Blood is a connective tissue that contains three key elements: plasma (55-60%), formed elements (cells, 40-45%), and the remaining is a fluid matrix. Like sand floats on water and does not mix with it, red blood cells and white blood cells float in the plasma, which is colourless. Red blood cells are 4 to 6 million for every microlitre, and white blood cells are 4 to 11 thousand. The red blood cells give the blood a bright red colour due to the presence of haemoglobin.

Plasma is made of two components: water and protein. In plasma, 92-94% is water, and 6-8% is protein. There are three types of proteins in plasma-

1. Fibrinogen: Fibrinogen performs the function of blood coagulation. It helps in making a clot of blood to stop bleeding from wounds.
2. Globulins: Globulins perform the function of defence mechanisms. It helps to make antibodies that are chemical against the diseases.
3. Albumins: Albumins perform the function of balancing osmotic. It helps in maintaining water and ions in the body. Water and ions help you in signal conduction, control your muscles.

Plasma also has minerals, glucose, and amino acids.

Lymphocytes are the cells that attack bacteria from the intestines in the blood fluid. Lymphocytes contain protein and fat. Lymphocytes also help remove interstitial fluid from tissues, absorb and transport fatty acids from the digestive system, and transport White Blood Cells (WBC) to and from lymph nodes into the bones. Lymph nodes are small, soft, round or bean-shaped structures containing immune cells and work as filters for harmful substances. They are located on various body parts on the neck, groin, armpit, centre of the abdomen, and chest. It makes immune cells fight infections and remove foreign material, bacteria, and cancer cells from the body. When lymph recognises bacteria, it starts making more white blood cells, making the lymph nodes swell. The swollen nodes many times can be felt on the groin, under the arms, and neck.

There are two types of circulatory systems. 

• Closed circulatory system:

The closed circulatory system has capillaries. There is no direct contact with tissue. Examples of closed circulatory systems are mammals, birds, reptiles, earthworms. In a closed circulatory system, blood flow speed is high because blood knows the path.

• Open circulatory system

The open circulatory system has no capillaries. There is direct contact with tissue. Examples of open circulatory systems are Arthropoda, Urochordata. In an open circulatory system, blood flow speed is low.

In a single circulation system, deoxygenated blood goes from the heart to the gills. In gills, oxygen is absorbed by the deoxygenated blood to start the circulation again, and the deoxygenated blood changes into oxygenated blood. The oxygenated blood goes to tissues, and from tissues, deoxygenated blood goes to the heart. Body Fluids And Circulation Ncert Chapter 18 Biology Class 11 has explained the concept of Circulatory Pathways of blood.

In double circulation, to complete one cycle of blood, the same blood passes twice through the heart to complete one cycle of blood. The deoxygenated blood pump from the right ventricle goes to the pulmonary artery and from the pulmonary artery goes to the lungs. In the lungs, carbon dioxide of deoxygenated blood is exchanged from oxygen. After that come back to the heart through the pulmonary vein.

In the second part of the cycle, oxygenated blood from the left ventricle is passed through the aorta to body parts. In body parts, oxygen is exchanged by carbon dioxide through diffusion, and then the deoxygenated blood goes back to the heart through Vena-cava. The process repeats continuously. Body Fluids And Circulation Ncert Chapter 18 Biology Class 11 has included double circulatory Pathways of blood.

The heart is made of muscle organs called cardiac muscle. The function of the heart is to pump blood to the parts of the body. There are internal and external factors that enable the heart to work on its own. External factors are hormones, ions, and the autonomic nervous system. The cardiac cycle is a cycle of the circulation of blood. It has two parts: systole and diastole. Systole is a contraction of cardiac muscles, and diastole is relaxation of cardiac muscles. The cardiac cycle is regulated automatically by the senatorial node and atrioventricular node. An increase or decrease in cardiac output is due to variation in the cardiac cycle.

The heart is the most important organ of our body and originates from the mesodermal germ layer. It is located in the mediastinum above the diaphragm. The heart is on the left side and also tilted towards the left. The approximate size of the heart is similar to a closed fist. The heart is myogenic, which means it depends on muscles to operate its functions.

The heart has four chambers- two atria and two ventricles. Atria are small and are irregular folds of muscles and ventricles are large and project muscles.

The pericardium is the first layer of the heart. It further has three layers: parietal pericardium, Fibrous pericardium and visceral pericardium. Between them, there is a pericardial cavity filled with pericardial fluid. The function of the pericardium is to reduce the friction between the heart and surrounding organs.

The second layer of the heart is the myocardium. It is made of cardiac muscles. The myocardium is the thickest layer.

The third and the innermost layer of the heart is the endocardium. It helps to pump blood.

Heart Valves

There are three types of heart valves.

• Tricuspid Valve: Tricuspid Valve is made of three muscle flaps. It guards the opening between the right ventricle and right atrium.
• Bicuspid Valve (Mistral Valve): Bicuspid Valve is made of two muscle flaps. It guards the opening between the left ventricle and the left atrium.
• Semilunar Valve: The heart has two semilunar valves that regulate the entry of blood into the pulmonary artery and the aorta. Semilunar Valves guard the opening between the left atrium and the left ventricle.

The cardiac cycle is 0.8 seconds. Therefore, the heart beats approximately 72 times per minute.

Body Fluids And Circulation Class 11 NCERT Solutions has included heart functions.

1. Why is Blood Considered a Connective Tissue?

Connective Tissues connect different body systems with their scattered cells throughout an extracellular matrix. Blood is a connective tissue with an extracellular matrix with red blood cells, white blood cells, protein, and platelets. Blood removes the waste products, transports nutrients and oxygen to all body parts, and connects the body systems.

2. Why is the Human Heart called Myogenic?

In the human heart, the contraction is initiated by the heart muscle, the sinoatrial node located in the right atrium. It controls the heartbeat and the contraction. It is hence known as the pacemaker. The heart beats in a rhythm due to its inherent nature. It is why the heart is known as Myogenic. The hearts of molluscs and vertebrates are also myogenic.

3. What is the lymphatic system?

The lymphatic system is a network of vessels, tissues, and organs that work to move a watery colourless fluid called lymph back into our bloodstream (our circulatory system).