Circulation in Mammals and Other Vertebrates

Circulation in Mammals

Two Zones:

  • Pulmonary Circuit: The pulmonary circuit is responsible for transporting oxygen-poor, carbon dioxide-rich blood from the right side of the heart to the lungs, where carbon dioxide is exchanged for oxygen. Once the blood is oxygenated, it returns to the left side of the heart as oxygen-rich blood, ready to be distributed to the body.

  • Systemic Circuit: The systemic circuit delivers oxygen and nutrient-rich blood from the left side of the heart to all body tissues. In this circuit, oxygen is delivered to cells for cellular respiration, and as blood moves through the tissues, it collects waste products, such as carbon dioxide and urea, which are returned to the right side of the heart to be expelled through the pulmonary circuit.

Heart Functions

Main Functions:

  • Generating Blood Pressure: The heart creates pressure through its contraction, which is necessary for blood to flow through the circulation system against the resistance of the blood vessels.

  • Routing Blood and Separating Circuits: It simultaneously routes blood through two distinct circuits (pulmonary and systemic), ensuring that oxygenated and deoxygenated blood do not mix.

  • Ensuring One-Way Flow: Heart valves (atrioventricular and semilunar) ensure blood flows in only one direction, preventing backflow during contraction.

  • Regulating Supply: The heart adjusts the rate and force of contractions based on the body's metabolic needs, increasing blood flow during activities (exercise) and reducing it during rest.

Heart Anatomy

External Structure:

  • Atria: The upper chambers of the heart that receive blood returning from the veins. The right atrium collects deoxygenated blood from the body through the superior and inferior vena cavae, while the left atrium receives oxygenated blood from the lungs through the pulmonary veins.

  • Ventricles: The more muscular lower chambers that pump blood out of the heart. The right ventricle sends blood to the lungs, while the left ventricle propels blood into the aorta for distribution throughout the body.

  • Auricles: Flap-like extensions of atria that increase the atrial volume and help accommodate varying blood volumes.

  • Chordae Tendineae: Strong, thin connective tissue strings that anchor the atrioventricular valves to the papillary muscles, preventing the valves from inverting and ensuring proper valve function during ventricular contraction.

Cardiac Muscle Characteristics

  • Muscle Composition: Cardiac muscle is composed of actin and myosin proteins, forming sarcomeres. Though striated, cardiac muscle fibers are shorter and more branched compared to the long fibers seen in skeletal muscle.

  • Cell Structure: Each cardiac muscle cell contains one or two central nuclei and is interconnected by junctions called intercalated disks, enabling efficient communication and synchronized contraction.

  • T-tubules: Located near Z disks, T-tubules allow for a coordinated contraction by rapidly transmitting action potentials throughout the cardiac muscle.

  • Calcium Handling: Cardiac myocytes rely on calcium diffusion from both the sarcoplasmic reticulum (SR) and the extracellular space for contraction, leading to slower and prolonged muscle contractions compared to skeletal muscle.

Conducting System of the Heart

Key Components:

  • Sinoatrial Node (SA): Often referred to as the heart's pacemaker, the SA node is located in the right atrium. It generates electrical impulses that initiate heartbeats and maintain rhythm.

  • Atrioventricular Node (AV): Positioned at the junction of the atria and ventricles, the AV node transmits impulses from the atria to the ventricles through the AV bundle.

  • Purkinje Fibers: These are specialized fibers that emerge from the AV bundle to distribute electrical impulses throughout the ventricular myocardium, leading to coordinated contraction of the ventricles.

Action Potential Timing:

  • Delay from SA to AV Node: A brief delay of 0.04 seconds occurs at the AV node, resulting in a total delay (including the time taken for atrial contraction) of 0.15 seconds. This ensures complete atrial contraction before ventricular contraction occurs.

Electrical Properties & Action Potentials

  • Membrane Potential: The resting membrane potential is characterized by low permeability to sodium (Na+) and calcium (Ca2+) ions and high permeability to potassium (K+), which establishes a negative interior electric potential.

  • Action Potential Duration: The duration of action potentials in cardiac muscle ranges between 200-500 milliseconds, significantly longer than the 2 milliseconds observed in skeletal muscle, allowing for sustained contractions and preventing tetanus.

  • Refractory Period: During this time, the cardiac muscle cannot be re-excited, ensuring orderly and effective contractions without risking overlap.

Phases of Action Potentials:

  • Depolarization: Upon stimulation, Na+ channels open, causing a rapid influx of sodium, and Ca2+ channels open slowly, contributing to the rising phase of the action potential.

  • Early Repolarization: Na+ channels begin to close, and some K+ channels open, resulting in a slight decrease in membrane potential.

  • Plateau Phase: Continued calcium influx during this phase is critical for muscle contraction and delays repolarization.

  • Final Repolarization: Ca2+ channels close, and more K+ channels open, facilitating the return to the resting membrane potential.

Cardiac Cycle

Definition: The cardiac cycle encompasses all events associated with the heart's pumping action, starting from the onset of contraction to the next contraction.

Phases:

  • Systole: The contraction phase where the heart pumps blood out of the chambers; atrial systole occurs first followed by ventricular systole.

  • Diastole: The relaxation phase where the heart chambers fill with blood in preparation for the next contraction.

Heart Rate Variability: Newborns average heart rate variability can range from 0.25-3 seconds, while athletes typically have an average of around 1 second, with the general population averaging 0.7-0.8 seconds between beats.

Cardiac Output & Stroke Volume Regulation

  • Cardiac Output (CO): Calculated as CO = HR x SV (heart rate times stroke volume), indicating the volume of blood the heart pumps per minute.

  • Stroke Volume: The amount of blood ejected from the heart per contraction, influenced by end-diastolic volume (EDV) and end-systolic volume (ESV).

  • Starling’s Law: This physiological principle states that the stroke volume increases as the end-diastolic volume increases, due to the elastic stretch of cardiac muscle fibers, enhancing the force of contraction.

Regulation Mechanisms

  • Intrinsic Regulation: The heart possesses inherent functional characteristics that allow it to function autonomously without external neural or hormonal inputs.

  • Extrinsic Regulation: The autonomic nervous system, including sympathetic and parasympathetic branches, and hormones (like epinephrine) regulate heart function and blood pressure:

    • Sympathetic Stimulation: This increases both heart rate and the force of contraction, preparing the body for 'fight or flight'.

    • Parasympathetic Stimulation: Reduces heart rate with minimal impact on stroke volume, promoting a state of rest and recovery.

Blood Flow & Vascular Compliance

  • Blood Flow Regulation: Blood flow to different tissues is proportionate to metabolic needs and can rapidly alter in response to local changes in conditions, such as high CO2 levels or physical activity.

  • Vascular Compliance: The ability of blood vessels to stretch and accommodate changes in blood volume; veins exhibit significantly greater compliance than arteries, allowing them to hold more blood with less pressure.

Capillary Dynamics

  • Fluid Exchange: The exchange of fluids within capillaries is driven by the Net Filtration Pressure (NFP), which compares hydrostatic pressure to osmotic pressure across the capillary membrane.

  • Excess Fluid Management: Conditions like edema arise due to increased capillary permeability or decreased plasma protein concentration, leading to fluid accumulation in tissues.

Long-term Blood Pressure Regulation

  • Blood Volume Control: Long-term regulation of blood pressure is primarily managed through kidney function, where adjustments in filtration and reabsorption dictate blood volume and pressure over extended periods.

  • Aldosterone Mechanism: Aldosterone influences blood volume by reabsorbing sodium (Na+) back into the bloodstream; this process raises blood volume and consequently blood pressure over hours to days.

Physiological Adaptations in Circulation Across Species

  • Mammals: Adaptations in cardiac output in mammals illustrate that output increases as body size decreases; thus, smaller animals have higher heart rates to accommodate their rapid metabolism.

  • Amphibians & Reptiles: Differences in heart anatomy among amphibians and reptiles reflect their evolutionary adaptations for prior oxygen supply; for instance, many possess a three-chambered heart which affects oxygen delivery efficiency.