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 haemoglobin (Hb) in red blood cells and is transported to body tissues.
3. In tissues, O₂ is released from haemoglobin 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 haemoglobin (as carbaminohaemoglobin).
   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.

Blood

Components of Blood

1. Plasma (~55% of volume): Fluid part of blood; mostly water with dissolved nutrients, hormones, and waste.
2. Red blood cells (40–45% of volume): Also called erythrocytes; carry oxygen from the lungs to tissues and carbon dioxide back.
3. Haematocrit: The percentage of red blood cells in blood. Endurance athletes typically show higher values.
4. White blood cells (leucocytes): Defend the body against infection.
5. Platelets: Essential for blood clotting after injury.

Oxygen and Carbon Dioxide Transport in Blood

1. Oxygen transport: Only a small fraction dissolves in plasma. Most binds to haemoglobin inside red blood cells. In the lungs, where oxygen partial pressure is high, it binds easily. In active muscle, where partial pressure is lower, it detaches and diffuses into the tissue.
2. Carbon dioxide transport: About 7–10% dissolves in plasma, 20% binds to haemoglobin as carbaminohaemoglobin, and 70% travels as bicarbonate ions (HCO₃⁻) in plasma.
3. Total blood volume: Approximately 5 litres in a 70 kg adult.

Blood Vessels and Circulation

Arteries, Capillaries and Veins

1. Arteries: Large diameter with thick muscular walls. Carry blood away from the heart under high pressure and branch into smaller arterioles.
2. Capillaries: Very narrow vessels with very thin walls. Form a branching network through tissues and are the site of exchange between blood and tissues.
3. Veins: Carry mostly deoxygenated blood back toward the heart. Less muscular than arteries, contain valves to prevent back-flow, and operate at lower pressure.

Pulmonary and Systemic Circulation