Revision: Class 11 >> Body Fluids and Circulation NEET (UG) Body Fluids and Circulation

A closed blood circulatory system is one in which blood flows entirely within blood vessels, as seen in humans and other vertebrates.

An open blood circulatory system is found in animals like insects, where blood flows freely through body cavities without confined blood vessels.

Blood is a fluid that circulates in the heart, arteries, capillaries, and veins of vertebrate animals. It transports nutrients and oxygen throughout the body and removes waste.

The plasma from which the protein fibrinogen has been removed is called serum. 

A Blood Recipient is a person who receives blood during a transfusion, requiring compatibility with the donor's blood group.

Rhesus factor is a hereditary protein present in red blood cells. If present, the individual is Rh-positive; if absent, they are Rh-negative.

Antibodies are proteins present in blood plasma that react against specific antigens not found on the individual's own red blood cells, playing a key role in blood group compatibility.

A person with blood group AB is called a universal recipient because they can receive blood from all major blood groups (A, B, AB, and O) without risk of incompatibility.

Blood transfusion is the process of introducing blood from a healthy donor into the bloodstream of a patient, typically through a vein, often during surgery or after heavy blood loss.

Antigens are specific proteins present on the surface of red blood cells that determine an individual’s blood group (e.g., Antigen A or Antigen B).

Lymph is the fluid formed from tissue fluid that enters lymph vessels. It helps in returning excess fluid to the blood and plays a role in immunity.

1. The heart's cyclical repetition of successive events, known as the cardiac cycle, is made up of the contraction and relaxation of the ventricles and atria during diastole.
2. A cardiac cycle lasts about 0.8 seconds. The trial systole (0.1 second), ventricular systole (0.3 second), and full cardiac diastole (0.4 second) are the phases of the heart cycle. 

1. Cardiac output is the volume of blood that the heart pumps out in a minute. It is computed by multiplying heart rate (the number of beats per minute) by stroke volume, which is the amount of blood pumped by each ventricle per minute.
2. A healthy person's heart beats 72 times per minute, pumping out roughly 70 millilitres of blood with each beat. As a result, the average cardiac output is five litres, or 5000 mL.

Double circulation is the process in which blood passes through the heart twice during one complete cycle—once for pulmonary circulation (to and from the lungs) and once for systemic circulation (to and from the body).

• ABO System: Introduced by Karl Landsteiner (1900). Based on the presence/absence of antigen A and B on RBCs, four groups: A, B, AB, O.
• Universal Donor & Acceptor: AB = universal acceptor (both antigens, no antibodies); O = universal donor (no antigens, both antibodies).
• Rh Factor: Discovered by Landsteiner & Wiener (1940). 80–85% people are Rh⁺; the rest are Rh⁻.
• HDN (Erythroblastosis Foetalis): When a Rh⁻ mother carries a Rh⁺ foetus, she produces anti-Rh antibodies (after the first delivery), which attack subsequent Rh⁺ foetuses.
• Prevention: Rh⁻ mother is injected with anti-D antibodies during all pregnancies with Rh⁺ foetus to prevent HDN.

• Lymphatic system consists of lymph, lymphatic vessels, organs and tissues. The word "lymph" means clear water.
• Lymph is similar to blood but lacks RBCs, platelets and some proteins. It contains mainly WBCs (lymphocytes), 94% water and 6% solids (proteins & fats).
• Lymph is formed from tissue fluid and flows through lymph vessels by contraction of nearby muscles. It drains into veins via thoracic duct and right lymphatic duct.
• Lymph supplies nutrients and O₂ to deep tissues where blood cannot reach, and drains excess tissue fluid, waste and proteins back into the bloodstream.
• Lymph absorbs fats from the intestine and defends the body by removing bacteria and housing immune cells (lymphocytes).

• ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. The machine used to record it is called an electrocardiograph.
• P-wave — Represents depolarisation of atria, causing contraction of both atria.
• QRS Complex — Represents depolarisation of the ventricles, initiating ventricular contraction. Contraction begins immediately after Q → marks the start of systole.
• T-wave — Represents repolarisation of ventricles. End of T-wave = end of systole.
• Key intervals — P-Q interval = 0.16 sec | Q-T interval = represents full ventricular activity | RR interval = time between two heartbeats.
• Important distinction — Electrocardiograph = the instrument | Electrocardiogram = the recording/graph produced.