5.6 Animal Organ Systems Overview
Key Takeaways
- The **nervous system** transmits signals via neurons; an **action potential** is an all-or-nothing depolarization triggered when the membrane reaches threshold, propagating via voltage-gated Na⁺ and K⁺ channels.
- Circulatory systems are **open** (hemolymph in tissue spaces - insects, most mollusks) or **closed** (blood confined to vessels - annelids, cephalopods, vertebrates); vertebrate hearts evolve from 2-chambered (fish) to 3-chambered (amphibians, most reptiles) to 4-chambered (birds, mammals, crocodilians).
- Gas exchange surfaces match habitat: **gills** in aquatic animals, **lungs** in most tetrapods, **tracheal systems** of branching tubes that deliver air directly to tissues in insects, and **skin** in amphibians and earthworms.
- Excretory organs across phyla include **flame cells/protonephridia** (flatworms), **metanephridia** (annelids), **Malpighian tubules** (insects), and **kidneys** with functional units called **nephrons** (vertebrates).
- Immunity has two arms: **innate** (rapid, non-specific - barriers, phagocytes, complement) and **adaptive** (slower, antigen-specific, with memory) - the latter uses **B cells** that secrete antibodies (humoral immunity) and **T cells** that kill infected cells or coordinate the response (cellular immunity).
Nervous System
Animals coordinate behavior with networks of neurons that signal via electrical impulses (action potentials) and chemical messages (neurotransmitters).
The Action Potential in Four Steps
- Resting potential (≈ -70 mV) - the Na⁺/K⁺ ATPase pumps 3 Na⁺ out and 2 K⁺ in; K⁺ leak channels let K⁺ trickle out, leaving the inside negative.
- Depolarization - a stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in. If the membrane reaches threshold (~ -55 mV), an all-or-nothing action potential fires.
- Repolarization - Na⁺ channels inactivate; voltage-gated K⁺ channels open, K⁺ flows out, restoring negative interior.
- Hyperpolarization and recovery - K⁺ channels close slowly, briefly making the interior more negative than -70 mV. The Na⁺/K⁺ pump resets ion gradients.
Signals jump between neurons at synapses, where neurotransmitters (acetylcholine, glutamate, GABA, dopamine, serotonin) diffuse across the gap and bind receptors on the post-synaptic cell. In vertebrates, the CNS = brain + spinal cord; the PNS = everything else.
Endocrine System
Hormones are chemical messengers that travel through the blood (or hemolymph) to distant target cells.
- Peptide/protein hormones (insulin, growth hormone) bind surface receptors and trigger second-messenger cascades.
- Steroid hormones (cortisol, testosterone, estrogen) cross membranes and bind intracellular receptors that act as transcription factors.
- The pituitary (controlled by the hypothalamus) is the master gland in vertebrates; it releases TSH, ACTH, FSH, LH, GH, prolactin, ADH, and oxytocin.
- Insect endocrine highlights: ecdysone triggers molting; juvenile hormone keeps larvae from maturing too early.
Circulatory System
Two basic plans:
| System | Description | Examples |
|---|---|---|
| Open | A heart pumps hemolymph into open body cavities (sinuses); no distinction between blood and interstitial fluid | Most arthropods, most mollusks |
| Closed | Blood stays inside vessels and is pumped through capillaries to exchange materials | Annelids, cephalopods (squids/octopuses), all vertebrates |
Vertebrate Heart Evolution
| Animal | Chambers | Circulation |
|---|---|---|
| Fish | 2 (one atrium + one ventricle) | Single circuit - blood passes through gills, then body |
| Amphibians, most reptiles | 3 (two atria + one ventricle) | Double circuit, but oxygenated and deoxygenated blood mix in the single ventricle |
| Crocodilians, birds, mammals | 4 (two atria + two ventricles) | Fully separated pulmonary (lung) and systemic (body) circuits - higher metabolic efficiency |
In the human cardiac cycle, the SA node fires → both atria contract simultaneously → impulse delays at the AV node → both ventricles contract → all chambers relax in diastole and refill.
Respiratory System
Different habitats demand different gas-exchange surfaces:
| Structure | Found In | How It Works |
|---|---|---|
| Skin (cutaneous) | Earthworms, amphibians | Diffusion across moist surface; requires moisture |
| Gills | Fish, aquatic invertebrates | Counter-current flow maximizes O₂ extraction from water |
| Tracheal system | Insects | Branching air tubes (tracheae) open at spiracles and deliver O₂ directly to tissues - bypasses blood for gas transport |
| Lungs | Most tetrapods | Internal sacs with high surface area; gas exchange at alveoli in mammals |
| Book lungs | Spiders, scorpions | Stacked plates inside an abdominal chamber |
In mammals, O₂ binds hemoglobin in red blood cells; CO₂ travels mostly as bicarbonate (HCO₃⁻) in plasma.
Digestive System
- Incomplete (gastrovascular) gut - single opening serves as both mouth and anus. Found in cnidarians and flatworms.
- Complete gut (alimentary canal) - separate mouth and anus, allowing specialized regions. Found in nematodes onward.
In vertebrates, digestion happens in stages: mouth (mechanical + amylase) → stomach (HCl + pepsin for proteins) → small intestine (most digestion and absorption; surface area boosted by villi and microvilli) → large intestine (water and electrolyte absorption).
Ruminants (cows, sheep, deer) have a four-compartment stomach (rumen, reticulum, omasum, abomasum) and rely on microbial symbionts that ferment cellulose - which animals cannot digest enzymatically.
Excretory System
Excretion removes nitrogenous waste and balances water/salt. Across phyla:
| Structure | Animal Group | Function |
|---|---|---|
| Flame cells (protonephridia) | Flatworms | Move waste-laden fluid out of body via beating cilia |
| Metanephridia | Annelids (earthworms), mollusks | Each segment has a tubule with cilia that draws coelomic fluid in for filtration |
| Malpighian tubules | Insects, arachnids | Tubules empty uric acid into the gut, conserving water - key for terrestrial life |
| Kidneys with nephrons | Vertebrates | Filter blood at the glomerulus in Bowman's capsule; reabsorb water/solutes in tubules; secrete waste |
Nitrogen waste form correlates with environment:
- Ammonia (NH₃) - cheap to make but very toxic and water-soluble; aquatic animals (fish).
- Urea - less toxic, requires water to excrete; mammals, amphibians.
- Uric acid - solid paste, water-conserving; birds, reptiles, insects.
Immune System
Vertebrate immunity comes in two coordinated arms:
| Feature | Innate Immunity | Adaptive Immunity |
|---|---|---|
| Specificity | Non-specific - general patterns | Pathogen-specific antigens |
| Speed | Immediate (minutes to hours) | Days on first exposure, faster on re-exposure |
| Memory | No memory | Memory cells - secondary response is faster and stronger |
| Components | Skin, mucus, phagocytes (macrophages, neutrophils), complement, inflammation, NK cells | B cells (antibodies - humoral) + T cells (cell-mediated) |
B cells mature in bone marrow, recognize antigens via membrane-bound antibodies, and (when activated) differentiate into plasma cells secreting antibodies (humoral immunity).
T cells mature in the thymus:
- Helper T cells (CD4+) coordinate the response by activating B cells and cytotoxic T cells.
- Cytotoxic T cells (CD8+) kill virus-infected or cancerous cells directly.
Memory B and T cells generated during the primary response circulate for years, enabling a much faster and stronger secondary response when the same antigen reappears - the basis of vaccination.
Skeletal and Muscular Systems
Three skeletal strategies:
- Hydrostatic skeleton - fluid-filled cavity that resists compression. Used by cnidarians, flatworms, annelids (e.g., earthworms move by alternating circular and longitudinal muscle contractions against coelomic fluid).
- Exoskeleton - external chitinous covering in arthropods (must molt to grow) or calcareous shell in mollusks.
- Endoskeleton - internal bony or cartilaginous framework in echinoderms and vertebrates; allows large body size without molting.
Vertebrate skeletal muscle is striated and contracts by the sliding filament mechanism - actin and myosin filaments slide past each other powered by ATP, triggered by Ca²⁺ release from the sarcoplasmic reticulum.
Reproductive System
- Asexual reproduction - one parent; offspring are genetic clones (budding in hydra, parthenogenesis in some insects and lizards). Fast but no genetic variation.
- Sexual reproduction - two parents and gamete fusion (meiosis + fertilization). Slower but generates variation that fuels adaptation.
Fertilization strategies:
- External fertilization - gametes released into water; common in fish, amphibians, and many aquatic invertebrates. High gamete numbers compensate for low success rates.
- Internal fertilization - sperm delivered inside the female. Required for terrestrial reproduction in reptiles, birds, mammals, and most arthropods. Usually paired with parental care or amniotic eggs.
In mammals, fertilization occurs in the fallopian tube, and the resulting blastocyst implants in the endometrium (uterine lining) about 6-10 days later. Implantation anywhere else (tube, ovary, cervix) is an ectopic pregnancy.
Which combination of features correctly characterizes the innate (non-adaptive) arm of vertebrate immunity?
Which excretory structure is correctly matched with the animal group that uses it AND its primary function?