Cardiac Physiology and Blood Vessel Anatomy: Study Notes

Electrocardiogram (ECG/EKG)

Overview of ECG

The electrocardiogram (ECG or EKG) is a diagnostic tool that records the electrical activity of the heart using electrodes placed on the skin. Changes in the ECG pattern can indicate abnormal heart activity, aiding in the diagnosis of cardiac conditions.

• Arrhythmia: Refers to an abnormal heart rhythm detected by changes in the ECG pattern.

• P wave: Represents atrial depolarization.

• QRS complex: Indicates ventricular depolarization; masks atrial repolarization.

• T wave: Corresponds to ventricular repolarization.

ECG Intervals

• PR interval: From the beginning of the P wave to the beginning of the QRS complex. If longer than 200 ms, may indicate damage to the conducting pathway or AV node.

• QT interval: From the beginning of the QRS complex to the end of the T wave. Represents the time for one cycle of ventricular depolarization and repolarization.

Autonomic Control of Heart Rate

Pacemaker Activity

The SA node establishes the heart rate by generating pacemaker potentials, which initiate action potentials at a rate of approximately 100 beats per minute. Parasympathetic nervous system (PNS) activity typically slows this rate to about 70 beats per minute.

• AV node: Depolarizes 40–60 times per minute if not stimulated by the SA node.

• To change heart rate, the time to reach threshold potential must be altered.

Autonomic Regulation

• Parasympathetic stimulation: Releases acetylcholine, opens potassium channels, and slows the rate of depolarization.

• Sympathetic stimulation: Releases norepinephrine, binds to beta-1 receptors, opens channels that increase the rate of depolarization.

• Cardiac centers in the medulla oblongata monitor and control these processes.

Other Influences on Heart Rate

• Baroreceptor reflexes: Maintain balance between sympathetic and parasympathetic activity.

• Body temperature and ion levels (e.g., K+, Ca2+) can affect heart rate.

• Input from the cerebrum or hypothalamus (e.g., emotional states) can also influence heart rate.

Cardiac Output and Cardiodynamics

Cardiac Output (CO)

Cardiac output is the best overall indicator of peripheral blood flow and is determined by heart rate and stroke volume.

• Stroke Volume (SV): Amount of blood pumped out of the ventricle during one heartbeat.

• Formula:

$SV = EDV - ESV$

$CO = HR \times SV$

• EDV: End-diastolic volume (volume of blood in ventricle at end of filling)

• ESV: End-systolic volume (volume of blood remaining after contraction)

Factors Affecting Cardiac Output

• Venous return: Amount of blood returning to the right atrium via vena cava and coronary sinus. Influenced by blood volume, muscular activity, and rate of blood flow through capillaries.

• Filling time: Duration of ventricular diastole. Slower heart rate increases filling time and EDV.

• Preload: Amount of myocardial stretching before contraction. Greater EDV leads to greater stroke volume (Frank-Starling law of the heart).

• Contractility: Force produced during a contraction at a given preload. Increased by sympathetic stimulation and hormones; decreased by drugs (e.g., beta blockers).

• Afterload: Tension required to open semilunar (SL) valves and eject blood. Increased afterload decreases stroke volume and increases ESV.

Cardiac Output Variation

Cardiac output varies to meet metabolic demands. Heart rate can increase by 250%, and stroke volume can almost double during exercise.

Condition	Cardiac Output (L/min)
Heart failure	~0–5
Average resting	~5
Heavy exercise (normal range)	~20–25
Peak trained athlete	~40

Blood Vessels

Circulatory Circuits

Blood vessels form a network connecting the heart to peripheral tissues via two main circuits:

• Pulmonary circuit: Transports blood between heart and lungs for gas exchange.

• Systemic circuit: Transports blood between heart and the rest of the body.

• Each circuit begins and ends with the heart.

Direction of Blood Flow

• Arteries: Always carry blood away from the heart.

• Veins: Always carry blood toward the heart.

• Arteries and veins are connected via capillaries.

Anatomy of Blood Vessels

Layers of Blood Vessels

Blood vessels have three main layers:

• Tunica intima (interna): Smooth inner layer that reduces friction.

• Tunica media: Middle layer of smooth muscle; responsible for constriction and dilation.

• Tunica externa (adventitia): Outer protective layer; anchors vessel to surrounding tissue.

Vasa Vasorum

Large vessels are too thick for diffusion of nutrients from the lumen. Small vessels called vasa vasorum supply nutrients to the vessel wall itself.

Subclassification of Blood Vessels

Arteries

• Elastic arteries: Large, resilient; accommodate heart's beating. Internal elastic membrane present. Examples: pulmonary trunk, aorta, major branches.

• Muscular arteries: Medium-sized; larger tunica media. Distribute blood to skeletal muscles and internal organs.

• Arterioles: Smallest arteries; little to no tunica externa.

Veins

• Large veins: Examples include superior vena cava, inferior vena cava, and major branches. Diameter: 2–9 mm.

• Venules: Smallest venous vessels.

Capillaries

General Features

Capillaries are the only vessels that allow exchange between blood and interstitial fluids. Their thin walls, composed of endothelial cells and a basement membrane, facilitate rapid diffusion. Tunica media and tunica externa are absent. The average diameter is not much larger than a single red blood cell (RBC).

Types of Capillaries

• Continuous capillaries: Complete lining of endothelial cells; found throughout the body except in epithelia and cartilage. Allow diffusion of water, small solutes, and lipid-soluble materials; prevent loss of blood cells or plasma proteins; use vesicular transport for some materials.

• Fenestrated capillaries: Contain pores in the endothelial lining; allow rapid exchange of water and larger solutes. Found in choroid plexus, hypothalamus, pituitary gland, pineal gland, thyroid gland, intestinal tract, and kidneys.

• Sinusoids: Resemble fenestrated capillaries but are flattened and irregularly shaped; have gaps between endothelial cells and little to no basement membrane. Permit free exchange of water and even larger molecules (e.g., plasma proteins). Found in liver, bone marrow, spleen, and many endocrine organs.

Capillary Type	Structure	Function	Location
Continuous	Complete endothelial lining	Diffusion of water, small solutes, lipid-soluble materials; restricts proteins and cells	Most tissues except epithelia, cartilage
Fenestrated	Pores in endothelium	Rapid exchange of water and larger solutes	Choroid plexus, endocrine glands, intestines, kidneys
Sinusoidal	Irregular shape, gaps, little basement membrane	Free exchange of water, large molecules	Liver, bone marrow, spleen, endocrine organs