The Circulatory System

Major and Specific Objectives

• Evolutionary Significance: Discuss the transition from open to closed circulatory systems.

• Medical Relevance: Investigate the implications of circulatory diseases and bloodborne parasites.

• Comparative Anatomy: Describe circulation patterns across different vertebrates and specifically within mammals.

• Structural Mechanics: Analyze the functions of the mammalian heart, blood vessels, and blood components.

Fundamental Principles of Material Exchange

• Environmental Interaction: Every organism must exchange materials with its environment.

• Cellular Level Exchange: This exchange ultimately takes place at the cellular level.

• Unicellular vs. Multicellular Organisms:

  • Unicellular organisms, exchanges occur directly with the environment

  • Complex multicellular organisms, most cells are not in direct contact with the environment, necessitating specialized internal transport systems.

Comparative Circulatory Systems: Open and Closed

Basic Components: All circulatory systems possess three essential components:     

1. Circulatory Fluid: Blood or hemolymph.

2. Blood Vessels: A set of tubes.     

3. Muscular Pump: The heart.

Open Circulatory System:

• Found in insects, other arthropods, and most mollusks

• Mechanism: The circulatory fluid, called hemolymph, bathes the organs directly

• There is no distinction between blood and interstitial fluid.

Closed Circulatory System:

• Mechanism: Blood is strictly confined to vessels and is distinct from the interstitial fluid

• Efficiency: Closed systems are more efficient at transporting fluids to specific tissues and cells.

Invertebrate Circulation: Gastrovascular Cavities

• Diversity: Invertebrates show a wide range of circulatory diversity corresponding to their body size.

• Simple Organisms: Cnidarians (e.g., jellies) have a body wall only $2$ cells thick.

• Gastrovascular Cavity: This cavity serves a dual purpose: digestion and distribution (circulation) of substances throughout the body.

  • Example: Cnidarians have elaborate gastrovascular systems including a circular canal, mouth, and radial canals, which can span distances such as $5\,cm$.

Vertebrate Circulation

• Cardiovascular System: Vertebrates utilize a closed system consisting of blood vessels and a heart with $2$ to $4$ chambers.

• Blood Vessel Pipeline:

1. Arteries: Carry blood away from the heart to smaller vessels called arterioles

2. Capillaries: Tiny vessels that serve as the site of chemical exchange between blood and interstitial fluid

3. Venules: Collect blood from capillaries and lead to larger veins

4. Veins: Return blood to the heart.

Evolutionary Patterns in Vertebrate Hearts

• Fishes: Two-chambered heart: $1$ ventricle and $1$ atrium. Pathway: Blood pumped from the ventricle goes to the gills to pick up $O_2$ and dispose of $CO_2$.

• Amphibians: Three-chambered heart: $2$ atria and $1$ ventricle. Forked Artery: The ventricle pumps blood into an artery that splits output into the pulmocutaneous circuit (skin/lungs) and the systemic circuit (rest of body).

• Reptiles: Possess double circulation with a pulmonary circuit and systemic circuit.

  • Turtles, Snakes, and Lizards: Three-chambered hearts.

  • Crocodilians: Four-chambered hearts.

• Mammals and Birds: Four-chambered heart: The ventricle is completely divided into separate left and right chambers

  • Efficiency: The left side receives and pumps only oxygen-rich blood, while the right side handles only oxygen-poor blood

  • Endothermy: A powerful four-chambered heart is an essential adaptation for the high metabolic demands of an endothermic lifestyle.

The Mammalian Heart: Detailed Anatomy

Separation and Protection:

1. Pericardium: A double-layered membrane separating the heart from the lungs and chest wall

2. Septum: A thick muscular wall dividing the right and left sides.

Chambers and Their Roles:

1. Right Atrium: Top right chamber; collects deoxygenated blood from the Vena Cava and pushes it to the right ventricle

2. Right Ventricle: Bottom right chamber; pumps deoxygenated blood to the pulmonary artery

3. Left Atrium: Top left chamber; collects oxygenated blood from the pulmonary vein and pushes it to the left ventricle

4. Left Ventricle: Bottom left chamber; the myocardium (muscle) is $3 \times$ thicker on this side because it must pump oxygenated blood to the aorta for systemic distribution around the entire body and brain.

Valves (One-Way Flow Controllers):

1. Tricuspid Valve: Three cusps/flaps between the right atrium and right ventricle

2. Pulmonary Valve: Three cusps; allows flow from the right ventricle into the pulmonary artery

3. Mitral Valve (Bicuspid): Two cusps; allows flow from the left atrium into the left ventricle

4. Aortic Valve: Three cusps; allows flow from the left ventricle into the aorta

5. Chordae Tendineae: Fibrous strands attaching valve cusps to the heart wall.

Vessels and Layers:

1. Superior/Inferior Vena Cava: Return deoxygenated blood from the body to the heart

2. Aorta: The main artery of the body

3. Pulmonary Artery: Carries deoxygenated blood to each lung

4. Pulmonary Veins: Carry oxygenated blood from lungs to the left atrium

5. Coronary Artery: Located on the heart surface; carries nutrients and oxygen to the heart muscle itself

6. Endocardium: A smooth interior lining only $1$ cell thick.

Maintenance of the Heart’s Rhythmic Beat

• The Cardiac Cycle: The rhythmic cycle of contraction and relaxation.

  • Systole: The contraction or pumping phase

  • Diastole: The relaxation or filling phase.

• Cardiac Metrics:

  • Heart Rate (Pulse): Measured in beats per minute

  • Cardiac Output: The volume of blood pumped into systemic circulation per minute.

• Electrical Signaling:Self-Excitability: Some cardiac cells contract without nervous system signals.

  • Sinoatrial (SA) Node: The "pacemaker" that sets timing and rate

  • Signaling Pathway: SA node - Atrioventricular (AV) node - bundle of His - Purkinje fibers (triggers ventricular contraction)

  • Influencing Factors: Heart rate is affected by nerves, hormones, body temperature, and physical exercise

  • Electrocardiogram (ECG/EKG): Records electrical impulses traveling during the cardiac cycle.

Blood Vessel Structure and Function

General Layers: All vessels have three similar layers (Endothelium, Smooth Muscle/Elastin, Connective Tissue).

Vessel Differentiation:

1. Arteries: Thick, highly elastic walls with large radii to accommodate high pressure from the heart. Function as pressure reservoirs

2. Arterioles: Half a million vessels ($500,000$); highly muscular and well-innervated. Act as primary resistance vessels to determine blood distribution

3. Capillaries: $10 \text{ billion}$ vessels; very thin-walled with a large total cross-sectional area. The site of gas and nutrient exchange

4. Veins: Thinner walls than arteries; highly distensible with large radii. Function as blood reservoirs. Blood return is assisted by venous valves and skeletal muscle action.

Blood Pressure and Capillary Mechanics

Hydrostatic Pressure: Blood pressure is the force blood exerts against vessel walls.

• Systolic Pressure: The highest pressure, occurring during ventricular systole.

• Diastolic Pressure: The lower pressure, occurring during diastole.

Regulation of Capillary Flow: Contraction of smooth muscle in arteriole walls.

• Precapillary Sphincters: Control blood flow between arterioles and venules.

Exchange Forces: Fluids are driven out of capillaries at the arteriole end and into capillaries at the venule end by the difference between blood pressure and osmotic pressure.

Lymphatic System: Returns fluid leaked from capillary beds back to the blood and aids in body defense.

Composition and Function of Blood

• Class: Specialized connective tissue.

• Distribution: Cellular elements occupy approximately $45\%$ of blood volume; the remaining is liquid plasma.

• Plasma Components: $90\%$ water. Solutes: Electrolytes (inorganic salts/dissolved ions) and plasma proteins (influence pH, osmotic pressure, and viscosity). Protein Functions: Lipid transport, immunity, and clotting.

• Cellular Elements:

  • Red Blood Cells (Erythrocytes): Transport oxygen via hemoglobin. Mammalian RBCs lose their nucleus and organelles; amphibian RBCs retain the nucleus

  • White Blood Cells (Leukocytes): Function in defense via phagocytosis of bacteria or antibody production

  • Platelets: Cell fragments involved in the clotting process.

Stem Cells and Hemostasis

• Regeneration: Blood cells wear out and are constantly replaced.

• Pluripotent Stem Cells: Erythrocytes, leukocytes, and platelets all originate from a single population of stem cells located in the red marrow of bones.

• Clotting Mechanism: Triggered by damage to the vessel endothelium. A complex cascade of reactions converts the protein fibrinogen into fibrin, creating the structural mesh of a blood clot.