Cardiovascular System: Structure, Function, and Regulation

Cardiac Muscle vs. Skeletal Muscle

Structural and Functional Differences

• Cardiac muscle cells are branched, interconnected, and contain one or two centrally located nuclei. They are joined by intercalated discs, which allow rapid electrical communication and synchronized contraction.

• Skeletal muscle cells are long, cylindrical, multinucleated, and lack intercalated discs. They contract independently.

• Unique features of cardiac muscle:

  • Automaticity (can generate their own action potentials)

  • Rhythmic contractions

  • Fatigue resistance

Pathway of Blood Through the Heart and Circulatory System

Blood Flow Sequence

• Deoxygenated blood enters the right atrium via the superior and inferior vena cava.

• Passes through the tricuspid valve into the right ventricle.

• Pumped through the pulmonary valve into the pulmonary arteries to the lungs.

• Oxygenated blood returns via pulmonary veins to the left atrium.

• Passes through the bicuspid (mitral) valve into the left ventricle.

• Pumped through the aortic valve into the aorta and systemic circulation.

Intrinsic Cardiac Conduction System (ICCS) and EKG Interpretation

Components and EKG Waves

• ICCS includes the Sinoatrial (SA) node, Atrioventricular (AV) node, Bundle of His, Right and Left Bundle Branches, and Purkinje fibers.

• EKG Peaks and Valleys:

  • P wave: Atrial depolarization

  • QRS complex: Ventricular depolarization

  • T wave: Ventricular repolarization

Cardiac Cycle: Systole and Diastole

Phases of the Cardiac Cycle

• Systole: Contraction phase; blood is ejected from the heart.

• Diastole: Relaxation phase; heart chambers fill with blood.

Factors Affecting Stroke Volume and Heart Rate

Regulation of Cardiac Output

• Stroke volume is affected by preload, contractility, and afterload.

• Heart rate is influenced by autonomic nervous system, hormones, fitness level, and age.

Tachycardia vs. Bradycardia

Definitions and Clinical Significance

• Tachycardia: Heart rate above 100 beats per minute (bpm).

• Bradycardia: Heart rate below 60 bpm.

• Both can be physiological or pathological.

Homeostatic Imbalances of the Heart

Effects of Aging and Disease

• Common imbalances include arrhythmias, heart failure, and myocardial infarction.

• Aging increases risk due to decreased elasticity and efficiency.

Types of Arteries

Classification and Anatomical Differences

• Elastic arteries: Large, close to the heart (e.g., aorta); stretch and recoil.

• Muscular arteries: Medium-sized; distribute blood to organs.

• Arterioles: Smallest; regulate blood flow into capillaries.

Types of Capillaries

Structure and Location

• Continuous capillaries: Most common; found in muscle, skin, brain.

• Fenestrated capillaries: Have pores; found in kidneys, intestines, endocrine glands.

• Sinusoidal capillaries: Large gaps; found in liver, bone marrow, spleen.

Blood Pressure Measurement

Understanding Systolic and Diastolic Values

• Systolic pressure: Pressure during ventricular contraction.

• Diastolic pressure: Pressure during ventricular relaxation.

• Measured in mmHg; normal adult BP is about 120/80 mmHg.

Factors Affecting Blood Flow Resistance

Determinants of Vascular Resistance

• Blood viscosity

• Vessel length

• Vessel diameter (most significant)

Blood Pressure Equation

Components and Their Effects

• Blood pressure is determined by cardiac output, stroke volume, and peripheral resistance.

Equation:

$BP = CO \times SV \times PR$

• CO: Cardiac Output

• SV: Stroke Volume

• PR: Peripheral Resistance

Venous Return Mechanisms

How Blood Returns to the Heart

• Muscle pump: Skeletal muscle contractions help push blood toward the heart.

• Respiratory pump: Changes in thoracic pressure during breathing assist venous return.

• Valves in veins prevent backflow.

Short-Term Control of Blood Pressure

Baroreceptors, Chemoreceptors, and Hormonal Regulation

• Baroreceptors: Detect changes in blood pressure and initiate reflexes to maintain homeostasis.

• Chemoreceptors: Respond to changes in blood chemistry (O2, CO2, pH).

• Hormonal control: Epinephrine, norepinephrine, and antidiuretic hormone (ADH) can rapidly alter BP.

Long-Term Control of Blood Pressure

Renal Mechanisms

• Kidneys regulate blood volume via water and salt excretion or retention.

• Renin-angiotensin-aldosterone system (RAAS) increases BP by promoting vasoconstriction and sodium retention.

Autoregulation of Blood Flow

Local Control Mechanisms

• Myogenic control: Vascular smooth muscle responds to changes in pressure.

• Metabolic control: Local chemical changes (e.g., low O2, high CO2) cause vasodilation.

Vasomotor Center

Role in Blood Pressure Regulation

• Located in the medulla oblongata.

• Regulates diameter of blood vessels via sympathetic nervous system.

Types of Circulatory Shock

Classification and Features

• Hypovolemic shock: Due to blood or fluid loss.

• Cardiogenic shock: Due to heart failure.

• Vascular (distributive) shock: Due to abnormal vasodilation (e.g., septic shock).

Hypertension and Its Effects

Definition and Consequences

• Hypertension: Chronic high blood pressure (usually >140/90 mmHg).

• Can lead to heart disease, stroke, kidney damage, and vascular complications.

Capillary Exchange: Hydrostatic and Osmotic Pressure

Mechanisms of Fluid Movement

• Capillary hydrostatic pressure: Pushes fluid out of capillaries into tissues.

• Colloid osmotic pressure: Pulls fluid back into capillaries due to plasma proteins.

Example: Edema occurs when hydrostatic pressure exceeds osmotic pressure, causing excess fluid accumulation in tissues.