Scherer Impact

Cardiovascular Physiology Overview

A Student-Centered Guide to Cardiovascular Function

Dean J. Scherer, DC

“Teach the mind. Restore the body. Inspire the heart.”

About the Author

Dean J. Scherer, DC

Professor of Anatomy and Physiology
Oklahoma State University – Oklahoma City

Doctor of Chiropractic (Cleveland Chiropractic College)
Over 30 years of experience in clinical practice and higher education

Founder, Scherer Impact
Composer and Producer – Scherer Impact Music & Media Studio

Dr. Scherer brings a unique integration of physiology, clinical insight, and teaching innovation, helping students understand complex systems through clear reasoning, analogies, and real-world application.

From the Scherer Impact Library

SchererImpact.org

A Student-Centered Guide to Cardiovascular Function

About the Author

Table of Contents

Chapter 1 — Foundations of the Circulatory System

Chapter 2 — Pressure, Flow, and Resistance

Chapter 3 — The Heart: Structure and Organization

Chapter 4 — Electrical Coordination of the Heart

Chapter 5 — Mechanical Events of the Cardiac Cycle

Chapter 6 — Cardiac Output

Chapter 7 — Measurement of Cardiac Function

Chapter 8 — Overview of the Vascular System

Chapter 9 — Arteries

Chapter 10 — Arterioles

Chapter 11 — Capillaries and Exchange

Chapter 12 — Veins and Venous Return

Chapter 13 — The Lymphatic System

Chapter 14 — Regulation of Mean Arterial Pressure

Chapter 15 — Baroreceptor Reflex

Chapter 16 — Long-Term Blood Pressure Control

Chapter 17 — Special Circulations

Chapter 18 — Shock and Hypotension

Chapter 19 — Cardiovascular Responses to Posture

Chapter 1 Foundations of the Circulatory System

The Circulatory System as an Integrated Transport Network

The circulatory system is best understood not as a collection of isolated parts, but as a continuous, pressure-driven transport network designed to sustain cellular life. It consists of three essential components:

The heart, which generates pressure
The blood vessels, which provide pathways
The blood, which serves as the transport medium

A useful way to conceptualize this system is as a living logistics network. The heart acts as a central pumping station, the vessels as highways, and the blood as a carrier of oxygen, nutrients, hormones, and waste products. Unlike any human-designed system, however, this network operates continuously, without interruption.

Composition and Function of Blood Plasma

Blood plasma is composed primarily of water (~90%), but its physiological importance lies in the substances dissolved within it. These include:

Glucose, which provides metabolic fuel
Amino acids, which serve as building blocks for proteins
Lipids, transported via lipoproteins
Hormones, which function as chemical messengers
Vitamins and electrolytes

Plasma functions as a multi-purpose transport medium, simultaneously supporting metabolism, communication, and homeostasis.

Blood Volume and Hematocrit

In the average adult, total blood volume is approximately 5 liters, divided into:

Plasma (~55%)
Formed elements (~45%)

The proportion of blood volume occupied by erythrocytes is termed the hematocrit. This value is critical because it directly influences both:

Oxygen-carrying capacity
Blood viscosity

A helpful analogy is to imagine a jar filled with water and sand. The sand represents the Formed Elements (Erythrocytes-RBC’s, Leukocytes-WBC’s, Thrombocytes-Platelets), and the water represents plasma. The hematocrit is simply the percentage of the jar occupied by sand.

Cellular Components and Their Origin

All blood cells originate from multipotent hematopoietic stem cells in the bone marrow. These stem cells differentiate into three major cell types:

Erythrocytes (Red Blood Cells)

Erythrocytes are specialized for oxygen transport. Their defining features include:

Absence of a nucleus, allowing maximal hemoglobin content
Biconcave shape, increasing surface area
Flexibility, enabling passage through narrow capillaries

Hemoglobin, the iron-containing protein within erythrocytes, binds oxygen in the lungs and releases it in peripheral tissues based on pressure gradients.

Leukocytes (White Blood Cells)

Leukocytes function in immune defense, identifying and eliminating pathogens. They represent the body’s mobile defense system.

Platelets

Platelets are cell fragments involved in clot formation. They play a crucial role in repairing vascular injury and preventing blood loss.

Bulk Flow vs Diffusion

A critical concept in cardiovascular physiology is that long-distance transport occurs via bulk flow rather than diffusion.

Diffusion is effective only over very short distances.
Bulk flow allows rapid movement across the entire body.

This distinction explains why a circulatory system is necessary in larger organisms.

Circulatory Circuits

The circulatory system is organized into two major circuits:

Pulmonary Circuit

Blood travels from the heart to the lungs and back, allowing gas exchange.

Systemic Circuit

Blood is delivered from the heart to the body and returns deoxygenated.

These circuits operate in series, ensuring continuous oxygenation and delivery.

Portal Systems

A portal system consists of two capillary beds arranged in sequence. The classic example is the hepatic portal system, where blood from the gastrointestinal tract passes through the liver before entering systemic circulation.

This arrangement allows for:

Nutrient processing
Detoxification

It functions similarly to a customs checkpoint, where substances are inspected before distribution.

Chapter 1 Integration

The circulatory system is a:

Pressure-driven system
Closed-loop network
Multi-functional transport mechanism

At its core, it exists to ensure that:👉 Every cell receives what it needs, when it needs it

Chapter 2 Pressure, Flow, and Resistance

The Fundamental Relationship

Blood flow through the circulatory system is governed by a simple but powerful relationship:

Flow increases with pressure difference.
Flow decreases with resistance.

This relationship mirrors electrical systems and highlights that the cardiovascular system functions as a fluid circuit.

Understanding Resistance

Resistance is determined by three factors:

Vessel length
Blood viscosity
Vessel radius

Among these, radius is overwhelmingly the most important.

The Power of Radius (r⁴ Relationship)

Resistance is inversely proportional to the fourth power of radius. This means:

A small decrease in radius produces a massive increase in resistance.
A small increase in radius results in a significant decrease in resistance.

This exponential relationship makes vessel diameter the primary regulator of blood flow.

Arterioles: The Control Point of the System

Arterioles are known as resistance vessels because they:

Contains smooth muscle
Can rapidly change diameter
Control the distribution of blood flow.

Through vasoconstriction and vasodilation, arterioles determine where blood goes and how much reaches each tissue.

Integration

At a systems level:

Pressure drives flow
Resistance limits flow
Radius controls resistance

👉 The entire circulatory system can be understood as a continuously adjustable flow network

Chapter 3 The Heart: Structure and Organization

Structural Layers of the Heart

The heart wall consists of three layers:

Epicardium – protective outer layer
Myocardium – contractile muscle
Endocardium – smooth inner lining

These layers allow the heart to function as an efficient, low-friction pump.

Chambers and Flow Pathway

The heart contains four chambers:

Right atrium
Right ventricle (Pulmonary Circuit Pump)
Left atrium
Left ventricle (Systemic Circuit Pump)

Blood flows through the heart in a precise sequence, ensuring separation of oxygenated and deoxygenated blood.

Valves and One-Way Flow

Valves prevent backflow and ensure efficient pumping:

Tricuspid valve
Pulmonary valve (Pulmonary Semilunar valve)
Mitral valve (Bicuspid Valve)
Aortic valve (Aortic Semilunar valve)

Without valves, blood flow would be inefficient and chaotic.

Electrical vs Mechanical Systems

The heart contains:

Conducting cells → electrical system
Contractile cells → mechanical system

This separation allows coordinated contraction.

Coronary Circulation

The heart requires its own blood supply via the coronary arteries. Because of the wall thickness and rapid flow, oxygen cannot diffuse from the chamber blood.

Failure of the coronary circulation leads to myocardial infarction.

Autonomic Control

The heart is self-paced but modulated by:

Sympathetic input → increases rate and force
Parasympathetic input → decreases rate.

Chapter 3 Integration

The heart is:

A dual pump
Electrically coordinated
Mechanically efficient
Self-regulated but modifiable

Chapter 4 Electrical Coordination of the Heart

Conduction Pathway

Electrical activity follows a precise sequence:

SA node
Atria (Internodal Pathway)
AV node (delay)
Bundle of His (AV Bundle)
Left & Right Bundle branches
Purkinje fibers

This ensures proper timing of contraction.

Importance of AV Node Delay

The AV node delay allows:

Atrial contraction to complete
Ventricles to fill before contraction (preload)

Without this delay, cardiac output would decrease significantly.

Cardiac Action Potentials

Cardiac muscle action potentials differ from neurons:

Long duration
Plateau phase
No summation or tetanus

This prevents sustained contraction (no summation-stops tetany), which would be fatal.

Pacemaker Activity

The SA node generates spontaneous depolarization due to:

Funny sodium channels
Calcium influx

It sets the rhythm because it depolarizes fastest.

ECG Interpretation

The ECG reflects electrical activity:

P wave → atrial depolarization
QRS complex → ventricular depolarization
T wave → ventricular repolarization

Chapter 4 Integration

The heart’s electrical system ensures:

Timing
Coordination
Efficiency

👉 Electrical order = mechanical efficiency

Chapter 5 Mechanical Events of the Cardiac Cycle

Systole and Diastole

Systole → contraction and ejection
Diastole → relaxation and filling

These phases alternate continuously.

Phases of the Cardiac Cycle

Ventricular filling
Isovolumetric contraction
Ventricular ejection
Isovolumetric relaxation

Each phase is defined by valve status, changes in pressure, and changes in volume.

Pressure and Volume Relationships

Blood flows according to pressure gradients. Key insight:

👉 Most ventricular filling occurs passively, before atrial contraction.

Stroke Volume

Stroke volume represents the amount of blood ejected per beat and is determined by:

End-diastolic volume (EDV)
End-systolic volume (ESV)

Pulmonary vs Systemic Circulation

Systemic circulation operates at high pressure.
Pulmonary circulation operates at low pressure to protect lung tissue.

Heart Sounds

S1 → AV valve closure
S2 → Semilunar valve closure

Murmurs indicate turbulent flow, often due to valve dysfunction.

Chapter 5 Integration

The cardiac cycle is a:

Pressure-driven sequence
Valve-coordinated process
Volume-regulated system

Chapter 6 Cardiac Output

Cardiac Output as System Throughput

Cardiac output (CO) is the volume of blood pumped by one ventricle per minute and is one of the most important functional measures of the cardiovascular system.

It is defined by the relationship:

Cardiac Output = Heart Rate × Stroke Volume

This equation reflects a fundamental systems concept:👉 The heart can increase output either by beating faster or by pumping more blood per beat.

A useful analogy is a delivery system:

Heart rate = number of delivery trucks leaving per minute
Stroke volume = number of packages per truck
Cardiac output = total packages delivered per minute

Regulation of Heart Rate

Heart rate is primarily controlled at the SA node, where the rate of spontaneous depolarization determines how frequently action potentials are generated.

Sympathetic Stimulation

Sympathetic activation increases heart rate by:

Increasing sodium and calcium permeability
Accelerating the rate of pacemaker depolarization

This causes the SA node to reach threshold more quickly.

Parasympathetic Stimulation

Parasympathetic input slows heart rate by:

Increasing potassium permeability (hyperpolarization)
Decreasing the rate of depolarization

This delays the threshold and reduces firing frequency. This causes the SA node to reach threshold more slowly.

Stroke Volume and Its Determinants

Stroke volume is influenced by three major factors:

1. Preload (End-Diastolic Volume)

Preload refers to the amount of ventricular filling before contraction.

The key concept is:👉 More filling → more stretch → stronger contraction

This relationship is explained by the Frank-Starling mechanism, which reflects the length–tension properties of cardiac muscle.

The heart automatically adjusts its output to match venous return, functioning as a self-regulating pump.

2. Contractility

Contractility refers to the intrinsic strength of cardiac muscle independent of preload.

It is primarily determined by:

Intracellular calcium levels
Sympathetic stimulation

Increased contractility allows the heart to:

Eject more blood
Reduce end-systolic volume

3. Afterload

Afterload is the resistance the ventricle must overcome to eject blood.

It is closely related to arterial pressure.

Key relationship:

Increased afterload → decreased stroke volume.

This can be understood as pushing against resistance:

Low resistance → easy ejection
High resistance → reduced output

Integration of Cardiac Output

Cardiac output reflects the combined effects of:

Electrical control (heart rate)
Mechanical factors (stroke volume)
Vascular resistance (afterload)

The heart continuously adjusts these variables to meet the body's metabolic demands.

Chapter 6 Integration

The heart functions as an adaptive pump that:

Matches output to input (venous return)
Adjusts force and rate dynamically
Responds to both neural and intrinsic signals

👉 Cardiac output is the functional expression of the entire cardiovascular system

Chapter 7 Measurement of Cardiac Function

Evaluating Pump Performance

Understanding cardiac function requires tools that assess:

Efficiency
Structure
Blood flow

Three primary methods are used clinically.

Ejection Fraction

Ejection fraction (EF) is the percentage of blood ejected from the ventricle during each contraction.

It provides a direct measure of pump efficiency.

Normal EF: ~55–70%
Reduced EF indicates impaired contractility.

Conceptually:

A healthy heart empties most of its volume.
A failing heart leaves a large residual.

Echocardiography

Echocardiography uses ultrasound to visualize the heart in real time.

It allows assessment of:

Chamber size
Wall motion
Valve function
Blood flow patterns

This technique provides a dynamic view of cardiac function, making it one of the most widely used diagnostic tools.

Cardiac Angiography

Cardiac angiography involves injecting contrast dye into the coronary circulation.

It is used to:

Identify blockages
Assess blood flow through the coronary arteries.

This method is essential for diagnosing conditions such as:

Coronary artery disease
Myocardial infarction

Chapter 7 Integration

These diagnostic tools correspond to different aspects of cardiac function:

Ejection fraction → performance
Echocardiography → structure and motion
Angiography → blood supply

👉 Together, they provide a complete picture of cardiac health

Chapter 8 Overview of the Vascular System

Structural Organization of Blood Vessels

All blood vessels share a common structural plan consisting of three layers:

Tunica intima → smooth inner lining
Tunica media → smooth muscle layer
Tunica externa → connective tissue support

Despite this shared structure, vessels differ significantly based on function.

Functional Classification of Vessels

Arteries

Arteries are high-pressure vessels with thick walls and elastic properties.

They function to:

Transport blood away from the heart.
Maintain pressure during diastole.

Arterioles

Arterioles are the primary resistance vessels.

They regulate:

Blood flow distribution
Total peripheral resistance

Capillaries

Capillaries are the site of exchange.

Their structure (one cell thick) allows:

Rapid diffusion of gases
Movement of nutrients and waste

Veins

Veins are low-pressure vessels with high compliance.

They function to:

Return blood to the heart.
Serve as blood reservoirs.

Pressure Changes Across the System

Blood pressure decreases progressively as blood moves through the system:

High in the arteries
Drops significantly across arterioles
Low in capillaries
Very low in veins

The greatest pressure drop occurs across arterioles due to high resistance.

Pulmonary vs Systemic Circulation

The pulmonary circulation operates at much lower pressure than the systemic circulation.

This is essential because:

Lung capillaries are delicate.
High pressure would cause fluid leakage and impair gas exchange.

Chapter 8 Integration

The vascular system is a:

Pressure gradient system
Functionally specialized network

👉 Structure always matches function in each vessel type

Chapter 9 Arteries

Arteries as Pressure Reservoirs

Arteries do more than transport blood—they store energy.

During systole:

Arteries expand as blood is ejected.

During diastole:

Arteries recoil, maintaining flow.

This phenomenon smooths pulsatile flow into continuous movement.

Compliance

Compliance refers to the ability of a vessel to stretch.

High compliance → large volume change with small pressure change
Low compliance → stiff vessel

With aging:

Arteries become less compliant.
Systolic pressure increases

Arterial Pressure

Arterial pressure fluctuates with the cardiac cycle:

Systolic pressure → peak during contraction
Diastolic pressure → lowest during relaxation

Pulse pressure is the difference between these values.

Mean Arterial Pressure

Mean arterial pressure (MAP) represents the average pressure driving blood flow.

It is weighted toward diastole because:

The heart spends more time in relaxation than in contraction

Blood Pressure Measurement

Blood pressure is measured using the auscultatory method.

Korotkoff sounds arise from turbulent flow as the cuff pressure is released.

First sound → systolic pressure
Disappearance → diastolic pressure

Chapter 9 Integration

Arteries:

Store and release energy
Maintain pressure
Ensure continuous flow

👉 They transform intermittent pumping into steady circulation

Chapter 10 Arterioles

Arterioles as Control Points

Arterioles are the most important regulators of blood flow.

They determine:

Distribution of blood to tissues
Total peripheral resistance

Why Arterioles Dominate Resistance

Due to their small radius, arterioles create the greatest resistance in the system.

This causes:

The largest pressure drop
Fine control of downstream flow

Vasoconstriction and Vasodilation

Arterioles regulate flow through changes in diameter:

Vasoconstriction → increased resistance → reduced flow
Vasodilation → decreased resistance → increased flow.

Local Control of Blood Flow

Tissues regulate their own blood flow in response to metabolic needs.

Key signals include:

Low oxygen
High carbon dioxide
Increased hydrogen ions
Adenosine

These factors cause vasodilation, increasing blood supply.

Myogenic Response

Arterioles respond to changes in pressure:

Increased pressure → constriction
Decreased pressure → dilation

This helps maintain stable blood flow.

Neural and Hormonal Control

Sympathetic stimulation causes vasoconstriction.

Hormones such as:

Angiotensin II
Vasopressin

also influences vessel diameter.

Endothelial Function

The endothelium releases substances that regulate vessel tone:

Nitric oxide → vasodilation
Endothelin → vasoconstriction

Chapter 10 Integration

Arterioles are:

The primary resistance vessels
The key regulators of blood distribution
Controlled by local, neural, and hormonal signals

👉 If the heart is the pump, arterioles are the decision-makers

Chapter 11 Capillaries and Exchange

The Capillaries: Where Function Becomes Reality

Everything the cardiovascular system does leads to this moment:

👉 Exchange at the capillary level

Capillaries are the smallest vessels in the body, composed of a single layer of endothelial cells. This minimal barrier allows rapid movement of substances between blood and tissues.

Think of capillaries as:

“The delivery dock where oxygen and nutrients are handed off, and waste is picked up.”

Types of Capillaries

Capillaries vary based on permeability:

Continuous capillaries → tight control (muscle, brain)
Fenestrated capillaries → moderate permeability (kidneys, intestines)
Discontinuous capillaries (sinusoids) → highly permeable (liver)

Structure determines function.

Diffusion: The Primary Mechanism

Most exchange occurs by diffusion, driven by concentration gradients.

Oxygen diffuses from blood → tissue.
Carbon dioxide diffuses from tissue → blood.

This process is rapid and efficient over short distances.

Bulk Flow and Starling Forces

Fluid movement across capillaries is governed by Starling forces, which include:

Hydrostatic pressure (Pc) → pushes fluid OUT.
Interstitial pressure (Pi) → pushes fluid IN
Plasma oncotic pressure (πc) → pulls fluid IN
Interstitial oncotic pressure (πi) → pulls fluid OUT.

Filtration vs Reabsorption

At the arterial end → filtration dominates (fluid leaves the capillary)
At the venous end → reabsorption dominates (fluid returns)

This creates a dynamic balance.

Clinical Insight: Edema

Edema occurs when filtration exceeds reabsorption.

Causes include:

Increased hydrostatic pressure (heart failure)
Decreased plasma proteins (liver disease)
Increased permeability (inflammation)

Chapter 11 Integration

Capillaries are where:

Oxygen is delivered
Nutrients are exchanged
Waste is removed

👉 This is the true purpose of circulation

Chapter 12 Veins and Venous Return

Veins as Blood Reservoirs

Veins contain most of the body’s blood volume (~60–70%).

They are:

Highly compliant
Low pressure
Capable of storing large volumes

The Challenge of Venous Return

Unlike arteries, veins must return blood to the heart without the strong pressure of arterial blood flow.

This requires assistance.

Mechanisms of Venous Return

1. Skeletal Muscle Pump

Muscle contractions compress veins and push blood toward the heart.

2. Respiratory Pump

Inhalation decreases thoracic pressure, drawing blood upward.

3. Venous Valves

Prevent backflow and ensure one-way movement.

Sympathetic Control

Sympathetic stimulation causes:

Venoconstriction
Increased venous return

This increases preload and cardiac output.

Chapter 12 Integration

Venous return determines:👉 How much the heart can pump

No return = no output

Chapter 13 The Lymphatic System

Recovering Lost Fluid

Not all filtered fluid is reabsorbed by capillaries.

The lymphatic system returns this excess fluid to circulation.

Functions of the Lymphatic System

Returns fluid to the blood
Transports lipids
Provides immune defense

Flow of Lymph

Lymph flows through:

Lymph vessels
Lymph nodes
Back to venous circulation via the lymphatic ducts

Clinical Insight: Lymphedema

Blockage of lymphatic drainage leads to fluid accumulation.

Chapter 13 Integration

The lymphatic system:👉 Maintains fluid balance and prevents edema

Chapter 14 Regulation of Mean Arterial Pressure

The Central Equation

Mean arterial pressure is defined as:

👉 MAP = Cardiac Output × Total Peripheral Resistance

This is the core equation of cardiovascular physiology.

Two Ways to Control MAP

1. Cardiac Output

Heart rate
Stroke volume

2. Total Peripheral Resistance

Primarily controlled by arterioles.

Short-Term vs Long-Term Control

Short-term → nervous system
Long-term → kidneys

Chapter 14 Integration

MAP is the result of:

Pump function
Vessel resistance

Chapter 15 Baroreceptor Reflex

Rapid Blood Pressure Control

Baroreceptors detect changes in pressure in:

Carotid sinus
Aortic arch

Reflex Response

If blood pressure drops:

↑ Heart rate
↑ Contractility
↑ Vasoconstriction

If blood pressure rises:

Opposite occurs

Chapter 15 Integration

Baroreceptors provide:👉 Second-to-second regulation of blood pressure

Chapter 16 Long-Term Blood Pressure Control

The Role of the Kidneys

The kidneys regulate:

Blood volume
Sodium balance

Key Hormonal Systems

Renin-Angiotensin-Aldosterone System (RAAS)

Increases blood volume
Causes vasoconstriction

Antidiuretic Hormone (ADH)

Increases water reabsorption

Chapter 16 Integration

Kidneys determine:👉 Long-term blood pressure stability

Chapter 17 Special Circulations

Organ-Specific Blood Flow

Different organs regulate flow differently:

Brain → constant flow
Heart → metabolic control
Skin → temperature regulation

Chapter 17 Integration

Blood flow is:👉 Tailored to organ function

Chapter 18 Shock and Hypotension

Definition of Shock

Shock is a condition of:👉 Inadequate tissue perfusion

Types of Shock

Hypovolemic
Cardiogenic
Distributive

Consequences

Reduced oxygen delivery
Organ failure

Chapter 18 Integration

Shock = failure of:

Cardiac output
Blood pressure
Tissue perfusion

Chapter 19 Cardiovascular Responses to Posture

The Challenge of Standing

When standing:

Blood pools in the lower extremities
Venous return decreases

Compensation

Baroreflex increases:

Heart rate
Vasoconstriction

Orthostatic Hypotension

Occurs when compensation fails.

Chapter 19 Integration

Posture challenges:👉 The ability to maintain perfusion

Closing Reflection

The study of cardiovascular physiology is more than understanding pressure, flow, and resistance.

It is the study of how life is sustained at the cellular level—how every heartbeat serves as a moment of delivery, connection, and renewal.

Each concept you have learned is not isolated, but part of an integrated system working continuously to support life.

As you move forward, remember:

Physiology is not just something to memorize—it is something to understand
The human body is not a collection of parts—it is a coordinated system
Every mechanism you study has purpose, direction, and meaning

Whether you go on to care for patients, continue your education, or simply deepen your understanding of the human body, carry this forward:

You are not just learning how the body works.
You are learning how life is maintained.

Teach the mind. Restore the body. Inspire the heart.

— Professor Dean J. Scherer

04/04/2026