Cardiovascular System

Cardiovascular System Introduction

Heart Anatomy and Function

The heart consists of two pumps in series:

1. The right ventricle propels blood to the pulmonary circulation.

2. The left ventricle propels blood to the systemic circulation.

Key anatomical structures include:

• Superior Vena Cava (SVC)

• Inferior Vena Cava (IVC)

• Right Atrium (RA)

• Right Ventricle (RV)

• Lungs

• Left Atrium (LA)

• Left Ventricle (LV)

• Aorta

The heart valves ensure unidirectional blood flow:

• Atrioventricular (AV) valves: Tricuspid (between RA and RV) and Mitral/Bicuspid (between LA and LV).

• Pulmonary valve: Between RV and pulmonary artery.

• Aortic valve: Between LV and aorta.

The volume output of the left-hand side (LHS) is equal to the right-hand side (RHS). However, the pressure generated by the LHS is approximately 4 times greater than the RHS. Typical pressures are ~120 mmHg for the LHS versus ~30 mmHg for the RHS.

Typical cardiac output is around $5 \text{ liters/min}$, which equates to approximately $70 \text{ ml/beat}$.

Blood Circulation

Deoxygenated blood returns to the heart via the venae cavae. Gas exchange occurs in the capillary beds of the lungs within the pulmonary system. Blood within the venous circulation is at low pressure. Blood leaving the heart enters the circulation at high pressure. Gas exchange also takes place in the capillary beds of all body tissues.

Exercise and Cardiac Function

During exercise, cardiac output (CO) increases to meet the increased metabolic needs of the body, particularly due to increased blood flow requirements in skeletal muscle.

At rest, CO is approximately $5 \text{ L/min}$. During maximal exercise, CO can reach approximately $25-30 \text{ L/min}$.

The equation for cardiac output is:

$\text{Cardiac output} = \text{Heart rate} \times \text{Stroke volume}$

Where:

• Cardiac output is measured in liters per minute.

• Heart rate is measured in beats per minute.

• Stroke volume is measured in milliliters per beat.

Several factors influence cardiac output during exercise:

• Venous return: Increases due to compression of veins by contracting skeletal muscle, which increases CO.

• SNS activity: Increased by higher brain centers that initiate exercise and by receptors in exercising muscles, leading to an increased heart rate and CO.

• PNS activity: Inhibited during exercise, which also contributes to an increased heart rate.

• Cardiac hypertrophy: Develops with prolonged training, causing the left ventricle to enlarge and increase stroke volume.

Factors Affecting Cardiac Output

Cardiac output is influenced by several interacting factors:

• Heart Rate: Affected by autonomic innervation, fitness level, and age.

• Stroke Volume: Affected by heart size, gender, and duration of contraction/contractility.

• Preload (Venous Return): Increased venous return increases cardiac output.

• Afterload: Influenced by hormones, chemoreceptors, and baroreceptors.

• Contractility: Affected by SNS and PNS reflexes.

Exercise activates skeletal muscle, which in turn affects these factors.

Blood Vessel Types

Aorta & Large Arteries

• Blood moves rapidly through the aorta and its arterial branches. These branches become thinner as they approach the periphery.

• Pressures in larger arteries are only slightly lower than in the aorta.

• Structure: Thick-walled, highly elastic.

• Function: Distribution (conduit), handles high pressure reservoir, forward propulsion.

• The greatest pressure drop occurs across the terminal segment of small arteries and the arterioles.

Arterioles

• Contraction of the circular muscle of small arterioles permits regulation of tissue blood flow and helps to control arterial blood pressure.

• Change from pulsatile (intermittent ejection from heart) to steady blood flow.

• Structure: Thick-walled, muscular, highly innervated.

• Function: Tissue distribution of flow, variable resistance.

Capillaries

• Many capillaries arise from each arteriole.

• Blood flow and velocity drop significantly across the capillary bed.

• Walls are only one cell thick.

• Structure: Thin-walled (single cell), lacking elastic tissue and smooth muscle.

• Function: Exchange.

Venules and Veins

• On return to the heart from the capillaries, blood passes through venules and then veins.

• Return blood back to the heart (venous return).

• Pressure decreases progressively until blood reaches the heart.

• Structure: Thin-walled, some muscle and innervation, valves (unidirectional).

• Function: Reservoir (capacitance); at rest, approximately 60% of blood volume is in the veins.

Blood Vessel Anatomy Comparison

Blood Flow Velocity and Cross-Sectional Area

• Aorta > Arteries > Arterioles > Capillaries (Individual vessel diameters).

• Capillaries < Venules < Veins (Individual vessel diameters).

• Aorta < Arteries < Arterioles < Capillaries (Total cross-sectional areas).

• Capillaries > Venules > Veins (Total cross-sectional areas).

• Bloodstream velocity depends on cross-sectional area.

$\text{Constant flow rate (CO)} = \text{blood flow velocity} \times \text{cross-sectional area}$

Determinants of Blood Flow (Q)

• Flow (Q) refers to the volume of fluid that passes a given cross-section of the vessel per unit time.

• Fluid velocity is inversely proportional to cross-sectional area – the larger the area, the lower the velocity.

• If flow rate is constant: velocity is inversely proportional to cross-sectional area.

• i.e. slowest in capillaries

• $\text{Flow rate} = \frac{\text{volume}}{\text{unit time}}$

• $\text{Velocity} = \frac{\text{distance}}{\text{unit time}}$

• $\text{Constant flow rate (CO)} = \text{blood flow velocity} \times \text{cross-sectional area}$

Relationship between Pressure and Flow (Q)

Poiseuille’s Law describes the factors affecting flow rate through a vessel:

• PRESSURE: Flow is proportional to the difference between inflow and outflow pressure $(P<em>i – P</em>o)$.

• LENGTH: Flow is inversely proportional to the length of the vessel (L).

• RADIUS: Flow is proportional to the radius of the vessel to the fourth power $(r^4)$.

• VISCOSITY: Flow is inversely proportional to the viscosity of the blood $\eta$.

The equation for flow (Q) is:

$Q = \frac{\pi (P<em>i – P</em>o)r^4}{8 \eta L}$

Which reads as flow is proportional to the change in Pressure multiplied by the fourth power of the radius divided by 8 times the fluid viscosity times the Length.

Peripheral Resistance

In electrical theory, resistance = voltage drop / electric current flow $(R=V/I)$. In the vasculature, the equation is:

$R = \frac{P<em>i - P</em>o}{Q}$

• Flow is directly related to the pressure difference.

• Flow is inversely related to the resistance $(1/r^4, l, \eta)$.

• The most important variable is vessel radius.

• Arterioles have the greatest capacity to change their radius and therefore influence resistance.