Cardiovascular Physiology

Overview

The cardiovascular system is essential for the transport of vital substances, removal of wastes, and maintenance of homeostasis in multicellular organisms. This study guide covers the structure and function of circulatory systems, the properties of blood vessels, and the mechanisms of fluid exchange, with a focus on comparative anatomy and physiology.

Circulatory Systems

Open vs. Closed Circulatory Systems

Circulatory systems are classified as open or closed based on the containment and flow of circulatory fluid.

• Open Circulatory System: Circulatory fluid (hemolymph) bathes organs directly in an open cavity (hemocoel). Found in most invertebrates, such as insects and mollusks.

• Closed Circulatory System: Blood is confined within vessels, allowing for higher pressure and more efficient transport. Found in vertebrates, cephalopods, and annelids.

Key Features:

• One or more pumps (hearts) provide the driving force for fluid movement.

• One-way valves ensure unidirectional flow.

• A system of tubes, channels, or spaces through which fluid moves.

• An aqueous circulatory fluid containing dissolved vital substances.

Example: Insects have an open circulatory system paired with a tracheal system for gas exchange, while mammals have a closed circulatory system.

Circulatory Fluid

The type of fluid and its distribution are central to circulatory system function.

• Hemolymph: Found in open systems; bathes organs in sinuses.

• Blood and Interstitial Fluid/Lymph: Found in closed systems; blood circulates in vessels, and exchange occurs at capillaries.

Respiratory Pigments:

• Hemoglobin: Main pigment in vertebrates, binds oxygen.

• Hemocyanin: Found in some invertebrates (mollusks, arthropods).

• In vertebrates, pigments are within cells; in invertebrates, they may be dissolved in fluid.

Additional info: Oxygen has low solubility in water; most is transported bound to respiratory pigments.

Open Circulatory System of Insects

Insects utilize a combination of open circulation and a tracheal system for gas exchange.

• Hemolymph circulates in sinuses, bathing organs.

• Tracheal system delivers oxygen directly to tissues via air sacs and tubes.

Note: The circulatory system in insects is not used for gas transport; this is handled by the tracheal system.

Structure and Function of Blood Vessels

Types of Blood Vessels

Blood vessels are specialized for transport, exchange, and regulation of blood flow and pressure.

• Arteries: Carry blood away from the heart; thick walls with elastic and fibrous tissue; act as pressure reservoirs.

• Arterioles: Smaller branches of arteries; major sites of variable resistance; regulate blood flow to tissues.

• Capillaries: Thin-walled vessels; sites of exchange between blood and interstitial fluid.

• Venules: Collect blood from capillaries; thin walls.

• Veins: Return blood to the heart; thin walls, large lumen; act as volume reservoirs; contain one-way valves.

Comparing Arteries and Veins

Properties of Capillaries

Capillaries are specialized for exchange due to their thin walls and large surface area.

• Single layer of endothelial cells.

• Junctions and fenestrations allow movement of fluids and solutes.

• Specific transport mechanisms for glucose, amino acids, and larger molecules (transcytosis).

• Specialized capillaries (e.g., in kidneys, intestines) have larger gaps for extensive exchange.

Cardiovascular Circuits

Systemic vs. Pulmonary Circuits

The mammalian heart supports two main circuits:

• Systemic Circuit: Carries oxygenated blood from the left heart to the body and returns deoxygenated blood to the right heart.

• Pulmonary Circuit: Carries deoxygenated blood from the right heart to the lungs and returns oxygenated blood to the left heart.

Additional info: The pulmonary circuit operates at lower pressure than the systemic circuit.

Blood Pressure and Flow

Key Pressure Terms

• Systolic Pressure: Peak pressure during ventricular contraction.

• Diastolic Pressure: Minimum pressure during ventricular relaxation.

• Pulse Pressure: Difference between systolic and diastolic pressure.

• Mean Arterial Pressure (MAP): Average pressure driving blood flow; calculated as:

Example: For a blood pressure of 120/80 mmHg, mmHg.

Determinants of Blood Pressure

• Cardiac Output (CO): Volume of blood pumped per minute.

• Vascular Resistance: Resistance to flow in blood vessels.

• Blood Volume: Total volume of blood in the system.

Regulation of Vascular Resistance

• Sympathetic Nervous System: Norepinephrine causes vasoconstriction (increased resistance).

• Hormonal Regulation: Epinephrine, angiotensin II, and atrial natriuretic peptide (ANP) modulate vessel diameter.

• Local Factors: Paracrine signals and metabolic activity can cause vasodilation or vasoconstriction.

Capillary Exchange and Bulk Flow

Mechanisms of Exchange

• Diffusion: Movement of gases (O2, CO2) and small molecules down concentration gradients.

• Bulk Flow: Mass movement of fluid driven by pressure gradients.

• Transcytosis: Transport of larger molecules via vesicles.

Filtration and Absorption

Fluid movement across capillaries is determined by hydrostatic and osmotic pressures.

• Hydrostatic Pressure (P): Pushes fluid out of capillaries.

• Colloid Osmotic Pressure (π): Pulls fluid into capillaries due to plasma proteins.

• Net Filtration Pressure:

• Filtration occurs at the arterial end; absorption at the venous end.

Example: Net flow out of capillaries is about 3 L/day, returned by the lymphatic system.

The Lymphatic System

Structure and Function

The lymphatic system returns excess interstitial fluid to the circulatory system and plays a role in immune defense and fat absorption.

• Lymphatic Capillaries: One-cell-thick walls; allow entry of fluid when interstitial pressure is high.

• Lymph Vessels: Contain valves and smooth muscle; fluid propelled by muscle contraction.

• Lymph Nodes: Filter lymph and house immune cells.

• Specialized Capillaries (Lacteals): Absorb fats from the intestine.

Edema and Lymphedema

Edema is the accumulation of excess interstitial fluid, often in the extremities, due to impaired return to the circulatory system.

• Causes: Increased capillary filtration, decreased absorption, lymphatic blockage, or venous insufficiency.

• Lymphedema: Chronic swelling due to lymphatic vessel damage or blockage (e.g., after cancer treatment or parasitic infection).

• Treatment: Compression, elevation, diuretics, and antiparasitic drugs (for filariasis).

Example: Elephantiasis is a severe form of lymphedema caused by parasitic blockage of lymph vessels.

Summary Table: Open vs. Closed Circulatory Systems

Key Terms

• Hemolymph: Circulatory fluid in open systems.

• Capillary: Smallest blood vessel; site of exchange.

• Edema: Swelling due to excess interstitial fluid.

• MAP (Mean Arterial Pressure): Average pressure driving blood flow.

• Transcytosis: Transport of large molecules across endothelial cells.

• Lymphatic System: Network returning interstitial fluid to blood.