15.7. SUMMARY

Vertebrates circulate blood, a fluid connective tissue, in their body, to transport essential substances to the cells and to carry waste substances from there. Another fluid, lymph (tissue fluid) is also used for the transport of certain substances.

In living organisms, blood plays a vital role as a connective tissue. It is not just a simple liquid; rather, it serves the crucial purpose of transporting oxygen, nutrients, hormones, and other essential substances to cells throughout the body. Additionally, blood helps carry waste products away from cells to be excreted. Alongside blood, lymph, which is derived from interstitial fluid, aids in transporting certain substances, particularly immune cells.

Imagine blood as a delivery service in a city. Just like delivery trucks bring food and supplies to homes while taking away garbage, blood delivers essential nutrients and oxygen to every cell in our body while removing waste products.

Blood comprises of a fluid matrix, plasma and formed elements. Red blood cells (RBCs, erythrocytes), white blood cells (WBCs, leucocytes) and platelets (thrombocytes) constitute the formed elements.

Blood has two main components: plasma and formed elements. Plasma is the liquid portion that makes up about 55% of blood, while the formed elements include red blood cells, white blood cells, and platelets. Red blood cells are responsible for transporting oxygen, white blood cells are crucial for immune defense, and platelets play a key role in blood clotting.

Think of blood as a fruit smoothie. The smoothie base is like plasma, providing a liquid medium, while the fruits and additives represent different cells: the vibrant blueberries (RBCs) provide energy and color, while the tiny seeds (platelets) act like the nutrients that help with repair and growth.

Blood of humans are grouped into A, B, AB and O systems based on the presence or absence of two surface antigens, A, B on the RBCs. Another blood grouping is also done based on the presence or absence of another antigen called Rhesus factor (Rh) on the surface of RBCs.

Human blood is categorized into four main types: A, B, AB, and O. This classification is based on the presence or absence of specific antigens on the surface of red blood cells. Additionally, the Rh factor determines whether someone's blood type is positive or negative, which is crucial for safe blood transfusions. Mismatched blood types can lead to serious complications, making it essential to match donors and recipients accurately.

Consider blood types as different flavors of ice cream. Just like how vanilla and chocolate cannot be combined without affecting the flavor, certain blood types need to be matched carefully for transfusions to avoid adverse reactions.

All vertebrates and a few invertebrates have a closed circulatory system. Our circulatory system consists of a muscular pumping organ, heart, a network of vessels and a fluid, blood.

In vertebrates, the closed circulatory system ensures that blood is contained within vessels as it flows throughout the body. This system consists of the heart, which pumps blood, and a network of blood vessels—arteries, veins, and capillaries—throughout the body. This structure allows for a more efficient and regulated distribution of blood compared to an open circulatory system.

Think of a closed circulatory system like a network of highways. The heart is like a gas station that pumps fuel into the highways (blood vessels), allowing for quick and direct transport of resources to various destinations (organs and tissues) without spilling out.

Heart has two atria and two ventricles. Cardiac musculature is auto-excitable. Sino-atrial node (SAN) generates the maximum number of action potentials per minute (70-75/min) and therefore, it sets the pace of the activities of the heart. Hence it is called the Pacemaker.

The human heart is made up of four chambers: two atria and two ventricles. The unique structure of the heart allows it to pump blood effectively. The sino-atrial node (SAN) acts as the pacemaker by generating electrical signals that trigger heart contractions. This auto-excitable property enables the heart to beat rhythmically without needing external signals.

Imagine the SAN as the conductor of an orchestra. Just like a conductor leads musicians to play in harmony at the right time, the SAN ensures that the heart beats in a coordinated manner, allowing blood to flow efficiently.

The action potential causes the atria and then the ventricles to undergo contraction (systole) followed by their relaxation (diastole). The systole forces the blood to move from the atria to the ventricles and to the pulmonary artery and the aorta. The cardiac cycle is formed by sequential events in the heart which is cyclically repeated and is called the cardiac cycle.

The cardiac cycle consists of a series of events that occur in the heart to pump blood. It includes two main phases: systole, when the heart muscles contract to pump blood out, and diastole, when the heart relaxes and fills with blood. This cycle is repeated multiple times a minute, allowing for continuous blood circulation throughout the body.

Think of the cardiac cycle like a well-choreographed dance routine. Each dancer must know when to step in (systole) and when to take a breather (diastole), ensuring the performance flows seamlessly and rhythmically, just like the running of the heart.

We have a complete double circulation, i.e., two circulatory pathways, namely, pulmonary and systemic are present. The pulmonary circulation starts by the pumping of deoxygenated blood by the right ventricle which is carried to the lungs where it is oxygenated and returned to the left atrium. The systemic circulation starts with the pumping of oxygenated blood by the left ventricle to the aorta which is carried to all the body tissues.

In double circulation, the blood passes through the heart twice during one complete circuit through the body. The first pathway is pulmonary circulation, where deoxygenated blood travels from the heart to the lungs for oxygenation. The second pathway, the systemic circulation, sends oxygen-rich blood from the heart to the rest of the body. This arrangement allows for efficient oxygenation and nutrient transport to tissues.

Imagine a two-part relay race, where one runner (pulmonary circulation) takes the baton (blood) to a station (lungs) to pick up refreshments (oxygen) before passing it to the next runner (systemic circulation) who races off to deliver it to all the other stations (body tissues). This ensures that every part gets what they need to keep going.

Though the heart is autoexcitable, its functions can be moderated by neural and hormonal mechanisms.

While the heart can beat on its own due to its auto-excitable tissues, external factors can influence its rate and strength of contractions. The autonomic nervous system can either increase or decrease heart activity through signals. Hormones released from glands can also adjust how forcefully the heart pumps, ensuring that body demands are met, such as during exercise.

Consider the heart as a car's engine. While the engine can run on its own, the accelerator (neural signals) and brakes (hormonal signals) regulate how fast or slow it operates depending on the road conditions (body's activity level), ensuring a smooth ride.

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