Chapter 42 - Animal Circulation and Gas Exchange

Five Steps of Gas Exchange from Environment → Cells

1. Ventilation: movement of air or water over a respiratory surface (i.e. lungs or gills)

2. Diffusion at respiratory surface: O2 moves into blood and CO2 moves out (both down their partial pressure gradients)

3. Circulation: transports O2 and CO2 throughout the body via the bloodstream

4. Diffusion at the tissues: O2 leaves the blood and enters cells, CO2 enters the blood from cells.

5. Cellular respiration: uses O2 to produce ATP (generates CO2 as a waste product)

Functional Link Between Respiratory and Circulatory Systems

• Respiratory system: responsible for gas exchange b/w organism and external environment.

• Circulatory system: transports oxygen from respiratory surfaces to tissues, returns carbon dioxide back to those surfaces

• Continuous blood flow maintains steep partial pressure gradients by removing oxygen and delivering carbon dioxide.

Partial Pressure: Driver of Gas Exchange

The effective availability of gas molecules for diffusion

• Gases diffuse based on differences in partial pressure, not overall concentration.

• Oxygen and carbon dioxide move from regions of higher partial pressure to lower partial pressure.

• At high altitudes, reduced atmospheric pressure lowers oxygen partial pressure, decreasing diffusion into the lungs.

Gas Exchange in Air vs. Water

• Air: higher concentration of oxygen and is less dense and viscous than water

• Water: less dissolved oxygen, significantly denser → making ventilation cost more energy

  • Aquatic organisms must move large volumes of water to obtain sufficient oxygen; require highly efficient gas-exchange structures such as gills.

  • Increasing temp and salinity reduce O2 availability in water, but mixing helps improve

Fick’s Law

• Diffusion rate = k × A × (P₂ − P₁) divided by D.

• The constant k represents the effects of temperature and gas solubility.

• Surface area (A) determines how much area is available for gas exchange. •

• The partial pressure gradient (P₂ − P₁) drives diffusion across the surface.

• Thickness (D) reduces diffusion rate as it increases, meaning thinner surfaces enhance diffusion.

Fick’s Law Example of High Diffusion Rate

• P1 = 5

• P2 = 15

• A = 2

• D = 2

• Large partial pressure difference drives diffusion

  • Although surface area is relatively low, the stronger partial pressure gradient drives diffusion more efficiently

Respiratory Structures: Optimizing Fick’s Law

• Gills maximize surface area and maintain gradients through countercurrent exchange

• Lungs increase surface area through extensive branching and thin alveolar membranes

• Tracheal systems minimize diffusion distance by delivering air directly to cells

Importance of Respiratory Surfaces Being Moist/Thin

• Gases must dissolve in water before diffusing (moisture)

• Thinness helps minimize diffusion distance

• Often increase risk of water loss; many terrestrial animals have internal respiratory structures to reduce dehydration

Gills: Maximizing Oxygen Uptake

• Contain numerous filaments and lamellae that greatly increase surface area

• Gill epithelium is extremely thin → minimizes diffusion distance

• Continuous water flow maintains a strong oxygen gradient across the surface

Countercurrent Exchange

• Occurs when blood flows in the opposite direction of water across gill surfaces

• Helps maintains a consistent partial pressure gradient along the entire exchange surface

  • O2 diffuses into blood across the entire length of the gill

Insect Tracheal System

• Tracheae: network of air-filled tubes that open to the exterior through spiracles

• Branch extensively into tracheoles that reach individual cells

• Oxygen diffuses directly from tracheoles into cells without the need for blood transport

• Bypasses the circulatory system

Insects: How Do They Ventilate?

• Via rhythmic contractions and relaxations of muscles

• Spiracles relax → tracheal volume increase → internal pressure drops → air flows in

• Spiracles contract/close → tracheal volume decreases → internal pressure increases → air is forced out

  • Boyle’s Law (inverse relationship of pressure and volume)

Positive vs. Negative Pressure Ventilation

• Positive: forces air into the lungs by increasing pressure in the oral cavity

  • Seen in amphibians such as frogs

• Negative: draws air into the lungs by expanding the chest cavity and lowering internal pressure

  • Seen in mammals

  • More efficient and supports higher metabolic demands

Regulation of Breathing Rate

• Controlled by the medullary respiratory center in the brain

• Primary stimulus for breathing: elevated CO2 levels in blood.

  • CO2 reacts w/ H2O to make H+, lowering pH

  • A detectable change that is sensed → increases breathing rate + depth to restore balance

Open vs. Closed Circulatory Systems

• Open: allow hemolymph to leave vessels and directly contact tissues

  • Operate at low pressure

  • Have limited ability to direct flow

• Closed: keep blood confined within vessels

  • Allows higher pressure and faster flow

  • Enable precise regulation of blood distribution to tissues.

Insects: Exception to the Open System

• Insects have an open circulatory system but do not rely on it for oxygen transport.

• Their tracheal system delivers oxygen directly to tissues

• This bypasses the limitations of low-pressure hemolymph flow.

Arteries, Capillaries, and Veins

• Arteries have thick, elastic walls that allow them to withstand and maintain high pressure as blood leaves the heart.

• Capillaries have extremely thin walls that facilitate exchange of gases, nutrients, and wastes.

• Veins have thinner walls and contain valves that prevent backflow as blood returns to the heart.

Pathway of Blood to the Heart

• Deoxygenated blood enters the right atrium from the body

• Blood moves to the right ventricle and is pumped to the lungs.

• Oxygenated blood returns to the left atrium.

• Blood moves to the left ventricle and is pumped to the body.

Pulmonary vs. Systemic Circuits

• The pulmonary circuit carries blood between the heart and the lungs for gas exchange.

• The systemic circuit carries blood between the heart and body

• This separation allows efficient oxygenation and delivery

Evolution of Heart Chambers

• Fish: two chambers and a single circuit, limiting pressure and flow

• Amphibians and reptiles: three chambers and two circuits, allowing some mixing of blood.

• Birds, crocodilians, and mammals: four chambers and two circuits, fully separating oxygenated and deoxygenated blood.

• Increased chamber number improves oxygen delivery and supports higher metabolism.

Systole vs Diastole

• Systole: cardiac cycle phase where muscle contracts → pumps blood out of the chambers.

• Diastole: cardiac cycle phase where muscle relaxes and the chambers fill with blood.

Blood Pressure

• Determined by cardiac output (HR x stroke volume) and resistance within blood vessels.

• Resistance: influenced by vessel diameter (smaller = more resistance)

• BP decreases as blood moves through capillary beds b/c of increased cross-sectional area.

• Regulated by baroreceptors located in major arteries/heart

Structure + Function of Hemoglobin

• Protein found in RBCs that consists of four polypeptide chains

• Each chain contains a heme group with an iron ion that can bind one oxygen molecule

  • Each hemoglobin can carry 4 O2 molecules

• Significantly increases the oxygen-carrying capacity of the blood

Cooperative Binding

• Binding of one oxygen molecule increases the affinity of hemoglobin for additional oxygen molecules

• Causes conformational changes in the hemoglobin protein after each oxygen binds

• So, O2 binding becomes progressively easier

• Produces a sigmoidal oxygen dissociation curve

Cooperative Binding Importance

• Allows hemoglobin to load oxygen efficiently in the lungs where oxygen partial pressure is high

• It also allows hemoglobin to release oxygen efficiently in tissues where partial pressure is lower.

• Small changes in oxygen partial pressure result in large changes in hemoglobin saturation.

• This makes oxygen delivery highly responsive to tissue demand.

Bohr Shift

• Occurs when hemoglobin’s affinity for oxygen decreases which promotes oxygen release.

• Caused by increased CO2 levels, decreased pH, and increased temperature.

• The shift ensures that more oxygen is delivered to tissues that need it most.

What Happens to CO2 in the Blood?

• Most CO2 diffuses into RBCs and is converted into bicarbonate ions (HCO3-) by carbonic anhydrase.

• This reaction also produces hydrogen ions which bind to hemoglobin

• Converting carbon dioxide into bicarbonate helps maintain a diffusion gradient for continued CO₂ uptake

CO2 in the Lungs

• Bicarbonate ions and hydrogen ions recombine to form carbon dioxide and water

• Carbon dioxide then diffuses from blood → alveoli → expelled during exhalation

• As CO2 leaves blood → pH rises → hemoglobin affinity for oxygen increases

Hagen-Poiseuille

• Blood flow increases with a greater pressure difference between the ends of a vessel.

• Flow increases dramatically with increases in vessel radius because radius is raised to the fourth power.

  • *Most influential factor

  • Small changes in radius can have large effects on flow rate

• Blood flow decreases with increased viscosity and increased vessel length.

Endotherms: Requiring More Efficient Systems

• Endotherms maintain body temperature through metabolic heat production, which requires high energy expenditure.

• This increased metabolic rate creates a greater demand for oxygen and removal of carbon dioxide.

• As a result, endotherms require highly efficient respiratory and circulatory systems.

• These systems typically include large surface areas for gas exchange and high pressure circulation.