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=40-41.pdf
Chapter 42: Circulation and Gas Exchange
42.1: Circulatory systems link exchange surfaces with cells throughout the body
Need for Circulatory Systems
Exchange with Environment
Cells require O₂ + nutrients
Must remove CO₂ + wastes
Exchange occurs across plasma membrane
Diffusion
Random molecular movement
Effective only over short distances
Time ∝ distance² → diffusion too slow for large bodies
Example: Glucose diffuses rapidly over micrometers, not centimeters
Limitation
Unicellular organisms: direct exchange with environment
Multicellular organisms: too many internal cells → need transport system
Adaptations for Exchange
Simple body plan: flat or thin bodies (cnidarians, flatworms)
Complex body plan: internal circulatory system developed
Gastrovascular Cavities (Simple Systems)
Definition Central cavity for digestion + distribution
Examples
Hydra / jellies: cavity branches through body (radial canals)
Planarian flatworms: highly branched cavity (short diffusion distance)
Function
Fluid bathes both inner + outer tissue layers
Nutrients diffuse short distance to all cells
Open vs Closed Circulatory Systems
Open Circulatory System
Fluid = hemolymph (mixes with interstitial fluid)
Flow: heart → sinuses → organs → heart (via pores)
Organisms: arthropods (grasshoppers), some molluscs (clams)
Advantages: low energy cost, hydrostatic functions (spider leg extension)
Disadvantages: lower pressure, slower flow
Closed Circulatory System
Fluid = blood, confined to vessels
Flow: heart → arteries → capillaries → veins → heart
Organisms: annelids (earthworms), cephalopods, vertebrates
Advantages: higher pressure → efficient O₂/nutrient delivery
Regulation: blood directed to specific organs
Single vs Double Circulation
Single Circulation
Organisms: fish, sharks, rays
Pathway: heart → gills → body → heart
Limitation: pressure drop after gills → slower systemic flow
Adaptation: swimming motion aids flow
Double Circulation
Organisms: amphibians, reptiles, birds, mammals
Two Circuits:
Advantage: maintains high pressure in systemic circuit
Result: vigorous blood flow to organs
42.2: Coordinated cycles of heart contraction drive double circulation in mammals
Mammalian Circulation
Pulmonary Circuit (Right Side)
1 Right ventricle pumps → pulmonary arteries
2 Lung capillaries: O₂ loads, CO₂ unloa
Pulmonary veins → left atrium
Systemic Circuit (Left Side)
4 Left ventricle pumps → aorta
5Branches to head/arms capillarie
6 Branches to abdominal organs + legs
Gas exchange: O₂ diffuses to tissues; CO₂ enters blood
7 Blood returns via:
Both → right atrium → right ventricle
The Mammalian Heart: Structure and Function
Chambers
Atria (2): thin walls, receive blood
Ventricles (2): thick walls, pump blood
Left ventricle: stronger → pumps to whole body
Right ventricle: less force → shorter pulmonary loop
Blood Pathway
Right atrium → AV valve → right ventricle → semilunar valve → pulmonary artery
Left atrium → AV valve → left ventricle → semilunar valve → aorta
Valves Prevent Backflow
AV valves: between atria & ventricles
Semilunar valves: at heart exits (pulmonary artery, aorta)
Support fibers: prevent AV valves from inverting
The Cardiac Cycle
hases
Diastole: chambers relax + fill with blood
Systole: chambers contract + pump blood
Steps
1 trial + Ventricular Diastole: blood flows in freely (0.4 s)
2 Atrial Systole: atria contract → push remaining blood to ventricles (0.1 s)
3 Ventricular Systole: ventricles contract → eject blood to arteries (0.3 s)
Cardiac Output
Formula: heart rate × stroke volume
Typical human: 70 beats/min × 70 mL = ~5 L/min
Equals total blood volume
Maintaining the Heart’s Rhythmic Beat
Autorhythmic Cells
Contract without external nerve signal
Continue beating even in tissue culture
Pacemaker: Sinoatrial (SA) Node
Location: wall of right atrium near superior vena cava
Function: sets rate & timing of contractions
Generates electrical impulses
Signal Pathway
1 SA node fires → atria contract
2 Signal delayed at AV node → allows atria to empty
3 Bundle branches carry impulses to heart apex
4 Purkinje fibers spread signal → ventricles contract together
Electrical Recording
Electrocardiogram (ECG/EKG): measures heart’s electrical currents
Shows sequence of atrial & ventricular activity
Regulation of Pacemake
Sympathetic nerves: ↑ heart rate
Parasympathetic nerves: ↓ heart rate
Hormones: epinephrine → speeds rate
Temperature: +1 °C → +10 beats/min
Example: fever → faster heart rate
42.3: Patterns of blood pressure and flow reflect the structure and arrangement of blood vessel
Blood Vessel Structure & Function
Capillaries
Only endothelium + basal lamina
Exchange site: gases, nutrients, wastes
Diameter ≈ single RBC → single-file flow
Arteries / Arterioles
Thick smooth muscle + elastic connective tissue
Elastic recoil maintains pressure between beats
Vasoconstriction / Vasodilation (nerves + hormones) → route blood
Blood Pressure Patterns
Source of pressure
Ventricular systole generates pressure
Elastic arterial recoil sustains flow during diastole
Gradient
Highest: aorta/arteries → falls across arterioles/capillaries → near zero in veins
Systolic vs Diastolic
Systolic = peak during ventricular contraction (palpable pulse)
Diastolic = pressure during ventricular relaxation
Regulation
Vasoconstriction ↑ arterial pressure; Vasodilation ↓ arterial pressure
Nitric oxide (NO) → vasodilation; Endothelin → vasoconstriction
Coordinated with cardiac output to stabilize pressure
ravity, Veins, and Return to Heart
BP measurement: arm at heart level
Standing: brain BP < heart BP; fainting lowers head to heart level → restores flow
Very long necks: require high systolic pressure near heart
Venous return aids
Capillary Bed Control
Only a fraction of capillaries perfused at once
Control mechanisms
Arteriolar tone (constrict/dilate)
Precapillary sphincters open/close entry to beds
Exchange Across Capillary Walls
Mechanisms
Diffusion: O₂, CO₂, small solutes (through cells or pores)
Vesicular transport: select macromolecules (endo/exocytosis)
Forces (Starling forces)
Blood pressure pushes fluid out
Osmotic pressure of plasma proteins pulls fluid in
Net effect: slight fluid loss from capillaries to tissues
Lymphatic System: Fluid & Protein Return
Lymph recovers lost fluid/proteins
Movement
Valves + rhythmic vessel contraction + skeletal muscle activity
42.4: Blood components function in exchange, transport, and defense
Blood Composition & Roles
Vertebrate blood = connective tissue
Plasma (~55%) — liquid matrix
Cellular elements (~45%)
Core functions
Transport: O₂/CO₂, nutrients, wastes, hormones
Homeostasis: osmotic balance, pH buffering, temperature
Defense & repair: immune cells, antibodies, clotting
Plasma
Water (solvent)
Ions (electrolytes): Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻
Plasma proteins
Albumin: osmotic balance, pH buffer
Immunoglobulins (antibodies): defense
Apolipoproteins: lipid transport
Fibrinogen: clotting
Serum = plasma minus clotting factors
Cellular Elements
Erythrocytes (RBCs)
Biconcave discs → high surface area
No nucleus or mitochondria → more hemoglobin; anaerobic metabolism
Hemoglobin-rich
Gas exchange: O₂ loads in lungs; unloads in tissues; some CO₂ carried
Leukocytes (WBCs) — defense & immunity
Types: neutrophils, eosinophils, basophils, monocytes, lymphocytes (B, T)
Actions: phagocytosis; adaptive responses (B/T cells)
Locations: blood and interstitial fluid/lymph; numbers rise during infection
Platelets — clotting
Anucleate fragments (~2–3 μm) from marrow megakaryocytes
Provide factors + surface for coagulation; release signals
Hematopoiesis (Blood Cell Formation)
Stem cells → progenitors
Lymphoid progenitors → B cells, T cells
Myeloid progenitors
Blood Clotting (Coagulation)
Rapid sequence
Platelet adhesion → platelets activate & aggregate (temporary plug)
Clotting cascade
Control
Positive feedback: thrombin amplifies its own generation
Anticlotting factors prevent spontaneous clots
Pathology: thrombus = intravascular clot; hemophilia = defective cascade
42.5: Gas exchange occurs across specialized respiratory surfaces
Partial Pressure Gradients Drive Gas Exchange
Partial pressure
Net diffusion: high Pᵍᵃˢ → low Pᵍᵃˢ
Calculating Pₒ₂
Atmospheric pressure: 760 mmHg
O₂ fraction ≈ 21% → Pₒ₂ ≈ 160 mmHg
P_CO₂ at sea level
Gases dissolved in liquids
At equilibrium: Pᵍᵃˢ (air) = Pᵍᵃˢ (water)
Concentration differs by solubility (O₂ far less soluble in water)
Warm/salty water holds less O₂
Respiratory Media (Air vs Water)
Air
21% O₂; low density/viscosity → easy to move
Breathing can be less efficient
Water
Much less O₂ per liter; denser & more viscous
Aquatic animals expend more energy to ventilate
→ Evolution of highly efficient exchange surfaces
Respiratory Surfaces: General Principles
Surfaces are large and thin
Simple animals: body surface sufficient (sponges, cnidarians, flatworms)
Many animals: specialized organs (skin, gills, tracheae, lungs)
Gills in Aquatic Animals
Ventilation
Move gills through water or water over gills
Examples: cilia (clams), appendages (crustaceans), jetting (cephalopods), mouth/operculum pumps (fish), ram ventilation (swimming with mouth open)
Tracheal Systems in Insects
Layout:
Tracheae open to outside; branch into tracheoles throughout body
Moist tips for diffusion directly to cells
Chitin rings keep tubes open
Bioenergetic adaptations
Flight muscle pumping ventilates tracheae
Tracheoles closely apposed to mitochondria in active tissues
Lungs (Localized Respiratory Organs)
General
Infolded body surface; subdivided into many pockets
Circulatory system bridges lungs tissues
Usage varies: amphibians (skin + small lungs), reptiles/birds/mammals (lungs primary)
Mammalian respiratory system
Air path: nasal cavity → pharynx → larynx (vocal cords) → trachea → bronchi → bronchioles → alveoli
Mucus escalator: cilia + mucus trap/clear particulates
Alveoli = gas exchange sites
Thin, moist epithelium + dense capillary web
O₂ dissolves in film → diffuses into blood; CO₂ diffuses out
Vulnerability: no cilia; macrophages patrol; particulates (smoking/vaping, dust) → inflammation & loss of function
Surface tension & surfactant
Tiny alveoli would collapse without help
Surfactant (phospholipids + proteins) reduces surface tension
Clinical note: RDS of preterm infants = surfactant deficiency
42.6: Breathing ventilates the lungs
Purpose
Maintains high O₂ and low CO₂ at exchange surface
Breathing = inhalation + exhalation (ventilation)
Amphibians — Positive Pressure Breathing
Air pushed into lungs by buccal pumping
Floor of mouth lowers → air in → rises → air forced into lungs
Exhalation by elastic recoil + body wall compression
Birds — Unidirectional Flow
Air sacs act as bellows → air flows one way through lungs (parabronchi)
Two breathing cycles:
1st inhalation: air → posterior sacs
1st exhalation: posterior sacs → lungs
2nd inhalation: lungs → anterior sacs
2nd exhalation: air out
Fresh air never mixes with used air → very efficient
Mammals — Negative Pressure Breathing
Air pulled in by expanding thoracic cavity
Inhalation: diaphragm contracts ↓ ; ribs expand ↑ → air in
Exhalation: muscles relax → air out
Pleural membranes link lungs + chest wall
Exercise uses extra muscles to increase volum
Lung Volumes
Tidal volume: ~500 mL
Vital capacity: ~3–5 L
Residual volume: air left after exhalation
Mixing reduces alveolar Pₒ₂ → less efficient than birds
Control of Breathin
Medulla oblongata: sets rhythm (via pH sensors)
Aorta / carotids: detect low O₂ or pH drop
Pons: modifies pattern
Stretch receptors limit over-inflation
Concept 42.7 — Adaptations for Gas Exchange Include Pigments That Bind and Transport Gases
Coordination of Circulation & Gas Exchange
O₂ diffuses from alveoli → blood; CO₂ diffuses from blood → alveoli
Blood leaving lungs: high O₂ / low CO₂
Tissues: O₂ out of blood, CO₂ in → returns to lungs for exchange
Respiratory Pigments
Low O₂ solubility in water → need for pigments
Function: bind O₂ reversibly → transport large amounts
Increase O₂ carrying capacity > 60×
CO₂ Transport
~7% dissolved in plasma
~23% bound to Hb (→ carbamino compounds)
~70% as bicarbonate (HCO₃⁻) via carbonic anhydrase
In lungs: ↓ CO₂ → carbonic acid → CO₂ + H₂O → exhaled