The Role of the Cardiovascular System

1. The cardiovascular system, also known as the circulatory system, is responsible for the continuous circulation of blood throughout the body.
2. It delivers essential substances such as oxygen, nutrients, and hormones while removing metabolic waste.

Functions of the Cardiovascular System in Transport

Transport of Nutrients

1. After digestion, nutrients like glucose, amino acids, and fatty acids are absorbed in the small intestine and enter the hepatic portal vein, which carries them to the liver for processing.
2. The liver regulates the distribution of these nutrients into the bloodstream for cellular use.
3. Glucose is transported via the blood to cells for ATP production through cellular respiration.
4. Amino acids are used for protein synthesis, and fatty acids are utilized for energy storage or cell membrane formation.

Transport of Hormones

1. The endocrine system releases hormones into the bloodstream to regulate body functions.
2. Hormones are chemical messengers that bind to specific receptors on target organs.

Examples of hormones transported via the bloodstream

1. Insulin: Secreted by the pancreas to regulate blood sugar levels.
2. Adrenaline (epinephrine): Released by the adrenal glands in response to stress, increasing heart rate and blood flow.
3. Thyroxine (T4): Produced by the thyroid gland, regulating metabolism and body temperature.

Gas Exchange

1. Oxygen (O₂) is inhaled into the lungs, where it diffuses into capillaries surrounding the alveoli.
2. O₂ binds to hemoglobin (Hb) in red blood cells and is transported to body tissues.
3. In tissues, O₂ is released from hemoglobin and diffuses into cells for aerobic respiration, producing ATP.
4. Carbon dioxide (CO₂), a waste product of metabolism, diffuses out of cells into the bloodstream:
   1. 7-10% dissolves in plasma.
   2. 20% binds to hemoglobin (as carbaminohemoglobin).
   3. 70% is converted into bicarbonate ions (HCO₃⁻) for transport.
5. CO₂ is expelled through the lungs during exhalation.

Heat Distribution and Thermoregulation

Blood plays a key role in maintaining homeostasis by redistributing heat.

1. Vasodilation (Heat Loss): Blood vessels widen, increasing blood flow to the skin to release excess heat (e.g., sweating).
2. Vasoconstriction (Heat Retention): Blood vessels narrow, reducing heat loss by redirecting blood to vital organs (e.g., in cold environments).

Waste Removal

1. Carbon dioxide is transported via blood and expelled through the lungs.
2. Urea, a waste product of protein metabolism, is filtered out by the kidneys and excreted in urine.
3. Lactic acid, produced during anaerobic respiration, is carried to the liver for breakdown into glucose via the Cori cycle.

Components of the Cardiovascular System in Transport

Variability in Cardiovascular Responses

1. The cardiovascular system adapts dynamically to maintain oxygen delivery, nutrient transport, and waste removal based on physiological demands.
2. Heart rate (HR), stroke volume (SV), cardiac output (Q), blood pressure (BP), and blood redistribution vary due to several factors, such as:
   1. Age
   2. Sex differences
   3. Body size
   4. Fitness level
   5. Type and intensity of physical activity

Key Cardiovascular Variables

Heart Rate (HR)

1. Resting HR: 60–80 bpm (lower in trained individuals).
2. Exercise HR: Can rise up to 200 bpm in intense activity.
3. Recovery HR: Returns to normal post-exercise (faster recovery = better fitness).

Factors Influencing HR

1. Age: Maximal heart rate decreases with age.
2. Sex: Females tend to have a higher resting heart rate compared to males.
3. Fitness level: Athletes usually have a lower resting heart rate due to higher stroke volume and efficiency.
4. Activity type and intensity: High-intensity activities cause a rapid increase in HR.

Stroke Volume (SV)

1. At rest, SV is around 70 mL per beat.
2. During exercise, SV can increase to 100–120 mL per beat.
3. Trained individuals have a higher stroke volume, meaning their heart pumps more blood per beat.

Factors Influencing SV

1. Fitness level: Trained individuals tend to have higher stroke volume due to the larger ventricular size and improved heart contractility.
2. Exercise intensity: During exercise, stroke volume increases to meet the higher demand for oxygen.
3. Body position: Lying down tends to increase stroke volume compared to standing.

Cardiac Output (Q)

1. At rest, cardiac output is about 5 L/min.
2. During exercise, it can increase to 20–40 L/min, depending on intensity and fitness.

Q=HR×SV

Factors Influencing CO

1. Fitness level: Trained athletes often have higher cardiac output due to increased stroke volume, compensating for lower heart rate at rest.
2. Exercise intensity: Cardiac output increases significantly with exercise intensity, ensuring sufficient oxygen delivery to muscles.

At Rest vs. During Exercise

Blood Pressure (BP)

BP is expressed as:

BP= Systolic Pressure/Diastolic Pressure

Readings:

1. Normal BP: 120/80 mmHg
2. Hypertension (High BP): 140/90 mmHg
3. Hypotension (Low BP): <90/60 mmHg

BP and Exercise

1. Systolic BP increases during exercise due to greater cardiac output.
2. Diastolic BP remains stable or slightly decreases.
3. Aerobic training lowers resting BP over time.

Factors Influencing BP

1. Exercise intensity: Higher intensity results in higher systolic BP.
2. Fitness level: Athletes generally have a lower resting BP and may experience smaller increases in BP during exercise.
3. Age and sex: Older individuals tend to have higher resting BP, and women may have lower BP than men at rest.

Blood Redistribution During Exercise

During exercise, blood is redistributed to prioritize working muscles and vital organs (such as the heart and lungs), while blood flow to other areas (such as the digestive system) is reduced.

1. Vasoconstriction: Blood vessels to non-essential organs constrict (e.g., digestive organs) to reduce blood flow.
2. Vasodilation: Blood vessels to the muscles dilate, increasing blood flow to the active muscles, providing oxygen and nutrients.
3. Sympathetic Nervous System Activation: Increases vasoconstriction to non-exercising organs and promotes vasodilation to working muscles.

The Cardiac Cycle

1. The cardiac cycle refers to the sequence of events that occurs in the heart during one complete heartbeat.
2. It involves the contraction and relaxation of the heart chambers (the atria and ventricles) to pump blood throughout the body.

Key Phases of the Cardiac Cycle

Diastole

1. This is the relaxation phase of the cardiac cycle, during which the heart fills with blood. The atria contract to push blood into the ventricles.
2. The AV valves (tricuspid and mitral) are open, and blood flows from the atria into the ventricles.
3. The heart is at rest, preparing for the next contraction.

Systole

1. This is the contraction phase, where the ventricles contract to pump blood into the arteries.
2. The semilunar valves (pulmonary and aortic) open as the ventricles contract, and blood is ejected from the ventricles to the lungs (via the pulmonary artery) or the rest of the body (via the aorta).

End of Systole

The ventricles relax, and the cycle begins again with diastole.

During Exercise

1. Heart Rate (HR) increases, causing the cardiac cycle to speed up. This means the heart beats faster, and both diastole and systole shorten.
2. However, during intense exercise, diastolic phase (ventricular filling) can shorten, which may limit the amount of blood entering the heart.
3. This is why stroke volume (the amount of blood pumped per beat) can be more significant than heart rate for maintaining cardiac output during exercise.