Revision Notes: The Circulatory System | Basic Science for JSS 3 PDF Download

Our body organs rely on circulating body fluids for their functioning. 

• White blood cells protect the body from diseases by engulfing bacteria that may enter the body.
• Antibodies produced by the blood neutralize poisonous substances or kill germs that enter the body.
• Blood transports digested food from the alimentary canal to the tissues.
• It carries excretory materials from the tissues to the liver, kidneys, or skin for elimination.
• Blood helps maintain a uniform body temperature by distributing heat.
• Hemoglobin in red blood cells (RBCs) combines with oxygen to form oxyhemoglobin, which delivers oxygen to the tissues.

Composition of Blood: Blood consists of plasma and blood corpuscles. 
Plasma: Plasma is a light yellow-colored alkaline liquid that primarily consists of:

• Water (90–92%)
• Proteins (7–8%)
• Inorganic Salts (1%)
• Other Substances (trace amounts)

Cellular Components of Blood

Blood consists of three types of cellular elements:
1. Red Blood Cells (RBCs) or Erythrocytes

• Structure and Size: RBCs are small, disc-shaped structures with a diameter of about 7 micrometers (µm).
• Production: In adults, RBCs are produced in the bone marrow of long bones, such as the ribs, breastbone, and the ileum of the hip girdle. In children, RBC production occurs in the bone marrow of all bones until the age of 5.
• Nucleus: Mature RBCs do not have a nucleus.
• Lifespan: The average lifespan of an RBC is about 120 days.

2. Haemoglobin

• Function: Haemoglobin is a respiratory pigment found in the stroma of RBCs. It plays a crucial role in transporting oxygen.
• Oxyhaemoglobin: Haemoglobin binds with oxygen to form a temporary compound called oxyhaemoglobin, which delivers oxygen to tissues.
• Carboxyhaemoglobin: Haemoglobin has a strong affinity for carbon monoxide (CO). When it binds with CO, it forms a stable compound called carboxyhaemoglobin, which reduces the blood's capacity to transport oxygen. This can be life-threatening.

3. White Blood Cells (WBCs) or Leucocytes

• Characteristics: WBCs are amoeboid in shape and are larger than RBCs.
• Production: WBCs are produced in the bone marrow, lymph nodes, and, in some cases, in the liver and spleen.
• Lifespan: The average lifespan of a WBC is about 2 weeks.

Types of WBCs: WBCs are classified into two categories based on their shape and characteristics:

• Granular: These WBCs have granules in their cytoplasm.
• Agranular: These WBCs do not have visible granules in their cytoplasm.

Disorders:

• Leukaemia: A type of cancer characterized by a significant increase in the number of WBCs at the expense of RBCs.
• Leucopenia: An abnormal decrease in the number of WBCs.

Functions of WBCs

• Phagocytosis: WBCs, particularly neutrophils, engulf and digest foreign substances, especially bacteria, as a defense mechanism against infections.
• Inflammation: Inflammation is the body's response to injury and localized infection. Leucocytes, especially monocytes, migrate through blood vessel walls (diapedesis) to the site of infection and help eliminate germs.
• Antibody Formation: WBCs, particularly lymphocytes, produce antibodies that neutralize or kill germs, playing a crucial role in the immune response.

Different Types of White Blood Cells

Blood Platelets (Thrombocytes)

Blood platelets are tiny, oval or round structures without a nucleus, found floating in the blood. They are produced from megakaryocytes in the red bone marrow, and their lifespan is approximately 3 to 5 days. Blood platelets play a crucial role in the process of blood clotting.

Clotting of Blood (Coagulation)

When there is an injury, the cells at the site of the wound and the platelets begin to break down and release a substance called thrombokinase or thromboplastin. This substance, along with calcium ions, helps to convert prothrombin (a protein present in the plasma) into thrombin. Thrombin then acts on fibrinogen, a soluble protein in the plasma, and converts it into insoluble fibrin.

• Fibrin forms threads that create a meshwork at the site of the wound.
• This meshwork traps blood cells, forming a solid mass known as a clot or thrombus.
• As the blood cells are trapped, the blood shrinks, and the excess plasma is squeezed out as a clear liquid.

Blood Transfusion

• Blood transfusion is the process of injecting blood into the body of patients undergoing surgery.
• Karl Landsteiner, a German biochemist, was the first to suggest that the blood of different individuals varies.
  

There are several systems of blood grouping, but the ABO system and the Rh system are the most important.

ABO System

• According to the ABO blood group system, there are four blood groups: A, B, AB, and O.
• O type blood can be given to persons of all types of blood, i.e., O, A, B, and AB. Hence, a person with O type of blood is called a universal donor.
• A person with AB type of blood can receive blood from all types, i.e., AB, A, B, and O. Hence, such a person is called a universal recipient.

Compatibility and Incompatibility in the ABO System

Rh System

• The blood of most people contains a substance called Rh factor. Rh stands for Rhesus, our common primate ancestor in which this factor was first discovered.
• When the blood of an Rh positive (Rh+) individual is transfused into a person lacking the Rh factor, the blood of the recipient develops antibodies against the Rh factor, which may even lead to death.

Tissue Fluid (Also Known as Intercellular Fluid)

• When blood circulates through the capillaries in tissues, the plasma from leucocytes leaks out and surrounds the cells. This fluid is referred to as tissue fluid or intercellular fluid.
• Cells take in oxygen and other nutrients from this fluid and release carbon dioxide back into it.

Lymph and the Lymphatic System

• Most of the tissue fluid is absorbed by a network of vessels called lymphatic vessels, and this fluid is then called lymph.
• Lymphatic vessels transport the lymph to lymph nodes, where it is filtered.
• From the lymph nodes, the lymph is transported back through lymphatic vessels and eventually enters the vena cava, just before it reaches the right atrium of the heart.

Cellular Component: Predominantly lymphocytes 
Non-cellular Component: Water (94%), Solids (6%)

• The lymphatic system provides nutrition and oxygen to areas where blood cannot reach.
• It also helps in draining excess tissue fluid and metabolic waste products.
• Lymphocytes and monocytes present in the lymph play a crucial role in the body’s defense mechanisms.

The Spleen

The spleen is a large lymphatic organ, roughly the size of a clenched fist. It has a reddish-brown color and is located in the abdomen, behind the stomach and above the left kidney.
Functions of the Spleen

• Acts as a blood reservoir.
• Produces lymphocytes.
• Destroys worn-out red blood cells (RBCs).
• In embryos, it produces red blood cells (RBCs).

The Circulatory System

• In our body, blood circulates in a closed manner through blood vessels all the time. This type of blood circulation is called a closed vascular system. 
• In contrast, animals like insects have an open vascular system where blood mostly flows through open spaces.
•  The human blood circulatory system consists of the heart, arteries, veins, and blood capillaries.

The Human Heart

• Location: The heart is situated in the center of the chest, between the lungs and above the diaphragm.
• Dimensions: The heart measures approximately 12 cm in length and 9 cm in width.
• Size: In adults, the heart is roughly the size of a fist.
• Covering: The heart is covered by a double membrane called the pericardium. This membrane contains a lubricating fluid known as pericardial fluid, which protects the heart from mechanical injuries.
• Chambers of the Heart: The heart consists of two upper chambers called atria and two lower chambers known as ventricles.

Blood Vessels Entering the Heart: 

• Superior Vena Cava: Brings deoxygenated blood from the head, chest, and arms to the right atrium. 
• Inferior Vena Cava: Carries deoxygenated blood from the abdomen and legs to the right atrium. 
• Pulmonary Veins: Transport oxygenated blood from the lungs to the left atrium.

Blood Vessels Leaving the Heart: 

• Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs for oxygenation. 
• Aorta: Distributes oxygenated blood from the left ventricle to the rest of the body.
• Coronary Arteries: Two arteries that branch from the base of the aorta, supplying blood to the heart muscles (cardiac muscles).

Heart Valves: 

• Tricuspid Valve: Located between the right atrium and right ventricle. 
• Bicuspid Valve: Found between the left atrium and left ventricle. 
• Pulmonary Semilunar Valves: Positioned at the opening of the right ventricle into the pulmonary artery. 
• Aortic Semilunar Valves: Located at the opening of the left ventricle into the aorta.

Blood Circulation in the Heart

Blood circulation in the heart is driven by the alternating contraction and relaxation of its chambers. Contraction is referred to as systole, while relaxation is called diastole. The complete sequence of events in one heartbeat is known as the cardiac cycle.
Atrial Systole and Ventricular Diastole

• During this phase, blood flows from the atria into the ventricles.
• The tricuspid and bicuspid valves open, allowing blood to enter the corresponding ventricles.

Ventricular Systole and Atrial Diastole

• The tricuspid and bicuspid valves close, preventing the backflow of blood into the atria.
• Both semilunar valves open, and blood is pumped from the right ventricle into the pulmonary artery and from the left ventricle into the aorta.

Ventricular Diastole

• At the end of ventricular systole, the ventricles begin to relax.
• For a brief period, both the atria and ventricles are in diastole, known as joint diastole.
• The pulmonary and aortic semilunar valves close to prevent the backflow of blood into the ventricles.

Cardiac muscles contract rhythmically in response to self-generated impulses.
The pacemaker, known as the sino-atrial node (SA node), is located in the upper wall of the right atrium. It generates an impulse that triggers atrial systole. This impulse quickly reaches the atrio-ventricular node (AV node), located at the bottom of the right atrium, which initiates ventricular systole.
The heart rate varies among different species; generally, smaller animals have faster heart rates.

Blood Vessels

• Blood vessels are like branched tubes that carry blood from the heart to every part of the body. Arteries are the vessels that take blood away from the heart to different organs. Veins are the vessels that bring blood back from the organs to the heart.
• Capillaries are very tiny tubes, about 8 micrometers (µm) wide.

Pulmonary and Systemic Circulation

• Pulmonary circulation involves the lungs. It begins with the pulmonary artery, which carries deoxygenated blood to the lungs. In the lungs, this blood gets oxygenated, and the pulmonary veins carry the oxygenated blood back to the heart.
• Systemic circulation refers to the circulation throughout the body. The aorta receives blood from the heart and distributes it to various parts of the body. Veins then collect deoxygenated blood from these body parts and return it to the heart.

Hepatic Portal System

• The veins from the stomach and intestine merge to form the hepatic portal vein before delivering blood to the posterior vena cava.
• This vein splits into capillaries within the liver and then forms a new hepatic vein.
• The portal vein is unique because it starts and ends with capillaries.
• In the liver, excess nutrients are stored, toxic substances are detoxified, and excess amino acids are broken down.

Pulse

Pulse refers to the rhythmic expansion and elastic recoil of an artery's wall during ventricular systole.

Blood Pressure

Blood pressure is the force exerted by blood against the walls of arteries as it flows through them.
There are two types of blood pressure:

• Systolic Pressure : This is the maximum pressure achieved when the heart contracts and pumps blood into the arteries.
• Diastolic Pressure : This is the minimum pressure observed when the heart is in a relaxed state, between beats.

The normal blood pressure range for an adult human is 100–140 mm for systolic pressure and 60–80 mm for diastolic pressure.
A sphygmomanometer is an instrument used to measure blood pressure.