Digestive, Circulatory, Excretory, Nervous, and Muscular Systems: Core Concepts

Digestion & Nutrition

Macromolecules & Diets

Macromolecules are essential nutrients required for energy, growth, and cellular function. Diets are composed of carbohydrates, proteins, and fats, each serving distinct biological roles.

• Carbohydrates (Sugars, Starch): Primary energy source. Found in high amounts in herbivore fruits, grains, and omnivore diets.

• Proteins: Required for growth, repair, and enzymes. "Essential amino acids" are those the body cannot synthesize and must be obtained from diet (e.g., meat, eggs, beans). A "complete" protein contains all essential amino acids.

• Fats/Lipids: Long-term energy storage, insulation, cell membranes.

• Chemical Energy (from food): Used for movement, including biosynthesis, active transport, and cell growth.

Digestive System Structures & Journey of Food

The digestive system breaks down food mechanically and chemically, allowing absorption of nutrients.

• Mouth: Mechanical digestion (chewing increases surface area). Chemical digestion of starch by salivary amylase.

• Stomach: Chemical digestion of proteins by pepsin (activated from pepsinogen by HCl). Food becomes chyme.

• Small Intestine: Primary site of digestion and absorption.

  • Duodenum: First part. Enzymes (from intestine & pancreas) digest fats, proteins, carbohydrates. Bile (liver) emulsifies fats.

  • Jejunum & Ileum: Nutrient absorption. Villi and microvilli increase surface area.

• Large Intestine: Reabsorbs water and electrolytes. Houses symbiotic bacteria. Forms feces for excretion.

Comparative Digestive Anatomy

• Gastrovascular Cavity: A single opening for ingestion and egestion. Found in simple animals (e.g., cnidarians, flatworms).

• Alimentary Canal: A tube with a mouth and anus, allowing for specialized regions for digestion and absorption. Found in Annelids, Arthropods, Chordates.

• Adaptations: Carnivores have sharp teeth for tearing and shorter digestive tracts. Herbivores have flat teeth for grinding, longer tracts, and often a large cecum for fermenting plant material. Omnivores have a mix.

Circulation & Gas Exchange

Blood and Blood Vessels

Circulatory systems transport nutrients, gases, and wastes throughout the body.

• Hemolymph: Blood is contained within a closed circulatory system (vessels). Hemolymph is the fluid in an open circulatory system that directly bathes tissues (e.g., in insects).

• Arteries: Carry blood away from the heart.

• Capillaries: Site of gas and nutrient exchange with tissues. Small diameter and thin walls.

• Veins: Carry blood back to the heart.

Gas Exchange Surfaces

• Lungs: Main organ for gas exchange in vertebrates.

• Counter-current Exchange: Blood and water flow in opposite directions, maximizing O2 uptake (e.g., fish gills).

Circulatory Pathways

• Fish (2-chambered heart): Atrium → Ventricle → Gills (O2 picked up) → Body → Atrium.

• Mammals/Birds (4-chambered heart): Double circuit. Oxygenated blood: Left Atrium → Left Ventricle → Body. Deoxygenated blood: Right Atrium → Right Ventricle → Lungs → Left Atrium.

Oxygen Transport

• Hemoglobin: In red blood cells, carries O2.

• Oxygen Saturation Curve: Shows how readily hemoglobin binds/releases O2 based on partial pressure of O2 (pO2). Low pO2 in active tissues causes more O2 to be released.

• Bohr Shift: At low pH (from CO2 buildup in active tissues), the O2 saturation curve shifts to the right, meaning hemoglobin releases more O2 at a given pO2. This is adaptive for supplying working muscles.

Osmoregulation and Excretion

Nitrogenous Waste

Excretory systems remove metabolic wastes and regulate water and salt balance.

• Ammonia (NH3): Very toxic; requires lots of water for dilution. Least energetically expensive to produce. Found in aquatic animals.

• Urea: Less toxic; requires less water. Energetically costly to produce. Found in mammals, amphibians.

• Uric Acid: Least toxic; least water needed. Most energetically expensive to produce. Found in birds, insects, reptiles.

Vertebrate Nephron (Kidney Functional Unit)

• Bowman's Capsule: Filtration of plasma from blood.

• Proximal Tubule: Reabsorption of water, nutrients, ions.

• Loop of Henle:

  • Descending Limb: Permeable to water. Filtrate becomes more concentrated.

  • Ascending Limb: Impermeable to water. Filtrate becomes more dilute as salts are reabsorbed.

• Distal Tubule: Fine-tuning of salt and water balance.

• Collecting Duct: Reabsorbs water under influence of antidiuretic hormone (ADH). The longer the loop of Henle, the greater the ability to concentrate urine.

Other Nephridia

• Protonephridium: A network of dead-end tubules with flame cells. Found in Platyhelminthes (flatworms).

• Metanephridium: Open, ciliated funnels (nephrostomes) that collect coelomic fluid. Found in each segment of Annelids (earthworms).

Nerves

Neurons and Nerve Signals

The nervous system transmits electrical and chemical signals for rapid communication and response.

• Neuron vs. Nerve: A neuron is a single nerve cell. A nerve is a bundle of many neurons.

• Ion Gradients: Resting cell: High K+ inside, High Na+ and Ca2+ outside.

• Parts of a Neuron: Dendrites, Cell Body (Soma), Axon, Axon terminals.

Action Potential (AP) Propagation

• Resting Potential: ~ -70 mV. Established by Na+/K+ pumps and selective permeability.

• Threshold: The membrane potential at which voltage-gated Na+ channels open, triggering an AP.

• Depolarization: Na+ channels open, Na+ enters the cell, making the inside more positive.

• Repolarization: K+ channels open, K+ exits the cell, restoring negative charge.

• Hyperpolarization: K+ channels stay open too long, briefly making the cell more negative than resting potential.

Synaptic Transmission

• Neurotransmitters: Chemicals released from axon terminals that cross the synaptic cleft to bind receptors on the next cell.

• Excitatory (EPSP): Depolarizes the cell (e.g., Na+ channels open), making an AP more likely.

• Inhibitory (IPSP): Hyperpolarizes the cell (e.g., Cl- channels open), making an AP less likely.

• Summation: Multiple signals from one neuron in quick succession (temporal) or multiple neurons at the same time (spatial) can add together to trigger an AP.

Central Nervous System (CNS)

• Schwann Cells: Glial cells in the peripheral nervous system (PNS) that form myelin sheaths, which insulate axons and speed AP conduction.

• CNS: The brain and spinal cord.

Muscles

Three Muscle Types

Muscle tissue enables movement and force generation in animals.

• Skeletal: Striated, voluntary. Attached to bone.

• Cardiac: Striated, involuntary. Found only in the heart.

• Smooth: Non-striated, involuntary. Found in the gut, blood vessels, and other internal organs.

Sliding Filament Model of Contraction

Muscle contraction occurs when actin and myosin filaments slide past each other, powered by ATP.

1. Signal: Nerve impulse releases ACh, triggering an AP in the muscle fiber.

2. Sarcoplasmic Reticulum: Releases Ca2+ in response to AP.

3. Ca2+ Binds Troponin: Causing tropomyosin to move, exposing actin binding sites.

4. Cross Bridge: Myosin heads attach to actin.

5. Power Stroke: Myosin hydrolyzes ATP to ADP + Pi, re-energizing and returning to its original position.

6. Detachment: A new ATP binds to myosin, causing it to detach from actin.

7. Relaxation: When stimulation stops, Ca2+ is pumped back into the SR, troponin re-covers the binding sites, and the muscle relaxes.

Key Equations

• Resting Membrane Potential: $V_{rest} \approx -70\,\text{mV}$

• ATP Hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

Table: Comparison of Nitrogenous Wastes

Waste Type	Toxicity	Water Needed	Energy Cost	Examples
Ammonia	High	High	Low	Aquatic animals
Urea	Medium	Medium	Medium	Mammals, amphibians
Uric Acid	Low	Low	High	Birds, reptiles, insects