Cardiovascular System in Humans

Cardiovascular System in Humans 




  • Circulatory patterns in animals are classified into two types: open and closed. 
  • In an open circulatory system, found in arthropods and mollusks, blood is pumped by the heart into open spaces or body cavities called sinuses. 
  • In a closed circulatory system, found in annelids and chordates, including vertebrates, blood is circulated through a closed network of blood vessels, allowing for more precise regulation of fluid flow. 
  • Humans have a closed circulatory system, also known as the cardiovascular system. 
  • The cardiovascular system consists of the heart, blood vessels (arteries, veins, and capillaries), and blood. 
  • The heart is a muscular chambered organ responsible for pumping blood throughout the body. 
  • Human hearts have four chambers: two atria (upper chambers) and two ventricles (lower chambers). 
  • Blood vessels form a network that transports blood to and from the heart and throughout the body, facilitating the exchange of nutrients, gases, and waste products with body tissues.


Human Circulatory System 





  • The human circulatory system, also known as the blood vascular system, comprises a muscular chambered heart, a network of closed branching blood vessels, and blood, the circulating fluid.


Heart Anatomy:

  • The heart, a mesodermally derived organ, is located in the thoracic cavity between the lungs, slightly tilted to the left. 
  • It is protected by a double-walled membranous bag called the pericardium, which encloses pericardial fluid. 
  • The heart has four chambers: two smaller upper chambers called atria and two larger lower chambers called ventricles. 
  • The inter-atrial septum separates the right and left atria, while the inter-ventricular septum separates the right and left ventricles. 
  • Each septum is provided with an opening to allow communication between the chambers of the same side.


Valves and Septa:

  • Valves in the heart allow the flow of blood only in one direction, preventing backward flow. 
  • The tricuspid valve guards the opening between the right atrium and the right ventricle, while the bicuspid or mitral valve guards the opening between the left atrium and the left ventricle. 
  • Semilunar valves are present at the openings of the ventricles into the pulmonary artery and the aorta. 
  •  The atrio-ventricular septum separates the atrium and the ventricle of the same side.


Cardiac Musculature and Nodal Tissue:

  • The entire heart is composed of cardiac muscles, with the ventricular walls thicker than those of the atria. 
  • Specialized cardiac musculature, known as nodal tissue, is distributed throughout the heart. 
  • Specialized nodal tissue, including the sino-atrial node (SAN) and the atrio-ventricular node (AVN), is distributed within the heart. 
  • The SAN is located in the upper right corner of the right atrium, while the AVN is situated in the lower left corner of the right atrium near the atrio-ventricular septum. 
  • From the AVN, the atrio-ventricular bundle (AV bundle) extends through the atrio-ventricular septa and divides into right and left bundles upon reaching the top of the interventricular septum. 
  • These bundles give rise to minute fibers throughout the ventricular musculature, known as purkinje fibers. 
  • Together with the right and left bundles, these fibers form the Bundle of His. 
  • The nodal musculature can generate action potentials without external stimuli, a property known as autoexcitability. 
  • The SAN serves as the pacemaker of the heart, generating the maximum number of action potentials (typically 70-75 per minute) and regulating the rhythmic contractile activity. 
  • As a result, the human heart beats at an average rate of 70-75 times per minute.


Cardiac Cycle 




  • The cardiac cycle describes how the heart functions in a rhythmic sequence of events, consisting of systole (contraction) and diastole (relaxation) of both the atria and ventricles.


Here's a breakdown of the cardiac cycle:

  • Joint Diastole: At the beginning of the cycle, all four chambers of the heart are relaxed. The tricuspid and bicuspid valves are open, allowing blood from the pulmonary veins and vena cava to flow into the left and right ventricles respectively, while the semilunar valves are closed. 
  •  Atrial Systole: The sino-atrial node (SAN) generates an action potential, causing both atria to contract simultaneously (atrial systole), increasing the flow of blood into the ventricles by about 30%. 
  • Ventricular Systole: The action potential is conducted to the ventricles via the atrio-ventricular node (AVN) and AV bundle, leading to ventricular contraction (ventricular systole). As the ventricles contract, the atria undergo relaxation (diastole). Ventricular systole increases ventricular pressure, causing the closure of tricuspid and bicuspid valves to prevent backflow of blood into the atria. 
  • Ejection Phase: As ventricular pressure further increases, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing blood to be ejected into the circulatory pathways. 
  • Ventricular Diastole: The ventricles relax (ventricular diastole), and ventricular pressure falls, leading to the closure of semilunar valves to prevent backflow of blood into the ventricles. As ventricular pressure declines further, the tricuspid and bicuspid valves open again, allowing blood to flow freely into the ventricles. 
  • Repetition of the Cycle: The SAN generates a new action potential, and the cycle repeats.


During each cardiac cycle, two prominent sounds, often referred to as "lub" and "dub," can be heard through a stethoscope. The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves, while the second heart sound (dub) is associated with the closure of the semilunar valves. These sounds are clinically significant for diagnostic purposes.


Each ventricle pumps out approximately 70 mL of blood during a cardiac cycle, known as the stroke volume. The cardiac output, which is the volume of blood pumped out by each ventricle per minute, averages around 5 liters in a healthy individual. Both the stroke volume and heart rate (number of beats per minute) contribute to the cardiac output, which can vary depending on factors such as physical activity and overall health.







An Electrocardiograph (ECG) is a machine used to obtain an electrocardiogram, which is a graphical representation of the heart's electrical activity during a cardiac cycle.


  • Standard ECG Setup: To obtain a standard ECG, a patient is connected to the machine with three electrical leads—one to each wrist and to the left ankle—to continuously monitor heart activity. For a more detailed evaluation, multiple leads are attached to the chest region. 
  • Components of an ECG: Each peak in the ECG is labeled with a letter from P to T, corresponding to specific electrical activities of the heart. 
  • P-Wave: Represents the electrical excitation (or depolarization) of the atria, leading to their contraction. 
  • QRS Complex: Represents the depolarization of the ventricles, initiating ventricular contraction (systole). The contraction begins shortly after the Q wave. 
  • T-Wave: Represents the repolarization of the ventricles, returning them to a normal state. The end of the T-wave marks the end of systole. 
  • Heart Rate Determination: By counting the number of QRS complexes that occur in a given time period, one can determine the heart rate of an individual. 
  •  Clinical Significance: ECGs obtained from different individuals typically have a similar shape for a given lead configuration. Any deviation from this shape indicates a possible abnormality or disease, making ECGs clinically significant for diagnosing heart conditions.


Double Circulation 

Double circulation refers to the two distinct pathways through which blood flows in the human body, each serving a specific purpose:


Pulmonary Circulation:

  • Blood pumped by the right ventricle enters the pulmonary artery, leading to the lungs. 
  • In the lungs, deoxygenated blood becomes oxygenated through the process of respiration. 
  • Oxygenated blood is then carried back to the heart via the pulmonary veins, entering the left atrium. 
  • This pathway ensures that blood receives oxygen and releases carbon dioxide, completing the cycle of pulmonary circulation.


Systemic Circulation:

  • Oxygenated blood from the left ventricle is pumped into the aorta, the body's main artery. 
  • Arteries, arterioles, and capillaries distribute oxygenated blood to tissues throughout the body, delivering nutrients and oxygen. 
  • Deoxygenated blood, along with waste products like carbon dioxide, is collected by venules and veins, eventually returning to the right atrium via the superior and inferior vena cava. 
  • Systemic circulation facilitates the exchange of gases and nutrients with body tissues, while removing waste products for elimination.


Hepatic Portal System:

  • This system is a unique vascular connection between the digestive tract and the liver. 
  • The hepatic portal vein carries blood from the intestines to the liver before it enters the systemic circulation. 
  • It allows for the processing of nutrients and toxins absorbed by the digestive system before they reach the rest of the body.


Coronary Circulation:

  •  A specialized network of blood vessels exclusively dedicated to supplying and draining blood from the heart muscle. 
  • It ensures that the cardiac muscle receives adequate oxygen and nutrients to function properly, while also removing waste products.







Regulation of Cardiac Activity:

  • The heart operates under precise regulation to ensure its proper function. This regulation occurs through intrinsic mechanisms, primarily involving specialized muscle tissues known as nodal tissue, which give the heart its myogenic nature.


1. Intrinsic Regulation:

  • The heart's activity is mainly self-regulated by nodal tissue, allowing it to beat rhythmically without external input. This myogenic control ensures that the heart can maintain a consistent pace and rhythm.


2. Extrinsic Regulation:

  • Additionally, the cardiac activity can be modulated by external factors, primarily through the autonomic nervous system (ANS). 
  • Sympathetic Nervous System (SNS):

Neural signals from the sympathetic nerves, a component of the ANS, can increase the heart rate, enhance the strength of ventricular contractions, and consequently elevate cardiac output. This response is typically associated with "fight or flight" situations, where the body needs increased blood flow and oxygen delivery to support heightened activity levels.


  • Parasympathetic Nervous System (PNS):

Conversely, signals from the parasympathetic nerves, another part of the ANS, can decrease the heart rate and slow the conduction of action potentials, leading to a reduction in cardiac output. This response is often associated with rest and relaxation, where the body's energy demands are lower.


3. Hormonal Regulation:

  • Hormones released from the adrenal medulla, such as adrenaline and noradrenaline, can also influence cardiac activity. These hormones can increase the heart rate and enhance cardiac output, particularly in response to stress or excitement.


Disorders of Circulatory System

  • The circulatory system, composed of the heart, blood vessels, and blood, plays a crucial role in maintaining the body's overall health and function. However, various disorders can affect this system, leading to serious health complications.  
  • These disorders often interfere with the normal flow of blood, impairing the delivery of oxygen and nutrients to tissues and organs throughout the body.  
  • Here, we'll explore some common disorders of the circulatory system and their impact on human health.


High Blood Pressure (Hypertension):

  • Hypertension is when blood pressure is consistently higher than normal (120/80 mm Hg). 
  • It's measured by systolic (pumping) pressure over diastolic (resting) pressure. 
  • A reading of 140/90 mm Hg or higher indicates hypertension. 
  • It increases the risk of heart diseases and affects vital organs like the brain and kidneys.


Coronary Artery Disease (CAD):

  •  CAD, also known as atherosclerosis, affects the vessels supplying blood to the heart muscle.
  • It's caused by deposits of calcium, fat, cholesterol, and fibrous tissues, narrowing the artery lumen.



  • Also called 'angina pectoris,' it's a symptom of acute chest pain when the heart muscle doesn't receive enough oxygen. 
  • It can occur at any age but is more common among middle-aged and elderly individuals. 
  • Angina results from conditions affecting blood flow to the heart.


Heart Failure:

  • Heart failure occurs when the heart can't pump blood effectively to meet the body's needs. 
  • It's sometimes known as congestive heart failure due to symptoms like lung congestion. 
  • Heart failure isn't the same as cardiac arrest (heart stops beating) or a heart attack (sudden heart muscle damage due to inadequate blood supply).