Body Fluids and Circulation

Circulatory Pathways

Exchange of gases and nutrients in animals happens through specialized systems. When you look at the animal kingdom chart, you begin to see how circulatory pathways have evolved. They are the most primitive at the bottom of the animal kingdom and we move up the different phyla, we begin to see complex systems beginning to form. You can clearly see the difference in arthropods, molluscs as well as in chordates and annelids. Let us look into how this exchange happens in chordates, especially humans.

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What are Circulatory Pathways?

Every other cell, tissue, and organ in the body is impacted to a great extent by the circulatory system of the body. This system is the most complex system, with the three main components of blood, blood vessels, and heart. The blood travels through the entire body through the arteries and veins. This is how circulation occurs in the body.

The pattern through which circulation happens is called the Circulatory pathway.  This pathway can be classified into two types – The open system of circulation and the closed system of circulation.

The open system of circulation is clearly visible in the two phyla Arthropoda and Mollusca. Here the blood that is pumped by the heart goes through large vessels into the open body cavities. But, when you see two other prominent phyla, Phylum Annelida and Phylum Chordata, you can see that there is a closed circulatory system. Blood is always circulated through closed blood vessels here. This closed pathway is said to be more advantageous, as the blood flow can be regulated with precision.

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Circulatory systems in Vertebrates

Vertebrates display some advanced features when compared to the non-vertebrates. All vertebrate animals have a chambered heart. In fishes, it is a two-chambered heart whereas, in amphibians and reptiles, it is a three-chambered heart. The exceptions here are the crocodiles, which along with birds and mammals have a four-chambered heart.

On one hand, fishes show single circulation while on the other, amphibians and reptiles show incomplete double circulation. Birds and mammals have two separate circulatory pathways, displaying double circulation.

(Source: Wikipedia)

Human Circulatory System

It is a complicated system that works efficiently in order for the other systems to function smoothly. To get a better understanding of this system, it is crucial to know about the structure of the heart, heartbeat and a few other important aspects. Let us explore them one by one.

Types of Circulation

The main components of the human circulatory system are the 4-chambered heart, the complex network of blood vessels- arteries and veins and blood. In humans, blood travels through two types of pathways – the pulmonary circulation (pathway) and the systemic circulation (pathway).

  • Pulmonary circulation – Here the blood without oxygen, called as the deoxygenated blood travels from the right side of the heart to the lungs. In the lungs, exchange of oxygen and carbon dioxide takes place, and the blood now becomes oxygenated (with oxygen).
  • Systemic circulation – Here, oxygenated blood, i.e. blood with oxygen travels from the left side of the heart to different areas of the body. Here, at different organ sites, there is an exchange of gases, nutrients, and waste through interstitial fluids. The deoxygenated blood now goes back to the right side of the heart.

Structure of the Heart

The human heart is roughly about the size of a closed fist. It is the body’s circulatory pump that pumps blood to various organs in the body. It weighs around 280 to 340 grams in men and around 230 to 280 grams in women. Heart is situated in the thoracic cavity, tilted slightly to the left side of the body.

The heart is encased in a double-walled sac called the pericardium. This protects the heart, and also anchors it inside the chest, between the two lungs. Between the outer and inner pericardial layer, the pericardial fluid is present which lubricates the heart.

There are a total of 4 chambers in the human heart – the two upper chambers called the right atrium and the left atrium, and the two lower chambers called the right ventricle and left ventricle. The right atrium and the right ventricle together may be called the right heart. The left atrium with the left ventricle together can be called as the left heart.  Both the sides of the heart are separated by a muscular wall called the septum. Even the ventricles and the atria are separated by muscular septa.

Chambers of the Heart

The chambers of the heart are connected through a small opening in the septa. These openings are guarded by valves. There are different types of valves. They are:

  • Tricuspid valve – Formed by three muscular flaps or cusps, guarding the opening between the right atrium and right ventricle.
  • Bicuspid valve or mitral valve – It guards the opening in between the left atrium and the left ventricle.
  • Aortic valve – Controls the blood flow out of the ventricles and is present in between the aorta and left ventricle
  • Pulmonary valve – Controls blood flow out of the ventricles and is present in between the pulmonary artery and right ventricle.

There are many blood vessels that go in and out of the heart. Veins, arteries, and capillaries form this intense network. Veins carry deoxygenated blood to the right side of the heart. Arteries carry oxygenated blood away from the heart to different parts of the body. The longest artery that leaves the heart is called the aorta. In between the vein and arteries, small tube-like vessels called the capillaries form a network.

The cardiac muscles make up the heart. The ventricular walls are thicker than the atrial walls. The cardiac muscles have a special nodal tissue called the Sino-atrial node and the atrioventricular node. There is also a bundle of nodal fibres called the atrioventricular bundle (AV bundle). These branch out into right and left bundles. The right and left bundles further branch out into minute fibres called the Purkinje fibres, throughout the ventricular musculature of the heart. These fibres along with right and left bundles are known as the bundle of His.

The most distinguishing feature of this nodal musculature is that it has the ability to generate electrical impulses, without any external stimuli.


What is a heartbeat and how does it happen?

In the simplest terms, heartbeat means the pulse of the heart. Pulse or Heart rate is the number of times the heart beats per minute. (A rhythmic contraction and relaxation of the chambers of the heart)

You must know this fact that our heart beats at an average rate of 72 beats per minute. How does this rhythmic beat of the heart occur?

The four chambers of the heart i.e. the atria and the ventricles work together. They contract and relax alternately to pump blood through the heart.  There are some electrical impulses that trigger the heartbeat. These impulses are the power source that makes heartbeat possible.

These impulses start at the Sinoatrial node, which is known as the heart’s natural pacemaker. When this electrical activity starts to spread through the muscular walls of the atria, it results in contraction. The Atrioventricular node slows the electrical signals before it enters the ventricles. And so, the atria contract first, followed by the ventricular contraction. The ventricles contract after receiving the impulse through the bundle of His.

Cardiac Cycle

You now know what a heartbeat is and the triggers that trigger the heart to beat. But, did you know that there are a few events that take place during one heartbeat? This is nothing but the cardiac cycle. The duration of the cardiac cycle is about 0.8 seconds and each ventricle pumps about 70 ml of blood, which is called the stroke volume.


(Source: Wikipedia)

There are three phases of a cardiac cycle. They are:

  • Atrial systole
  • Ventricular systole
  • Relaxation

During the atrial systole phase, the atria contract and push the blood into the ventricles. Here, the atrioventricular valves and semilunar valves play a major role. The former valves stay open while the semilunar valves stay closed to stop the arterial blood from re-entering the heart. The ventricles at this stage are in diastole.

In the next phase, the ventricles contract and push blood into the aorta and the pulmonary trunk. The pressure with which the ventricles push blood forces the semilunar valves to open. The atrioventricular valves close. Thus blood flows from ventricles into the arteries. The atria are now in a state of diastole during this phase.

During the relaxation phase, the four chambers of the heart are in diastole. Blood goes into the heart from the veins.  During this time, the ventricles fill up to about 75% of their capacity.  They get completely filled after the atria enter systole. At this juncture, the cardiac muscle cells of the ventricles repolarize and prepare for the next round of depolarization and contraction.

Cardiac Output

Did you know that each ventricle pumps out an average of 5litres of blood every minute? This is nothing but the cardiac output. Our bodies have this great capability to alter the heart rate, stroke volume, and cardiac output.

The cardiac output of man resting and that of a running athlete is different. The athlete’s cardiac output will be much higher.


ECG means an electrocardiogram, which measures the electrical activity of the heart at rest. It gives information about the heart rate, heart rhythm, and enlargement of the heart if any and also evidence of any previous heart attacks.

(Source: Wikipedia)

Solved Questions For You

Q: Why is the Sinoatrial node called the pacemaker?

Ans: The heart is made up of cardiac muscles. A specialized nodal tissue called the Sinoatrial node is present in the upper part of the right atrium. This nodal musculature has the ability to generate electrical impulses, without any external stimuli. The electrical impulse then triggers a sequence of events leading to the rhythmic beating of the heart. The Sinoatrial node is responsible for initiating and maintaining the rhythmic beats of the heart. For this very reason,  it is called as the natural pacemaker.

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