Endocrine System and Human Reproduction: Key Concepts

Posted on Dec 20, 2024 in Biology

Key Points for Exam on the Endocrine System

Types of Chemical Communication

• Endocrine signaling: Uses hormones to regulate metabolism and homeostasis over a longer duration, requiring blood supply for transmission.
• Neurohormone signaling: Bridges the nervous and endocrine systems, where hormones are secreted into the bloodstream by neurons.

Hormone Properties and Functions

• Water-soluble hormones:
  • Stored in vesicles, released via exocytosis.
  • Travel freely in blood and bind to membrane receptors, triggering rapid responses.
• Lipid-soluble hormones:
  • Released by diffusion, require carrier proteins in blood.
  • Bind intracellular/nuclear receptors, altering gene expression for longer-term effects.

Hypothalamic-Pituitary-Endocrine Axis

• Posterior pituitary:
  • Releases neurohormones (e.g., oxytocin and ADH) stored in axon terminals.
  • Regulated by stimuli such as osmolarity changes or blood pressure.
• Anterior pituitary:
  • Releases tropic hormones (e.g., ACTH, TSH) under hypothalamic control via the hypothalamic-hypophysial portal system.
  • Tropic hormones regulate target glands like the thyroid, adrenal cortex, and gonads.

Feedback Mechanisms

• Negative feedback:
  • Maintains homeostasis by inhibiting hormone release when sufficient levels are reached.
  • Example: Thyroid hormones (T3/T4) inhibit TRH and TSH secretion.
• Positive feedback:
  • Enhances hormone release in response to stimuli, often for short-term processes.
  • Example: Oxytocin release during childbirth.

Key Hormonal Examples

• Insulin/Glucagon (Pancreas): Regulate blood glucose levels.
• ADH (Posterior Pituitary): Increases water reabsorption in kidneys to control osmolarity.
• Cortisol (Adrenal Cortex): Mobilizes energy stores during stress, increases blood glucose.
• T3/T4 (Thyroid): Regulate metabolism, thermogenesis, and growth.

Disorders

• Hyperthyroid goiter (e.g., Grave’s disease): Caused by overactivation of TSH receptors, leading to excessive T3/T4.
• Hypothyroid goiter (e.g., iodine deficiency): Low T3/T4 triggers high TSH, causing thyroid enlargement.

Reproduction

Asexual vs. Sexual Reproduction

• Asexual:
  • Offspring are genetically identical (e.g., budding, parthenogenesis).
• Sexual:
  • Produces genetically diverse offspring through gametogenesis, mating, and fertilization.

Gametogenesis

• Spermatogenesis:
  • Continuous process in testes producing four haploid sperm from one germ cell.
• Oogenesis:
  • Produces one haploid egg and polar bodies; meiosis pauses until puberty and fertilization.

Fertilization

• Species-Specific Mechanisms:
  • Sperm-egg recognition involves bindin proteins and glycoprotein receptors.
• Polyspermy Prevention:
  • Fast block: Na⁺ depolarization prevents additional sperm fusion.
  • Slow block: Cortical granule exocytosis forms a fertilization envelope.

Sex Determination

• Humans: XX (female), XY (male).
• SRY gene on the Y chromosome initiates male development.

Reproductive Cycles

• Ovarian Cycle:
  • Follicular Phase: FSH and LH stimulate follicle growth; ovulation occurs at day 14.
  • Luteal Phase: Corpus luteum secretes estrogen and progesterone.
• Uterine Cycle:
  • Proliferative Phase: Estrogen promotes endometrial growth.
  • Secretory Phase: Progesterone supports vascularization for implantation.
  • Menstruation: Endometrial shedding if no pregnancy occurs.

Pregnancy Hormones

• hCG: Maintains corpus luteum, preventing menstruation.
• Progesterone: Sustains endometrium and suppresses new ovarian cycles.

Contraception and Contragestation

• Hormonal Contraceptives: Mimic estrogen and progesterone to inhibit ovulation.
• Plan B (Levonorgestrel): High-dose progesterone prevents fertilization or implantation.
• Mifepristone: Blocks progesterone receptors, inducing endometrial breakdown.

Integrative Physiology

Homeostasis and System Interactions

• Organ systems interact to maintain physiological states, e.g., Renin-Angiotensin-Aldosterone System (RAAS) regulates blood pressure by integrating kidney, liver, and vascular functions.
• Baroreceptor reflex controls blood pressure by adjusting heart rate, stroke volume, and vessel radius through autonomic signaling (parasympathetic and sympathetic systems).

Acid-Base Balance

• pH homeostasis is essential for protein function and cellular activity.
• Regulated through:
  • Buffers: E.g., bicarbonate in plasma.
  • Ventilation: Adjusts CO₂ levels to stabilize pH.
  • Renal system: Type A cells secrete H⁺; Type B cells secrete HCO₃⁻ for balance.

Endocrine System

Chemical Communication

• Local (paracrine/autocrine): Quick and short-lasting effects.
• Neural: Rapid, precise control.
• Endocrine: Slower but longer-lasting effects via hormones.

Hypothalamic-Pituitary Axis

• Posterior pituitary releases ADH and oxytocin.
• Anterior pituitary secretes tropic hormones (e.g., ACTH, TSH) to regulate adrenal, thyroid, and gonadal activity.

Hormone Types

• Water-soluble: Bind to surface receptors (e.g., insulin, epinephrine).
• Lipid-soluble: Diffuse into cells, altering gene expression (e.g., cortisol, thyroxine).

Examples of Regulation

• RAAS: Activated by low BP; increases blood pressure via angiotensin II.
• Thyroid hormones (T3/T4): Regulate metabolism and thermogenesis.

Mechanism behind myogenic function of heart

• C. They possess voltage-gated If channels highly permeable to sodium at “resting” Vm

True statement about cardiac muscle cells

• D. Gap junctions allow depolarization to travel directly to neighboring cells

Pulmonary valve regurgitation’s effect on blood flow

• A. Blood flows from the pulmonary arteries into the right ventricle

Common feature among all muscle types

• C. Contraction results from the interaction between myosin and actin proteins

During one twitch of skeletal muscle

• D. Relaxation occurs more rapidly because it requires active transport of calcium ions

Cause of rigor mortis in muscles post-mortem

• D. Myosin cannot release actin after the power stroke without ATP

Core Functions

Excretory System: Regulates fluid volume, osmolarity, and ionic composition of extracellular fluid (ECF) by removing excess solutes, maintaining fluid balance, and influencing blood pressure.

Processes

Filtration: Non-selective fluid and solute movement from blood to filtrate.

Reabsorption: Selective retrieval of solutes and water from filtrate back into blood.

Secretion: Addition of specific solutes from blood to filtrate.

Key Concepts

Osmolarity: Movement of water toward areas of higher solute concentration (osmosis).

Osmoregulators: Actively regulate osmolarity different from the environment (most vertebrates).

Osmoconformers: Match osmolarity with environment (marine animals).

Nephron Structure & Function

Blood Flow: Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries.

Filtration: Occurs in the glomerulus, creating filtrate in Bowman’s capsule.

Tubule Sections

Proximal Convoluted Tubule (PCT): Reabsorbs most solutes and water.

Loop of Henle: Creates osmotic gradient for water reabsorption.

Distal Convoluted Tubule (DCT) & Collecting Duct: Adjust water and ion reabsorption; collecting duct permeability influenced by ADH (antidiuretic hormone).

Glomerular Filtration Rate (GFR)

Factors Affecting GFR: Hydrostatic pressure, osmotic pressure, and capsule pressure.

Regulation: Maintained by adjusting blood pressure in glomerular capillaries (vasoconstriction/dilation of arterioles).

Regulation of Blood Pressure and Osmolarity

ADH: Released in response to high osmolarity or low blood pressure, increasing water reabsorption in the kidneys by adding aquaporins to the collecting duct cells.

Aldosterone: Promotes Na+ reabsorption in DCT and collecting duct, enhancing water retention and increasing blood pressure.

Water Conservation in Mammals

Loop of Henle’s Countercurrent Mechanism: Helps concentrate urine by maintaining a high osmolarity gradient in the kidney medulla, allowing water reabsorption.

Urine Concentration: Achieved by ADH action in the collecting duct, resulting in the excretion of a small, concentrated urine volume.

Filtration, Reabsorption, and Secretion Equation

Excretion Rate = Amount Filtered – Amount Reabsorbed + Amount Secreted

Digestion Process

Steps: Ingestion → Digestion →

Absorption → Elimination

Peristalsis: Smooth muscle contractions move food through the gut; sphincters regulate passage between sections.

Regions

Mouth: Salivary amylase begins carbohydrate digestion.

Stomach:

Parietal cells produce acid (HCl) to denature proteins and activate pepsinogen.

Chief cells secrete pepsinogen for protein digestion.

Mucus cells protect stomach lining.

Enzyme Action in Small Intestine

Liver and Gallbladder: Produce and release bile to emulsify fats.

Pancreas: Releases digestive enzymes (e.g., trypsinogen, lipase) and bicarbonate to neutralize stomach acid.

Small Intestine: Enzymes like lactase for carbohydrate breakdown; absorption of nutrients occurs here.

Nutrient Absorption

Mechanisms: Diffusion, active transport, and facilitated diffusion.

Monosaccharides: Transported via SGLT (with Na+) and GLUT carriers.

Hepatic Portal System: Absorbed nutrients go to the liver for storage or processing.

Metabolism Phases

Absorptive State (post-meal): Anabolism (building) of glycogen, proteins.

Postabsorptive State (fasting): Catabolism (breakdown) of glycogen and fats to release energy.

Metabolic Regulation

Insulin (from pancreatic beta cells): Lowers blood glucose by promoting glucose uptake and storage.

Glucagon (from alpha cells): Raises blood glucose by stimulating glycogen breakdown.

Energy and Storage

Carbohydrate Metabolism: Glucose →

Glycogen (storage in liver/muscle); glycolysis and citric acid cycle for ATP production.

Control: Blood glucose homeostasis maintained by insulin and glucagon balance.

Sensory Systems (Topic 2.1)

1. Neural Networks:

   • Afferent (Sensory) Neurons: Transmit input from sensory receptors to CNS.
   • Efferent (Motor) Neurons: Relay signals from CNS to effectors (muscles, glands).
   • Interneurons: Facilitate integration and coordination within CNS.

2. Reflex Arc:

   • Sequence: Stimulus → Receptor → Afferent neuron → CNS (Integration) → Efferent neuron → Effector → Response.

3. Sensory Transduction:

   • Conversion of stimuli (light, sound, pressure, etc.) into neural signals.
   • Mechanism:
     • Stimulus alters membrane potential via ion channel activity.
     • Direct Mechanism: Ion channels open/close directly.
     • Indirect Mechanism: Involves secondary messengers and neurotransmitter release.

4. Mechanoreception:

   • Detects physical deformation (stretch, vibration, pressure).
   • Example:
     • Baroreceptors: Stretch-sensitive afferents in blood vessels regulate blood pressure.
     • Hair Cells: Mechanoreceptors in auditory and vestibular systems (e.g., cochlea, neuromast organs).

5. Auditory Transduction:

   • Pathway:
     1. Sound waves → Tympanic membrane vibration.
     2. Ossicles amplify vibration → Cochlear fluid movement.
     3. Hair cells in cochlea deflect → Stereocilia movement opens ion channels.
     4. Depolarization triggers neurotransmitter release → Auditory nerve.

6. Phototransduction:

   • Rods: Dim light, black-and-white vision (contain rhodopsin).
   • Cones: Bright light, color vision (multiple photopigments).
   • Dark: Na+ channels open (depolarized); continuous neurotransmitter release.
   • Light: Photon absorption → cGMP breakdown → Na+ channels close (hyperpolarized).

Muscle Systems (Topic 2.2)

1. Muscle Types:

   • Skeletal Muscle:
     • Voluntary; striated.
     • Works in antagonistic pairs: Flexors (bend joints) vs. Extensors (straighten joints).
   • Cardiac Muscle:
     • Involuntary; striated; intercalated discs with gap junctions.
     • Long plateau phase in action potentials.
   • Smooth Muscle:
     • Involuntary; non-striated; regulated by Ca2+.

2. Skeletal Muscle Contraction:

   • Steps:
     1. Neuromuscular Junction:
        • Motor neuron AP → ACh release → Muscle AP.
     2. Excitation-Contraction Coupling:
        • AP travels via T-tubules → Ca2+ release from SR → Binds to troponin → Tropomyosin shift exposes actin binding sites.
     3. Sliding Filament Mechanism:
        • Myosin binds actin → Power stroke (ATP hydrolysis).
        • ATP binding detaches myosin → Cycle repeats if Ca2+ remains.
     4. Relaxation:
        • Ca2+ reuptake into SR by ATPase → Tropomyosin blocks actin.

3. Force Generation:

   • Recruitment: Increasing motor units for stronger contractions.
   • Summation: APs increase frequency → Unfused or complete tetanus.
   • Length-Tension Relationship: Optimal sarcomere length maximizes cross-bridges.

Circulatory Systems (Topic 2.3)

1. Types:

   • Open: Hemolymph bathes tissues directly (e.g., invertebrates).
   • Closed: Blood confined to vessels; exchanges with interstitial fluid (e.g., vertebrates).

2. Human Circulatory Pathway:

   • Pulmonary Circuit: R atrium → R ventricle → Pulmonary artery → Lungs → Pulmonary vein → L atrium.
   • Systemic Circuit: L ventricle → Aorta → Body → Vena cava → R atrium.

3. Cardiac Cycle:

   • Phases:
     1. Atrial Systole: Atria contract; fill ventricles.
     2. Ventricular Systole: Ventricles contract; blood ejected.
     3. Diastole: Relaxation; chambers fill passively.
   • Heart Sounds:
     • S1: AV valves close (start systole).
     • S2: Semilunar valves close (start diastole).

4. Electrical Conduction:

   • SA Node (pacemaker) → AV Node → Bundle of His → Purkinje fibers → Ventricular contraction.

5. Regulation of Blood Pressure:

   • Mean Arterial Pressure (MAP):
     • MAP = CO × TPR (Cardiac Output × Total Peripheral Resistance).
   • Baroreceptor Reflex:
     • Sensors in carotid/aortic walls → CV center → Adjust heart rate and vessel radius.

6. Key Equations:

   • Poiseuille’s Law:
     • Resistance ∝ L × η / r⁴ (Length, Viscosity, Radius⁴).
   • Flow:
     • Q ∝ ΔP / Resistance.
   • Stroke Volume:
     • SV = EDV − ESV (End-Diastolic Volume − End-Systolic Volume).

Bio 303 Exam Cheat Sheet

Homeostasis

• Components: Sensor → Integrator → Effector
• Feedback Types:
  • Negative Feedback: Restores set point.
  • Positive Feedback: Amplifies deviation.
• Key Concept: Fluctuates around the set point, not a fixed state.

Membrane Proteins & Transport

• Passive Transport:
  • Glucose Transport: Follows concentration gradient.
  • Simple Diffusion: High → low gradient; governed by Fick’s Law.
• Active Transport:
  • Na/K ATPase: Uses ATP to move ions against gradients.
  • Na-Glucose Co-Transporter: Uses Na gradient for glucose transport.
• Electrochemical Gradient:
  • Chemical gradient (diffusional force) vs. Electrical gradient (electrical force).
• Osmosis:
  • Diffusion of water via aquaporins.
  • Key Terms:
    • Molarity: Mole/L of solvent.
    • Osmolarity: Total solute particles/L.
    • Tonicity: Net flow of water based on solute and permeability.

Neurophysiology

• Membrane Potential:
  • Resting: -70 mV; maintained by Na/K ATPase and K leak channels.
  • Depolarization: Membrane becomes more positive.
  • Hyperpolarization: Membrane becomes more negative.
• Action Potential:
  • Threshold: Minimum voltage to open voltage-gated channels.
  • Refractory Periods:
    • Absolute: No 2nd action potential possible.
    • Relative: 2nd possible but harder.
• Synaptic Transmission:
  • Electrical: Rapid, gap junction-mediated.
  • Chemical: Slower, uses neurotransmitters (NT).
  • Removal: Reuptake, enzymatic degradation, or diffusion.

Sensory Systems

• Mechanoreception:
  • Blood pressure: Baroreceptors detect stretch.
  • Hearing: Hair cells transduce sound to electrical signals.
• Photoreception:
  • Rods: Dim light; grayscale.
  • Cones: Bright light; color vision.
  • Light activates rhodopsin → changes membrane potential.

Muscle Physiology

• Types:
  • Smooth: Involuntary; gap junctions.
  • Skeletal: Voluntary; striated.
  • Cardiac: Involuntary; striated.
• Skeletal Muscle Contraction:
  • Steps:
    1. Neuron AP → Muscle AP.
    2. Ca release from SR → contraction.
    3. Relaxation: Ca pumped back to SR.
  • Mechanisms:
    • Twitch: One contraction-relaxation cycle.
    • Summation: Increased tension with closer stimuli.
    • Tetanus: Sustained tension with high-frequency stimuli.

Key Equations

• Fick’s Law: Diffusion rate ∝ Surface Area × Concentration Gradient × Membrane Permeability.
• Diffusion Time: T=x22DT = \frac{x^2}{2D}.