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The circulatory system, also known as the cardiovascular system, is a vast network of organs and blood vessels that acts both as a delivery and waste removal system for the body. WHICH ORGANS ARE INVOLVED IN THE CIRCULATORY SYSTEM? WHAT DOES THE HEART AND CIRCULATORY SYSTEM LOOK LIKE? WHAT COMMON DISEASES CAN AFFECT THE HEART AND THE CIRCULATORY SYSTEM? 1: Superior and inferior vena cava Right Atrium Tricuspid valve 2: Right Ventricle Pulmonary valve.

Learning Objectives

By the end of this section, you will have completed the following objectives:

• Describe an open and closed circulatory system
• Describe interstitial fluid and hemolymph
• Compare and contrast the organization and evolution of the vertebrate circulatory system.

In all animals, except a few simple types, the circulatory system is used to transport nutrients and gases through the body. Simple diffusion allows some water, nutrient, waste, and gas exchange into primitive animals that are only a few cell layers thick; however, bulk flow is the only method by which the entire body of larger more complex organisms is accessed.

Circulatory System Architecture

The circulatory system is effectively a network of cylindrical vessels: the arteries, veins, and capillaries that emanate from a pump, the heart. In all vertebrate organisms, as well as some invertebrates, this is a closed-loop system, in which the blood is not free in a cavity. In a closed circulatory system, blood is contained inside blood vessels and circulates unidirectionally from the heart around the systemic circulatory route, then returns to the heart again, as illustrated in Figure 1a. As opposed to a closed system, arthropods—including insects, crustaceans, and most mollusks—have an open circulatory system, as illustrated in Figure 1b. In an open circulatory system, the blood is not enclosed in the blood vessels but is pumped into a cavity called a hemocoel and is called hemolymph because the blood mixes with the interstitial fluid. As the heart beats and the animal moves, the hemolymph circulates around the organs within the body cavity and then reenters the hearts through openings called ostia. This movement allows for gas and nutrient exchange. An open circulatory system does not use as much energy as a closed system to operate or to maintain; however, there is a trade-off with the amount of blood that can be moved to metabolically active organs and tissues that require high levels of oxygen. In fact, one reason that insects with wing spans of up to two feet wide (70 cm) are not around today is probably because they were outcompeted by the arrival of birds 150 million years ago. Birds, having a closed circulatory system, are thought to have moved more agilely, allowing them to get food faster and possibly to prey on the insects.

Figure 1. In (a) closed circulatory systems, the heart pumps blood through vessels that are separate from the interstitial fluid of the body. Most vertebrates and some invertebrates, like this annelid earthworm, have a closed circulatory system. In (b) open circulatory systems, a fluid called hemolymph is pumped through a blood vessel that empties into the body cavity. Hemolymph returns to the blood vessel through openings called ostia. Arthropods like this bee and most mollusks have open circulatory systems.

Circulatory System Variation in Animals

The circulatory system varies from simple systems in invertebrates to more complex systems in vertebrates. The simplest animals, such as the sponges (Porifera) and rotifers (Rotifera), do not need a circulatory system because diffusion allows adequate exchange of water, nutrients, and waste, as well as dissolved gases, as shown in Figure 2a. Organisms that are more complex but still only have two layers of cells in their body plan, such as jellies (Cnidaria) and comb jellies (Ctenophora) also use diffusion through their epidermis and internally through the gastrovascular compartment. Both their internal and external tissues are bathed in an aqueous environment and exchange fluids by diffusion on both sides, as illustrated in Figure 2b. Exchange of fluids is assisted by the pulsing of the jellyfish body.

Figure 2. Simple animals consisting of a single cell layer such as the (a) sponge or only a few cell layers such as the (b) jellyfish do not have a circulatory system. Instead, gases, nutrients, and wastes are exchanged by diffusion.

For more complex organisms, diffusion is not efficient for cycling gases, nutrients, and waste effectively through the body; therefore, more complex circulatory systems evolved. Most arthropods and many mollusks have open circulatory systems. In an open system, an elongated beating heart pushes the hemolymph through the body and muscle contractions help to move fluids. The larger more complex crustaceans, including lobsters, have developed arterial-like vessels to push blood through their bodies, and the most active mollusks, such as squids, have evolved a closed circulatory system and are able to move rapidly to catch prey. Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptation during evolution and associated differences in anatomy. Figure 3 illustrates the basic circulatory systems of some vertebrates: fish, amphibians, reptiles, and mammals.

As illustrated in Figure 3a Fish have a single circuit for blood flow and a two-chambered heart that has only a single atrium and a single ventricle. The atrium collects blood that has returned from the body and the ventricle pumps the blood to the gills where gas exchange occurs and the blood is re-oxygenated; this is called gill circulation. The blood then continues through the rest of the body before arriving back at the atrium; this is called systemic circulation. This unidirectional flow of blood produces a gradient of oxygenated to deoxygenated blood around the fish’s systemic circuit. The result is a limit in the amount of oxygen that can reach some of the organs and tissues of the body, reducing the overall metabolic capacity of fish.

In amphibians, reptiles, birds, and mammals, blood flow is directed in two circuits: one through the lungs and back to the heart, which is called pulmonary circulation, and the other throughout the rest of the body and its organs including the brain (systemic circulation). In amphibians, gas exchange also occurs through the skin during pulmonary circulation and is referred to as pulmocutaneous circulation.

As shown in Figure 3b, amphibians have a three-chambered heart that has two atria and one ventricle rather than the two-chambered heart of fish. The two atria (superior heart chambers) receive blood from the two different circuits (the lungs and the systems), and then there is some mixing of the blood in the heart’s ventricle (inferior heart chamber), which reduces the efficiency of oxygenation. The advantage to this arrangement is that high pressure in the vessels pushes blood to the lungs and body. The mixing is mitigated by a ridge within the ventricle that diverts oxygen-rich blood through the systemic circulatory system and deoxygenated blood to the pulmocutaneous circuit. For this reason, amphibians are often described as having double circulation.

Figure 3. (a) Fish have the simplest circulatory systems of the vertebrates: blood flows unidirectionally from the two-chambered heart through the gills and then the rest of the body. (b) Amphibians have two circulatory routes: one for oxygenation of the blood through the lungs and skin, and the other to take oxygen to the rest of the body. The blood is pumped from a three-chambered heart with two atria and a single ventricle.

Most reptiles also have a three-chambered heart similar to the amphibian heart that directs blood to the pulmonary and systemic circuits, as shown in Figure 4a. The ventricle is divided more effectively by a partial septum, which results in less mixing of oxygenated and deoxygenated blood. Some reptiles (alligators and crocodiles) are the most primitive animals to exhibit a four-chambered heart. Crocodilians have a unique circulatory mechanism where the heart shunts blood from the lungs toward the stomach and other organs during long periods of submergence, for instance, while the animal waits for prey or stays underwater waiting for prey to rot. One adaptation includes two main arteries that leave the same part of the heart: one takes blood to the lungs and the other provides an alternate route to the stomach and other parts of the body. Two other adaptations include a hole in the heart between the two ventricles, called the foramen of Panizza, which allows blood to move from one side of the heart to the other, and specialized connective tissue that slows the blood flow to the lungs. Together these adaptations have made crocodiles and alligators one of the most evolutionarily successful animal groups on earth.

In mammals and birds, the heart is also divided into four chambers: two atria and two ventricles, as illustrated in Figure 4b. The oxygenated blood is separated from the deoxygenated blood, which improves the efficiency of double circulation and is probably required for the warm-blooded lifestyle of mammals and birds. The four-chambered heart of birds and mammals evolved independently from a three-chambered heart. The independent evolution of the same or a similar biological trait is referred to as convergent evolution.

Figure 4. (a) Reptiles also have two circulatory routes; however, blood is only oxygenated through the lungs. The heart is three chambered, but the ventricles are partially separated so some mixing of oxygenated and deoxygenated blood occurs except in crocodilians and birds. (b) Mammals and birds have the most efficient heart with four chambers that completely separate the oxygenated and deoxygenated blood; it pumps only oxygenated blood through the body and deoxygenated blood to the lungs.

Section Summary

In most animals, the circulatory system is used to transport blood through the body. Some primitive animals use diffusion for the exchange of water, nutrients, and gases. However, complex organisms use the circulatory system to carry gases, nutrients, and waste through the body. Circulatory systems may be open (mixed with the interstitial fluid) or closed (separated from the interstitial fluid). Closed circulatory systems are a characteristic of vertebrates; however, there are significant differences in the structure of the heart and the circulation of blood between the different vertebrate groups due to adaptions during evolution and associated differences in anatomy. Fish have a two-chambered heart with unidirectional circulation. Amphibians have a three-chambered heart, which has some mixing of the blood, and they have double circulation. Most non-avian reptiles have a three-chambered heart, but have little mixing of the blood; they have double circulation. Mammals and birds have a four-chambered heart with no mixing of the blood and double circulation.

Circulatory system diseases cover a vast array of different abnormalities and disorders that affect the way the body circulates blood. Circulatory system disorders can lead to decreased perfusion of blood throughout the body, threatening the healthy function of tissue and organs.

The human circulatory system is a complex network of blood vessels, varying in size, working in tandem with the rhythmic pumping of the heart. Essential for keeping the body working optimally, its main purpose is to carry oxygen, nutrients, electrolytes, and hormones, via the blood, making sure that all your bodily processes get what they need to be able to function as they should, with the end goal of keeping you healthy and alive.

Complications affecting the circulatory system can arise from a number of different factors, including genetics, lifestyle, and even infection that could threaten your health or even your life.

Anatomy of the circulatory system

There are two blood vessel systems in the body, arterial and venous. Arteries are tasked with carrying blood away from the heart and to all reaches of the body, from the top of your head to the tips of your toes. Veins transport the blood from the body’s tissues back to the lungs to become re-oxygenated again via pulmonary circulation. This blood is then sent to the heart to be pumped back into the arterial vascular system.

The anatomy of the circulatory system consists of a network of blood vessels that resembles the branches of a tree, extending to every corner of the human body. None of this would be useful, however, without the pumping action of the heart, as it works to make sure blood is pumped with enough force to reach the most remote places in the body.

The human heart is made up of four chambers: the right and left atriums and the right and left ventricles. Each one of these compartments helps to pump deoxygenated blood from the venous system of the body to the lungs to become oxygenated; then pump it back out through the main aorta. This then travels through some larger and smaller arteries into the capillary network (fine branching blood vessels). The heart plays a vital role in the circulatory system with any abnormality potentially being life-threatening.

What causes circulatory system diseases?

Diseases of the circulatory system can present in many different forms. The most common diseases of the circulatory system tend to be a result of longstanding poor health and metabolic disease that take a toll on blood vessels over the years, only to create complications later in life. These may include diseases such as diabetes, atherosclerosis, and high blood pressure (hypertension). Common causes of circulatory problems can be classified into the five following groups:

Trauma

An example of trauma may involve penetrating injuries from knife wounds that damage blood vessels. This type of injury can cause major damage depending on the location of the cut. Blunt force trauma, as in the case of being hit by an object like a bat, can bruise blood vessels to the extent that a blood clot is formed, prohibiting blood flow and causing additional pain. Due to the abundance of varying kinds of blood vessels in the body, collateral circulation helps to still provide the affected part of the body receive oxygenated blood, but this does depend on the severity of the injury.

Aneurysms

Healthy blood vessels contract and expand to better handle varying blood flows. However, sometimes a localized weakness of the vessel wall can cause a portion to expand like a balloon, creating an aneurysm. If an aneurysm were to rupture, severe hemorrhaging will likely result and require immediate surgical repair.

Vascular malformation

A vascular malformation is characterized by an abnormal connection between veins and arteries. Knowing how the circulatory system operates, having such a connection shunts excess blood though small connecting vessel into the arterial system, flooding it with de-oxygenated blood. Depending on the severity of the case, vascular malformation can lead to patients experiencing pain, heaviness, increased temperature, and spontaneous bleeding.

Raynaud’s phenomenon/disease

This is a condition in which, during times of stress or in response to cold temperature, the blood vessels in the hand narrow or spasm, restricting blood flow. This is often seen as blue discoloration of the fingertips. The sensation of coldness, numbness, and tingling may also be present. Raynaud’s symptoms may also be seen in other distant parts of the body, such as the nose or toes.

Risk factors for circulatory system diseases

Some individuals are more likely to be at risk for developing circulatory system disease. The following are some of the risk factors that lead to the development of these conditions:

Modifiable risk factors (can be controlled, changed, or treated):

• Lack of exercise
• Being overweight
• Smoking
• Overuse of alcohol
• Elevated levels of stress
• Poor diet

Non-modifiable risk factors (cannot be controlled, changed, or treated):

• Advanced age
• Being male
• Family history of heart disease, stroke, high blood pressure, or high cholesterol
• Certain ethnicities

21 circulatory system diseases

High blood pressure

Also going by the term hypertension, this is a condition that is defined by the increased force required to pump blood through your arteries. It is often described as a disease without any presenting symptoms, but over time this excessive force can damage the heart and lead to stroke, heart disease, or kidney problems. High blood pressure does not always have to begin at the heart, as seen with atherosclerosis.

Atherosclerosis and coronary artery disease

Here, blood vessels narrow due to cholesterol plaque buildup on the walls of your arteries, eventually restricting blood flow. This means greater force is required for blood to pass through these narrow areas to be able to deliver adequate blood supply, causing increased blood pressure. If this blood vessel narrowing occurs in the vessels supplying the heart, it can trigger a heart attack.

Heart attack

This occurs when the heart does not receive enough blood due to a blocked coronary artery. If not remedied in time, the heart muscle can become permanently damaged and subsequently lead to heart failure or even sudden death. Typical symptoms of a heart attack include pain in the center or left side of the chest, pain that radiates to the jaw, shoulder, or arm, shortness of breath, nausea, sweating, irregular heartbeat, and/or loss of consciousness.

Heart failure

Also known as congestive heart failure, this condition occurs due to weakened or damaged heart muscle. This causes inefficient pumping of blood throughout the body, as the heart is not strong enough. Early symptoms of heart failure include fatigue, ankle swelling (edema), and an increased need to urinate at night. Later symptoms may include rapid breathing, chest pain, and loss of consciousness.

Stroke

A stroke occurs due to the blockage of a blood vessel within the brain reducing oxygenated blood supply and possibly causing permanent brain damage. It is most commonly caused by a blood clot that originated in another part of the body, such as the heart, then travelling through the arterial system to the brain and causing a blockage (embolic stroke) there. Strokes can also occur due to excessive bleed (hemorrhagic stroke), as seen in the case of brain aneurysms. Strokes are a serious condition, with every minute upon onset proving vital for reversing the symptoms of blood clots in the brain.

Related: Understanding stroke rehabilitation: Exercise tips for stroke recovery

Aortic Aneurysm

This is a condition involving the major artery stemming from the heart, called the aorta. When part of the aorta weakens, it can bulge and potentially rupture. The aorta is the largest blood vessel in the body and carries blood to your abdomen, legs, and pelvis. Rupturing aortic aneurysms can cause heavy bleeding and require immediate medical attention.

Peripheral artery disease (PAD)

Occurring in the peripheral extremities, such as the arms and legs, this condition is essentially atherosclerosis. PAD is characterized by reduced blood flow leading to symptoms such as leg cramps, a foot or leg sore that doesn’t heal, and redness or other skin color changes.

Mitral prolapse

The mitral valve separates the left atrium from the left ventricle in the heart. It is a one-way valve that allows a certain volume of blood into the left ventricle in tandem with the heartbeat. Mitral prolapse occurs when the flaps of the valve do not close properly, allowing for blood to regurgitate backward into the left atrium. While the condition is mostly harmless, some cases may require surgical correction. Mitral prolapse can be distinguished by a unique heart murmur.

Angina pectoris

Referring to pain in the chest, this condition is a specific type of chest pain that is related to the heart. It is often accompanied by shortness of breath, fatigue, and nausea. A diagnosis of angina signifies that not enough blood is reaching the heart muscles. Angina pain patients often take nitroglycerine pills, which help to dilate blood vessels, to relieve the pain.

Arrhythmia

The heart follows a certain rhythmic action that is required to adequately ensure enough blood is pumped out of it. The classic “lub-dub” sounds that emanate from the heart are actually caused by contacting heart muscles and closing of heart valves. If the heart loses this rhythmic action, due to any number of different heart pathology, it will be unable to pump blood out effectively. Arrhythmias often present with fatigue, shortness of breath, and chest pain.

Ischemia

This medical term refers to tissue not getting enough oxygenated blood supply, which leads to tissue damage. This can occur in the heart or any other type of bodily tissue. Most of the time, ischemia is a temporary problem leading to pain and discomfort. However, there are cases where ischemia occurring over a longer period of time can cause serious tissue damage and dysfunction, sometimes even irreversible.

Varicose veins

Varicose veins are visible veins that may look dark purple or blue in color, usually in the legs and feet. These enlarged and discolored veins may not pose any immediate health concerns to some patients and can be more of a cosmetic problem, looking unsightly or unattractive. However, some individuals experience aching pain and discomfort and this could signal a higher risk for other circulatory problems. Varicose veins are thought to be a result of prolonged standing or walking that increases the pressure in the veins of the lower body, with the effects of gravity mostly to blame. Dysfunction of tiny valves in the blood vessels themselves has also been seen to play a role. Other risk factors include age, sex, family history, and obesity.

Related: Varicose veins natural treatment: How to get rid of spider veins naturally

Chronic venous insufficiency

This condition is characterized by pooling blood in the lower extremities, as it has become difficult for the blood vessels to return blood to the heart. Chronic venous insufficiency can be the result of obesity, a history of varicose veins, deep vein thrombosis, sedentary lifestyle, long periods of sitting or standing, being over 50 years old, being female, or being pregnant. Symptoms often include swelling in the lower legs or ankles, aching feeling in the legs, and development of varicose veins.

Endocarditis

Endocarditis is the result of an infection of the endocardium layer of the heart, which lines the heart chambers and heart valves. The condition occurs when bacteria infect another part of your body and spread to your bloodstream, granting access to infect the heart. If not promptly treated, endocarditis can damage or destroy the heart valves and can even lead to life-threatening complications.

Acute coronary syndrome

This syndrome consists of a range of different conditions associated with sudden restricted blood flow to the heart muscle. These may include myocardial infarction (MI) and unstable angina. Acute coronary syndrome may not only lead to cell death, but also, because it reduces blood flow, it can alter heart function drastically. This is a medical emergency. Symptoms include difficulty breathing, feeling nauseous, sweating, tightness, pressure, or pain in the chest, and pain in the jaw, neck, back, arms, and/or stomach.

Pulmonary valve stenosis

This is a condition of the valve that separates the pulmonary artery from the right ventricle. It is the access pathway for deoxygenated blood to reach the heart to become reoxygenated again. Deformity of the pulmonary valve can cause blood to back up in the heart and the venous circulatory system, leading to symptoms such as shortness of breath, chest pain, and loss of consciousness.

Thrombophlebitis

This inflammatory process causes the development of blood clots that block one or more veins. The legs are usually the most common extremity involved. Superficial thrombophlebitis often appears as redness and swelling in the affected area. If the condition occurs deeper beneath the skin, it may trigger a condition called deep vein thrombosis.

Temporal Arteritis

This condition affects the arteries that supply the head and brain with blood. They can become inflamed and damaged, leading to symptoms, such as a severe headache or blurry vision. Nearly a quarter-million Americans are thought to have the condition, with almost all patients being over the age of 50 years. If temporal arteritis is left untreated, it can cause an aneurysm, a stroke, or even death.

Ventricular tachycardia

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This is a type of arrhythmia caused by an abnormal electrical signal to the lower chambers of the heart. The condition is often characterized by irregular ventricular contraction, causing a heartbeat of greater than 100 beats per minute that throws it out of sync with the rest of the heart. Ventricular tachycardia can lead to sudden cardiac arrest.

Related: Ventricular arrhythmia: Meaning, types, causes, treatment, and complications

Congenital heart defects

In the womb, a baby’s heart may develop incorrectly, leading to heart dysfunction and additional health problems early in life. There are several types of congenital heart defects, ranging from mild to severe in symptomatology.

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Cardiomyopathy

This condition affects the muscles of the heart. There are four main types of cardiomyopathy: dilated; hypertrophic; ischemic; and restrictive. These variations all cause the heart to have difficulty pumping and delivering blood to the rest of the body, often leading to heart failure.

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Related: Enlarged heart (cardiomegaly): Causes, symptoms, diagnosis and treatment