Cardiovascular System: Heart Function and Blood Flow

Posted on Aug 27, 2025 in Biology

The Cardiovascular System

The cardiovascular system is responsible for circulating blood throughout the body, delivering oxygen and nutrients while removing waste products. Key components include the heart and blood vessels.

The Heart

The heart is a muscular organ with four chambers that pumps blood. It contains structures like the chordae tendineae, endocardium, myocardium (with intercalated disks), and epicardium.

  • The heart has 4 chambers.
  • The left side pumps oxygen-rich blood.
  • The right side pumps oxygen-poor blood.

Heart Valves

Valves ensure unidirectional blood flow:

  • Tricuspid (atrioventricular) valve
  • Bicuspid (mitral) valve
  • Pulmonary (semilunar) valve: before the lungs
  • Aortic (semilunar) valve: before the aorta

Heart Nodes and Conduction

The heart’s electrical activity is regulated by specialized nodes:

  • Sinoatrial (SA) node (approx. 70 bpm)
  • Atrioventricular (AV) node (approx. 50 bpm) – the slowest due to nodal delay
  • Bundle of His
  • Purkinje fibers (approx. 30 bpm)

Cardiac Action Potential

The action potential in cardiac cells involves:

  • Rapid depolarization (influx of Na+ ions)
  • Peak of action potential (Na+ channels close, transient K+ channels open)
  • Repolarization (completion of K+ efflux)

Electrocardiogram (ECG)

The ECG records the heart’s electrical activity:

  • P wave: Atrial depolarization
  • QRS complex: Ventricular depolarization
  • T wave: Ventricular repolarization

Key Cardiac Terms

  • End Diastolic Volume (EDV): Volume of blood in a ventricle at the end of diastole (maximum volume).
  • Isovolumetric Ventricular Contraction: Period during ventricular contraction when all valves are closed, and no blood enters or leaves the ventricles.
  • End Systolic Volume (ESV): Volume of blood remaining in a ventricle at the end of systole.
  • Stroke Volume (SV): Volume of blood ejected from a ventricle during one contraction (SV = EDV – ESV).
  • Isovolumetric Ventricular Relaxation: Period during ventricular relaxation when all valves are closed, and no blood enters or leaves the ventricles, leading to a decrease in chamber pressure.

Valve Damage and Stethoscope Patterns

  • Stenotic Valve: A valve that does not open fully, causing a “whistling” sound as blood is forced through.
  • Insufficient Valve: A valve that does not close fully, causing a “swishing” sound due to backflow.

Stethoscope sounds:

  • Lub: Closure of AV valves.
  • Dup: Closure of semilunar valves.
  • A “whistle” or “swish” between Lub and Dup indicates a problem during systole.
  • A “whistle” or “swish” after Lub and Dup indicates a problem during diastole.

Cardiac Output

Cardiac Output (CO) is the volume of blood pumped by the heart per minute:

CO = Heart Rate (HR) x Stroke Volume (SV)

Autonomic Nervous System Influence

  • Parasympathetic Nervous System: Decreases heart rate and cardiac output (calming effect).
  • Sympathetic Nervous System: Increases heart rate, cardiac output, venous return, end-diastolic volume, and stroke volume.

Frank-Starling Mechanism

The Frank-Starling mechanism describes the relationship between the stretch of cardiac muscle fibers and the force of contraction. The strength of contraction is based on the amount of blood received.

  • Sympathetic stimulation shifts the Frank-Starling curve upward and to the left.
  • Heart failure shifts the Frank-Starling curve downward and to the right, often compensated by increased blood volume (due to salt and water retention by the kidneys).

Circulatory System Components

The circulatory system includes:

  • Arteries
  • Arterioles
  • Capillaries (have the greatest total cross-sectional area due to their sheer number)
  • Venules
  • Veins

Blood Flow Dynamics

Blood flow is determined by the pressure gradient and resistance:

Flow Rate = Pressure Gradient / Resistance

Factors influencing resistance include:

  • Blood viscosity (due to plasma proteins and red blood cells)
  • Vessel length
  • Vessel radius (has a significant impact, proportional to the 4th power)

Elastin in Arteries

Elastin is a protein that provides stretching and flexibility, primarily found in arteries. Arteries act as a pressure reservoir, smoothing out blood flow.

Blood Pressure Regulation

  • Pulse Pressure: The difference between systolic and diastolic blood pressure.
  • Mean Arterial Pressure (MAP): The average arterial pressure during a cardiac cycle. MAP = Diastolic Pressure + (1/3 x Pulse Pressure). This is monitored and regulated by blood pressure reflexes.

Sphygmomanometer

A sphygmomanometer is used to measure blood pressure. Sounds are typically heard between 120 mmHg (systolic) and 80 mmHg (diastolic) during cuff deflation.

Local Intrinsic Controls of Blood Flow

Local factors influencing blood flow include:

  • Oxygen concentration (O2 conc.)
  • Carbon dioxide concentration (CO2 conc.)
  • pH
  • Potassium ion concentration (K+ conc.)
  • Osmolarity
  • Adenosine
  • Prostaglandins
  • Histamine release
  • Temperature
  • Myogenic response

Capillary Exchange

Exchange of substances between blood and tissues occurs via diffusion and bulk flow across endothelial cells (which have pores).

  • Ultrafiltration: Bulk flow of fluid out of capillaries into tissues (positive pressure).
  • Reabsorption: Bulk flow of fluid from tissues into capillaries (negative pressure).

Forces in Bulk Flow

  • Capillary Blood Pressure (BP): Forces fluid out of capillaries.
  • Plasma Colloid Osmotic Pressure: Draws water into capillaries.
  • Interstitial Fluid Hydrostatic Pressure: Pushes fluid into capillaries.
  • Interstitial Fluid Colloid Osmotic Pressure: Draws water out of capillaries.

Venous Return

Venous return (blood returning to the heart) is influenced by:

  • Sympathetic activity
  • Skeletal muscle activity
  • Venous valves
  • Respiratory activity
  • Cardiac suction

Baroreceptor Reflex

The baroreceptor reflex responds to changes in arterial blood pressure by adjusting the rate of neuronal firing. This reflex alters the activity of parasympathetic and sympathetic neurons in the cardiovascular control center to maintain blood pressure homeostasis.