Circulatory Systems and the Heart

How animals move blood or haemolymph around the body, and how hearts generate the pressure and flow that match delivery to the demands of tissues.

Definition

A circulatory system is the arrangement of pump, fluid, and vessels that transports gases, nutrients, wastes, hormones, and heat through an animal's body, and the heart is the muscular pump whose rhythmic contraction generates the pressure that drives that flow.

Scope

This topic covers the comparative physiology of internal transport: open and closed circulatory systems, single and double circuits, the structure and electrical activity of vertebrate and invertebrate hearts, the cardiac cycle, and the regulation of cardiac output and blood pressure. It addresses how vessel resistance and capacitance distribute flow and how circulation is matched to body size, activity, and respiratory strategy. Coverage is comparative and mechanistic rather than clinical.

Core questions

• What distinguishes open from closed circulatory systems, and what are their trade-offs?
• How does the heartbeat originate and spread, and how is the cardiac cycle organised?
• How is cardiac output regulated to match the changing needs of the body?
• Why did double circulation evolve in birds and mammals, and what advantage does it give?

Key theories

Frank–Starling relationship

The force of cardiac contraction increases with the degree to which the heart is filled, so within limits the heart automatically pumps out the volume of blood returned to it, matching output to venous return.

Myogenic origin of the heartbeat

In vertebrate hearts the rhythm arises within specialised pacemaker muscle cells rather than from nerves, and the impulse spreads through the cardiac muscle to coordinate contraction, with nervous and hormonal input modulating rate and force.

Mechanisms

Closed systems confine blood to vessels and can sustain high pressures and fine control of distribution, while open systems bathe tissues in haemolymph at lower pressure. Vertebrate hearts beat myogenically: pacemaker cells depolarise spontaneously and the impulse spreads through conducting tissue and muscle, producing the coordinated cardiac cycle of filling and ejection. Stroke volume is set by filling (the Frank–Starling mechanism) and contractility, and heart rate by autonomic and hormonal input, so cardiac output equals their product. Arteries, arterioles, capillaries, and veins differ in resistance and compliance to regulate pressure and distribute flow, and the fish single circuit, amphibian and reptilian intermediate hearts, and the fully divided double circulation of birds and mammals represent successive solutions matched to gas exchange and metabolic demand.

Clinical relevance

Comparative cardiac physiology, including the extreme bradycardia of diving animals and the high outputs of small endotherms, illuminates the principles behind cardiovascular performance and its regulation. This entry is educational reference material and not medical guidance.

History

William Harvey's demonstration of the circulation of the blood in 1628 founded cardiovascular physiology, and Frank and Starling later established the length–tension basis of cardiac output. Comparative physiology extended these principles across the diversity of animal hearts and circuits, relating circulatory design to body size, lifestyle, and respiratory strategy.

Key figures

• William Harvey
• Otto Frank
• Ernest Starling
• Knut Schmidt-Nielsen

Seminal works

• hill2016
• randall2002
• schmidtnielsen1997

Frequently asked questions

What is the difference between open and closed circulation?

In closed systems blood stays within vessels and returns to the heart through them; in open systems the pump empties haemolymph into body spaces that bathe the tissues directly before it drains back.

What is the Frank–Starling mechanism?

It is the heart's tendency to contract more forcefully when it is filled more, so that it pumps out roughly as much blood as it receives without needing external control.