Cardiovascular, Blood, and Respiratory Physiology: Exam 3 Study Guide

Cardiovascular Physiology

Action Potential of a Cardiac Contractile Cell

The action potential in cardiac contractile cells is essential for coordinated heart contractions. It differs from skeletal muscle and neuron action potentials due to its prolonged plateau phase.

• Phase 0 (Depolarization): Rapid influx of Na+ through voltage-gated sodium channels.

• Phase 1 (Initial Repolarization): Brief efflux of K+ as some potassium channels open.

• Phase 2 (Plateau): Ca2+ enters via voltage-gated calcium channels, balancing K+ efflux and prolonging depolarization.

• Phase 3 (Repolarization): Calcium channels close, more potassium channels open, restoring the resting membrane potential.

• Phase 4 (Resting): Stable resting potential maintained by K+ permeability.

Example: The plateau phase prevents tetanus in cardiac muscle, ensuring rhythmic contractions.

Comparison of Contractile Cardiac, Autorhythmic Cardiac, and Skeletal Muscles

Muscle types differ in structure, function, and electrical properties. Table 14.3 compares these characteristics.

Feature	Contractile Cardiac	Autorhythmic Cardiac	Skeletal Muscle
Action Potential Duration	Long (200 ms)	Short (150 ms)	Very short (1-2 ms)
Initiation	Stimulated by autorhythmic cells	Spontaneous (pacemaker)	Neural input required
Calcium Source	Sarcoplasmic reticulum & extracellular	Extracellular	Sarcoplasmic reticulum
Refractory Period	Long	Long	Short
Contraction Type	All-or-none	Rhythmic	Graded
Additional info:	Cardiac cells are interconnected by intercalated discs	Autorhythmic cells set heart rate	Skeletal muscle can be tetanized

Mechanical Events of the Cardiac Cycle

The cardiac cycle describes the sequence of events during one heartbeat, including contraction (systole) and relaxation (diastole).

• Atrial Systole: Atria contract, pushing blood into ventricles.

• Ventricular Systole: Ventricles contract, ejecting blood into arteries.

• Ventricular Diastole: Ventricles relax, filling with blood from atria.

• Valves: AV valves open during diastole, semilunar valves open during systole.

Example: The cardiac cycle ensures unidirectional blood flow and efficient circulation.

Interpretation of ECGs: Four Questions to Ask

Electrocardiograms (ECGs) record the electrical activity of the heart. Interpretation involves systematic analysis.

• 1. Is there a normal rhythm? Check for regular P waves and QRS complexes.

• 2. Is the heart rate normal? Calculate beats per minute from the ECG trace.

• 3. Are all waves present and normal? Assess P, QRS, and T waves for abnormalities.

• 4. Is conduction normal? Evaluate intervals (PR, QRS, QT) for delays or blocks.

Example: A prolonged QRS interval may indicate ventricular conduction issues.

Blood Flow and Control of Blood Pressure

Baroreceptor Reflex

The baroreceptor reflex is a rapid negative feedback mechanism that regulates blood pressure.

• Baroreceptors: Located in the carotid sinuses and the aortic arch, sense changes in blood pressure.

• Response: Increased pressure triggers decreased heart rate and vasodilation; decreased pressure triggers increased heart rate and vasoconstriction.

• Neural Pathway: Signals sent to the medulla oblongata, which adjusts autonomic output.

Example: Standing up quickly activates the baroreceptor reflex to prevent fainting.

Risk Factors for Cardiovascular Disease (CVD)

CVD risk factors are conditions or behaviors that increase the likelihood of heart and blood vessel disease.

• Non-modifiable: Age, gender, genetics.

• Modifiable: High blood pressure, high cholesterol, smoking, obesity, physical inactivity, and diabetes.

• Additional info: Lifestyle changes can significantly reduce CVD risk.

Blood

Complete Blood Count (CBC): Six Tests

A CBC is a common diagnostic test that evaluates the components of blood.

Test	Purpose
Red Blood Cell Count (RBC)	Measures oxygen-carrying cells
White Blood Cell Count (WBC)	Assesses immune function
Hemoglobin (Hb)	Evaluates oxygen transport capacity
Hematocrit (Hct)	Determines proportion of RBCs in blood
Platelet Count	Assesses clotting ability
Mean Corpuscular Volume (MCV)	Measures average size of RBCs

Example: Low hemoglobin may indicate anemia.

Mechanics of Breathing

Four Lung Volumes and Four Lung Capacities

Lung volumes and capacities are measurements used to assess respiratory function.

• Lung Volumes:

  • Tidal Volume (TV): Air inhaled/exhaled in a normal breath.

  • Inspiratory Reserve Volume (IRV): Maximum air inhaled after normal inspiration.

  • Expiratory Reserve Volume (ERV): Maximum air exhaled after normal expiration.

  • Residual Volume (RV): Air remaining after maximal exhalation.

• Lung Capacities:

  • Inspiratory Capacity (IC): $IC = TV + IRV$

  • Functional Residual Capacity (FRC): $FRC = ERV + RV$

  • Vital Capacity (VC): $VC = TV + IRV + ERV$

  • Total Lung Capacity (TLC): $TLC = TV + IRV + ERV + RV$

Example: Reduced vital capacity may indicate restrictive lung disease.

Elastance vs. Compliance

Elastance and compliance describe the mechanical properties of the lungs.

• Compliance: The ability of the lungs to stretch; high compliance means easy expansion.

• Elastance: The ability of the lungs to recoil after stretching; high elastance means strong recoil.

• Relationship: Inverse; diseases like emphysema increase compliance but decrease elastance.

Example: Fibrosis decreases compliance, making breathing more difficult.

Ventilation

Ventilation refers to the movement of air in and out of the lungs, driven by pressure differences.

• Inspiration: Diaphragm contracts, thoracic volume increases, pressure decreases, air enters.

• Expiration: Diaphragm relaxes, thoracic volume decreases, pressure increases, air exits.

• Minute Ventilation: $\text{Minute Ventilation} = \text{Tidal Volume} \times \text{Respiratory Rate}$

Example: During exercise, both tidal volume and respiratory rate increase.

Gas Exchange and Transport

Reflex Control of Ventilation

Ventilation is regulated by reflexes responding to changes in blood gases and pH.

• Central Chemoreceptors: Located in the medulla, respond to CO2 and pH changes.

• Peripheral Chemoreceptors: Located in carotid and aortic bodies, respond to O2, CO2, and pH.

• Neural Pathways: Signals sent to respiratory centers to adjust breathing rate and depth.

Example: High CO2 levels trigger increased ventilation to restore normal gas balance.