Cardiac Automaticity and Blood Circulation
Cardiac Automaticity
Some specialized cells in the heart can maintain a rhythmic beat automatically, without needing external signals from the body. This property is known as cardiac automaticity.
Cardiac Cycle (Heartbeat)
The cardiac cycle is an alternating sequence of contraction (systole) and relaxation (diastole) of the heart, which pumps blood from the veins to the arteries.
A cardiac cycle consists of the following stages:
- Atrial Diastole: Blood enters the atria through the pulmonary veins and vena cava.
- Atrial Systole: The atria contract. The atrioventricular valves open, and blood flows into the ventricles.
- Ventricular Systole: The ventricles contract. The atrioventricular valves close (first heart sound), and the semilunar valves open, causing blood to flow into the arteries (pulmonary and aorta).
- Ventricular Diastole: Once blood has left the ventricles, they relax, and the semilunar valves close (second heart sound).
Venous Hydrostatic Pressure
Venous pressure at rest is defined as:
Venous Pressure = Hydrostatic Pressure + Dynamic Pressure
Hydrostatic pressure is the weight of the blood column at a given point. This can be significant, as the “zero” point is at the level of the right atrium.
Dynamic pressure is the energy exerted by the left ventricle during systole.
Circulation: Systemic and Pulmonary
The right side of the heart pumps deoxygenated blood from the tissues to the lungs, where it is oxygenated. The left side receives oxygenated blood from the lungs and pumps it through the arteries to the body tissues. This creates two types of circulation: pulmonary and systemic.
Systemic Circulation
Systemic circulation begins in the left ventricle, where oxygenated blood is pumped through the aorta and its branches to the capillary system. Deoxygenated blood from the capillaries flows into the vena cava (superior and inferior), which drain into the right atrium.
Pulmonary Circulation
In pulmonary circulation, deoxygenated blood from the body enters the right atrium through the vena cava. When the right atrium contracts, blood is pushed through the tricuspid valve into the right ventricle. The right ventricle then contracts, pumping blood to the lungs through the pulmonary artery.
Blood
Blood is a liquid connective tissue that carries oxygen and nutrients to the body’s cells and removes waste products. It also plays a vital role in the immune system and other essential functions.
The amount of blood in a person varies with age, weight, sex, and height. An adult typically has between 4.5 and 6 liters of blood.
Blood Components
- Red Blood Cells (Erythrocytes): Responsible for oxygen transport. They are biconcave discs and are so small that each cubic millimeter contains four to five million. They lack a nucleus and are considered dead cells.
- White Blood Cells (Leukocytes): Play a crucial role in the immune system, performing phagocytosis (engulfing pathogens) and producing antibodies. They are larger than red blood cells but less numerous (about seven thousand per cubic millimeter). They are living cells that can move and exit capillaries to destroy microbes and dead cells.
- Plasma: The liquid component of blood. It is salty and yellow and carries the other blood components, nutrients, and waste products. When blood coagulates, plasma becomes serum.
- Platelets (Thrombocytes): Cell fragments that help stop bleeding by forming blood clots.
Blood Formation
Blood cells are formed in the bone marrow through a process called hematopoiesis.
Blood Groups
A blood group is a classification of blood based on the presence or absence of specific antigens on the surface of red blood cells and in the blood serum.
The two major blood group systems are ABO and Rh factor.
An antigen is a molecule (usually a protein or polysaccharide) on the cell surface that can trigger an immune response.
- Type A: Individuals with type A blood have the A antigen on their red blood cells and anti-B antibodies in their serum.
- Type B: Individuals with type B blood have the B antigen on their red blood cells and anti-A antibodies in their serum.
- Type O: Individuals with type O blood have neither A nor B antigens on their red blood cells but have both anti-A and anti-B antibodies in their serum.
- Type AB: Individuals with type AB blood have both A and B antigens on their red blood cells and neither anti-A nor anti-B antibodies in their serum.
Another important blood characteristic is the Rh factor. Approximately 85% of the population has the Rh factor on their red blood cells (Rh-positive), while 15% do not (Rh-negative).
The Rh factor is often combined with the ABO type. O-positive is the most common blood type, although the prevalence of other types, such as A and B, can reach 80% in certain populations.