lecture_23_KM_ch48a_for_class

Learning Objectives

• Compare and contrast open and closed circulatory systems.

• Describe single and double circulations in vertebrates.

• List the four components of blood; describe their composition and function in detail.

• Describe the structure of the vertebrate heart, including variations among different species.

• Outline the cardiac cycle sequence: diastole vs. systole, and the physiological significance of each phase.

• Explain structural and functional distinctions among arteries, arterioles, capillaries, venules, and veins, including how these differ in various types of circulatory systems.

Primary Functions of the Circulatory System

The circulatory system serves multiple vital functions:

• Transport of Materials:

  • Nutrient Delivery: Supplies essential nutrients, such as glucose, amino acids, and vitamins, to cells for metabolism.

  • Oxygen Supply: Transports oxygen absorbed in the lungs to tissues throughout the body.

  • Waste Removal: Carries carbon dioxide and metabolic waste products away from cells for excretion through the lungs and kidneys.

  • Hormonal Distribution: Ensures hormones produced by glands are delivered efficiently to target organs.

Types of Circulatory Systems

Open Circulatory System

• Found in arthropods (e.g., insects, arachnids) and some mollusks (e.g., snails, octopuses).

• Basic Components:

  • Fluid: Hemolymph, which circulates freely within body cavities.

  • Vessels: Blood vessels open into a hemocoel (body cavity) to bathe tissues directly.

  • Hearts: Can have one or multiple heart-like structures.

• Functioning:

  • The heart pumps hemolymph into sinuses surrounding organs; relaxation allows hemolymph to return to the heart via openings called ostia.

  • Nutrients and wastes exchange through diffusion across cell membranes directly with the hemolymph.

• Efficiency: While metabolically inexpensive, it limits the ability to regulate flow selectively; hemolymph cannot be directed specifically where needed most.

Closed Circulatory System

• Found in earthworms, cephalopods (like squids and octopuses), and all vertebrates.

• Key Features:

  • Blood is distinct and separate from interstitial fluid, allowing precise control of flow and composition.

  • Blood is pumped under pressure through a network of vessels by one or more muscular hearts.

  • Solutes, including oxygen and carbon dioxide, are exchanged via diffusion across vessel walls.

  • Contains infection-fighting leukocytes, as well as platelets for blood clotting.

• Arrangement of Closed Circulatory Systems:

  • Two Major Arrangements:

    • Single Circulation: Seen in fishes, where one circuit (i.e., the heart to gills to body) operates with a single atrium and ventricle.

    • Double Circulation: Observed in crocodiles, birds, and mammals with two distinct circuits (pulmonary and systemic), improving efficiency in oxygen delivery.

Single Circulation in Fishes

• Blood flows from the heart to the gills, where it is oxygenated, and then travels directly to the body.

• Efficiency: Due to lower pressure and flow rates, oxygen delivery is less efficient compared to systems with double circulation, resulting in less active metabolism.

Double Circulation in Crocodiles, Birds, and Mammals

• Separation of Blood Types:

  • Pulmonary Circulation: Blood is pumped to the lungs for gas exchange and returns to the heart oxygenated.

  • Systemic Circulation: Oxygenated blood is distributed to the body, with deoxygenated blood returning to the heart.

• Heart Structure:

  • Comprises two atria and two ventricles, with one-way valves preventing backflow.

  • A septum separates left and right sides of the heart, ensuring oxygen-rich and oxygen-poor blood do not mix.

Blood Composition

• Components of Blood:

  • Plasma: The liquid matrix (~55% of blood) containing water, electrolytes, nutrients, hormones, proteins, gases, and waste products.

  • Leukocytes (White Blood Cells): Variety of cell types (e.g., lymphocytes, neutrophils) that protect against infection and form part of the immune system.

  • Erythrocytes (Red Blood Cells): Carry oxygen bound to hemoglobin; lack a nucleus in mammals, maximizing space for hemoglobin.

  • Platelets (Thrombocytes): Cell fragments involved in hemostasis and blood clot formation to prevent blood loss.

Blood Clotting Process

1. Injury Connect: A blood vessel ruptures, initiating clotting.

2. Platelet Activation: Platelets adhere at injury sites and release chemicals attracting more platelets.

3. Fibrin Formation: Fibrinogen converts to fibrin, forming a mesh that stabilizes the platelet plug and ultimately seals the wound.

The Vertebrate Heart

• Structure: All vertebrates have at least one atrium and one ventricle; many have multiple chambers.

• Blood Flow: Blood enters through veins into the atrium, flows through atrioventricular (AV) valves to ventricles, and exits through semilunar valves to arteries.

• Myogenic Heart: Characterized by the ability to generate its own electrical impulses, differing from the neurogenic hearts of arthropods.

• Pacemaker Role: The sinoatrial (SA) node initiates action potentials, regulating the heart's rhythmic contraction.

Phases of Cardiac Cycle

• Atrial Phase: The SA node activates, leading to atrial contraction and pushing blood into ventricles.

• Ventricular Phase: The electrical impulse travels to the AV node, causing the ventricles to contract and pump blood into the arteries.

• Diastole: The relaxation phase of the heart; chambers refill with blood, pressure is lowest.

• Systole: The contraction phase of the heart; pressure is highest as blood is expelled from the ventricles.

Blood Vessels and Gas Exchange

• Capillaries: Microscopic vessels with thin walls that facilitate gas and nutrient exchange with tissues.

• Arteries and Arterioles: Conduct blood away from the heart; arterioles regulate blood flow to capillary beds through vasoconstriction and vasodilation.

• Venules and Veins: Return blood to the heart; veins have thinner walls than arteries and rely on skeletal muscle action and valves to prevent backflow.

Regulation of Blood Pressure

• Blood Pressure: The force exerted by circulating blood on vessel walls, higher in arteries than in veins.

• Resistance: Refers to factors that slow blood flow, influenced chiefly by vessel diameter and length.

• Cardiac Output (CO): The total volume of blood pumped by the heart per minute; CO influences blood pressure and is affected by heart rate and stroke volume.

Adaptive Functions of Closed Circulatory Systems

• The system can rapidly adapt to varying conditions (e.g., exercise) through vasodilation and vasoconstriction, optimizing blood flow.

• Hormonal Influence: Hormones, such as epinephrine, can increase heart rate, stroke volume, thus increasing cardiac output.

• Baroreceptors: Specialized stretch receptors located in arteries monitor blood pressure and provide feedback to the brain to maintain homeostasis.