Animal Physiology: Nutrition, Digestion, Respiration, Circulation, and Excretion

Posted on Nov 29, 2024 in Biology

Animal Physiology: Key Systems

Nutrition

The process by which living organisms obtain energy and matter for growth, development, and vital functions. Animals use energy from organic matter catabolism.

  • Ingestion and digestion of food to release nutrients.
  • Distribution of nutrients to body cells.
  • Absorption of oxygen and transport to cells.
  • Cell metabolism
  • Waste removal from cells.
  • Waste excretion.

Digestion

Macromolecule transformation into smaller, absorbable molecules.

  • Physical/Mechanical: Fragmenting food, mixing with secretions.
  • Chemical: Enzyme breakdown of molecules.

Digestion without a Digestive System

Porifera perform intracellular digestion using choanocytes (cells with flagella and collars). Food enters through pores, is filtered, and digested within the choanocytes via endocytosis and lysosomal enzymes. Undigested residues are expelled by exocytosis.

Digestion in a Cavity (Gastrovascular)

Food enters a gastrovascular cavity (single opening). Enzymes break down food; some intracellular digestion occurs. Cnidarians and flatworms have branched cavities for efficient nutrient distribution.

Digestion in a Tube

Animals with bilateral symmetry have a digestive tube with two openings (mouth and anus). This allows for efficient digestion of large food amounts. Most vertebrates and invertebrates have a basic scheme:

  1. Receiving region
  2. Storage and transport region
  3. Grinding and initial digestion region
  4. Final digestion and absorption region
  5. Water absorption and waste expulsion region
Receiving Region

Mouth, pharynx, and associated structures adapted to food type. Examples include:

  • Butterflies/moths: Espiritrompa (proboscis)
  • Mosquitoes: Piercing-sucking mouthparts
  • Crustaceans: Claws
  • Spiders: Mouthpart for injecting enzymes
  • Cephalopods: Horny beak
  • Starfish: Stomach eversion
  • Gastropods: Radula
  • Sea urchins: Aristotle’s lantern
  • Vertebrates: Teeth and tongue (birds/turtles: beak)
  • Mammals: Teeth, tongue, salivary amylase (ptialina).
Region of Storage and Transportation

Esophagus transports food. Annelids, insects, and birds have a crop (esophageal dilation) for temporary storage.

Region of Grinding and Initial Digestion

Stomach stores and mixes food with digestive juices. Animals without teeth may have a gizzard (muscular forestomach) with stones for grinding. Examples include:

  • Crustaceans: Gastric mill
  • Insects: Chitinous teeth
  • Ruminants: Rumen, reticulum, omasum, abomasum (fermentation and cud chewing).
Region of Digestion and Absorption of Nutrients

Small intestine: Chemical digestion and nutrient absorption. Secretions from intestinal cells, liver, and pancreas (vertebrates). Adaptations for increased surface area include:

  • Annelids: Typhlosole
  • Sharks: Spiral valve
  • Other vertebrates: Villi and microvilli.
Region of Water Absorption and Expulsion of Undigested Food

Large intestine: Water reabsorption. Undigested material forms feces, expelled through the anus (or cloaca in some animals).

Respiratory Function

Gas exchange between organism and environment. Cellular respiration (mitochondria) oxidizes organic matter for energy.

  • Gas exchange via diffusion (moist surface, large surface area, thin membrane).

Breathing without Respiratory Organs

Sponges, cnidarians, flatworms, nematodes: Gas exchange through body surface. Oligochaetes and hirudinea have enhanced skin breathing with capillary networks.

Breathing in Aquatic Environments

Gills (branchae): Branched, vascularized body surface extensions. Can be external (e.g., polychaetes) or internal (e.g., bivalves, cephalopods).

Breathing in Terrestrial Environments

Tracheae (insects): System of tubes connecting air to cells. Lungs (vertebrates): Branched, vascularized sacs.

Tracheal Breathing

Tracheae branch into tracheoles, reaching cells. Air enters through spiracles (with valves in insects).

Lung Breathing

Lungs are internal, vascularized cavities. Air enters through airways (nose/mouth, pharynx, larynx, trachea, bronchi). Fish swim bladders are modified lungs.

Circulatory Function

Distributes nutrients, oxygen, and removes waste. Components include:

  • Blood (transport fluid)
  • Blood vessels
  • Heart (pump)

Open Circulatory Systems

Blood leaves vessels, fills spaces between cells. Arthropods have hearts with ostioles (suction holes).

Closed Circulatory Systems

Blood remains in vessels (annelids, cephalopods, vertebrates). Blood flows through arteries, arterioles, capillaries, venules, and veins.

  • Arteries: Thick, muscular, elastic.
  • Arterioles: Thin smooth muscle, precapillary sphincters.
  • Capillaries: Thin walls, exchange of nutrients/wastes.
  • Venules: Collect blood from capillaries.
  • Veins: Return blood to heart, thinner muscle layer, valves.

Closed systems have higher blood pressure, efficient nutrient delivery and waste removal. Selective irrigation directs blood to needed areas.

  • Simple circulation: Blood completes a circuit before returning to the heart (annelids, cephalopods, fish).
  • Double circulation: Pulmonary (lungs) and systemic (body) circuits (mammals: tetraventricular heart).

Excretory Function

Removes waste (carbon dioxide, water, nitrogenous waste) to maintain homeostasis.

Homeostasis

Maintaining a stable internal environment (salt concentration, pH, etc.). Involves nervous, endocrine, circulatory, and other systems.

Excretory Structures

Tubes collect fluid, adjust composition via reabsorption and secretion. Vertebrates have kidneys (nephrons: glomerulus, Bowman’s capsule, renal tubule).

Osmoregulation in Vertebrates

Terrestrial vertebrates lose water; mammals excrete urea. Nephrons (loop of Henle) recover water, produce hypertonic urine.