BIO 104: The Cardiovascular System – Study Guide (Chapter 11)

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The Cardiovascular System

Blood Vessels

The cardiovascular system is composed of a network of blood vessels that transport blood throughout the body. Understanding the structure and function of these vessels is essential for grasping how blood circulates and how the heart maintains homeostasis.

Vessel Wall Structure

Tunica intima: The innermost layer, lined by endothelium (simple squamous epithelium), providing a smooth surface for blood flow.
Tunica media: The middle layer, composed of smooth muscle; it is the thickest layer in arteries and is responsible for regulating vessel diameter.
Tunica externa: The outermost connective tissue layer, providing structural support and protection.

Key fact: The thickest layer of an arterial wall is the tunica media, not the tunica externa.

Vessel Types & Functions

Arteries: Carry blood away from the heart; have thick, elastic walls and the highest blood pressure.
Arterioles: Small arteries that regulate blood flow into capillaries via vasoconstriction and vasodilation.
Capillaries: Sites of gas and nutrient exchange; walls are only one cell thick (endothelium only).
Venules: Small veins that collect blood from capillaries.
Veins: Return blood to the heart; contain one-way valves to prevent backflow and have the lowest pressure.

Venous Return Mechanisms

Blood returning through veins must overcome gravity and low pressure. Three main mechanisms assist venous return:

Skeletal muscle contraction: Squeezes veins, pushing blood toward the heart.
One-way valves: Prevent backflow of blood.
Respiratory pump: Breathing-related pressure changes draw blood toward the thorax.

Note: Peristalsis does not assist venous return; it is a mechanism used in the gastrointestinal tract.

Capillary Exchange

At the arterial end of capillaries: Fluid moves out via filtration (hydrostatic pressure exceeds osmotic pressure).
At the venous end of capillaries: Fluid moves in via osmosis (osmotic pressure pulls fluid back).
Edema results when this balance is disrupted.

Vascular Control

Vessel diameter is regulated by the autonomic nervous system (ANS).
Vasoconstriction: Narrows the lumen, increases resistance and blood pressure.
Vasodilation: Widens the lumen, decreases resistance; occurs in response to high blood pressure.

Cardiac Output and Blood Pressure

Cardiac output and blood pressure are critical for maintaining tissue perfusion and homeostasis. They are regulated by neural and hormonal mechanisms.

Key Formulas

Cardiac Output (CO):
Stroke Volume (SV): Volume of blood ejected per ventricular beat.

Blood Pressure Concepts

Highest blood pressure is found in the arteries (directly from ventricular ejection).
Systolic pressure (contraction) is greater than diastolic pressure (relaxation) in healthy individuals.
Pressure gradient: Arteries → Arterioles → Capillaries → Venules → Veins.

Autonomic & Hormonal Control of Heart Rate

Sympathetic nervous system: Increases heart rate and stroke volume; activated during stress or exercise.
Parasympathetic nervous system: Decreases heart rate; releases acetylcholine at the SA node.
Norepinephrine/Epinephrine: Released by the adrenal medulla; increases heart rate (sympathomimetic effect).
Acetylcholine: Released by parasympathetic nerves; decreases heart rate.

Homeostatic Responses

Response to Hypotension (low BP): Increased heart rate, vasoconstriction, and increased force of ventricular contraction occur together as compensation.
Response to Decreased Stroke Volume/Blood Loss: Blood pressure and cardiac output decrease; heart rate increases reflexively to maintain homeostasis.
Anaphylactic Shock: Triggered by severe allergic reaction; massive histamine release causes widespread vasodilation and a dramatic drop in blood pressure.
Sympathetic Activation (anger, fear, exercise): Increases cardiac output, heart rate, vasoconstriction, and blood pressure simultaneously.

Path of Blood Through the Heart

The heart pumps blood through a specific sequence of chambers and valves, ensuring oxygenation and systemic delivery.

Superior/Inferior Vena Cava: Deoxygenated blood from the body enters the heart.
Right Atrium: Receives systemic venous return.
Tricuspid Valve (AV): Opens into the right ventricle.
Right Ventricle: Pumps blood to the pulmonary circuit.
Pulmonary Semilunar Valve: Opens into the pulmonary trunk.
Pulmonary Trunk → Pulmonary Arteries: Carries deoxygenated blood to the lungs.
Lung Capillaries: Gas exchange occurs (CO₂ out, O₂ in).
Pulmonary Veins: Return oxygenated blood to the heart.
Left Atrium: Receives pulmonary venous return.
Bicuspid/Mitral Valve (AV): Opens into the left ventricle.
Left Ventricle: Pumps blood to the systemic circuit; most muscular chamber.
Aortic Semilunar Valve: Opens into the aorta.
Aorta → Systemic Arteries: Delivers oxygenated blood to the body.

Note: Only veins return blood to the heart; arteries always carry blood away from the heart.

Pericardium

The pericardium is a double-layered serous membrane that surrounds and protects the heart.

Pericardium: Anchors the heart, reduces friction, and prevents overfilling.
Pericardial fluid: Lubricates and reduces friction during heartbeat.

Comparison of membrane types:

Serous membrane: Lines internal cavities; produces watery fluid (e.g., pericardium, pleura, peritoneum).
Mucous membrane: Lines passages open to the exterior (e.g., respiratory, GI tracts).
Synovial membrane: Lines joints.
Cutaneous membrane: Skin.

Heart Valves

Heart valves ensure unidirectional blood flow through the heart by opening and closing in response to pressure changes.

Valve Types

Atrioventricular (AV) valves: Between atria and ventricles; have chordae tendineae; open/close based on pressure changes.
Tricuspid valve: Right AV valve; between right atrium and right ventricle; 3 cusps.
Bicuspid (Mitral) valve: Left AV valve; between left atrium and left ventricle; 2 cusps; prevents backflow into left atrium.
Semilunar valves: Between ventricles and great arteries; no chordae tendineae; cup-shaped.
Pulmonary semilunar valve: Right ventricle to pulmonary trunk.
Aortic semilunar valve: Left ventricle to aorta.

How Valves Work

Valves open and close in response to pressure changes within the heart (not nerve signals or direct muscle contraction).
AV valves open when atrial pressure > ventricular pressure.
Semilunar valves open when ventricular pressure > arterial pressure.
Chordae tendineae (attached to papillary muscles) prevent AV valves from flipping into atria.

Heart Sounds

First heart sound ("lub"): AV valves close at the start of ventricular systole.
Second heart sound ("dub"): Semilunar valves close at the end of ventricular systole.

Quick Reference: Which Valve Does What?

Valve	Location	Chordae Tendineae?
Tricuspid	R. Atrium → R. Ventricle	YES
Bicuspid/Mitral	L. Atrium → L. Ventricle	YES
Pulmonary Semilunar	R. Ventricle → Pulmonary Trunk	NO
Aortic Semilunar	L. Ventricle → Aorta	NO

Cardiac Conduction System & Arrhythmias

The cardiac conduction system coordinates the heartbeat through a sequence of electrical impulses, ensuring efficient pumping.

Conduction Pathway — In Order

SA Node (Sinoatrial): Located in the right atrium; primary pacemaker; initiates each heartbeat.
AV Node (Atrioventricular): Located in the right atrium; delays the impulse briefly to allow atria to finish contracting.
AV Bundle (Bundle of His): Carries impulse from AV node into interventricular septum.
Bundle Branches: Right and left branches conduct impulse down septum toward apex.
Purkinje Fibers: Located in the ventricles; spread impulse through ventricular walls; contraction begins at apex and moves superiorly to push blood toward semilunar valves.

Note: Chordae tendineae are not part of the cardiac conduction system; they are structural connective tissue.

ECG Waves

P wave: Atrial depolarization; immediately followed by atrial systole (atria contract).
QRS complex: Ventricular depolarization; ventricular systole begins.
T wave: Ventricular repolarization (ventricles relaxing).

Cardiac Arrhythmias

Tachycardia: Persistent resting heart rate ≥ 100 bpm; can result from hyperactivity of the SA node.
Bradycardia: Persistent resting heart rate ≤ 60 bpm.
Ventricular fibrillation: Uncoordinated, chaotic electrical activity in ventricles; no effective pumping.
Asystole: Complete absence of electrical and mechanical activity in the heart; flatline.

The Cardiac Cycle

The cardiac cycle describes the sequence of events in one heartbeat, including contraction and relaxation of the atria and ventricles.

Phases

Atrial systole: Atria contract (initiated by SA node signal); AV valves open; blood pushed into ventricles.
Ventricular systole: Ventricles contract; AV valves close (1st heart sound); semilunar valves open; blood ejected.
Diastole: All chambers relax and refill; the longest phase during rest; passive ventricular filling occurs.

Valve Status During Each Phase

Phase	AV Valves (Tricuspid & Bicuspid)	Semilunar Valves (Aortic & Pulmonary)
Atrial diastole / Passive ventricular filling	OPEN	CLOSED
Atrial systole	OPEN	CLOSED
Ventricular systole	CLOSED (1st sound)	OPEN
End of ventricular systole	CLOSED	CLOSED (2nd sound)

During Atrial Diastole / Passive Ventricular Filling

AV valves are open; semilunar valves are closed.
All heart chambers are relaxed.
Blood flows passively from atria into ventricles.

During Ventricular Systole

Blood is ejected from the ventricles (not atria, not entering ventricles).
AV valves close (bicuspid and tricuspid) — creates 1st heart sound.
Semilunar valves open (aortic and pulmonary).

High-Yield Summary Table

Use this table for last-minute review before quizzes or exams:

If the question asks about...	Remember...
Simple squamous epithelium lining vessels	Endothelium (tunica intima layer)
Thickest arterial layer	Tunica MEDIA (not externa)
Venous return — NOT a mechanism	Peristalsis (that's GI tract)
Fluid OUT of capillary, arterial end	Filtration
Vessels return blood to heart	Veins only
Highest blood pressure	Arteries
High BP vessel response	Vasodilation
Pacemaker of the heart	SA node
AV node function	Delays impulse (atria finish first)
Purkinje fiber location	Ventricles
Not part of conduction system	Chordae tendineae
Decreases heart rate	Acetylcholine (parasympathetic)
Increases heart rate	Norepinephrine / Adrenal medulla (sympathetic)
P wave followed by	Atrial systole
Tachycardia definition	HR ≥ 100 bpm persistently
Asystole	Complete absence of electrical + mechanical activity
Longest cardiac phase (at rest)	Diastole
Atrial systole initiated by	SA node signal
Ventricles contract apex → superior	Forces blood toward semilunar valves
Blood from right ventricle → next	Pulmonary trunk
Blood from left atrium → next	Bicuspid/Mitral valve → left ventricle
Semilunar valves — chordae tendineae?	NO — only AV valves have them
2nd heart sound	Semilunar valves closing
Pericardium membrane type	Serous membrane
Pericardium prevents	Overfilling of the heart
Cardiac output formula	HR × SV