Body Fluids and Circulation – Complete NEET Notes, Concepts

1. Transportation of Substances in Human Body – Blood and Lymph Explained

In all living organisms, it is essential to supply each cell with important substances like oxygen (O₂), nutrients, and other essentials. At the same time, waste products or toxic substances formed during cell functions must be removed regularly to maintain the health of tissues. That’s why an efficient transport system is necessary for moving useful materials to the cells and taking wastes away from the cells.

Different types of animals have developed different ways for this internal transport. In simple animals such as sponges and coelenterates, water from the surrounding environment flows through their body cavities, allowing direct exchange of substances with the cells. However, complex organisms, including humans, use special body fluids to perform this function.

The most important body fluid used for transportation in humans and other higher organisms is blood. It plays a central role in carrying oxygen, nutrients, hormones, and waste materials throughout the body. Along with blood, another important fluid called lymph also helps in transporting certain substances, especially fats and immune cells.

In this chapter we will explore the composition of blood and lymph, their functions, and the mechanism of blood circulation in detail with 🥰Learn Sufficeint Notes🥰. Understanding these concepts is key to learning how our body stays healthy and functional by constantly moving essential substances where they’re needed.

2. What is Blood ? – Composition and Function of Human Blood

Blood is a vital connective tissue in the human body that plays a key role in maintaining life. It is made up of two main parts: a fluid matrix called plasma, and a variety of formed elements such as red blood cells (RBCs), white blood cells (WBCs), and platelets. This special fluid is responsible for transporting oxygen, nutrients, hormones, and waste products to and from different parts of the body. Because of its complex composition and essential role in circulation, blood is often called the lifeline of the body.

2.1 Plasma in Blood – Composition and Functions of Blood Plasma

Plasma is the straw-colored, slightly thick (viscous) fluid that makes up around 55% of human blood. It mainly consists of 90–92% water, which helps dissolve and carry various substances. About 6–8% of plasma is made of proteins, including three major types – fibrinogen, globulins, and albumins. Each of these has a specific and vital function: fibrinogen is essential for blood clotting, globulins support the body’s immune defense, and albumins help maintain osmotic balance in the blood.

Besides proteins, plasma also contains small but important minerals like sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), bicarbonate (HCO₃⁻), and chloride ions (Cl⁻). Additionally, glucose, amino acids, and lipids are present, as they are constantly being transported to different parts of the body. Plasma also carries inactive clotting factors, which become active when clotting is needed. When these clotting factors are removed from plasma, the remaining clear fluid is called serum. Overall, plasma acts as the transport medium for nutrients, hormones, waste, and more in the circulatory system.

2.2 Formed Elements in Blood – Detailed Explanation

1. Red Blood Cells (RBCs) – Oxygen Carriers of the Body

Red blood cells (RBCs), also known as erythrocytes, are the most abundant cells in human blood and are mainly responsible for carrying oxygen to all body tissues. In a healthy adult male, the count of RBCs ranges from 5 to 5.5 million per mm³ of blood. These cells are produced in the red bone marrow and are shaped like biconcave discs, allowing maximum surface area for gas exchange. Unlike many cells, RBCs in mammals do not have a nucleus, which helps in more space for haemoglobin—a red-colored, iron-rich protein that binds with oxygen and carbon dioxide. The normal haemoglobin level in a person is about 12 to 16 grams per 100 ml of blood, which plays a crucial role in transporting respiratory gases. The average life span of an RBC is around 120 days, after which they are broken down in the spleen, often referred to as the graveyard of RBCs.

2. White Blood Cells (WBCs) – Body’s Defense Warriors

White blood cells (WBCs), or leucocytes, are colorless and nucleated cells that form a key part of the immune system. Unlike RBCs, they are less in number, typically 6,000 to 8,000 per mm³ of blood, and have a shorter life span. Their main function is to defend the body against infections and foreign substances. WBCs are classified into two groups: granulocytes (which have visible granules) and agranulocytes (which don’t). Granulocytes include neutrophils, eosinophils, and basophils, while agranulocytes include monocytes and lymphocytes. Among them, neutrophils are the most abundant (60–65% of total WBCs) and basophils are the rarest (0.5–1%). Neutrophils and monocytes work as phagocytic cells, meaning they engulf and destroy pathogens. Basophils release chemicals like histamine, serotonin, and heparin, which help in inflammatory reactions. Eosinophils (2–3%) fight parasitic infections and are active in allergic responses. Lymphocytes (20–25%) exist as B and T cells, both of which help in building specific immunity against various diseases.

3. Platelets – Blood Clotting Agents

Platelets, also called thrombocytes, are small cell fragments that originate from megakaryocytes, special large cells found in the bone marrow. A normal blood sample contains about 1,50,000 to 3,50,000 platelets per mm³. Though they are not full cells, platelets play a life-saving role in the body by participating in the clotting process. When there is an injury or bleeding, platelets release clotting substances that help seal the wound and prevent excessive blood loss. If the platelet count drops, it can lead to clotting disorders, causing uncontrolled bleeding even from minor injuries. This makes platelets a critical component in maintaining the body’s internal balance.

3. Blood Groups – Understanding ABO and Rh Blood Grouping in Humans

Even though human blood looks the same in all individuals, it actually differs at the molecular level, which is why blood grouping is essential. Scientists have classified blood into different types based on specific antigens and antibodies present on the surface of red blood cells (RBCs). The two most widely recognized systems used globally for this purpose are the ABO blood group system and the Rh factor grouping. These classifications are extremely important in blood transfusions, organ transplants, and pregnancy care, as mismatches in blood groups can cause serious complications. By identifying the correct blood group, doctors ensure safe and compatible transfusions, making blood grouping a life-saving diagnostic tool in medicine.

3.1 Rh Blood Grouping in Humans – Importance, Pregnancy Complications, and Prevention

In addition to the ABO blood group system, there is another important antigen called the Rh factor, named after the Rhesus monkey, in which it was first discovered. This Rh antigen is found on the surface of red blood cells (RBCs) in about 80% of the human population, and such individuals are referred to as Rh-positive (Rh⁺). Those who do not have this antigen are called Rh-negative (Rh⁻). If an Rh⁻ person receives Rh⁺ blood, their immune system reacts by producing antibodies against the Rh antigen, which can lead to dangerous transfusion reactions. Hence, it is very important to match Rh groups during blood transfusions.

A serious complication due to Rh incompatibility can also occur during pregnancy, when an Rh-negative mother is carrying an Rh-positive fetus. In the first pregnancy, this usually doesn’t cause harm because the placenta keeps the mother’s and baby’s blood separate. However, during childbirth, a small amount of Rh-positive fetal blood might enter the mother’s circulation, causing her immune system to start producing anti-Rh antibodies. In a second or later pregnancy, these antibodies can cross the placenta and attack the fetal red blood cells, leading to a dangerous condition called erythroblastosis foetalis. This can cause severe anemia, jaundice, or even death of the fetus if left untreated. Thankfully, this condition can be prevented by injecting anti-Rh antibodies (Rh immunoglobulin) into the Rh-negative mother immediately after the first delivery, which neutralizes any Rh-positive cells and prevents antibody formation.

3.2 ABO Grouping – Concept, Antigens, Antibodies, and Compatibility in Blood Transfusion

The ABO blood grouping system is based on the presence or absence of two specific surface antigens on the red blood cells (RBCs), namely antigen A and antigen B. These antigens are capable of inducing an immune response in the body. Correspondingly, the plasma of individuals naturally contains specific antibodies, known as anti-A and anti-B, that react against these antigens if they are foreign. Based on the combination of these antigens and antibodies, human blood is classified into four major groups – A, B, AB, and O. Individuals with blood group A have antigen A on their RBCs and antibody anti-B in their plasma. Those with blood group B have antigen B on their RBCs and antibody anti-A in their plasma. People with blood group AB possess both antigens A and B but lack both antibodies, making them universal recipients in terms of plasma compatibility. On the other hand, individuals with blood group O have neither antigen A nor B on their RBCs but have both antibodies (anti-A and anti-B) in their plasma, making them universal donors for red cells. During blood transfusion, it is crucial to match the donor’s blood group with the recipient’s. If incompatible blood is transfused, the recipient’s antibodies may react with the donor’s RBC antigens, leading to agglutination or clumping, and destruction of the red blood cells. This can result in serious complications, including kidney failure and even death. Hence, careful blood group matching is essential before any transfusion is performed.

4. Blood Clotting Mechanism

When a person experiences a cut or injury, the bleeding does not continue indefinitely. This stoppage of blood flow is due to a biological process known as blood coagulation or clotting. This is a protective mechanism of the body that prevents excessive loss of blood following an injury or trauma. Over time, a dark reddish-brown scum or clot forms at the wound site. This clot is scientifically referred to as a coagulum. It consists mainly of a network of thread-like protein structures called fibrins, in which dead and damaged blood cells are trapped.
Fibrins are produced from fibrinogen, an inactive protein that normally circulates in plasma. The enzyme responsible for converting fibrinogen into fibrin is called thrombin. However, thrombin itself is also not present in active form initially. It is derived from another inactive plasma protein called prothrombin. The conversion of prothrombin into thrombin requires a special enzyme complex known as thrombokinase (also called prothrombinase). This enzyme complex is not directly available but is formed through a cascade of enzymatic reactions, each involving a series of clotting factors that are normally present in an inactive form in the blood plasma.
When an injury occurs, it activates platelets, which are small blood components responsible for initiating clotting. These activated platelets release various clotting factors, which kickstart the formation of thrombokinase. Additionally, injured tissue cells at the site of trauma also release chemical substances that can trigger the clotting mechanism. One of the most critical components in this entire clotting cascade is calcium ions (Ca²⁺), which play an essential and irreplaceable role in enabling these biochemical conversions and interactions. Without calcium, the clotting process would not proceed efficiently.

5. Physiology of Tissue Fluid (Lymph)

Lymph, also known as tissue fluid, is a vital component of the human circulatory and immune systems. As blood flows through the thin-walled capillaries in various tissues of the body, a portion of the water along with many small, water-soluble substances is filtered out into the intercellular spaces. This extravascular fluid is known as interstitial fluid or tissue fluid. While it shares a mineral composition nearly identical to that of plasma, it lacks the larger plasma proteins and most of the formed elements like red and white blood cells which remain within the blood vessels. This tissue fluid plays a crucial intermediary role, as the exchange of essential substances such as oxygen, carbon dioxide, nutrients, and metabolic waste between blood and tissue cells occurs through it. The human body has a specialized network of lymphatic vessels that collect this interstitial fluid and gradually return it to the bloodstream via the major veins, thus maintaining fluid balance.
Once inside the lymphatic vessels, this fluid is called lymph. Lymph is a clear, pale fluid that contains a unique population of white blood cells known as lymphocytes. These lymphocytes are pivotal to the body’s immune responses, identifying and neutralizing pathogens like bacteria and viruses. Apart from its immune function, lymph also acts as a transport medium for nutrients and hormones, helping distribute them across the body. Additionally, the lymphatic system plays a critical role in fat absorption. Specialized lymphatic capillaries known as lacteals, located within the intestinal villi of the small intestine, absorb dietary fats and fat-soluble vitamins, which are then transported via lymph into the bloodstream. Thus, the lymphatic system not only maintains fluid homeostasis but also contributes significantly to nutrition and defense mechanisms in the human body.

6. Pathways of Circulation: Double and Single

The circulatory systems found in animals are of two main types: open circulatory system and closed circulatory system.

In an open circulatory system, the blood is pumped by the heart into large vessels that empty into open spaces or body cavities called sinuses. This type of system is found in arthropods and molluscs.
In contrast, a closed circulatory system is observed in annelids and chordates, where blood flows through a closed network of blood vessels. This type of system is more advantageous because the flow of blood can be precisely regulated and directed to specific tissues.

Types of Heart in Vertebrates

All vertebrates possess a muscular, chambered heart, but the number and arrangement of chambers vary:

Fishes have a 2-chambered heart, consisting of one atrium and one ventricle.
Amphibians and most reptiles (except crocodiles) have a 3-chambered heart with two atria and one ventricle.
Crocodiles, birds, and mammals possess a 4-chambered heart with two atria and two ventricles, which ensures complete separation of oxygenated and deoxygenated blood.

Circulatory Patterns

Single Circulation:
In fishes, the heart pumps deoxygenated blood to the gills for oxygenation. The oxygenated blood is then distributed to the body, and the deoxygenated blood from the body returns to the heart. This pattern involves only one cycle through the heart, hence termed single circulation.
Incomplete Double Circulation:
In amphibians and reptiles, the left atrium receives oxygenated blood from the lungs, skin, or gills, while the right atrium receives deoxygenated blood from the body. Both types of blood mix in the single ventricle, which then pumps out mixed blood. This is called incomplete double circulation because of partial mixing.
Double Circulation:
In birds and mammals, the left atrium receives oxygenated blood, and the right atrium receives deoxygenated blood. This blood is then pumped into their respective ventricles, and from there, sent to the body and lungs respectively. There is no mixing of oxygenated and deoxygenated blood, making it true double circulation.

6.1 Cycle of Cardiac Events

The cardiac cycle refers to the cyclical sequence of events that occur in the human heart during each heartbeat to ensure efficient blood circulation throughout the body. The cycle begins with a phase in which all four chambers of the heart—the two atria and two ventricles—are in a relaxed state, a phase known as joint diastole. During this stage, the tricuspid and bicuspid (mitral) valves remain open, allowing blood from the vena cava to flow into the right atrium and blood from the pulmonary veins to flow into the left atrium. From there, the blood passes directly into the right and left ventricles, respectively. At the same time, the semilunar valves—which guard the pulmonary artery and aorta—remain closed, preventing backflow of blood into the heart.
The process is initiated when the sino-atrial node (SAN), the natural pacemaker of the heart, generates an action potential. This electrical impulse stimulates both atria to contract simultaneously—a phase known as atrial systole—increasing blood flow into the ventricles by about 30%. Following this, the impulse is conducted to the atrioventricular node (AVN), then through the AV bundle (also called the bundle of His), which transmits the signal via Purkinje fibers across the ventricular musculature, triggering ventricular systole. As the ventricles contract, the atria relax—a phase known as atrial diastole.
The contraction of the ventricles increases intraventricular pressure, causing the tricuspid and bicuspid valves to close, thereby preventing the backflow of blood into the atria. As the pressure rises further, it forces the semilunar valves open, allowing blood to flow out of the right ventricle into the pulmonary artery and from the left ventricle into the aorta, thereby pushing blood into the systemic and pulmonary circulation.
Following this, the ventricles enter diastole, relaxing and reducing the ventricular pressure, which leads to the closure of the semilunar valves to prevent backflow from the arteries. As the pressure in the ventricles falls below that in the atria, the tricuspid and bicuspid valves reopen, and blood flows again from the atria to the ventricles. This returns the heart to the initial joint diastole, and the cycle is ready to repeat itself with the next SAN impulse.
Each complete cardiac cycle involves a systole and diastole of both atria and ventricles, and this cycle repeats about 72 times per minute in a normal adult, meaning each cycle lasts 0.8 seconds. During each cycle, each ventricle pumps approximately 70 mL of blood, which is termed the stroke volume. When multiplied by the heart rate, this gives the cardiac output. Hence, cardiac output is defined as the volume of blood pumped by each ventricle per minute, which is typically 5000 mL or 5 litres in a healthy adult. The body can regulate stroke volume and heart rate to adjust the cardiac output, which explains why trained athletes have significantly higher cardiac output compared to non-athletes.
Additionally, two distinct heart sounds are produced during each cardiac cycle and can be clearly heard using a stethoscope. The first heart sound (commonly referred to as “lub”) is produced by the closure of the atrioventricular valves (tricuspid and bicuspid), while the second heart sound (“dub”) is generated when the semilunar valves close. These sounds are clinically significant and are used to assess the functional condition of the heart.

6.2 Blood Circulation Mechanism in Humans

The human circulatory system, also referred to as the blood vascular system, is a highly specialized internal transport mechanism composed of three key components: a muscular chambered heart, a closed network of branching blood vessels, and blood, the circulating fluid that transports nutrients, gases, hormones, and waste products throughout the body. The heart, which is mesodermally derived, is located in the thoracic cavity, nestled between the two lungs and slightly tilted toward the left. It is approximately the size of a clenched fist and is safeguarded by a double-walled membranous sac known as the pericardium, which encloses the pericardial fluid to reduce friction during heartbeats.
The human heart consists of four chambers—two smaller upper chambers called the atria and two larger lower chambers called the ventricles. The right and left atria are separated by a thin muscular wall known as the inter-atrial septum, while the right and left ventricles are divided by a thick muscular wall called the inter-ventricular septum. Each atrium and its corresponding ventricle are separated by a fibrous tissue known as the atrioventricular septum, which contains openings that connect the chambers on the same side. These openings are guarded by valves to ensure unidirectional flow of blood. The right atrium and right ventricle are connected via the tricuspid valve, which consists of three muscular flaps or cusps, whereas the left atrium and left ventricle are connected through the bicuspid valve, also known as the mitral valve, which has two flaps.
The exit points of the right and left ventricles—into the pulmonary artery and aorta, respectively—are secured by semilunar valves. All the valves in the heart allow the flow of blood in only one direction, effectively preventing any backflow during contractions and relaxations. The walls of the heart are made entirely of cardiac muscle, with the ventricular walls being much thicker and stronger than those of the atria, due to the greater force required to pump blood out to the body and lungs.
Embedded within the heart muscle is a specialized type of tissue known as nodal tissue, which plays a critical role in regulating the rhythmic contractions of the heart. One important part of this nodal system is the sino-atrial node (SAN), located in the upper right corner of the right atrium. Another vital component is the atrioventricular node (AVN), situated in the lower left corner of the right atrium, near the atrioventricular septum. Extending from the AVN is a bundle of specialized fibers called the atrioventricular bundle (AV bundle), which passes through the atrioventricular septum and quickly divides into the right and left bundle branches. These further spread into fine fibers called Purkinje fibers, which are distributed throughout the ventricular muscle, enabling coordinated contractions.
A remarkable feature of the nodal musculature is its auto-excitability—it can generate action potentials without external stimuli. Among all parts of the nodal system, the SAN has the highest rate of impulse generation, producing 70–75 action potentials per minute. This inherent rhythmicity makes the SAN the natural pacemaker of the heart. As a result, the normal resting heart rate in a healthy adult is about 72 beats per minute, although it may vary slightly from person to person.

6.3 Electrocardiographic Tracing of Heartbeats

An electrocardiogram (ECG) is a diagnostic tool used to record the electrical activity of the heart during a complete cardiac cycle. It is commonly used in hospitals to monitor the functioning of the heart in real-time.

What is an ECG ?

An ECG is a graphical recording that shows how the heart’s electrical signals change during each heartbeat. These signals control the contraction and relaxation of heart muscles, ensuring the proper pumping of blood.

How is ECG Taken ?

To record a standard ECG:

The patient is connected to a machine using three electrical leads:
- One attached to each wrist
- One to the left ankle
These leads detect the tiny electrical changes on the skin that arise from the heart muscle’s electrical activity.
In clinical settings, multiple leads are often placed around the chest area for more detailed monitoring.

Waves of ECG and Their Meaning

Each ECG tracing consists of several waves, each representing a specific event in the cardiac cycle:

Wave	Description
P-wave	Indicates depolarization (electrical activation) of the atria, causing them to contract.
QRS Complex	Represents depolarization of the ventricles, initiating ventricular contraction (systole begins just after Q).
T-wave	Shows repolarization (return to resting state) of the ventricles, marking the end of systole.

Heart Rate from ECG

By counting the number of QRS complexes in a specific time interval, one can easily calculate the heart rate.

Clinical Significance

The shape and size of the ECG waves are generally similar in healthy individuals for the same lead configuration.
Any deviation in the waveform may indicate heart diseases or abnormalities, such as:
- Arrhythmias
- Myocardial infarction (heart attack)
- Conduction disorders

Thus, ECG is a non-invasive, quick, and valuable diagnostic tool widely used to assess heart health.

7. Blood Circulation Through Two Pathways

1. Fixed Path of Blood Flow through Blood Vessels
In the human circulatory system, blood moves along a definite and fixed pathway through specific blood vessels – arteries and veins. Each of these blood vessels is made up of three distinct layers. The innermost layer is known as the tunica intima, which is lined by squamous endothelium. Next is the middle layer, called tunica media, which consists of smooth muscle fibers and elastic tissues. Finally, the outermost layer, known as the tunica externa, is formed of fibrous connective tissue rich in collagen fibers. Notably, the tunica media is thinner in veins as compared to arteries, which is a key structural difference aiding in their respective functions. This structural organization allows for efficient transport and regulation of blood throughout the body.

2. Pulmonary Circulation – Right Ventricle to Lungs
When blood is pumped out from the right ventricle, it enters the pulmonary artery, which is unique as it carries deoxygenated blood. This blood is transported to the lungs, where it undergoes oxygenation. Once oxygen-rich, the blood is returned to the left atrium through the pulmonary veins. This entire loop of blood movement – from the right ventricle to the lungs and back to the left atrium – is known as the pulmonary circulation. It plays a crucial role in gas exchange, ensuring that the blood receives oxygen and expels carbon dioxide before it re-enters the main circulatory system.

3. Systemic Circulation – Left Ventricle to Body Tissues
After oxygenated blood enters the left ventricle, it is pumped into the aorta, the largest artery in the body. From the aorta, the oxygenated blood travels through a network of arteries, arterioles, and capillaries, reaching the body tissues. Here, oxygen and nutrients are delivered, and carbon dioxide and waste products are absorbed from the tissues. This now deoxygenated blood is collected by venules, which merge into veins and eventually drain into the vena cava – the main vein returning blood to the right atrium. This circulation route is called the systemic circulation, and it is essential for providing oxygen, nutrients, and removing metabolic wastes from the tissues.

4. Hepatic Portal System – Special Digestive-Liver Connection
An interesting and unique vascular arrangement known as the hepatic portal system exists between the digestive tract and liver. In this system, the hepatic portal vein collects nutrient-rich blood from the intestine and transports it directly to the liver. Before entering the systemic circulation, this blood is processed by the liver – detoxifying harmful substances and regulating nutrients. This mechanism is crucial for maintaining metabolic balance and protecting the body from toxins absorbed through food.

5. Coronary Circulation – Dedicated Pathway for the Heart
The heart, being a muscular organ that continuously pumps blood, requires its own supply of oxygen and nutrients. For this purpose, the body has a specialized coronary circulation system. This system consists of coronary arteries and veins that circulate blood to and from the cardiac muscles (myocardium). Through this dedicated vascular network, the heart muscles receive a constant and uninterrupted supply of oxygenated blood, ensuring efficient contraction and functioning of the heart itself.

8. Heart Activity Regulation Mechanism

The functioning of the human heart is primarily self-regulated by its specialized muscle tissue, which is why it is referred to as myogenic in nature. This intrinsic regulation is controlled by nodal tissues such as the sinoatrial (SA) node and atrioventricular (AV) node, enabling the heart to beat on its own without external commands. However, the heart’s rhythm and efficiency can also be modulated externally by the autonomic nervous system (ANS). A specific neural center located in the medulla oblongata of the brain plays a crucial role in monitoring and modifying cardiac activity. When sympathetic nerves—a division of the autonomic nervous system—are activated, they send impulses to the heart which lead to an increased heart rate, stronger ventricular contractions, and ultimately a higher cardiac output. In contrast, the parasympathetic nervous system, particularly through the vagus nerve, acts antagonistically to reduce the heart rate, slow down the conduction speed of action potentials, and consequently, lower the overall cardiac output. Beyond neural control, hormonal regulation also influences cardiac activity. The adrenal medulla secretes hormones like adrenaline and noradrenaline, which can significantly enhance heart rate and force of contraction, thereby boosting cardiac output during stress, excitement, or physical activity. This coordinated regulation ensures that the heart responds appropriately to the physiological demands of the body at all times.

9. Common Circulatory System Diseases

High Blood Pressure (Hypertension):
High blood pressure, also called hypertension, is a medical condition in which the blood pressure in the arteries remains consistently higher than normal. The normal blood pressure reading is considered to be 120/80 mm Hg, where 120 mm Hg is the systolic pressure (when the heart pumps blood) and 80 mm Hg is the diastolic pressure (when the heart relaxes). If multiple blood pressure readings for an individual are consistently at 140/90 mm Hg or higher, then the person is considered to be hypertensive. Persistent hypertension significantly increases the risk of developing cardiovascular diseases and can damage vital organs like the brain and kidneys over time. This condition often remains undiagnosed due to the absence of clear symptoms, making regular monitoring essential.
Coronary Artery Disease (CAD):
Coronary Artery Disease, commonly referred to as CAD or atherosclerosis, is a disorder in which the coronary arteries (the arteries that supply blood to the heart muscle) become narrowed or blocked. This narrowing is caused by the deposition of cholesterol, fat, calcium, and fibrous tissues on the inner walls of the arteries. These deposits form a plaque that reduces the lumen (inner diameter) of the arteries, restricting the blood flow to the heart. As a result, the heart muscles receive an insufficient supply of oxygen-rich blood, which can lead to chest pain (angina), shortness of breath, or even a heart attack. CAD is one of the leading causes of death globally and is directly linked to lifestyle choices, diet, lack of exercise, and stress.
Heart Failure: Heart failure is a chronic condition in which the heart is unable to pump blood efficiently to meet the metabolic demands of the body. This condition results in poor circulation, leading to fatigue, breathlessness, and fluid retention. Heart failure is often referred to as congestive heart failure (CHF) because one of the major signs is the congestion of the lungs, which causes difficulty in breathing. It is important to understand that heart failure is not the same as cardiac arrest (where the heart stops beating suddenly) or a heart attack (where a part of the heart muscle is damaged due to lack of blood supply). Heart failure usually develops gradually due to long-term conditions like hypertension, CAD, or previous heart attacks.
Angina (Angina Pectoris):
Angina, medically termed as angina pectoris, is a clinical symptom characterized by acute chest pain. This pain arises when the heart muscle does not receive enough oxygen due to reduced blood flow, often as a result of narrowed coronary arteries. The discomfort may also radiate to the shoulders, arms, neck, or jaw. Angina is more prevalent among middle-aged and elderly individuals, although it can occur at any age. Factors such as stress, physical exertion, or emotional disturbances can trigger angina attacks. Unlike a heart attack, angina does not cause permanent damage to the heart but indicates an underlying heart condition that needs medical attention.

Final Overview of Body Fluids and Circulation – Class 11 Biology Chapter Notes

Circulatory system is essential in vertebrates for the transportation of oxygen, nutrients, hormones, and for the removal of metabolic waste products. Two main body fluids are involved in this function: blood and lymph.

🔹 Blood Composition and Blood Groups

Blood is a fluid connective tissue composed of a liquid portion called plasma and cellular components known as formed elements. These formed elements include:

Red Blood Cells (RBCs or Erythrocytes) – Transport oxygen.
White Blood Cells (WBCs or Leucocytes) – Provide immune defense.
Platelets (Thrombocytes) – Help in blood clotting.

Blood is classified into different ABO groups based on the presence or absence of antigens A and B on the surface of RBCs. Another important classification is the Rh factor, which is based on the presence or absence of the Rhesus antigen.

🔹 Lymph (Tissue Fluid)

Lymph is a clear fluid derived from blood plasma. It lacks RBCs and has lower protein content. It helps in the transport of nutrients, fats, and immune cells, and also returns excess fluid from tissues back to the blood.

🔹 Types of Circulatory Systems

Open Circulatory System: Found in arthropods and mollusks.
Closed Circulatory System: Found in all vertebrates including humans; blood flows through a closed network of blood vessels.

🔹 Structure and Function of the Human Heart

The human heart is a muscular, four-chambered organ with two atria and two ventricles. It is myogenic in nature, meaning it can initiate its own contraction without external nervous input.

The Sino-atrial Node (SAN) acts as the natural pacemaker by generating the highest number of action potentials (70–75/min).
Electrical impulses from the SAN cause atrial systole, followed by ventricular systole, ensuring one-way blood flow.
This sequence of events is called the cardiac cycle, which lasts about 0.8 seconds and repeats approximately 72 times per minute in a healthy adult.

🔹 Cardiac Output and Stroke Volume

Stroke Volume: The amount of blood pumped out by one ventricle during each cardiac cycle (~70 mL).
Cardiac Output: Total volume of blood pumped by one ventricle per minute = Stroke Volume × Heart Rate = ~5 Litres/min.

🔹 Electrocardiogram (ECG)

The electrical activity of the heart can be recorded using an electrocardiograph. The output, called an electrocardiogram (ECG), is useful in detecting various heart abnormalities based on the P, QRS, and T waves.

🔹 Double Circulation in Humans

Humans have double circulation, which includes:

Pulmonary Circulation: Right ventricle pumps deoxygenated blood to lungs for oxygenation. Oxygenated blood returns to the left atrium.
Systemic Circulation: Left ventricle pumps oxygenated blood to the body. Deoxygenated blood returns to the right atrium.

🔹 Neural and Hormonal Regulation

Even though the heart is self-regulated, autonomic nervous system (ANS) and hormones (like adrenaline) can influence the heart rate and strength of contraction.

Sympathetic nerves increase heart rate and output.
Parasympathetic nerves (via vagus nerve) slow it down.

✅ Key Takeaways for NEET and Class 11 Exams:

Blood = Plasma + RBCs + WBCs + Platelets
SAN = Natural pacemaker
Double circulation = Pulmonary + Systemic circulation
ECG detects heart function
Cardiac output ≈ 5 litres/min
Lymph = tissue fluid, returns excess fluid to blood
Regulation by ANS and hormones