The Vascular System and Blood Flow
The Vascular System & Blood
The Vascular System
A network of vessels that transport blood throughout the body, divided into four main categories:
Arteries
Carry blood away from the heart to different organs.
- Very elastic, can easily stretch and recoil.
- Blood pressure is measured in arteries.
Arterioles
Regulate blood distribution to various tissues in the body.
- Have rings of smooth muscle that can constrict or relax and alter blood flow.
- Arteriole muscle is controlled chemically (Autoregulation) and through nervous system intervention.
Capillaries
Responsible for the exchange of gases and nutrients with the tissues via diffusion.
Veins
After capillaries, smaller venules become larger veins, which become the vena cava, returning blood to the heart.
Arteries
Elastic and have more muscle than veins. Carry oxygenated blood and nutrients away from the heart (giving a bright red color) to tissues requiring oxygen.
Veins
Carry deoxygenated blood towards the heart (giving a dark red color).
- Exceptions are the Pulmonary Veins.
- Have one-way valves to keep blood moving forward.
The Return of Blood From Veins To The Heart
The Skeletal Muscle Pump
Upon contraction of skeletal muscle, blood is pushed/massaged back to the heart.
The Thoracic Pump
Pressure in veins (in the chest) decreases while pressure in the veins (in the abdominal cavity) increases upon intake of breath. This pressure difference pushes blood from veins in the abdominal cavity into veins in the thoracic cavity.
The Nervous System
Sends a signal to the veins, causing venoconstriction. Veins also have muscle, allowing them to constrict and send more blood back to the heart.
Cardiac Cycle
The series of events that occurs through a single heartbeat. During this cycle, there are 2 events:
- Diastole: The relaxation phase, where the heart fills with blood.
- Systole: The heart muscle contracts and ejects blood.
During this cycle, there are dramatic changes in blood pressure, which propel the blood.
- Blood Pressure is the pressure that blood exerts on blood vessel walls as it travels through the body.
- Changes in blood pressure correspond to the phases of the heartbeat.
- Systolic Pressure: Maximum pressure in arteries when ventricles contract and push blood through (120 mmHg). Arteries stretch.
- Diastolic Pressure: Lowest pressure in arteries when ventricles are not contracting (80 mmHg). Arteries recoil.
Blood Pressure & Exercise
Blood pressure is affected by exercise, but changes depend on the type, duration, and intensity of the exercise.
Exercise Type: Acute Aerobic/Endurance
Blood Pressure: Increase in systolic pressure proportional to exercise intensity.
Exercise Type: Resistance Training
Blood Pressure: Short but very large increases in both systolic and diastolic pressure, proportional to intensity.
- Post Workout: Regardless of exercise type, individuals experience a prolonged period of hypotension after completion.
- Blood pressure is a commonly used indicator to measure overall cardiovascular health.
- Normal blood pressure (BP): 120 mmHg over 80 mmHg.
- Hypertension: BP greater than 140 mmHg over 90 mmHg. A modifiable risk factor for CVD (can be changed through lifestyle). Factors affecting BP: Diet & Aerobic Exercise.
Blood: The Fluid of Life
Blood is straw-colored with red pigments and has two main components:
- Plasma (55%): Fluid component of blood (mostly water) + nutrients + proteins + ions + gases.
- Formed Portion (45%): Solid portion of blood with 3 components:
- Red Blood Cells (Erythrocytes): Made in bone marrow. Transport O2 and CO2 in the blood. Transport nutrients and waste, and contain Hemoglobin.
- White Blood Cells (Leukocytes): Destroy foreign elements, critical in immune system function.
- Platelets: Regulate blood clotting.
Cardiovascular Dynamics
Deals with how the Cardiovascular System adapts to meet demands placed on it, such as during exercise.
The heart adjusts the amount of blood pumped by altering:
- Heart Rate (HR): Duration of each cardiac cycle. Increases with exercise.
- Stroke Volume (SV): Volume of blood ejected by the left ventricle in a single beat (mL).
SV is regulated by:
- LVEDV (Left Ventricle End Diastolic Volume)
- Aortic Pressure
- Strength of Ventricular Contraction
Changes in any of these affect SV.
SV = LVEDV – LVESV
- Left Ventricle End Diastolic Volume: Amount of blood in the left ventricle after the contraction of the left atrium.
- Left Ventricle End Systolic Volume: Amount of blood remaining in the left ventricle after the contraction of the ventricle.
- Note: The ventricle never empties completely.
Frank-Starling Law
High LVEDV leads to ventricular stretching, causing a more forceful contraction and ejecting more blood from the ventricle.
Cardiac Output (Q)
The volume of blood pumped out of the left ventricle in 1 minute (L/minute). Rest: 5-6 L/min, Exercise: 30 L/min.
HR + SV = Q
Ejection Fraction
The proportion of blood ejected from the left ventricle during a single heartbeat. Increases with exercise.
Cardiovascular Dynamics and Venous Return
The most important factor regulating Stroke Volume (SV) is the amount of blood returned to the heart, also known as Venous Return.
During exercise, venous return increases due to four main factors:
- Constriction of the Veins (Venoconstriction): Veins have small amounts of smooth muscle that contract during exercise, reducing blood in veins and directing it toward the heart.
- The Skeletal Muscle Pump: Working muscles squeeze veins, sending blood to the heart.
- The Thoracic Pump: Breathing creates pressure that forces blood through veins.
- Nervous Stimulation of the Heart: During exercise, this leads to increased HR and increased force of contraction in the heart.
Blood Flow Distribution
During exercise, working skeletal muscles have an increased need for O2. The cardiovascular system meets this need in two ways:
- Increase in Cardiac Output (Q)
- Redistribution of Blood Flow: Blood flow is increased to working muscles while being decreased to less active organs (e.g., stomach, intestines, kidneys). Blood flow depends on the intensity of the exercise. The higher the intensity, the more flow to skeletal muscles.
The Effects of Training on the Cardiovascular System
Regular Aerobic Training leads to many cardiovascular benefits, including:
- Alterations in Heart Structure: The heart grows in weight and dimensions. Ventricular volume increases along with ventricular muscle, likely due to the constant increase in venous return seen with training.
- Increased Capillaries to the Myocardium
- Increased Diameter of Coronary Arteries
- Increased Blood Volume (Plasma & Erythrocytes): Leads to increased venous return, increased ventricular stretching, increased SV, increased Q, and increased ventricular muscle.
- Bradycardia: The decrease of HR at rest and during sub-maximal exercise, but unchanged at maximal exercise (60 bpm or less). 100 bpm or more = Tachycardia.