

Increase tubular osmolarity → pull water into urine

Which hormones regulate water?

• ADH (vasopressin)

• Aldosterone

• ANP

Which hormones control glucose?

• Insulin

• Glucagon

• Cortisol

What hormones are involved in appetite & metabolism?

• Leptin: satiety signal

• Ghrelin: hunger signal

Which hormones regulate Ca²⁺?

• PTH: ↑ Ca²⁺

• Calcitriol (vitamin D): ↑ intestinal Ca²⁺ absorption

• Calcitonin: ↓ Ca²⁺ (minor role)

What hormones control growth?

• Growth hormone (GH): from pituitary

• IGF-1: from liver (mediates GH effects)

• Stimulates protein synthesis, cell proliferation

Effects of cortisol?

• ↑ blood glucose

• Suppresses immune system

• Enhances metabolism of fats and proteins

What is glycolysis?

• Location: cytosol

• Process: glucose → 2 pyruvate

• Net yield: 2 ATP + 2 NADH

• Anaerobic vs aerobic:

  • Anaerobic: pyruvate → lactate (regenerates NAD⁺)

  • Aerobic: pyruvate → enters mitochondria → Krebs cycle

What is gluconeogenesis?

• Formation of glucose from non-carbohydrate sources (lactate, amino acids, glycerol)

• Occurs mainly in liver

• Requires energy (ATP/GTP)

• Maintains blood glucose during fasting

What is lipogenesis?

• Conversion of excess glucose → fatty acids → triglycerides

• Occurs in liver and adipose tissue

• Stimulated by insulin

What is glycogenolysis?

• Breakdown of glycogen → glucose-1-phosphate

• Occurs in liver (for blood glucose) and muscle (for local use)

• Stimulated by glucagon and epinephrine

What is glycogenesis?

• Formation of glycogen from glucose

• Occurs in liver and muscle

• Stimulated by insulin

What is the Krebs (citric acid) cycle?

• Location: mitochondrial matrix

• Oxidizes acetyl-CoA → CO₂

• Produces:

  • NADH, FADH₂ → used in electron transport chain (ETC)

• Occurs only under aerobic conditions

Aerobic vs Anaerobic Metabolism?

• Aerobic:

  • Requires O₂

  • Occurs in mitochondria

  • Produces ~30–32 ATP per glucose

• Anaerobic:

  • No O₂

  • Occurs in cytosol

  • Produces 2 ATP + lactate

How are carbohydrates digested?

• Salivary and pancreatic amylase → polysaccharides → disaccharides

• Brush border enzymes → monosaccharides

• Absorbed via SGLT (glucose/galactose) and GLUT (fructose)

How are lipids digested?

• Bile salts emulsify fats

• Pancreatic lipase → fatty acids + monoglycerides

• Form micelles → enterocytes → chylomicrons → lymph (lacteals)

How are proteins digested?

• Stomach: pepsin

• Small intestine: pancreatic proteases (trypsin, chymotrypsin)

• Brush border peptidases → amino acids

How are nucleic acids digested?

• Pancreatic nucleases → nucleotides

• Broken into bases, sugars, phosphates → absorbed

What are G-protein coupled receptors (GPCRs)?

Membrane receptors that activate intracellular signaling

What are the four basic tissue types?

• Epithelial – protection, absorption, secretion

• Connective – support (bone, blood, fat)

• Muscle – contraction (skeletal, cardiac, smooth)

• Nervous – signaling

How does structure of tissues relate to function?

• Epithelial: tightly packed → barrier

• Muscle: elongated fibers → contraction

• Nervous: long processes → signal transmission

What is diffusion?

Movement from high → low concentration

What is osmosis?

• Water movement across membrane toward higher solute concentration

How is blood osmolality maintained?

• Regulated by ADH and thirst

• Albumin contributes to oncotic pressure → keeps water in blood vessels

Facilitated diffusion?

• Passive = no ATP

• Uses carrier proteins (e.g., GLUT)

Active transport?

• Requires ATP or ion gradient

• Moves against gradient

Simple diffusion?

• Directly through membrane (nonpolar molecules)

Protein channels?

Passive movement of ions through channels

What is transcellular transport?

Through cells (via membranes and transporters)

What is paracellular transport?

Between cells (through tight junctions)

How does the NMJ function?

• Action potential arrives at neuron

• Ca²⁺ enters neuron → ACh released

• ACh binds nicotinic receptors on muscle

• Na⁺ influx → depolarization (end plate potential)

• Triggers muscle action potential

What is the hierarchy of muscle structure?

• Muscle → whole organ

• Fascicle → bundle of fibers

• Muscle fiber (muscle cell) → single multinucleated cell

• Myofibril → contractile threads inside fiber

• Sarcomere → smallest functional unit

What are the differences between skeletal, cardiac, and smooth muscle?

Feature	Skeletal	Cardiac	Smooth
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No
Nuclei	Multinucleated	1–2	1
Cells	Long fibers	Branched	Spindle-shaped
Special features	NMJ	Intercalated discs	Dense bodies
Contraction	Fast	Rhythmic	Slow, sustained

What are slow-twitch (Type I) fibers?

• High endurance

• Many mitochondria

• Aerobic metabolism

• Fatigue-resistant

What are fast-twitch (Type II) fibers?

• High (force) and speed

• Anaerobic metabolism

• Fatigue quickly

What is excitation–contraction coupling and how does it work?

Link between muscle action potential and contraction.

Works by:

1. Action potential travels along sarcolemma

2. Enters T-tubules

3. Triggers Ca²⁺ release from SR

4. Ca²⁺ enables contraction

What determines oxygen diffusion into blood?

• Partial pressure gradient (PO₂)

• Surface area of alveoli

• Thickness of membrane

• Hemoglobin concentration

• Ventilation-perfusion matching (V/Q ratio)

How do humans adapt to low oxygen (hypoxia)?

• Increased ventilation (hyperventilation) → more O₂ intake

• Increased red blood cell production via EPO (erythropoietin)

• Increased capillary density → improved O₂ delivery

• Increased 2,3-BPG in RBCs → decreases hemoglobin affinity → enhances O₂ unloading

• Long-term: mitochondrial efficiency improves

What are oxygen-hemoglobin dissociation curves?

• Relationship between PO₂ (partial pressure of oxygen) and hemoglobin saturation

How is CO₂ transported in blood?

• Dissolved in plasma (~10%)

• Bound to hemoglobin (~20%)

• As bicarbonate (~70%)

Neutrophils

first responders, phagocytosis

Macrophages

phagocytosis + antigen presentation

Dendritic Cells

strongest antigen-presenting cells

B Cells

produce antibodies; memory

T Cells

• Helper (CD4⁺): coordinate response

• Cytotoxic (CD8⁺): kill infected cells

NK Cells

kill infected/tumor cells (innate)

What is innate immunity?

• Immediate, non-specific

• Includes barriers, phagocytes, inflammation

What is adaptive immunity?

Specific, slower, has memory

Humoral immunity?

B cells → antibodies

Cell-mediated immunity?

T cells → kill infected cells

What are formed elements?

• Red blood cells (erythrocytes): O₂/CO₂ transport

• White blood cells (leukocytes): immune defense

• Platelets (thrombocytes): clotting

What determines blood volume?

• Water intake/output

• Kidney regulation (ADH, aldosterone)

• Salt balance (Na⁺ retention)

• Plasma proteins (oncotic pressure)

• Blood loss or dehydration

What is albumin?

• Most abundant plasma protein

• Maintains oncotic (osmotic) pressure

• Keeps water in blood vessels

• Transports hormones, fatty acids, drugs

Where is most blood during exercise?

• Skeletal muscles (majority)

• Skin (thermoregulation)

• Heart (increased demand)

Where is most blood during sleep/rest?

• Digestive organs

• Kidneys

• Liver (metabolic processing)

What mechanisms control blood volume in the kidneys?

1. RAAS (Renin-Angiotensin-Aldosterone System)

   • ↑ Na⁺ and water retention

2. ADH (vasopressin)

   • Inserts aquaporins → ↑ water reabsorption

3. Aldosterone

   • ↑ Na⁺ reabsorption in DCT/collecting duct

4. ANP (atrial natriuretic peptide)

   • Promotes Na⁺ and water excretion

What is stroke volume (SV)?

• Volume of blood ejected per beat (from one ventricle)

• Depends on preload, contractility, and afterload

What is cardiac output?

• CO = HR × SV

• Total blood pumped per minute

What is the Frank-Starling mechanism?

• Increased venous return → increased ventricular filling (stretch)

• Stretch increases sarcomere length → optimal actin-myosin overlap

• Results in stronger contraction

What does the ECG (PQRST) represent?

• P wave: atrial depolarization

• QRS complex: ventricular depolarization

• T wave: ventricular repolarization

What are Starling forces? Its Key Forces?

Forces controlling fluid movement across capillaries

• Capillary hydrostatic pressure (pushes fluid out)

• Blood oncotic pressure (albumin pulls fluid in)

What are the main retinal components?

• Rods: low light (black/white vision)

• Cones: color vision (high acuity)

• Bipolar cells: relay signals

• Ganglion cells: send signal to brain via optic nerve

How is light processed?

Light passes through retinal layers → photoreceptors → bipolar → ganglion → brain

What is membrane potential?

• Electrical difference across membrane (~ -70 mV resting)

Oligodendrocytes?

CNS myelin formation

Schwann cells?

PNS myelin formation

Astrocytes?

• Blood-brain barrier support

• Nutrient regulation

• Ion balance

Microglia?

Immune defense of CNS

Ependymal cells?

Produce cerebrospinal fluid (CSF)

Unipolar neurons?

• Single process

• Common in sensory pathways

Bipolar neurons?

• 2 processes

• Example: retina, olfactory system