Open vs Closed Circulatory Systems

• The circulatory system distributes nutrients, gases, hormones, and waste products.
• It is composed of the heart, blood vessels, and blood.
• It helps with thermoregulation, immunity, and pH balance.

Open Circulatory System

• Found in invertebrates, such as arthropods and most mollusks.
• Hemolymph is pumped into sinuses and directly bathes the tissues.

Closed Circulatory System

• Present in annelids, cephalopods, and all vertebrates.
• Blood circulates entirely within blood vessels allowing efficient nutrient and gas exchange.

Comparison of Open and Closed Circulatory Systems

• Open circulatory systems have blood pumped into open body cavities, unlike closed systems.
• Open systems have hemolymph as the circulatory fluid, versus blood in closed systems.
• Open systems have heart-like structures, sinuses, and hemocoel, contrasting with the heart and blood vessels in closed ones.
• Pressure is low in open systems but high in closed ones.
• Open circulatory systems are less efficient and suitable for smaller, slower animals.
• Closed circulatory systems are more efficient for larger and active animals.
• Arthropods and mollusks have open systems versus vertebrates and cephalopods with closed systems.

High Metabolic Rates

• Closed circulatory systems support high metabolic rates required by animals with greater energy demands, like mammals and birds.

Heart Structure and Blood Flow

• Mammals and birds have a four-chambered heart, with two atria and two ventricles.
• The right side of the heart pumps deoxygenated blood to the lungs via the pulmonary circuit.
• The left side pumps oxygenated blood to the body via the systemic circuit.
• Valves (tricuspid, bicuspid, aortic, pulmonary) maintain unidirectional blood flow.

Comparative Anatomy of Vertebrate Circulation

• Fish have a heart with 2 chambers (1 atrium, 1 ventricle) and single circulation, so blood passes through the heart once per cycle.
• Amphibians possess a 3-chambered heart (2 atria, 1 ventricle) and double circulation where some mixing of oxygenated and deoxygenated blood occurs.
• Reptiles have a 3.5-chambered heart (partial septum) and double circulation with improved separation of oxygenated blood.
• Birds and mammals have a 4-chambered heart (2 atria, 2 ventricles) and full double circulation.
• Full double circulation allows for complete separation of oxygenated and deoxygenated blood.

Blood Vessels

• Arteries carry blood away from the heart and have thick muscular walls for high pressure.
• Capillaries are thin-walled vessels where the exchange of gases, nutrients, and waste occurs.
• Veins return blood to the heart and contain valves to prevent backflow.

Pathway of Blood Flow

• Deoxygenated blood flows from the body to the vena cava.
• Blood moves from the right atrium, through the tricuspid valve, to the right ventricle.
• Blood is then pumped through the pulmonary valve to the pulmonary artery and then to the lungs.
• Oxygenated blood returns from the lungs to the pulmonary veins.
• Blood flows from the left atrium, through the bicuspid (mitral) valve, to the left ventricle.
• Finally, blood exits the left ventricle through the aortic valve to the aorta and then to the body.

Memory trick

• "RA → LA via foramen ovale, and PA → aorta via ductus arteriosus."

Components of Blood

• Plasma makes up 55% of total blood volume and is a water-based solution containing proteins, nutrients, gases, and hormones.
• Water (90-92%) in plasma acts as a solvent and transporter.
• Plasma proteins (about 7%) include albumin, globulins, and fibrinogen.
• Albumin maintains osmotic pressure.
• Globulins are antibodies and transport proteins.
• Fibrinogen is involved in blood clotting.
• Solutes (1-2%) in plasma include ions (Na+, K+, Ca+), nutrients (glucose, amino acids), hormones, wastes, and gases (O2, CO2).
• Formed elements make up 45% of blood.
• Red blood cells (erythrocytes) transport oxygen using hemoglobin.
• White blood cells (leukocytes) defend against infection and disease.
• Platelets (thrombocytes) contribute to blood clotting and wound healing.