Circulation & Gas Exchange: Structure and Function in Animals

Objectives

• Understand the four steps of ventilation and the structures required for each.

• Know the difference between open and closed circulatory systems, including which animals possess each type and why.

• Identify the main parts of respiratory and circulatory systems and their functions.

• Explain the function of the four-chambered heart.

• Describe blood flow through the four-chambered heart and the cellular structures governing this flow.

• Understand the loading and unloading of oxygen on hemoglobin, including factors that help or hinder this process.

Circulation & Gas Exchange: Introduction

Overview of Gas Exchange in Animals

All animals require oxygen for cellular respiration and must expel carbon dioxide, a byproduct of this process. Gas-exchange organs are specialized to maximize the rate of O2/CO2 diffusion by:

• Presenting a large, thin surface area to the environment.

• Maintaining a steep partial-pressure gradient favoring entry of O2 and elimination of CO2.

Blood and Hemoglobin

• Blood is a specialized tissue that transports gases, nutrients, and wastes.

• Hemoglobin is an oxygen-carrying protein, highly efficient at taking up oxygen at respiratory surfaces and delivering it to tissues.

• Circulatory systems use pressure generated by the heart(s) to transport blood throughout the body.

Gas Exchange Steps

Four Steps of Gas Exchange

1. Ventilation: Movement of air or water through/across specialized gas-exchange organs.

2. Gas Exchange: Diffusion of CO2/O2 between air/water and blood at the ventilator surface.

3. Circulation: Transport of dissolved O2/CO2 throughout the body.

4. Cellular Respiration: Gas exchange in tissues, where O2/CO2 diffuse between blood and tissues.

Diffusion Time & Gas Exchange

Diffusion Efficiency and Animal Size

• Diffusion time is proportional to the square of distance; efficient only over small distances.

• Small/thin animals can exchange materials directly with the surrounding medium and often lack circulatory/respiratory systems due to their large surface area to volume ratio.

• Cnidarians and flatworms have elaborate gastrovascular cavities that function in both digestion and circulation.

Large Animals & Circulation

Components of Circulatory Systems

• Circulatory fluid

• Set of interconnecting vessels

• Muscular pump (heart)

There are two main types of circulatory systems: open and closed.

Open Circulatory Systems

Characteristics and Function

• Uses hemolymph instead of blood.

• Relatively low pressure via heart; low flow rates.

• Hemolymph not confined exclusively to vessels; direct contact with tissues.

• Most suitable for sedentary/slow-moving organisms with low O2 demand.

• Hemolymph may contain oxygen-carrying pigments, some cells, and clotting agents.

Pancrustaceans as Special Case

Exceptions to Open Systems

• Insects have a tracheal respiratory system that delivers oxygen directly to tissues.

• Crustaceans have a network of small vessels, allowing preferential delivery of hemolymph to tissues with highest O2 demands.

Closed Circulatory Systems

Characteristics and Function

• Blood is confined to vessels, forming a continuous circuit.

• High pressure generates high flow with precision.

• Found mostly in active lineages: vertebrates, annelids, cephalopods.

Blood Vessels & Closed Circulation

Classification of Blood Vessels

• Arteries: Tough/thick-walled vessels taking blood away from heart under high pressure.

• Veins: Thinner-walled vessels taking blood back to heart under lower pressure.

• Capillaries: 1-cell thick vessels connecting arterioles to venules; site of nutrient/gas/waste exchange.

• Aorta: Largest artery from heart.

• Arterioles: Contain sphincters regulating flow to tissues.

Venous Circulation

Mechanisms Governing Venous Return

• Skeletal muscular pump

• Internal valves

• Smooth muscle contractions

• Respiratory pump

• Venous volume adjustment

Capillaries

Structure and Function

• Extremely thin (1-cell layer)

• Often somewhat "leaky"

• Form dense networks throughout body

• Site of gas/nutrient/waste exchange between blood and tissues

• Made only of endothelium

Capillary Exchange Dynamics

Forces Governing Fluid Exchange

• Interstitial space: Area between cells filled with interstitial fluid.

• Fluid builds up in tissues due to:

  • Outward-directed hydrostatic force (created by heart pressure)

  • Inward-directed osmotic force (created by higher solute concentration in blood plasma)

Lymphatic System & Unclaimed Fluid

Role in Fluid Balance

• Interstitial fluid not reclaimed by capillaries is collected in the lymphatic system.

• Lymphatic ducts permeate all tissues, join to form larger vessels, and return excess interstitial fluid (lymph) to major veins entering the heart.

Vertebrate Circulation: General

Evolutionary Trends

• Number of distinct heart chambers increased over time.

• Fish have a single circuit servicing gills and body.

• Other vertebrates have separate circuits to lungs and body.

One Circuit versus Two: Details

Fish vs. Terrestrial Vertebrates

• Fish: Two-chambered heart, single circulatory circuit.

• Terrestrial vertebrates: Double pump system with pulmonary (lower pressure) and systemic (higher pressure) circuits.

Terrestrial Vertebrate Circulatory Details

Heart Structure and Circulation

• Four-chambered hearts (crocodiles, birds, mammals): Complete separation of pulmonary and systemic circuits.

• Three-chambered hearts (amphibians): Partial separation, with pulmocutaneous circuit.

• "Five-chambered" hearts (turtles, lizards): Bypass vessel allows shunting of blood between circuits under certain conditions.

Cardiac Circulation

Pulmonary and Systemic Circulation

• Pulmonary circulation: Blood returns from body, enters right atrium, moves to right ventricle, and is pumped to lungs.

• Systemic circulation: Blood returns from lungs, enters left atrium, moves to left ventricle, and is pumped to body.

• Systole: Contraction phase

• Diastole: Relaxation phase

Blood Pressure

Measurement and Phases

• Measured in systemic arterial circulation.

• Systolic blood pressure: Peak of ventricular ejection into aorta.

• Diastolic blood pressure: Prior to ventricular ejection.

Cardiac Measurements

Key Parameters

• Heart rate (HR): Number of beats per minute (typically ~72 bpm).

• Stroke volume (SV): Amount of blood pumped per ventricular contraction (~70 ml/beat).

• Cardiac output (CO): Volume of blood pumped into systemic circulation per minute.

Formula:

$CO = HR \times SV$

• "Lub-dup" sound of heartbeat caused by recoil of blood against valves.

• Heart murmur: Backflow of blood through defective valve.

Heart Beat Regulation

Electrical Control of Heart Contraction

• Pacemaker cells (SA Node): Initiate contraction, located in right atrium.

• Receive input from nervous system and chemical messengers in blood.

• Atrioventricular (AV) node: Sends signal through ventricles via cardiac muscle cells.

• Intercalated discs: Physical/electrical connections of cardiac muscle cells, containing gap junctions for direct electrical signal passage.

The EKG

Key Events in Heart's Electrical Activation

1. SA node originates signal.

2. Signal propagated over atria, causing simultaneous contraction/filling of ventricles.

3. Signal conducted to AV node, relaying signal to ventricles after complete filling.

4. Signal rapidly transmitted through both ventricles (via bundle of His & Purkinje fibers), causing ventricular contraction.

5. Final electrical event occurs as ventricles relax and cells recover.

Blood Pressure Changes

Regulation and Effects

• Blood pressure is the force blood exerts on vessel walls.

• Total cross-sectional area of blood changes in vessels; large area in capillaries causes blood pressure drop.

• Blood pressure drop decreases blood flow rate, allowing sufficient time for diffusion between tissues and blood in capillaries.

Blood Pressure Change Detection

Regulatory Mechanisms

• Blood movement is regulated at points throughout the circulatory system.

• Nervous system and chemical messengers control blood flow to tissues via contraction/relaxation of arteriolar sphincters.

• Falling blood pressure detected by baroreceptors in heart walls and major arteries.

Table: Comparison of Open and Closed Circulatory Systems

Feature	Open Circulatory System	Closed Circulatory System
Circulatory Fluid	Hemolymph	Blood
Pressure	Low	High
Vessel Confinement	Not confined; direct contact with tissues	Confined to vessels
Organisms	Arthropods, most mollusks	Vertebrates, annelids, cephalopods
Efficiency	Lower; suitable for sedentary/slow-moving animals	Higher; suitable for active animals

Table: Cardiac Measurements

Parameter	Definition	Typical Value
Heart Rate (HR)	Beats per minute	~72 bpm
Stroke Volume (SV)	Blood pumped per beat	~70 ml/beat
Cardiac Output (CO)	Blood pumped per minute	~5 liters/min

Additional info: These notes cover the essential concepts of animal circulation and gas exchange, including the structure and function of circulatory systems, heart anatomy, blood vessel classification, and regulatory mechanisms. The tables provide a concise comparison of open vs. closed circulatory systems and summarize key cardiac measurements for reference.