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 function, which is to return deoxygenated blood back to the heart, often against gravity, especially from the lower parts of the body.
2. Step 2: Recognize that veins have valves to prevent backflow, but they rely on external mechanisms to help push blood toward the heart since the pressure in veins is relatively low compared to arteries.
3. Step 3: Identify the role of skeletal muscle contractions (option a) in compressing veins during movement, which helps propel blood forward through the valves—this is often called the 'muscle pump.'
4. Step 4: Understand the respiratory pump (option c), which involves pressure changes in the thoracic cavity during breathing that help draw blood into the thoracic veins and the right atrium.
5. Step 5: Evaluate the options: decreasing blood pressure (option b) does not directly aid venous return; instead, the combination of skeletal muscle contractions and the respiratory pump (option d) effectively assists venous blood flow.

Key Concepts

Skeletal Muscle Contractions

Skeletal muscle contractions help propel blood through veins, especially in the limbs, by compressing the veins and pushing blood toward the heart. This mechanism is often called the 'muscle pump' and is crucial for venous return against gravity.

The Respiratory Pump

The respiratory pump involves pressure changes in the thoracic cavity during breathing. When you inhale, decreased thoracic pressure and increased abdominal pressure help draw venous blood toward the heart, aiding venous return.

Venous Blood Pressure and Flow

Venous blood pressure is generally low and does not decrease to drive blood flow; instead, mechanisms like muscle contractions and the respiratory pump assist venous return. Understanding that decreasing blood pressure alone does not transport venous blood is key.