Cardiovascular System: Blood Circulation & Function

• The cardiovascular system consists of the heart, blood vessels, and blood.

Blood Circulation Circuits

• Pulmonary circuit carries blood between the heart and lungs for oxygen and carbon dioxide exchange
• Systemic circuit transports blood between the heart and the peripheral tissues.
• Coronary circuit is a component of systemic circulation, supplying blood to the heart itself
• Blood travels in a series from arteries to arterioles to capillaries, then from venules to veins

Cardiovascular System Functions

• Transports blood, oxygen, and nutrients to body tissues.
• Removes carbon dioxide and metabolic waste.
• Transports nutrients from the GI tract.
• Delivers waste products to excretory organs.
• Protects the body from infections via white blood cells.
• Arteries have thick, muscular walls and small lumens to withstand high pressure, and typically carry oxygenated blood, except for the pulmonary artery.
• Veins have thin walls, large lumens, contain valves to prevent backflow and carry blood towards the heart under low pressure; they typically carry deoxygenated blood, except for the pulmonary veins.
• Capillaries are microscopic vessels with thin, fenestrated walls for gas, nutrient, and waste exchange; they handle slow-moving, low-pressure blood.
• Blood constitutes about 8% of human body weight and consists of plasma, red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
• Plasma makes up 55% of blood volume, and is a liquid that transports carbon dioxide, hormones, and metabolic waste.
• Red blood cells make up 45% of blood and are formed in the bone marrow to carry oxygen.
• White blood cells are created in the bone marrow and help protect the body from infections.
• Platelets are small cells that help clot the blood

Blood Functions

• Transports gases, nutrients, hormones, and waste products
• Facilitates substance exchange between tissues and eliminating organs
• Regulates acid-base balance through gas diffusion in the lungs
• Balances water between capillaries and intracellular/extracellular spaces
• Maintains constant body temperature
• Facilitates blood coagulation through a cascade of coagulation factors like fibrinogen/fibrin

Blood Cellular Components

• Erythrocytes transport oxygen via hemoglobin, lack a nucleus, and have a biconcave shape with a diameter of 8-10 µm
• Healthy adults have about 5 million erythrocytes per µl and express blood group antigens on their surface membrane
• Leukocytes have a nucleus with a normal concentration of 4,000 to 10,000 per µl and function in immune defense
• Lymphocytes (25-40% of leukocytes) mediate adaptive immune responses, including antibody production by B lymphocytes and direct cellular immune response by T lymphocytes.
• Monocytes (4-8% of leukocytes) perform phagocytosis and can differentiate into fixed macrophages (histiocytes) or free macrophages
• Platelets derive from megakaryocytes and function in hemostasis by adhering to damaged blood vessels and reacting with fibrin to form a clot within 1-3 minutes
• The physiological range for platelets is 150,000 – 400,000/µl

Blood Histology

• Blood consists of 45% cells (erythrocytes, leukocytes, and platelets) and 55% blood plasma (water, proteins, antibodies, and electrolytes).
• Red blood cells (RBCs) are 45% of the total volume of blood, and their percentage is called the hematocrit

Erythrocytes

• Do not have nuclei
• Filled with hemoglobin, an oxygen-carrying protein
• Reversibly bind oxygen and carbon dioxide
• Structure allows flexibility for movement through narrow capillaries
• Have a 120-day lifespan

Leukocytes

• White blood cells are classified as granulocytes or agranulocytes
• Neutrophils are 12-15 micrometers in diameter, make up 54-60% of circulating white blood cells, and have a lobed nucleus with 3-5 lobes
• Neutrophils secrete ECM-degrading enzymes for phagocytosis and are first responders to infection by chemotaxis.
• Eosinophils are 1-2% of leukocytes, roughly the same size as neutrophils, and have a bilobed nucleus
• They contain major basic proteins (MBPs), peroxidases, enzymes, and toxins to kill helminths and parasitic worms
• Eosinophils are triggered by allergies and release chemicals such as cytokines, lipid mediators, and chemokines
• Chronic inflammation occurs at sites such as the lungs and in the connective tissue of the intestinal lining.

Coronary Artery Disease

• Involves narrowing of arteries supplying the heart, caused by atherosclerosis (accumulation of fatty cholesterol plaque)
• Atherosclerotic buildup leads to reduced oxygen delivery to heart tissue
• This can lead to angina (painful spasming of coronary arteries) and myocardial infarction (heart attack), e.g. complete blockage of the anterior interventricular artery, known as a "widowmaker."
• Stent placement (percutaneous transluminal coronary angioplasty) and coronary artery bypass graft surgery are treatments for coronary artery disease.

An overview of the cardiovascular system focusing on blood circulation circuits (pulmonary, systemic and coronary) and key functions such as transport of oxygen, waste removal and infection control. Arteries and veins are structured differently to maintain blood pressure and prevent backflow.