Heart and Circulatory System

The heart and circulatory system together ensure that every cell in an animal’s body receives oxygen, nutrients, hormones, and that waste products such as carbon dioxide and urea are transported away. In this chapter we focus on the basic structure and function of the vertebrate (especially human) heart and circulation, and contrast this with some simpler and alternative circulatory systems in other animals.

Functions of the Circulatory System

The circulatory system is a transport system. Its main tasks in animals include:

• Transport of respiratory gases
• Oxygen from respiratory organs (e.g. lungs, gills) to tissues
• Carbon dioxide from tissues to respiratory organs
• Distribution of nutrients absorbed from the digestive system
• Removal of metabolic waste products to excretory organs (e.g. kidneys)
• Transport of hormones from endocrine glands to target organs
• Heat distribution and temperature regulation (especially in birds and mammals)
• Immune defense (via white blood cells and antibodies in the blood)
• Maintenance of internal environment (pH, ion composition, volume of body fluids)

These functions are made possible by three basic components:

1. Pump (the heart or its equivalents)
2. Transport medium (blood or hemolymph)
3. Vessels or spaces through which the fluid moves (arteries, veins, capillaries, body cavities)

Open and Closed Circulatory Systems

Animals have evolved different types of circulatory systems. Two major forms are common:

Open Circulatory System

Found in many invertebrates, such as most arthropods (insects, crustaceans) and many mollusks.

• The circulating fluid is called hemolymph, not clearly separated from interstitial fluid.
• A tubular heart (or several hearts) pumps hemolymph into large body cavities (hemocoels).
• Organs are directly bathed in hemolymph; there is no closed network of capillaries.
• Pressure is relatively low, and flow is slower and less strictly directed.
• Advantages:
• Structurally simple and energetically inexpensive to build and maintain.
• Sufficient for animals with lower metabolic demands or those relying heavily on tracheal systems (e.g. insects) for gas exchange.
• Disadvantages:
• Less efficient transport and regulation of flow to specific organs.
• Limited support for very high and rapidly changing metabolic rates compared to closed systems.

Closed Circulatory System

Found in vertebrates, annelid worms (e.g. earthworms), and some cephalopod mollusks (e.g. octopus).

• Blood remains enclosed within vessels and the heart at all times.
• Exchange with tissues occurs through thin-walled capillaries.
• Higher pressure can be maintained, allowing:
• Faster, more directed blood flow
• Fine regulation of blood distribution among organs
• Support of higher metabolic rates and active lifestyles
• In vertebrates, blood components are specialized:
• Red blood cells with hemoglobin for oxygen transport
• White blood cells for immune defense
• Platelets (in mammals) or thrombocytes (in other vertebrates) for blood clotting

The rest of this chapter focuses on the vertebrate closed circulatory system, especially that of humans as an example.

Structure of the Vertebrate Circulatory System

Main Types of Blood Vessels

Although detailed features of blood composition and gas transport are treated elsewhere, some basic vessel types are essential here:

• Arteries
• Carry blood away from the heart.
• Have relatively thick, elastic, and muscular walls.
• Can withstand and help smooth out high pressure from heart contractions.
• Arterioles
• Fine branches of arteries within organs.
• Their smooth muscle layer controls vessel diameter.
• Important in regulating blood flow to different tissues and in controlling blood pressure.
• Capillaries
• The smallest vessels; form dense networks in tissues.
• Very thin walls (often a single cell layer) for exchange of gases, nutrients, and wastes between blood and tissue fluid.
• Blood flow is slow here to maximize exchange.
• Venules and Veins
• Venules collect blood from capillaries and converge into larger veins.
• Veins carry blood back to the heart.
• Thinner walls and lower pressure than arteries.
• Often contain valves (especially in limbs) that prevent backflow and aid venous return.

Single and Double Circulation

The arrangement of blood flow differs among vertebrate groups:

• Single circulation (e.g. most fishes)
• Blood passes through the heart once per circuit.
• Path: heart → gills (gas exchange) → body tissues → heart.
• The heart generally has two main chambers: one atrium and one ventricle.
• After passing the gills, blood pressure drops, limiting the speed of flow to systemic tissues.
• Double circulation (amphibians, reptiles, birds, mammals)
• Blood passes through the heart twice per circuit:
• One loop to the respiratory organs (pulmonary or pulmocutaneous circulation)
• One loop to the rest of the body (systemic circulation)
• Allows separate pressure levels for lungs and body and supports higher metabolic rates.

In mammals and birds, the separation between oxygen-rich and oxygen-poor blood is complete; in amphibians and most non-crocodilian reptiles, the separation is partial.

The Human Heart: Structure

The human heart is a muscular, fist-sized organ located in the thoracic cavity between the lungs, slightly left of the midline.

Heart Chambers and Sides

• The heart consists of four chambers:
• Right atrium (RA)
• Right ventricle (RV)
• Left atrium (LA)
• Left ventricle (LV)
• The right side receives oxygen-poor (deoxygenated) blood from the body and pumps it to the lungs.
• The left side receives oxygen-rich (oxygenated) blood from the lungs and pumps it to the body.

Internally, a septum (interatrial and interventricular septa) separates right and left heart so that systemic and pulmonary blood are completely separated.

Valves of the Heart

Heart valves ensure that blood flows in only one direction:

• Atrioventricular (AV) valves
• Located between atria and ventricles.
• Right side: tricuspid valve (three leaflets).
• Left side: mitral or bicuspid valve (two leaflets).
• Prevent backflow from ventricles into atria during ventricular contraction.
• Chordae tendineae (tendon-like cords) and papillary muscles keep the valve leaflets from inverting.
• Semilunar valves
• Between ventricles and the large arteries leaving the heart.
• Pulmonary valve: between RV and pulmonary trunk.
• Aortic valve: between LV and aorta.
• Prevent backflow from arteries into ventricles when ventricles relax.

Valve closure and opening are passive: they respond to pressure differences across them.

Major Blood Vessels Connected to the Heart

• To and from the lungs (pulmonary circulation):
• Pulmonary trunk leaves the right ventricle; branches into left and right pulmonary arteries to the lungs.
• Four pulmonary veins (usually two from each lung) carry oxygenated blood to the left atrium.
• To and from the body (systemic circulation):
• Superior and inferior vena cava bring deoxygenated blood from the body into the right atrium.
• Aorta leaves the left ventricle and distributes oxygenated blood to all body regions.

Heart Wall and Coronary Circulation

The heart wall has three main layers:

• Endocardium: thin inner lining of the heart chambers and valves.
• Myocardium: thick layer of cardiac muscle responsible for pumping action.
• Epicardium: outer layer; part of the pericardium (the protective sac surrounding the heart).

Because the myocardium is thick and highly active, it needs its own blood supply:

• Coronary arteries branch off from the base of the aorta and supply the heart muscle with oxygen and nutrients.
• Cardiac veins collect deoxygenated blood from the myocardium and drain into the right atrium (often via the coronary sinus).

Blockage of coronary arteries reduces blood flow to heart muscle and can cause myocardial infarction (heart attack).

Pumping Cycle of the Heart (Cardiac Cycle)

The cardiac cycle is the sequence of events during one heartbeat. It consists of alternating phases of contraction (systole) and relaxation (diastole) of atria and ventricles.

Phases of the Cardiac Cycle (Simplified)

1. Atrial diastole, ventricular diastole
• Heart is relaxed; blood flows into atria from veins.
• AV valves are open; semilunar valves are closed.
• Blood passively fills ventricles.
2. Atrial systole
• Atria contract and push additional blood into ventricles.
• Completes ventricular filling just before ventricular contraction.
3. Ventricular systole (isovolumetric contraction)
• Ventricles begin to contract.
• Pressure rises in ventricles; AV valves close (first heart sound).
• Semilunar valves are still closed; volume in ventricles does not change yet.
4. Ventricular systole (ejection phase)
• Ventricular pressure exceeds pressure in aorta and pulmonary trunk.
• Semilunar valves open; blood is ejected into arteries.
• Atria are now relaxing and filling again.
5. Ventricular diastole (isovolumetric relaxation)
• Ventricles relax; pressure drops.
• Semilunar valves close (second heart sound) as arterial pressure becomes higher than ventricular pressure.
• All valves temporarily closed; ventricle volume constant.
6. Ventricular filling
• Once ventricular pressure falls below atrial pressure, AV valves open again.
• Blood flows from atria into ventricles, starting a new cycle.

Stroke Volume and Cardiac Output

Two important quantities describe heart performance:

• Stroke volume (SV): volume of blood ejected by one ventricle in a single beat.
• Heart rate (HR): number of heartbeats per minute.

Cardiac output (CO) is the volume of blood pumped by one ventricle per minute:

$$
\text{Cardiac output} = \text{Stroke volume} \times \text{Heart rate}
$$

For a typical adult at rest, approximate values might be:

• $SV \approx 70\ \text{mL/beat}$
• $HR \approx 70\ \text{beats/min}$

Thus:

$$
CO \approx 70\ \text{mL/beat} \times 70\ \text{beats/min} \approx 4900\ \text{mL/min} \approx 5\ \text{L/min}
$$

During exercise, both HR and SV can increase, leading to higher cardiac output to meet the body’s increased oxygen demands.

Electrical Control of the Heartbeat

Unlike skeletal muscles, the vertebrate heart can generate its own rhythmic contractions. This property is called myogenic automaticity.

Conduction System of the Human Heart

Specialized cardiac muscle cells form the heart’s conduction system:

• Sinoatrial (SA) node
• Located in the wall of the right atrium.
• Acts as the primary pacemaker: generates regular electrical impulses.
• Each impulse triggers one heartbeat.
• Atrioventricular (AV) node
• Located in the lower part of the right atrium, near the septum.
• Receives impulses from the atria.
• Introduces a slight delay, allowing ventricles to fill before contraction.
• Atrioventricular bundle (Bundle of His) and bundle branches
• Conduct impulses from the AV node into the interventricular septum.
• Divide into right and left bundle branches, running toward the apex of the heart.
• Purkinje fibers
• Spread throughout the ventricular walls from the apex upward.
• Rapidly conduct impulses to ventricular muscle cells, causing coordinated ventricular contraction from apex upward toward the base.

Pacemaker Potential and Autonomic Regulation

Cells of the SA node display spontaneous depolarization (pacemaker potential), so they reach threshold and fire regularly without external input.

However, the rate of firing is modulated by the autonomic nervous system and hormones:

• Sympathetic stimulation (e.g. via norepinephrine)
• Increases heart rate and force of contraction.
• Important in stress and exercise.
• Parasympathetic stimulation (mainly via the vagus nerve and acetylcholine)
• Decreases heart rate.
• Dominant during rest.

Adrenaline (epinephrine) from the adrenal medulla also increases heart rate and contractility.

Electrocardiogram (ECG)

The electrical activity of the heart can be recorded from the body surface as an electrocardiogram:

• P wave: depolarization of the atria.
• QRS complex: depolarization of the ventricles (atrial repolarization occurs at the same time but is obscured).
• T wave: repolarization of the ventricles.

ECG patterns help diagnose disturbances in rhythm and conduction.

Systemic and Pulmonary Circulation in Humans

In humans, the double circulatory system consists of:

• Pulmonary circulation
• Right ventricle → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → left atrium.
• Function: gas exchange (oxygen uptake, carbon dioxide release).
• Systemic circulation
• Left ventricle → aorta → arterial branches → organs and tissues → veins → venae cavae → right atrium.
• Function: supply tissues with oxygen and nutrients; carry away carbon dioxide and wastes.

Pressure differences:

• Systemic circulation operates at higher pressure (to reach all body tissues).
• Pulmonary circulation operates at lower pressure (to protect the delicate lung capillaries and facilitate gas exchange).

Blood Pressure and Its Regulation

What Is Blood Pressure?

Blood pressure is the force exerted by blood on the walls of blood vessels, usually expressed in arteries near the heart.

In humans, arterial pressure is typically given as two numbers, e.g. 120/80 mmHg:

• Systolic pressure (~120 mmHg): maximum pressure during ventricular systole (ejection).
• Diastolic pressure (~80 mmHg): minimum pressure during ventricular diastole.

Determinants of Blood Pressure

Blood pressure depends mainly on:

• Cardiac output (CO)
• Total peripheral resistance (TPR): the resistance of the vascular system, especially in small arteries and arterioles.

A simplified relation is:

$$
\text{Blood pressure} \propto \text{Cardiac output} \times \text{Total peripheral resistance}
$$

Changes in heart rate, stroke volume, vessel diameter, and blood volume all influence blood pressure.

Short-Term Regulation

Short-term adjustments are mainly handled by:

• Baroreceptors (pressure sensors) in the aorta and carotid arteries
• Detect changes in arterial pressure.
• Send signals to cardiovascular centers in the brainstem.
• Autonomic nervous system
• Sympathetic activation: increases heart rate, stroke volume, and arteriolar constriction → raises blood pressure.
• Parasympathetic activation: decreases heart rate → lowers blood pressure.

This feedback system keeps blood pressure relatively stable on a moment-to-moment basis, especially during posture changes or sudden activity.

Long-Term Regulation

Long-term regulation involves:

• Kidney function
• Adjusts blood volume by controlling how much water and salt are excreted or retained.
• Hormonal systems (e.g. renin–angiotensin–aldosterone system, antidiuretic hormone)
• Can alter both blood volume and vessel tone, thus influencing blood pressure over hours to days.

Microcirculation and Return of Blood

Capillary Exchange

In capillaries, blood pressure and osmotic pressure together govern fluid movement:

• At the arterial end of a capillary bed
• Blood pressure is relatively high.
• Net filtration: fluid (with dissolved substances) moves from blood into tissue fluid.
• At the venous end
• Blood pressure has dropped.
• Osmotic pressure of blood (due largely to plasma proteins) is relatively higher than that of tissue fluid.
• Net reabsorption: some fluid moves back into capillaries.

Not all filtered fluid is reabsorbed; excess is taken up by the lymphatic system, which returns it to the venous circulation.

Venous Return

Because venous pressure is relatively low, additional mechanisms assist blood return to the heart:

• Muscle pump
• Contraction of skeletal muscles compresses veins.
• One-way valves prevent backflow and help push blood toward the heart.
• Respiratory pump
• Changes in pressure in the thoracic cavity during breathing help draw venous blood toward the chest.
• Venoconstriction
• Sympathetic nervous system can cause veins to constrict, pushing blood toward the heart.

Variations in Circulatory Systems Among Vertebrates

While the human heart is a good model, other vertebrates show variations linked to their environment and metabolic demands.

Fish

• Two-chambered heart (one atrium, one ventricle).
• Single circulation: heart → gills → body → heart.
• Less separation between oxygen-rich and oxygen-poor blood.
• Adequate for aquatic life and ectothermic metabolism.

Amphibians

• Three-chambered heart (two atria, one ventricle).
• Double circulation (pulmocutaneous and systemic), but with incomplete separation in the ventricle.
• Some mixing of oxygenated and deoxygenated blood occurs.
• Gas exchange can also occur through skin in many species.

Non-Crocodilian Reptiles

• Typically three-chambered heart with a partially divided ventricle.
• Degree of separation varies between groups.
• Reduced mixing of blood compared with amphibians; supports higher activity.

Crocodilians, Birds, and Mammals

• Four-chambered heart with complete separation of right and left sides.
• Strict separation of systemic and pulmonary circulation.
• Enables high blood pressure in systemic circuit without overloading pulmonary circuit.
• Supports endothermy (internal heat production) and high, stable metabolic rates in birds and mammals.

Circulatory System and Homeostasis

The heart and circulatory system are central to maintaining internal stability:

• They rapidly distribute signals (hormones), adjusting organ function.
• They coordinate with respiratory, excretory, and digestive systems to maintain:
• Proper oxygen and carbon dioxide levels
• Stable pH
• Appropriate nutrient and ion balance
• Constant temperature (in endotherms)

Disturbances of heart or vessels (e.g. heart failure, vascular blockages, valve defects) can have widespread consequences, since virtually all tissues depend on adequate blood supply.

Understanding the basic structure and function of the heart and circulatory system provides the foundation for examining gas exchange in lungs and tissues and the transport of carbon dioxide, which are covered in the following chapters.