The circulatory system

Surface Area to Volume Ratio:

• Large organisms have a small surface area to volume ratio, making diffusion insufficient for transporting substances across their bodies.

• The distance for diffusion to reach the center of the organism is too great.

Double Circulatory System in Mammals:

• Mammals have a double circulatory system with two loops:

  • Pulmonary loop: Blood passes through the lungs to collect oxygen.

  • Systemic loop: Circulates oxygenated blood to the rest of the body.

Advantages of Double Circulation:

• • No mixing of oxygenated and deoxygenated blood ensures efficient oxygen delivery.

  • Blood can be pumped to the lungs and body under different pressures.

General structure of the circulatory system

Blood vessels

Arteries

• Carry blood away from the heart under high pressure.

• Key features:

  • Thick walls with elastic and muscle tissue to withstand high pressure.

  • Elastic tissue allows arteries to stretch and recoil.

  • Muscle tissue contracts to maintain pressure and regulate flow.

  • Small lumen helps maintain high pressure.

  • Smooth endothelial lining reduces friction.

Arterioles

• Transport blood from arteries to capillaries.

• Key features:

  • Lower pressure than arteries.

  • Thick muscle layer contracts to narrow the lumen and regulate blood flow.

  • Less elastic tissue compared to arteries.

Capillaries

• Site of exchange between blood and tissues.

• Key features:

  • Made of a single layer of endothelial cells for a short diffusion distance.

  • Small lumen forces red blood cells to squeeze, reducing diffusion distance.

  • Gaps between endothelial cells allow movement of water, white blood cells, and substances.

  • Provide a large surface area for exchange.

Venules

• Collect blood from capillaries and transport it to veins.

Veins

• Carry blood back to the heart under low pressure.

• Key features:

  • Thinner walls than arteries.

  • Blood flow aided by skeletal muscle contraction, compressing veins.

  • Valves prevent the backflow of blood.

The structure of arteries and veins

Formation of tissue fluid

Role of Tissue Fluid:

• Surrounds body cells, facilitating the movement of substances between cells and capillaries.

How Tissue Fluid is Formed

1. High Hydrostatic Pressure at Arterial End:

   • Forces water and small dissolved substances (e.g., oxygen, glucose) out of capillaries into tissue fluid.

   • Large proteins and blood cells remain in capillaries.

2. Exchange of Substances:

   • Oxygen and glucose diffuse from tissue fluid into cells.

   • Carbon dioxide and urea diffuse out of cells into tissue fluid.

   • Hormones, antibodies, and ions also move into tissue fluid for their functions.

3. Low Hydrostatic Pressure at Venule End:

   • As water leaves, hydrostatic pressure decreases in capillaries.

   • Water potential in blood plasma becomes lower than in tissue fluid.

   • Water re-enters capillaries by osmosis.

   • Carbon dioxide and waste products enter by diffusion.

4. Excess Water:

   • Drains into the lymphatic system and re-enters the blood via the superior vena cava.

The formation of tissue fluid

Risk factors and cardiovascular disease

Risk Factors for Cardiovascular Disease (CVD)

• Common risk factors:

  • High blood pressure (hypertension).

  • High levels of LDL cholesterol.

  • Smoking, lack of exercise, and an unhealthy diet.

  • Genetic predisposition or age.

Evaluating Conflicting Evidence

• CVD risk factors often involve complex interactions. For instance:

  • Some studies suggest moderate alcohol consumption reduces the risk of heart disease, while others warn of increased risks with excessive consumption.

  • Interpreting conflicting data involves considering sample size, methodology, and potential biases in studies.

Correlations and Causal Relationships

• Correlation does not equal causation: For example:

  • A correlation between high salt intake and high blood pressure does not necessarily mean salt is the sole cause. Other factors, such as genetics, may play a role.

• Causal relationships: It is important to identify evidence that directly links risk factors (e.g., smoking damages the endothelium, leading to atherosclerosis).

Definitions