The structure and function of the cardiovascular system

What are the major purposes of the cardiovascular system (CVS)?

Transport of oxygen and nutrients; removal of CO₂ and waste; delivery of hormones; temperature regulation; immune response support; pH and fluid balance stability.

What are the main components of the CVS?

Heart, blood vessels (arteries, veins, capillaries), and blood.

What are the two circuits of blood flow in the CVS?

Pulmonary circuit (deoxygenated blood from heart to lungs) and systemic circuit (oxygenated blood from heart to body tissues).

Describe the structure of the heart.

Four chambers (right atrium, right ventricle, left atrium, left ventricle); enclosed in a pericardial sac; wall has three layers: pericardium (outer), myocardium (muscle), endocardium (inner).

Describe the 3 layers of the heart

Pericardium - Lubricative outer layer
Myocardium - Middle layer that provides muscular contractions that eject blood from the heart chambers
Endocardium - Inner layer that protects inner lining of chambers and valves

List the main phases of the cardiac (heart) cycle.

Diastole (filling); atrial systole (atria contract); ventricular systole (ventricles contract).

Trace the flow of blood through the heart.

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

2. As the right atrium fills, blood passively enters the right ventricle through the open atrioventricular valve (tricuspid)

3. Atrial systole: The RA contracts, squeezing the extra blood into the RV (pressure was higher in the aorta than the ventricle)

4. The tricuspid valve closes once the RV is full to prevent backflow

5. Ventricular systole: The RV contracts, forcing blood out through the pulmonary (semilunar valve) and into the pulmonary artery

6. Deoxygenated blood goes to the lungs, gas exchange occurs —> blood gains oxygen and loses carbon dioxide

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

8. As the LA fills, blood passively flows into the left ventricle through the open mitral valve

9. Atrial systole: The LA contracts, forcing the rest of the blood into the LV

10. The mitral valve closes as the LV becomes full, preventing backflow into the atrium 

11. Ventricular systole: The LV contracts powerfully, forcing blood out aortic valve into the aorta (delivering oxygenated blood to the body and tissues)

In a healthy individual how long should the total cardiac cycle be?

0.8 secs

What are the times for the cardiac cycle?

Atrial and ventricular diastole (0.4 secs)
Atrial systolic and ventricular diastole (0.1 secs)
Ventricular systole and atrial diastole (0.3 secs)

What initiates and conducts the electrical impulse in the heart?

AP originates in the sinoatrial (SA) node; impulse travels through atria to atrioventricular (AV) node, bundle of His, bundle branches, and Purkinje fibers in the ventricular walls

What are arteries?

Blood travels away from the heart

• Have thick muscular and elastic walls

  • Walls can withstand and help maintain high pressure from the hearts pumpinh

  • Elastic fibres help them expand (during systole) and recoil (during diastole)

What are veins?

Blood travels towards the heart

• Thinner walls than arteries, less muscle and elastic tissue

• Have valves —> prevents backflow

• Operate under low pressure

What are capillaries?

Site of exchange between blood and tissues

• Walls are extremely thin (1 cell thick) to allow exchange 

What is systemic pressures?

Oxygenated blood from the left side of the heart to the body

What is the average resting arterial blood pressure?

120/80 mmHg (systolic/diastolic)

What is pulmonary pressure?

Deoxygenated blood from the right side of the heart to the lungs

What is the resting pulmonary arterial pressure?

25/10 mmHG

What is cardiac output and how is it calculated?

The volume of blood pumped by each ventricle per minute; calculated as stroke volume x heart rate.

What is the typical CO in a healthy adult?

5L/min

Define stroke volume

The amount of blood ejected by a ventricle with each heartbeat

What is the typical SV in a healthy adult?

70ml/beat

What happens to cardiac output, heart rate, and blood pressure during exercise?

Cardiac output and heart rate rise to meet oxygen demand

Systolic BP rises, changes little (systemic vasodilation)

What are the 3 factors that regulate at rest/during exercise?

End-diastolic volume - the volume of blood in the ventricles at the end of diastolic (preload)

Contractibility - the force of the contraction of the heart

Afterload - the resistance that the ventricles must overcome to eject blood

What is preload?

The initial stretching of the cardiac muscle fibres (ventricular walls) just before the contraction, caused by the volume of blood filling the ventricles at EDV

Name three mechanisms that aid venous return during exercise.

Venoconstriction
Muscle pump
Respiratory pump

Explain venoconstriction during exercise

It increases venous return by decreasing the diameter of the vessels and increasing the pressure within the vein. The end result of increased venous pressure is to drive blood back towards the heart

Explain the muscle pump during exercise

When muscles contract during exercise they compress veins and expedite the return of blood to the heart

Explain the respiratory pump during exercise

Enhances venous return by creating pressure changes in the thoracic cavity during deep, rapid breathing. Inhalation lowers thoracic cavity pressure and raises abdominal pressure, drawing more blood back to the heart. This effect is amplified during exercise as BR and depth increase

What is autoregulation in circulation?

The intrinsic ability of tissues to regulate their own blood flow in response to changes in local metabolites (e.g., ATP, nitric oxide), promoting vasodilation and ensuring increased blood supply to the working muscles during activity

What is the difference between arteries, veins, and capillaries?

Arteries carry blood away from heart, thick muscular walls; veins carry blood towards heart, thin walls, valves; capillaries are one cell thick, site of exchange.

What are the three main factors regulating stroke volume?

End-diastolic volume (preload); contractility; afterload.

What does the Frank-Starling law state?

The greater the stretch of cardiac muscle (preload), the greater the force of contraction (increases stroke volume).

What is the central command theory in exercise physiology?

The brain’s motor signal initiates increased heart rate during exercise, fine-tuned by feedback from muscle chemoreceptors, muscle mechanoreceptors, arterial baroreceptors to the cv control centre

What is a muscle chemoreceptor?

They are sensitive to increases in muscle metabolites (e.g., potassium, lactic acid), and send messages to higher brain centres to fine tune the CV responses to exercise

What is a muscle mechanoreceptors?

E.g., muscle spindles and GTO

They are sensitive to the force and speed of muscular movement. These send messages to higher brain centres to aid in modification of the CV responses to a given exercise task

What is a baroreceptors?

They are sensitive to change in arterial BP and sends afferent info back to the CV control centre to precisely adjust the CV activity during exercise

• Regulates arterial BP around an elevated systemic pressure during exercise

What happens to HR, SV, and CO during normal exercise?

HR rises quickly, then plateaus

SV increases initially, then gradually decreases (slight drop)
CO may plateau or rise less as HR increases are offset by greater SV reductions

What happens during hot humid exercise?

Cardiovascular drift

Explain cardiovascular drift

After prolonged exercise:
HR continues to drift upwards

SV falls more rapidly due to increased skin blood flow and sweat loss
CO may plateau or rise less as HR increases are offset by greater SV reductions

The progressive drift in HR is due to what?

The influence of rising body temperature on dehydration and a reduction in plasma volume

A drop in plasma volume reduces what?

Venous return to the heart and therefore SV

Is HR and BP typically higher in arm work or leg work?

Arm work

Why does arm work lead to higher HR and BP?

Smaller muscle mass in the arms means less vascular bed, so to deliver the same O2, HR must rise

Arm work causes greater peripheral resistance and less vasodilation compared to leg work, leading to higher systolic and diastolic BP
Enhanced sympathetic response (e.g., more vasoconstriction in non working muscles) during upper body work also contributes to the differences