Essential Physiology: GI, Renal, Endocrine, and Sensory Systems

Posted on Oct 19, 2025 in Medicine

The Digestive System: Structure and Function

Functions of the digestive system include: ingestion, secretion, mixing, propulsion, digestion, absorption, and defecation.

Layers of the GI Tract Wall

The mucosa is the innermost layer of the GI tract.
The Serosa is the outermost layer.
The Serosa is called the visceral peritoneum in portions of the GI tract suspended in the abdominopelvic cavity.
The Serosa is called the Adventitia in the esophagus.

Innervation and Secretions

The lower portion of the large intestine receives parasympathetic innervation from sacral spinal nerves.
The parotid salivary glands are located anterior and inferior to the ears.
Sympathetic stimulation decreases salivation.
Sublingual and submandibular glands are innervated by the facial nerve (CN VII).
The openings of the folds on the inner surface of the stomach are called gastric pits.
ECL (Enterochromaffin-like) cells in the gastric epithelium secrete Histamine and Serotonin.
Trypsin is the enzyme that activates other zymogens of pancreatic juice.
Bilirubin is produced as a derivative of the heme group in hemoglobin.
Enterokinase is attached to the membrane of the brush border cells.

Key GI Components and Functions

Stomach: Histamine, Chief cells (pepsinogen + gastric lipase), ECL cells, D cells (somatostatin), Gastric juice, Parietal cells (intrinsic factor + HCl), Gastric pits, G cells (gastrin).
Duodenum: Contains Enterokinase.
Jejunum: Important for the control of stomach emptying.
Ileum: Site of CCK release. CCK inhibits gastric emptying by inhibiting the pyloric pump. Absorbs bile salts, Vitamin B12, water, and electrolytes.
Pancreas: Secretes Trypsin, Elastase, and Bicarbonate. Contains sinusoids with large pores (fenestrae). Secretes insulin and glucagon.
Gallbladder: Stores and concentrates bile.

Additional Digestive Facts

Bile salts are needed for the absorption of lipids.
The liver is the principal site of iron storage and albumin production.
Increased distension of the duodenum decreases gastric emptying.
Parietal cells release intrinsic factor, a substance necessary for the absorption of Vitamin B12.
The pancreas arises from the embryological germ layer: endoderm.
Pepsin is more likely to be found in a patient suffering from reflux.
Colon: Mucus is secreted for lubrication. Fecal transit is inversely related to fiber content.
Path of Bile Salts: Liver → Gallbladder → Duodenum → Jejunum → Ileum → Liver (via enterohepatic circulation).

Renal System: Kidney Function and Nephron Physiology

Kidney Function

The kidneys regulate extracellular fluid (via urine formation), maintain blood pH, blood pressure, and solute concentration.

Afferent arterioles: Deliver blood into the glomeruli.
The kidneys are located in the retroperitoneal space.
The entrance of the kidney is called the Hilum.
The outermost covering of the kidney is the Cortex.
Each minor calyx receives urine from the Renal Papillae.
The basic functional unit of the kidney is the Nephron.
The last part of the nephrons are the collecting ducts.
Urine flows from the kidney to the bladder by peristalsis.
The external sphincter is under voluntary control.

Nephron Types

Cortical Nephrons (80%): Renal corpuscles lie in the outer cortex. They have a short Loop of Henle and are surrounded by peritubular capillaries.
Juxtamedullary Nephrons (20%): Renal corpuscles lie deep in the cortex. They have a long Loop of Henle and are surrounded by peritubular capillaries and the vasa recta. They are crucial for producing dilute urine.

Urine Formation Processes

Urine Output = Glomerular Filtration – Tubular Reabsorption + Tubular Secretion

Glomerular Filtration: Water and solutes in blood plasma move across the wall of glomerular capillaries into the glomerular capsule.
Tubular Reabsorption: Movement of water and solutes from the tubules back to the peritubular capillaries and vasa recta.
Tubular Secretion: Secretion of wastes from the blood into the urine.

Glomerular Structure and Regulation

The inner layer of the glomerular capsule presents cells called podocytes.
Sympathetic Nervous System Effect: Increased sympathetic stimulation constricts afferent arterioles, decreasing Glomerular Hydrostatic Pressure (GHP), Net Filtration Rate (NFR), Urine Volume, and Glomerular Filtration Rate (GFR).

Renal Autoregulation (2 Mechanisms)

Myogenic Mechanism: Stretching triggers contraction of smooth muscles in afferent arterioles. Rises in blood pressure (BP) lead to rises in GFR.
Tubuloglomerular Feedback: Involves the macula densa and juxtaglomerular cells.

Tubule Segments and Cell Types

Bowman’s Capsule: Filtration.
Proximal Convoluted Tubule: Always permeable to water (Aquaporin-1); major site of reabsorption of water and solutes.
Descending Loop of Henle: Highly permeable to water (Aquaporin-1).
Ascending Loop of Henle: Impermeable to water; primary active transport of NaCl.
Distal Convoluted Tubule: Water permeability depends on the body’s needs. Site of H+ secretion, regulated by aldosterone. Contains intercalated cells and principal cells.

Collecting Duct Cells

Principal cells: Regulated by ADH and Aldosterone.
Intercalated cells: Important for the homeostasis of blood pH.

The Endocrine System: Hormone Classification and Action

Hormone Chemical Classes

Proteins and Polypeptides: Anterior and posterior pituitary gland hormones, pancreas hormones, parathyroid gland hormone.
Steroids: Adrenal cortex hormones, ovaries, testes, and placenta hormones.
Tyrosine Derivatives (Amines): Thyroid hormones and adrenal medulla hormones.

Mechanism of Action and Synthesis

Polypeptide and Protein Hormones

Synthesized in the endoplasmic reticulum (prohormones are formed, which are finally activated).
Receptors are located on the plasma membrane.

Steroid Hormones

Synthesized from cholesterol; soluble in lipids.
Receptors are located in the cytoplasm.
Pregnenolone formation is the rate-limiting step.

Amine Hormones Derived from Tyrosine

Thyroid Hormones: Synthesized and stored in the thyroid gland.
Adrenal Medullary Hormones: Epinephrine and norepinephrine, stored as vesicles.
Receptors are located in the nucleus.
Thyroid gland produces T4, T3, and calcitonin.

Calcium Regulation

Calmodulin: Binds to Ca2+ (four Ca2+ sites); activates or inactivates protein kinases.
Parathyroid hormone (PTH): Increases calcium and phosphate absorption from bone.

Pituitary Gland and Hypothalamus

Pituitary secretion is controlled by the hypothalamus, connected via the pituitary stalk.
Magnocellular cells (in the hypothalamus) release ADH and oxytocin into the posterior pituitary.

Anterior Pituitary Hormones

All are secreted by the anterior pituitary:

GH (Growth Hormone): Targets the liver.
ACTH (Adrenocorticotropic Hormone): Targets the adrenal cortex; action is cortisol secretion.
TSH (Thyroid-Stimulating Hormone): Targets the thyroid gland.
PRL (Prolactin): Targets the mammary gland; inhibited by dopamine.
FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone): Target ovaries and testes.
MSH (Melanocyte-Stimulating Hormone): Targets the brain.

Posterior Pituitary Hormones

ADH (Antidiuretic Hormone) and Oxytocin.

Hormonal Feedback

Estrogen (females) and Testosterone (males) suppress GnRH, FSH, and LH.
Progesterone inhibits GnRH.
GnRH controls the secretion of FSH and LH.

Thyroid Hormones

Located below the larynx on each side of the anterior trachea.
Thyroxine = T4; Triiodothyronine = T3. Iodine is obtained from the diet.
Formation Steps: Iodine trapping, synthesis of thyroglobulin, oxidation of iodide, iodination of tyrosine, coupling of T1 and T2, secretion of thyroid hormone. Stored in target cells.
TSH (Thyrotropin) Action: Increases iodide pump activity, iodination of tyrosine, and proteolysis of thyroglobulin.
TRH (Thyrotropin-Releasing Hormone): A tripeptide that affects the anterior pituitary, increasing TSH output.
Action Mechanism: Activates nuclear transcription.
Functions: Increase ATP production, stimulate fat metabolism, and affect other endocrine glands.

Adrenal Glands

Adrenal Cortex: Produces steroid hormones.
Adrenal Medulla: Produces three catecholamines (epinephrine, norepinephrine, dopamine).

Adrenal Cortex Zones

Zona Glomerulosa (Outer): Secretes mineralocorticoids (e.g., aldosterone). Secretion is controlled by ECF concentration of Angiotensin II and K+.
Zona Fasciculata (Middle): Secretes Cortisol. Secretion is controlled by CRH-ACTH axis.
Zona Reticularis (Inner): ACTH regulated. Secretes weak androgens and estrogen.

Corticoid Functions

Mineralocorticoids (Aldosterone): Regulate blood pressure and volume. Promote excretion of H+ in urine and regulate levels of Na+, K+, and H+.
Glucocorticoids (Cortisol): CRH stimulates ACTH secretion, which controls the production of cortisol (negative feedback effects).

Pancreas Hormones (Islets of Langerhans)

A cells: Secrete glucagon.
B cells: Secrete insulin.
D cells: Secrete somatostatin, which inhibits insulin and glucagon.
F cells: Secrete pancreatic polypeptide.

Regulation of Pancreatic Hormones

Factors increasing Insulin: Acetylcholine, GIP (Gastric Inhibitory Peptide), Arginine, and Leucine.
Factors increasing Glucagon Secretion: Exercise, and meals high in protein.

Sensory Biology: Receptors and Perception

Classification of Sensory Receptors

Mechanoreceptors: Respond to mechanical compression of adjacent tissues.
Thermoreceptors: Respond to changes in temperature.
Nociceptors: Respond to damage or potential damage (do not adapt).
Electromagnetic Receptors: Respond to light.
Chemoreceptors: Respond to taste, smell, and chemical concentrations (e.g., CO2).

Receptor Location

Exteroceptors: Located on the external surface of the body; provide information about the environment.
Interoceptors: Located in blood vessels and muscles; monitor the internal environment.
Proprioceptors: Located in muscles, tendons, joints, and the inner ear; monitor body position.

Processing Sensations (4 Steps)

Stimulation of the sensory receptor.
Transduction of the stimulus.
Generation of a nerve impulse (First-order neuron).
Integration in the Central Nervous System.

Potential Types

Generator Potential: Occurs in free nerve endings of the first-order neuron (pain, temperature) or encapsulated first-order neurons (pressure, vibration, touch). An action potential is reached if the potential is large enough to reach the threshold.
Receptor Potential: Occurs in separate cells that synapse with the first-order sensory neuron (hearing, equilibrium). This triggers the release of neurotransmitters, producing a Postsynaptic Potential (PSP) and triggering an action potential.

Receptor Adaptation

Tonic Receptors: Transmit impulses to the brain continuously, providing information on muscle contraction status, pain, position, and chemical composition.
Phasic Receptors: React strongly to changes but quickly adapt. They have a predictive function.

Pain

Pain is subjective.

Acute Pain: Protective, well localized, reliable for diagnosis.
Chronic Pain: Not well localized; involves peripheral and central sensitization.
Nociceptor Types: Thermal, Mechanical, and Polymodal.
Sensitization: Decreases threshold, increases response, and develops spontaneous activity.
Hyperalgesia: Exaggerated response to noxious stimuli.
Allodynia: Response to stimuli that are normally innocuous.

Eyes and Vision

Ocular Structures

Cornea: Transparent coat, curved, and helps focus light.
Sclera: Opaque, fibrous, protective outer layer. Provides shape and attachment surfaces for extrinsic eye muscles.
Choroid: Highly vascular to supply nutrients and remove waste products. Melanin absorbs light rays, preventing reflection.
Ciliary Body: Secretes aqueous humor. Contains ciliary muscles, enabling the lens to change shape to focus on near and distant objects.
Iris: Pigmented muscular structure that controls the amount of light entering the eye.
Pupil: The hole in the center of the iris through which light continues passage.
Lens: Transparent, flexible, curved structure. Focuses light using refractive properties.
Retina: Layer of sensory neurons containing photoreceptors (rods and cones).
Fovea: Responsible for good visual acuity.
Blind Spot: Where sensory fibers bundle to form the optic nerve.

Photoreceptors Comparison

Cones

Day vision and color vision.
High acuity, low sensitivity.
Plasma membrane folds back and forth.
Little convergence (good resolution).
Three types (for color). Used for tasks like reading.

Rods

Night vision and shades of gray.
Low acuity, high sensitivity.
Contain Opsin and Retinal (forming Rhodopsin).
Free-floating discs.
Much convergence (poor resolution).

Accommodation

For near vision, the ciliary muscle contracts so that the suspensory ligaments become slack. This allows the lens to round up, which increases the strength of the lens.

Hearing and Auditory System

Functional Parts of the Ear

External Ear: Captures mechanical energy. (Includes Auricle, External Auditory Canal, Eardrum).
Middle Ear: Transmits energy to the receptor organ. (Air-filled; contains auditory ossicles: Malleus, Incus, Stapes).
Inner Ear: Transduces mechanical energy into an electrical signal. (Contains the labyrinth and perilymph).