Circulatory System: Definition and Function

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The circulatory system is the organ system composed of the heart, blood vessels, and blood that transports respiratory gases, nutrients, hormones, metabolic wastes, and heat throughout the body. Its core function is to maintain a continuous, one-way flow of blood that delivers oxygen and substrates to tissues and carries carbon dioxide and byproducts away from them.

Every cell in a vertebrate body sits within roughly a few cell widths of a capillary, and that proximity is not accidental. It is the physical result of a pump, a branching network of tubes, and a fluid that never stops moving. When any of those three components fails, the consequences appear far from the heart: a kidney stops filtering, a brain loses consciousness, a limb goes cold. Understanding the circulatory system means understanding how structure at each level (chamber, valve, vessel wall, shunt) maps onto the function of keeping tissue perfusion matched to tissue demand. This article covers the definition, the comparative anatomy of hearts across vertebrate classes, the fetal shunts that redirect blood before birth, and the species-level differences a veterinary student needs to recognize.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Core Definition and Components

The Three Essential Parts

The circulatory system definition rests on three structural components that cannot function independently:

1. Heart. A muscular pump with one or more chambers. Contraction generates pressure that drives blood into the arterial system.
2. Blood vessels. A closed network of arteries, arterioles, capillaries, venules, and veins. Arteries carry blood away from the heart under high pressure. Capillaries are single-endothelial-cell-thick exchange vessels. Veins return blood to the heart at low pressure and serve as a capacitance reservoir holding most of the blood volume.
3. Blood. A connective tissue consisting of plasma and formed elements (erythrocytes, leukocytes, thrombocytes or platelets). Blood is the transport medium: hemoglobin binds oxygen, plasma carries glucose, amino acids, lipids, hormones, and nitrogenous wastes.

What the System Actually Does

The circulatory system performs five broad functions:

• Gas transport. Oxygen from the lungs or gills binds hemoglobin in erythrocytes and is delivered to tissues. Carbon dioxide travels back to the respiratory surface, mostly as bicarbonate and carbamino compounds.
• Nutrient delivery. Absorbed monosaccharides, amino acids, fatty acids, vitamins, and minerals enter the bloodstream and reach every tissue.
• Hormone distribution. Endocrine glands secrete hormones into the blood, which carries them to distant target cells.
• Waste removal. Metabolic byproducts such as urea, creatinine, and bilirubin are carried to the kidneys, liver, or gills for excretion.
• Thermoregulation and immune surveillance. Blood redistributes heat and carries leukocytes, antibodies, and complement proteins to sites of infection or injury.

The Two-Circuit Principle

In birds and mammals, the circulatory system operates as two circuits in series. The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs and returns oxygenated blood to the left atrium. The systemic circuit carries oxygenated blood from the left ventricle to the body and returns deoxygenated blood to the right atrium. This arrangement keeps oxygenated and deoxygenated blood separate, which maximizes oxygen delivery to tissues. In fish, the circuits are in series but not separated: blood passes through the gills first, then through the body, losing pressure along the way.

Comparative Heart Anatomy Across Vertebrates

Fish: Two Chambers in Series

A fish heart has two chambers arranged in series: one atrium and one ventricle. Blood enters the atrium, passes to the ventricle, and is pumped to the gills. After gas exchange at the gills, blood flows directly to the systemic tissues at reduced pressure. This single-pass arrangement works well for ectothermic fish with relatively low metabolic demands, but it limits the pressure available for systemic perfusion.

Amphibians: Three Chambers with Mixing

Amphibians have two atria and one ventricle. The left atrium receives oxygenated blood from the lungs and skin. The right atrium receives deoxygenated blood from the body. Both empty into a single ventricle, where some mixing occurs. Despite this mixing, anatomical ridges and flow dynamics within the ventricle reduce the degree of mixing, and the conus arteriosus helps direct blood preferentially. Amphibians also supplement pulmonary gas exchange with cutaneous respiration, which reduces their dependence on perfectly separated circulation.

Reptiles: Partially Divided Ventricle

Most reptiles have two atria and a ventricle that is partially divided by a muscular septum. In lizards and snakes, the septum is incomplete, allowing some mixing. In crocodilians, the ventricle is nearly completely divided, with a small opening called the foramen of Panizza that permits some communication between the systemic circuits. This partial separation improves oxygen delivery compared with amphibians while retaining the ability to shunt blood away from the lungs during diving or breath-holding.

Birds and Mammals: Four Chambers with Complete Separation

Birds and mammals independently evolved four-chambered hearts with two atria and two ventricles. The septum between the ventricles is complete, so oxygenated and deoxygenated blood never mix. This allows the left ventricle to generate high systemic pressure while the right ventricle pumps at low pressure into the pulmonary circuit. The result is efficient, high-pressure systemic perfusion that supports the high metabolic rates of endothermy. Birds and mammals are the only vertebrate groups with complete separation, and this is a classic example of convergent evolution.

Summary Table: Heart Chambers and Circulation by Species Group

Terms Students Confuse

Fetal Circulation and the Transition at Birth

Why Fetal Circulation Is Different

The fetus does not use its lungs for gas exchange. Oxygen and nutrients come from the placenta via the umbilical vein. Because the lungs are fluid-filled and non-functional, blood must be redirected away from them and toward the systemic circulation. Three shunts accomplish this: the ductus venosus, the foramen ovale, and the ductus arteriosus.

Ductus Venosus

The ductus venosus is a fetal vessel that connects the umbilical vein to the inferior vena cava, allowing oxygen-rich blood from the placenta to bypass the liver and reach the heart directly [1]. The tone of the ductus venosus determines how much substrate-rich blood is streamed preferentially away from the liver and toward the heart [2]. In fetal sheep studied at 116 to 117 days gestational age (term is 150 days), hypoxemia alone did not change ductus venosus shunting, but the addition of alpha-adrenergic blockade significantly decreased shunting [2]. This shows that the ductus venosus is actively regulated, not a passive tube.

When the ductus venosus is absent or malformed, blood from the umbilical vein must follow alternative routes. These include extrahepatic shunts or intrahepatic shunts [1]. Prenatal diagnosis of these portosystemic venous shunts is important because they can lead to complications after birth [3]. In a study of intrahepatic portosystemic shunts diagnosed prenatally, spontaneous closure occurred in 96 percent of patients at a median age of 7.5 months [4]. Growth restriction affected 77 percent of that cohort [4].

Foramen Ovale

The foramen ovale is an opening in the interatrial septum that allows blood to flow directly from the right atrium to the left atrium in the fetus. This bypasses the pulmonary circuit. Oxygen-rich blood entering the right atrium from the inferior vena cava preferentially crosses the foramen ovale into the left atrium, then enters the left ventricle and is pumped to the brain and coronary arteries. After birth, increased left atrial pressure and decreased right atrial pressure close the foramen ovale functionally within minutes to hours, and anatomically over weeks to months.

Ductus Arteriosus

The ductus arteriosus connects the pulmonary artery to the descending aorta. It allows blood ejected from the right ventricle to bypass the lungs and enter the systemic circulation. After birth, rising oxygen tension and falling prostaglandin levels cause the ductus arteriosus to constrict and close. In some congenital heart defects, the ductus arteriosus must remain open to supply pulmonary blood flow. This is called duct-dependent pulmonary circulation. Ductal stenting has emerged as a critical intervention for neonates with duct-dependent pulmonary circulation, with a reported procedural success rate of 98.5 percent in one series of 124 neonates [5]. Virtual reality modeling from CT data has been used to plan ductus arteriosus stenting in complex cases [6].

The Pulmonary-to-Systemic Switch at Birth

At birth, three events occur almost simultaneously:

1. Lung inflation. The first breaths expand the lungs and reduce pulmonary vascular resistance.
2. Increased pulmonary blood flow. Blood now flows through the pulmonary circuit instead of bypassing it.
3. Shunt closure. The foramen ovale closes functionally, and the ductus arteriosus constricts. The ductus venosus also closes, redirecting portal blood through the liver.

This transition is not instantaneous. In preterm infants, the transitional circulation can be unstable. Low superior vena cava flow, independent of fetal shunts, has been proposed as a biomarker of circulatory impairment in this population [7]. The fetal circulation has unique shunts and shifting pressure-loading conditions that adult cardiovascular models do not fully capture [8].

Mermaid Diagram: Fetal Blood Flow and Shunt Closure

The following flowchart shows the path of fetal blood and the shunts that close after birth.

flowchart TD
    A[Placenta] --> B[Umbilical vein]
    B --> C[Ductus venosus]
    C --> D[Inferior vena cava]
    D --> E[Right atrium]
    E --> F[Foramen ovale]
    F --> G[Left atrium]
    G --> H[Left ventricle]
    H --> I[Aorta to brain]
    E --> J[Right ventricle]
    J --> K[Ductus arteriosus]
    K --> I
    I --> L[Body tissues]
    L --> M[Umbilical arteries]
    M --> A

Species Differences in Heart Rate and Cardiac Output

Scaling with Body Size

Heart rate scales inversely with body size across mammals. A mouse heart beats roughly 500 to 700 times per minute. A cat heart beats 140 to 220 times per minute. A horse heart beats 28 to 40 times per minute at rest. This inverse relationship reflects metabolic rate: smaller animals lose heat faster and need faster circulation to maintain body temperature and deliver oxygen.

Cardiac output is the product of heart rate and stroke volume. Stroke volume scales with heart size, which scales with body mass. The net result is that cardiac output scales with body mass to approximately the 0.75 power, matching the scaling of metabolic rate. This means a horse has a much larger absolute cardiac output than a cat, but a cat has a much higher cardiac output per gram of tissue.

Practical Implications

In veterinary practice, heart rate is one of the first parameters assessed during physical examination. Normal ranges vary widely by species:

• Dog: 60 to 160 beats per minute, depending on size and excitement.
• Cat: 140 to 220 beats per minute.
• Horse: 28 to 40 beats per minute at rest.
• Cow: 48 to 84 beats per minute.
• Bird: 200 to 600 beats per minute, depending on species and size.

These differences matter when interpreting auscultation findings, when calculating drug doses that affect heart rate, and when monitoring anesthesia. A heart rate that would be normal in a cat could indicate distress in a horse.

How the Circulatory System Is Studied and Observed

Physical Examination

Auscultation with a stethoscope reveals heart rate, rhythm, and the presence of murmurs. Palpation of peripheral pulses (femoral, dorsal pedal, lingual in some species) assesses pulse quality and strength. Capillary refill time and mucous membrane color provide rapid bedside information about perfusion.

Diagnostic Imaging

Echocardiography uses ultrasound to visualize chamber size, wall thickness, valve motion, and blood flow direction. Doppler echocardiography measures flow velocity and can detect shunts. In fetal medicine, phase contrast MRI and T2 MRI oximetry have been used to measure blood flow and oxygen saturation within the fetal circulation [2].

Computational Modeling

Computational models of the fetal circulation integrate physiological knowledge to capture developmental adaptations. Existing models are often adapted from adult cardiovascular frameworks, which do not fully account for fetal-specific features such as circulatory shunts, placental resistance, and shifting pressure-loading conditions [8]. Refining these models is an active area of research.

Laboratory Testing

Blood tests assess the transport function of blood: packed cell volume (PCV), total protein, blood glucose, blood urea nitrogen (BUN), and electrolyte concentrations. These values reflect whether the circulatory system is delivering nutrients and removing wastes effectively.

Clinical Relevance, Limitations and Common Mistakes

Clinical Relevance

Circulatory system function is central to every branch of veterinary medicine. Congenital heart defects, acquired valvular disease, arrhythmias, shock, and anemia all involve the circulatory system. Understanding fetal shunts is essential for interpreting prenatal ultrasound findings and for managing neonates with duct-dependent circulation [5][6]. Portosystemic shunts, whether intrahepatic or extrahepatic, are diagnosed prenatally and can affect growth and development [3][9][10]. In a study of umbilical-portal-systemic venous shunts, the ductus venosus was absent in 70 percent of cases [10].

Limitations

Species differences in heart anatomy and physiology mean that findings from one species do not always translate directly to another. Computational models built on adult frameworks may not capture fetal-specific features [8]. Individual cases require veterinary assessment, and this article is educational only.

Common Mistakes

1. Confusing arteries and veins with oxygen content. Arteries carry blood away from the heart, but the pulmonary artery carries deoxygenated blood. Veins carry blood toward the heart, but the pulmonary vein carries oxygenated blood.
2. Assuming all reptiles have three-chambered hearts. Crocodilians have a nearly four-chambered heart with a foramen of Panizza.
3. Thinking fetal shunts are abnormal. The foramen ovale, ductus arteriosus, and ductus venosus are normal fetal structures that close after birth.
4. Ignoring species differences in heart rate. A heart rate that is normal for a dog may be bradycardic for a cat.
5. Treating cardiac output as a fixed number. Cardiac output changes with metabolic demand, body size, and disease state.

Quick Review

• The circulatory system is the heart, blood vessels, and blood working together to transport gases, nutrients, hormones, and wastes.
• Fish have two-chambered hearts with single-circuit circulation. Amphibians have three chambers with mixing. Reptiles have partially divided ventricles. Birds and mammals have four-chambered hearts with complete separation.
• Fetal circulation uses three shunts: the ductus venosus, foramen ovale, and ductus arteriosus.
• At birth, lung inflation and falling pulmonary vascular resistance trigger shunt closure and the switch from parallel to series circulation.
• Heart rate scales inversely with body size. Cardiac output scales with metabolic rate.
• The ductus venosus is actively regulated by adrenergic tone, not a passive tube [2].
• Species differences in heart anatomy and heart rate are clinically important for physical examination, anesthesia, and drug dosing.

Frequently Asked Questions

What is the circulatory system?

The circulatory system is the heart, blood vessels, and blood working together to transport oxygen, nutrients, hormones, and wastes throughout the body.

How many chambers does a fish heart have?

A fish heart has two chambers: one atrium and one ventricle, arranged in a single circuit through the gills and then the body.

What are the three fetal shunts?

The three fetal shunts are the ductus venosus, the foramen ovale, and the ductus arteriosus. They redirect blood away from the non-functional fetal lungs and liver.

When do fetal shunts close?

Fetal shunts close after birth as pulmonary vascular resistance falls and oxygen tension rises. Functional closure of the foramen ovale occurs within minutes to hours, while anatomical closure takes weeks to months.

Why does a cat have a faster heart rate than a horse?

Heart rate scales inversely with body size because smaller animals have higher metabolic rates and need faster circulation to deliver oxygen and maintain body temperature.

Do all reptiles have three-chambered hearts?

No. Most reptiles have a partially divided ventricle, but crocodilians have a nearly four-chambered heart with a small opening called the foramen of Panizza.

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Sources

1. Absence of Ductus Venosus: A Comparison of 2 Distinctive Fetal Autopsy Cases and Embryologic Perspectives.
2. α-Adrenergic blockade impairs ductus venosus shunting during an acute period of fetal hypoxaemia without further deficits to cerebral oxygen delivery.
3. [[Importance of prenatally diagnosed portosystemic vascular shunts in clinical outcomes].](https://pubmed.ncbi.nlm.nih.gov/38329310/)
4. Intrahepatic portosystemic shunts, from prenatal diagnosis to postnatal outcome: a retrospective study.
5. Institutional experience with ductus arteriosus stenting in neonates with duct dependent pulmonary circulation: Procedural outcomes & mid-term follow up.
6. Virtual Reality in Neonatal Ductal Stenting: A New Era in Preprocedural Planning for Ductus Arteriosus Stenting in Complex Congenital Heart Defects with Duct-Dependent Pulmonary Circulation.
7. Dose-finding for dobutamine during transitional circulation in the very preterm infant: The study protocol.
8. Computational modelling of the fetal circulation: what could cardiovascular system models teach us about the heart-placenta axis?
9. Fetal Extrahepatic Portosystemic Venous Shunts: Prenatal Diagnosis Management and Therapy - 21 Years of Evolving Insights.
10. Perinatal outcomes in cases of umbilical-portal-systemic venous shunts: experience of a tertiary center.