Unit 3 Physiology Study Guide: Kidneys, Digestive System, Metabolism, and Immune System

Kidney Physiology

Homeostatic Functions of the Kidneys

The kidneys are essential organs responsible for maintaining internal balance (homeostasis) by regulating various physiological parameters.

• Regulation of Volume: Controls blood volume, which directly affects blood pressure.

• Regulation of Osmolarity: Maintains sodium (Na+) and water balance.

• Regulation of pH: Adjusts hydrogen ion (H+) and bicarbonate (HCO3-) levels.

• Removal of Wastes: Eliminates metabolic waste products from the body.

• Hormone Production: Synthesizes hormones such as erythropoietin and renin.

Nephron Structure and Function

Nephrons are the functional units of the kidneys, each consisting of a renal corpuscle and tubules.

• Renal Corpuscle: Includes Bowman's capsule and the glomerulus, a fenestrated capillary acting as a filtration sieve.

• Glomerular Filtration Rate (GFR): Influenced by blood pressure and the constriction of afferent/efferent arterioles.

• Tubular Processing: Filtrate undergoes reabsorption (from tubule to blood) and secretion (from blood to tubule).

Water Reabsorption and Vasopressin (AVP)

Water reabsorption is regulated by vasopressin (antidiuretic hormone, AVP), which responds to changes in osmolarity.

• Chemoreceptors: Located in the hypothalamus, monitor osmolarity.

• AVP Release: Increases as osmolarity rises, promoting water reabsorption.

• Aquaporin-2 (AQP2): Inserted into the apical membrane of collecting duct cells to facilitate water movement.

Glucose and Sodium Reabsorption: SGLT and GLUT

Glucose and sodium reabsorption in the nephron involves specialized transporters.

• SGLT (Sodium-Glucose Linked Transporter): Located on the apical side, moves Na+ down its gradient and glucose against its gradient.

• GLUT: On the basal side, transports glucose into the blood.

• Na-K-ATPase: Maintains sodium gradient by pumping Na+ out of the cell.

• Clinical Application: Farxiga (a drug) blocks SGLT for diabetes mellitus type 2 treatment.

Calcium Regulation

Calcium levels are managed by the parathyroid gland through parathyroid hormone (PTH).

• PTH Release: Triggered when Ca2+ levels are low.

• Actions: Promotes reabsorption of Ca2+ in the kidneys and release from bone.

pH Regulation

The kidneys regulate acid-base balance by controlling H+ and HCO3- excretion.

• Acidosis (pH < 7.35): Increased H+ secretion.

• Alkalosis (pH > 7.45): Increased HCO3- excretion.

Micturition (Urination)

Micturition is the process of urine storage and voiding, controlled by smooth and skeletal muscle.

• Storage: Detrusor muscle relaxed; internal and external sphincters contracted.

• Voiding: Stretch receptors activate the parasympathetic division; detrusor contracts, internal sphincter relaxes, external sphincter voluntarily relaxed.

Renin-Angiotensin-Aldosterone System (RAAS)

RAAS is a hormonal system regulating blood pressure and fluid balance.

• Low Blood Pressure: Stimulates renin release.

• Renin: Converts angiotensinogen to angiotensin I.

• ACE: Converts angiotensin I to angiotensin II.

• Angiotensin II Effects: Vasoconstriction, increased AVP and aldosterone, increased thirst, increased blood volume and pressure.

• ACE Inhibitors: Block conversion to angiotensin II; used to treat hypertension.

Digestive System Physiology

Functions of the Digestive System

The digestive system breaks down food and absorbs nutrients through coordinated processes.

• Digestion: Breakdown of biomacromolecules into monomers.

• Absorption: Movement of nutrients into circulation.

• Secretion: Release of enzymes, acid, bile, and mucus.

• Motility: Movement via smooth muscle (peristalsis, segmentation).

Mechanical vs. Chemical Digestion

Digestion occurs through mechanical and chemical means.

• Mechanical Digestion: Increases surface area without breaking chemical bonds (e.g., mastication).

• Chemical Digestion: Uses enzymes to break chemical bonds, producing absorbable monomers.

GI Tract Structure and Motility

The gastrointestinal tract is a multilayered tube facilitating digestion and movement.

• Smooth Muscle: Enables peristalsis (propulsion) and segmentation (mixing).

Oral Cavity and Stomach Digestion

• Oral Cavity: Saliva contains amylase for carbohydrate digestion; mastication increases surface area.

• Stomach: Acid (HCl) lowers pH, kills microbes, denatures proteins; pepsin digests proteins; mucus and bicarbonate protect lining.

Small Intestine: Digestion and Absorption

The small intestine is the primary site for digestion and absorption.

• Structure: Circular folds, villi, and microvilli maximize absorption.

• Pancreas: Secretes digestive enzymes and bicarbonate to neutralize stomach acid.

• Enzymes: Digest carbohydrates, proteins, nucleic acids.

Liver and Bile: Lipid Digestion

• Bile: Emulsifies lipids into micelles, increasing surface area for lipase activity.

• Lipid Digestion: Produces fatty acids and monoglycerides, absorbed into the lymphatic system.

Absorption of Nutrients

• Carbohydrates: Monosaccharides absorbed into blood.

• Proteins: Amino acids absorbed into blood.

• Nucleic Acids: Bases, sugars, phosphate absorbed into blood.

• Lipids: Fatty acids and glycerol absorbed into lymphatic system.

Water and Ion Absorption

• Ion Movement: Na+, Cl-, K+ absorption creates osmotic gradient; water follows via osmosis.

• Disruption: Cholera toxin opens Cl- channels, causing water retention in lumen and severe diarrhea.

Liver Functions

• Detoxification: Neutralizes drugs and toxins (CYP enzymes).

• Bile Production: Supports lipid digestion.

Metabolism and Energy Balance

Metabolic Pathways

Metabolism encompasses all chemical reactions in the body, divided into anabolic and catabolic pathways.

• Anabolism: Builds molecules and requires energy (e.g., glycogenesis, lipogenesis, protein synthesis).

• Catabolism: Breaks down molecules and releases energy (e.g., glycogenolysis, lipolysis, gluconeogenesis).

Hormonal Regulation of Metabolism

• Fed State (Insulin): Promotes glucose uptake, stimulates glycogenesis, lipogenesis, protein synthesis.

• Fasted State (Glucagon): Promotes glycogenolysis, stimulates gluconeogenesis and lipolysis.

Energy Storage

• Glycogen: Short-term, rapid access energy storage.

• Lipids: Long-term, energy-dense storage.

Immune System Physiology

Overview of the Immune System

The immune system is a multilayered defense mechanism protecting the body from foreign substances.

• Barriers: Physical (skin, mucus), mechanical (cilia, coughing), and chemical (stomach acid, enzymes).

• Innate Immunity: Rapid, non-specific defense; recognizes pathogen-associated molecular patterns (PAMPs) via pattern-recognition receptors (PRRs).

• Adaptive Immunity: Slower, highly specific; generates immune memory.

Immune Cell Types

• Leukocytes (White Blood Cells): Produced in bone marrow, differentiate into specialized cells.

• Lymphatic System: Returns fluid to circulation, filters lymph, exposes immune cells to pathogens.

Innate Immunity

• Granulocytes: Neutrophils (phagocytosis of bacteria), eosinophils (target parasites), basophils (release cytokines, promote inflammation).

• Natural Killer (NK) Cells: Monitor MHC I expression; destroy cells lacking MHC I.

• Inflammation: Cytokines coordinate immune cell activity, increase blood flow and cell delivery.

Adaptive Immunity

• B and T Lymphocytes: Each specific to one antigen; B cells mature in bone marrow, T cells in thymus.

• Antigen Presentation: Antigens presented on MHC II by phagocytic cells activate B and T cells.

• Clonal Expansion: Activated lymphocytes divide rapidly, differentiate into effector and memory cells.

• Effector Cells: Helper T cells (cytokines), cytotoxic T cells (destroy infected cells), plasma cells (produce antibodies).

• Memory Cells: Provide long-term immunity, enable faster secondary responses.

• Antibodies: Bind antigens, enhance phagocytosis (opsonization), agglutination, neutralization.

Blood Type and Immune Reactions

• Blood Type: Determined by surface antigens (A, B, Rh) on red blood cells.

• Antibodies: Target non-self antigens; incorrect transfusions cause agglutination and cell destruction.

Autoimmune Diseases and Allergies

• Autoimmune Disease: Failure of self-tolerance; immune system attacks self tissues.

• Allergies: Exaggerated response to harmless antigens; can cause anaphylaxis (treated with epinephrine).

Vaccines and Immunity

• Vaccines: Use antigens to train adaptive immunity; primary exposure creates memory cells, secondary exposure produces stronger response.

• Types: Inactivated, attenuated, subunit, mRNA.

• Passive Immunity: Immediate protection via transferred antibodies; temporary, no memory formation.

Antibody Types and Applications

• Polyclonal Antibodies: Target multiple antigens; produced by multiple B cell clones.

• Monoclonal Antibodies: Target a single antigen; used in diagnostics and treatment (e.g., antibody-mediated drug delivery for cancer).

Table: Comparison of Innate and Adaptive Immunity

Feature	Innate Immunity	Adaptive Immunity
Response Time	Minutes to hours	Days
Specificity	Non-specific	Highly specific
Memory	None	Long-term memory
Cell Types	Granulocytes, NK cells	B and T lymphocytes
Key Functions	Inflammation, phagocytosis	Antibody production, cytotoxicity

Table: Metabolic Pathways

Pathway	Type	Function
Glycogenesis	Anabolic	Glucose to glycogen
Lipogenesis	Anabolic	Glucose to lipid
Protein Synthesis	Anabolic	Amino acids to proteins
Glycogenolysis	Catabolic	Glycogen to glucose
Lipolysis	Catabolic	Fat to fatty acids
Gluconeogenesis	Catabolic	Non-carbohydrate to glucose

Key Equations

• Glomerular Filtration Rate (GFR): $\text{GFR} = \text{K}_f \times \left(\text{P}_\text{GC} - \text{P}_\text{BS} - \pi_\text{GC}\right)$ Where $\text{K}_f$ is the filtration coefficient, $\text{P}_\text{GC}$ is glomerular capillary pressure, $\text{P}_\text{BS}$ is Bowman's space pressure, and $\pi_\text{GC}$ is glomerular capillary oncotic pressure.

• Osmosis: $\text{Water movement} \propto \text{Osmotic gradient}$

• Metabolism: $\text{Metabolism} = \text{Anabolism} + \text{Catabolism}$

Example: ACE inhibitors are used clinically to treat hypertension by blocking the conversion of angiotensin I to angiotensin II, thereby reducing vasoconstriction and blood pressure.

Example: Cholera toxin disrupts ion transport in the intestine, leading to severe diarrhea due to impaired water absorption.

Example: Vaccines (such as mRNA vaccines) stimulate adaptive immunity, creating memory cells for long-term protection against pathogens.