6.1 Cardiovascular and Nervous Systems

Cardiovascular and Nervous Systems

The human body relies on complex, interconnected physiological systems to maintain homeostasis. For biomedical equipment technicians (BMETs) preparing for the CABT exam, a deep understanding of the cardiovascular system and the nervous system is essential. These systems generate the primary bioelectric signals—such as the electrocardiogram (ECG) and electroencephalogram (EEG)—measured and monitored by medical instrumentation.

Cardiovascular Anatomy and Chamber Function

The heart is a muscular organ composed of specialized cardiac muscle (myocardium) that functions as a dual-pump system. It is divided into four distinct chambers:

• Right Atrium (RA): A thin-walled chamber that receives deoxygenated blood from the systemic circulation via the superior vena cava (from the upper body) and inferior vena cava (from the lower body).
• Right Ventricle (RV): Receives blood from the right atrium and pumps it under low pressure (typically 15-25 mmHg systolic) through the pulmonary valve into the pulmonary artery toward the lungs.
• Left Atrium (LA): Receives oxygen-rich blood returning from the lungs via the four pulmonary veins.
• Left Ventricle (LV): A thick, highly muscular chamber that receives oxygenated blood from the left atrium and pumps it under high pressure (typically 100-120 mmHg systolic) through the aortic valve into the aorta to supply the entire systemic circulation.

Because the left ventricle must overcome the high resistance of the entire systemic vasculature, its muscular wall is approximately three times thicker than that of the right ventricle.

Heart Valves and Flow Control

To maintain unidirectional blood flow, the heart utilizes four primary valves, categorized into two groups based on structure and location:

AV valves are anchored by tendon-like cords called chordae tendineae, which connect to papillary muscles projecting from the ventricular walls. When the ventricles contract, the papillary muscles pull on the chordae tendineae, preventing the valve cusps from prolapsing (inverting) into the atria.

Systemic vs. Pulmonary Circulation

The cardiovascular network is divided into two closed-loop circuits:

1. Pulmonary Circulation: This loop begins in the right ventricle, which pumps deoxygenated blood into the pulmonary trunk. The pulmonary trunk branches into the left and right pulmonary arteries—the only arteries in the post-natal body that carry deoxygenated blood. In the capillaries of the lungs, carbon dioxide is released and oxygen is absorbed. The oxygenated blood returns via the pulmonary veins to the left atrium.
2. Systemic Circulation: This loop begins in the left ventricle, which ejects oxygenated blood into the aorta. The aorta branches into smaller systemic arteries, arterioles, and capillaries, delivering oxygen and nutrients to organs, muscles, and tissues. Deoxygenated blood, carrying carbon dioxide and waste, is collected by venules and veins, eventually entering the vena cava to return to the right atrium.

The Cardiac Conduction System

The heart possesses autorhythmicity, meaning it generates its own electrical impulses independently of the nervous system. The electrical signal spreads through a specialized conduction pathway to coordinate mechanical contraction:

• Sinoatrial (SA) Node: Located in the upper wall of the right atrium, the SA node is the primary pacemaker. It depolarizes spontaneously, initiating impulses at an intrinsic rate of 60 to 100 beats per minute (bpm).
• Atrioventricular (AV) Node: Located in the interatrial septum, the AV node receives the impulse from the atria. It introduces a critical delay of approximately 0.1 seconds (100 milliseconds). This delay ensures the atria have finished contracting and fully filled the ventricles before ventricular contraction begins. If the SA node fails, the AV node can act as a secondary pacemaker at an intrinsic rate of 40 to 60 bpm.
• Bundle of His (AV Bundle): The only electrical connection between the atria and the ventricles, passing through the non-conductive fibrous skeleton of the heart.
• Bundle Branches: The Bundle of His splits into the Right Bundle Branch (RBB) and Left Bundle Branch (LBB), which run down the interventricular septum toward the apex of the heart.
• Purkinje Fibers: Highly specialized, rapid-conducting fibers that extend from the bundle branches into the ventricular myocardium. They distribute the action potential throughout the ventricles almost simultaneously, triggering coordinated contraction from the apex upward. Their intrinsic pacing rate is 20 to 40 bpm.

BMETs must relate this conduction system directly to the electrocardiogram (ECG) waveform:

• P wave: Atrial depolarization (triggered by SA node firing).
• QRS complex: Ventricular depolarization (rapid conduction through Bundle of His and Purkinje fibers); atrial repolarization occurs simultaneously but is masked by the larger ventricular electrical signal.
• T wave: Ventricular repolarization.

Brain Regions and Autonomic Regulation

The central nervous system (CNS) consists of the brain and spinal cord. The brain is the control center of the body, divided into three primary functional regions:

• Cerebrum: The largest part of the brain, divided into left and right hemispheres. It controls higher cognitive functions, including reasoning, sensory processing (vision, hearing, touch), memory, speech, and voluntary motor movement.
• Cerebellum: Located under the occipital lobe of the cerebrum, it is responsible for muscle coordination, balance, posture, and fine motor control. It does not initiate movement but fine-tunes motor commands.
• Brainstem: Connecting the brain to the spinal cord, it consists of the midbrain, pons, and medulla oblongata. The brainstem contains vital autonomic centers that regulate life-support functions, including breathing rate, heart rate, and blood pressure.

The autonomic nervous system (ANS) adjusts cardiovascular and respiratory activities via two divisions: the sympathetic nervous system (which increases heart rate and contractility, raising cardiac output) and the parasympathetic nervous system (which decreases heart rate via the vagus nerve).

Action Potentials and Nerve Conduction

Neurons communicate through electrochemical signals called action potentials. This process is governed by the movement of ions across the cell membrane:

1. Resting Membrane Potential: At rest, a neuron maintains a negative internal charge of approximately -70 mV. This polarization is maintained by the active transport of the sodium-potassium pump (Na⁺/K⁺ ATPase), which pumps 3 sodium ions (Na⁺) out of the cell for every 2 potassium ions (K⁺) brought in, combined with passive leakage of K⁺.
2. Depolarization (Threshold): When a stimulus causes the membrane potential to rise to a threshold of approximately -55 mV, voltage-gated sodium channels open rapidly. Sodium (Na⁺) rushes into the cell, driven by chemical and electrical gradients, causing the membrane potential to spike to approximately +30 mV.
3. Repolarization: At the peak of the action potential, sodium channels close, and voltage-gated potassium channels open. Potassium (K⁺) rushes out of the cell, restoring the negative internal charge.
4. Hyperpolarization: Potassium channels close slowly, causing the membrane potential to briefly drop below the resting level to about -80 mV before the sodium-potassium pump restores the resting state of -70 mV.

In myelinated axons, the electrical impulse exhibits saltatory conduction, where the action potential jumps from one Node of Ranvier (uninsulated gap in the myelin sheath) to the next. Myelin, produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system, acts as an electrical insulator, increasing signal transmission speeds from under 2 m/s to over 100 m/s.

When the action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels. The influx of calcium (Ca²⁺) causes synaptic vesicles to fuse with the presynaptic membrane, releasing chemical messengers called neurotransmitters across the synaptic cleft to bind to receptors on the postsynaptic cell.