Body Fluids and Circulation Study Guide

Composition of Blood Plasma and Protein Functions

Blood plasma contains specific proteins that perform vital physiological functions. Fibrinogen is a primary protein required for the process of blood clotting or coagulation. Globulins are another major class of plasma proteins that are essentially involved in the defense mechanisms of the body. Albumins are responsible for maintaining the osmotic balance of the blood. Additionally, the plasma serves as a medium for transport, though certain waste products like urea are eliminated through the kidneys.

Characteristics and Specializations of Erythrocytes

Mammalian erythrocytes, or red blood cells, are specialized for the transport of respiratory gases. A defining characteristic of mature mammalian erythrocytes is that they are enucleated, meaning they lack a nucleus. They also lack other internal organelles such as mitochondria and the Golgi apparatus. The biological explanation for this lack of organelles is that it provides more internal space for hemoglobin, thereby maximizing the capacity for oxygen transport. The life span of these cells is limited, and once they are aged or damaged, they are destroyed in the spleen, which is famously referred to as the graveyard of erythrocytes.

Classification and Functions of Leukocytes

Leukocytes, or white blood cells, are categorized based on their structural and functional properties. Neutrophils are the most abundant type of blood cell among the leukocytes and are characterized by their phagocytic behavior, allowing them to engulf and destroy foreign organisms. Monocytes are also phagocytic and possess the ability to move out of the blood into tissue spaces to fight infections.

Basophils are involved in inflammatory responses and are known to secrete several important chemicals, including serotonin, heparin, and histamine. B-lymphocytes constitute approximately $20\%$ of the total blood cells and are central to the immune response of the body. Eosinophils are another type of leukocyte that play a role in resisting infections and are particularly associated with allergic reactions.

Blood Grouping and Universal Recipient Status

Blood typing is determined by the presence or absence of specific antigens on the surface of red blood cells. Individuals with the 'AB' blood group are identified as "Universal Recipients." This status is due to the absence of both anti-A and anti-B antibodies in their plasma. Because they lack these antibodies, they can receive blood from any donor group without triggering an immediate antibody-mediated destruction of the donor's RBCs.

The Mechanism of Blood Coagulation

Blood coagulation is a complex process designed to prevent excessive blood loss following an injury. When a blood vessel is damaged, a coagulum is formed. This coagulum consists of a network of threads made of fibrin, in which formed elements of the blood like destroyed RBCs are trapped. Fibrins are produced by the conversion of inactive fibrinogens present in the plasma. This transformation is catalyzed by an enzyme called thrombin. Thrombin itself is generated from an inactive precursor in the plasma known as prothrombin. The conversion of prothrombin to thrombin requires an enzyme complex known as thrombokinase.

Cardiac Anatomy and Heart Valves

The human heart contains several valves that ensure the unidirectional flow of blood and prevent backflow. The tricuspid valve is located between the right atrium and the right ventricle. The bicuspid valve, also frequently called the mitral valve, is situated between the left atrium and the left ventricle. Semilunar valves are positioned at the base of the large arteries leaving the heart, specifically between the right ventricle and the pulmonary artery, as well as between the left ventricle and the aorta.

Electrocardiography and Heart Activity

An Electrocardiogram (ECG) provides a graphical representation of the electrical activity of the heart during a cardiac cycle. The P-wave on an ECG represents the depolarisation of the atria, which leads to atrial contraction. The QRS complex represents the depolarisation of the ventricles, which initiates ventricular contraction and marks the beginning of systole. The T-wave represents the repolarisation of the ventricles, signifying their return from an excited state to a normal state. The atrio-ventricular nodes are responsible for generating action potentials that coordinate these contractions. During the cardiac cycle, blood moves because of pressure gradients, such as when increased ventricular pressure forces the semilunar valves to open.