Circulatory System: Open vs Closed

• Circulatory and respiratory systems often work together to deliver oxygen throughout the body.

Circulatory System Overview

• Includes the heart, blood vessels, and blood.

Open Circulatory System

• Used by less evolutionary advanced creatures.
• Lacks blood vessels.
• Features hemolymph, a mixture of circulatory and interstitial fluid.
• Found in insects, arthropods, and mollusks.
• Less efficient but requires less energy due to lower hydrostatic pressure.
• Supports additional functions in some invertebrates, like spiders using hydrostatic pressure to move their legs.

Closed Circulatory System

• Blood is confined to vessels and distinct from interstitial fluid.
• One or more hearts pump blood.
• Allows chemical exchange between blood, interstitial fluid, and body cells.
• Seen in earthworms, humans, and other mammals.
• Supports effective O2 and nutrient delivery in larger, active animals.
• Better regulation of blood distribution to organs.

Heart

• Mammalian heart has two atria (left and right) and two ventricles (left and right).
• The left atrium is on the anatomical right, and vice versa.
• Atria have thin walls and act as collection chambers for returning blood.
• The right atrium receives deoxygenated blood, while the left atrium receives oxygenated blood from the lungs.
• Ventricles pump blood, enabled by thicker, more muscular walls.
• One contraction and one relaxation constitutes a cardiac cycle, measured in beats per minute.
• Systole is the contraction phase, while diastole is the relaxation phase.

Valves

• Atrioventricular (AV) valves (2) are one-way doors anchored by strong fibers, opening during ventricle contraction and seal the door from the atrium to the ventricle.
• Semilunar valves (2) are located at the heart's exits and open during ventricle contraction.
• Valves prevent backflow of blood.
• The pulmonary artery carries deoxygenated blood from the right atrium to the lungs for oxygenation.
• Oxygenated blood from the left atrium goes to the organs.

Blood Vessels

• Feature a central lumen lined with endothelium, a layer of epithelial cells reducing fluid flow resistance.

Capillaries

• Smallest blood vessels with thin walls (endothelium + basal lamina).
• Site of chemical and gas exchange between blood and interstitial fluid.

Arteries

• Walls consist of two tissue layers surrounding the endothelium.
• Outer layer: connective tissue with elastic fibers & collagen.
• Inner layer: smooth muscle and elastic fibers.
• Thick, strong, and elastic, carrying blood away from the heart.

Veins

• Carry blood back to the heart.
• Similar structure to arteries but with thinner walls.
• Generally experience lower pressure.
• Have valves to maintain unidirectional blood flow.

Blood Composition

• Plasma (55%): liquid matrix containing ions, proteins, nutrients, waste, gases, and hormones.
• Cellular elements (45%): includes erythrocytes, leukocytes, and platelets.

Erythrocytes (RBCs)

• Red blood cells primarily responsible for O2 transport via hemoglobin.
• Biconcave shape.
• Lack nuclei and mitochondria.

Leukocytes (WBCs)

• White blood cells that fight infections.
• Digest microorganisms, debris, and dead cells.
• Found outside the circulatory system in interstitial fluid and the lymphatic system.
• Some have nuclei.

Platelets

• Specialized bone marrow cells involved in blood clotting.
• Lack nuclei.

Atherosclerosis

• Accumulation of lipids in blood vessels near the heart, leading to narrower passages.
• Can result in high blood pressure and heart attack.

Respiratory System

• Includes the trachea (windpipe) branching into two bronchi.
• Bronchioles: finer tubes branching from the bronchi.
• Terminal bronchiole: the end of the branching.

Alveoli

• Air sacs at the end of bronchioles where gas exchange occurs.
• Covered in capillaries.
• O2 dissolves from alveoli into capillaries.
• CO2 dissolves from capillaries into alveoli.