Blood is transported through the venous system by means of:
(a) Skeletal muscle contractions
(b) Decreasing blood pressure
(c) The respiratory pump
(d) A and C

Verified step by step guidance

1. Step 1: Understand the venous system's role in transporting blood. Veins carry blood back to the heart, often against gravity, and rely on mechanisms other than direct pumping by the heart.
2. Step 2: Analyze the role of skeletal muscle contractions. When skeletal muscles contract, they compress nearby veins, pushing blood toward the heart. This is known as the 'muscle pump' mechanism.
3. Step 3: Examine the concept of the respiratory pump. During inhalation, the pressure in the thoracic cavity decreases while abdominal pressure increases, creating a pressure gradient that helps move blood toward the heart.
4. Step 4: Evaluate the role of decreasing blood pressure. Blood pressure decreases as blood moves through the venous system, but this does not actively transport blood; instead, it is a passive result of circulation.
5. Step 5: Combine the mechanisms. Both skeletal muscle contractions and the respiratory pump actively assist in venous blood transport, making the correct answer 'a and c.'

Key Concepts

Venous Return

Venous return refers to the process by which blood is transported back to the heart through the veins. This process is crucial for maintaining adequate blood circulation and is influenced by several mechanisms, including skeletal muscle contractions and the respiratory pump, which help propel blood against gravity.

Skeletal Muscle Pump

The skeletal muscle pump is a mechanism that aids venous return during physical activity. When skeletal muscles contract, they compress nearby veins, pushing blood toward the heart. This is particularly important in the limbs, where blood must travel against gravity, especially when standing or exercising.

Respiratory Pump

The respiratory pump is a mechanism that enhances venous return during breathing. As the diaphragm contracts and the thoracic cavity expands during inhalation, the pressure in the thoracic cavity decreases, allowing blood to flow more easily from the veins into the heart. This process is vital for maintaining efficient circulation, especially during physical activity.