Table of Contents
Overview
Animals are active, energy-demanding organisms. They must:
- break down food (digestion),
- take up oxygen and release carbon dioxide (respiration),
- and distribute substances (transport) throughout the body.
This chapter gives an integrated overview of how these three systems work together in animals, with special emphasis on vertebrates and humans, without going into details that are treated in later subchapters.
Why Digestion, Respiration, and Transport Belong Together
All three systems are components of one large supply and disposal network:
- Digestive system: supplies nutrients (e.g., glucose, amino acids, fatty acids).
- Respiratory system: supplies oxygen and removes carbon dioxide.
- Circulatory (transport) system: distributes nutrients and oxygen to cells and removes waste to excretory organs.
Cells use these supplies mainly in cellular respiration (treated in detail elsewhere) to generate ATP. Here, the focus is how the organism as a whole secures and delivers the necessary substances to every cell.
General Principles of Digestion in Animals
Goals of Digestion
Digestion serves two main purposes:
- Mechanical breakdown of food into smaller pieces (e.g., chewing, gizzard grinding).
- Chemical breakdown of macromolecules into small, absorbable units:
- Proteins → amino acids
- Carbohydrates → monosaccharides
- Lipids → fatty acids and glycerol
- Nucleic acids → nucleotides
Only these small molecules can cross the intestinal epithelium into the body’s internal transport systems.
Intracellular vs. Extracellular Digestion
- Intracellular digestion
- Food particles are taken up by cells (e.g., phagocytosis in some protists, sponges).
- Digestive enzymes work in intracellular vesicles (lysosomes).
- Typical of primitive or very simple multicellular organisms; limited food size and efficiency.
- Extracellular digestion
- Food is digested in a body cavity outside the cells.
- Nutrient molecules are then absorbed by specialized epithelial cells.
- Enables digestion of larger prey and processing of higher food volumes; typical of most animals.
Many animals combine both: extracellular digestion in a cavity, followed by intracellular processing of absorbed molecules.
Simple vs. Complex Digestive Systems
- Gastrovascular cavity
- One opening (mouth = anus).
- Digestion and distribution of nutrients occur in the same cavity.
- Example: cnidarians (e.g., hydra, jellyfish).
- Limitation: food and waste share a single opening; less specialized, reduced flow-through.
- Complete digestive tract (alimentary canal)
- Two openings: mouth and anus.
- Food moves in one direction.
- Regional specialization is possible:
- Ingestion and mechanical breakdown (mouth, teeth, beak, radula).
- Storage and initial digestion (stomach or crop).
- Enzymatic digestion and absorption (intestine).
- Water reabsorption and feces formation (large intestine/cloaca).
- Found in most bilaterian animals.
Specialization According to Diet
Basic pattern is similar, but form and length of the digestive tract depend strongly on diet:
- Herbivores
- Often long intestines → more time and surface for digestion of plant cell walls.
- Specialized fermentation chambers with symbiotic microorganisms (e.g., rumen in cows, cecum in horses, rabbits).
- Teeth adapted for grinding (broad molars, reduced or absent canines).
- Carnivores
- Generally shorter intestines; animal tissue is easier to digest.
- Strong stomach acids, proteases.
- Teeth for grabbing and cutting (sharp canines, carnassials).
- Omnivores
- Intermediate traits; versatile teeth and digestive tract.
- Specialists
- E.g., nectar feeders with long tongues and reduced teeth; blood-feeding parasites with anticoagulants and specialized mouthparts.
In all cases, digestion aims to convert food into simple molecules that can be absorbed into blood or hemolymph.
Absorption and Internal Transport of Nutrients
Surface Enlargement and Absorption
To absorb nutrients efficiently, animals greatly increase the surface area of the intestinal epithelium:
- Folds, villi, and microvilli in vertebrate small intestine.
- Analogous structures or long, thin intestines in invertebrates.
Across this surface:
- Monosaccharides and amino acids enter capillaries.
- Fatty acids are assembled into lipoprotein particles and enter either lymphatic vessels (in vertebrates) or blood directly, depending on the group.
Link to the Circulatory System
After absorption:
- Nutrients enter a transport system:
- In vertebrates: closed circulatory system with heart and vessels (arteries, capillaries, veins).
- In many invertebrates: open circulatory system with hemolymph and hemocoel.
The circulatory system:
- Delivers nutrients to tissues.
- Works closely with respiratory structures to also deliver oxygen.
- Carries waste products (e.g., CO₂, urea) away for excretion.
General Principles of Respiration in Animals
Respiration, at the level of the whole organism, involves gas exchange between the environment and body fluids. The internal chemical steps of cellular respiration are handled in another chapter; here we focus on how gases get into and out of the body.
Basic Requirements for Gas Exchange
Gas exchange always relies on the same physical principles:
- Gases diffuse from high to low partial pressure.
- Diffusion is effective only over very short distances.
- Therefore, gas-exchange surfaces:
- Must be thin and moist.
- Must have a large surface area.
- Must be supplied by a transport medium (blood/hemolymph) if the animal is large.
Strategies Without Specialized Organs
Small or thin animals can often manage without dedicated respiratory organs:
- Flatworms, some cnidarians:
- Every cell is near the body surface.
- Gas exchange occurs directly through the skin (body surface respiration).
- Many small aquatic invertebrates:
- Entire body surface or simple external projections used for gas exchange.
Here, circulation may be very simple or absent; diffusion alone can suffice.
Specialized Respiratory Organs
As body size and metabolic rate increase, most animals develop specialized exchange surfaces:
- Gills for life in water:
- Thin, often feathery structures with high surface area.
- Must be ventilated by water flow (swimming, ciliary movement, or pumping movements).
- Typically associated with a circulatory system that collects oxygen from gills and transports it to tissues.
- Tracheal systems (insects, some other arthropods):
- Air-filled tubes (tracheae) open to the outside via spiracles.
- Tracheae branch into very fine tracheoles reaching almost every cell.
- Gas exchange occurs mostly directly between air and tissues; blood plays a lesser role in O₂ transport.
- Particularly suited for small-bodied, terrestrial arthropods.
- Lungs for life in air (terrestrial vertebrates, some snails, some arachnids):
- Internal cavities with large, moist surfaces.
- Ventilated by body movements (e.g., rib cage, diaphragm in mammals).
- Closely associated with blood vessels for efficient exchange.
These organs enable much higher oxygen uptake than diffusion alone and thus support more active lifestyles.
Circulation and Transport in Animals
Why Most Animals Need a Circulatory System
Diffusion alone is too slow over distances larger than a few tenths of a millimeter. A circulatory system provides convection—bulk flow of a fluid—that rapidly transports substances between organs.
Main tasks:
- Supply tissues with:
- Oxygen
- Nutrients
- Hormones and signaling molecules
- Remove:
- Carbon dioxide
- Nitrogenous wastes
- Metabolic by-products
- Distribute:
- Heat (in many vertebrates)
- Immune cells
Open vs. Closed Circulatory Systems
- Open circulatory systems (many arthropods, most mollusks):
- Circulatory fluid = hemolymph.
- Heart(s) pump hemolymph into body cavities (hemocoel).
- Organs are bathed directly in hemolymph; pressure is relatively low.
- Suitable for animals where very precise control of blood distribution or high pressures are not essential.
- Closed circulatory systems (vertebrates, some annelids, cephalopods):
- Blood remains in a continuous system of vessels.
- Higher pressures possible → faster, more directed flow.
- Capillaries enable fine control of delivery to individual tissues.
- Well suited to animals with high metabolic demands and active movement.
Despite these differences, both types integrate with respiratory structures (gills, lungs, skin) and excretory organs (e.g., kidneys) to form a unified transport network.
Respiratory Pigments
To transport more oxygen than simple dissolution in fluid allows, many animals use respiratory pigments—proteins that bind O₂ reversibly:
- Hemoglobin:
- Iron-containing pigment, often in blood cells (e.g., vertebrates) or dissolved in plasma (some invertebrates).
- Gives blood a red color when oxygenated.
- Hemocyanin:
- Copper-containing pigment in many crustaceans and mollusks.
- Often dissolved in hemolymph; oxygenated form is bluish.
These pigments increase the oxygen-carrying capacity of blood or hemolymph and improve the match between supply and metabolic demand.
Integration of Digestion, Respiration, and Transport
From Food and Air to Cells
In a typical vertebrate (for example, a human), the path of key substances can be summarized as:
- Nutrients
- Ingested and mechanically processed in the mouth.
- Chemically broken down by digestive enzymes in stomach and small intestine.
- Absorbed through intestinal epithelium into blood or lymph.
- Transported by circulatory system to tissues.
- Taken up by cells and used as building blocks or fuel.
- Oxygen
- Inhaled into lungs and reaches alveoli.
- Diffuses into blood because of partial pressure differences.
- Binds to hemoglobin in red blood cells.
- Transported via circulation to capillary beds in tissues.
- Diffuses into cells and is used in cellular respiration.
- Carbon dioxide and other wastes
- Produced by cells during metabolism.
- Diffuse into blood; transported as dissolved gas, bicarbonate, or bound to hemoglobin.
- Released in lungs and exhaled, or delivered to kidneys and other excretory organs.
- Water, salts, and nitrogenous wastes are further processed and excreted.
Thus, digestion provides the chemical substrates, respiration provides the oxidant (O₂) and removes CO₂, and the circulatory system links all organs involved.
Matching Supply to Demand
Animals alter the performance of these systems according to activity level and environment:
- Increased activity:
- Faster breathing (more ventilation).
- Increased heart rate and stronger cardiac output.
- Greater blood flow to working muscles; digestive organs may temporarily receive less.
- Rest or fasting:
- Lower breathing and heart rates.
- More blood directed to digestive organs shortly after feeding (postprandial state).
Hormones and nervous signals coordinate these adjustments so that oxygen, nutrients, and ATP production match the current needs of tissues.
Constraints and Adaptations
Because the three systems are linked, limitations in one often drive adaptations in the others:
- Air-breathing vertebrates evolved lungs and circulation patterns that keep oxygen-rich and oxygen-poor blood partly or fully separated.
- Animals with highly specialized diets adapted their digestive systems accordingly, sometimes relying on symbiotic microorganisms.
- Active flyers (e.g., birds, many insects) possess extremely efficient respiratory and circulatory adaptations to support high energy demands.
Summary
- Digestion, respiration, and internal transport form an integrated supply network in animals.
- Digestion transforms complex food into absorbable molecules; absorption delivers these to the transport system.
- Respiration provides oxygen and removes carbon dioxide through specialized surfaces (gills, tracheae, lungs, skin), relying on diffusion and often respiratory pigments.
- Circulatory systems—open or closed—move gases, nutrients, wastes, and signaling molecules between organs and tissues.
- Coordinated regulation ensures that the delivery of nutrients and oxygen keeps pace with the metabolic needs of the organism.