The Cardiovascular and Respiratory System

• Venous return in the deoxygenated blood returing to the heart.
• Starling's law states that 'stroke volume is dependent on venous return'.
• So, if venous return increases, stroke volume and cardiac output will also increase.
• If venous return decreases, stroke volume and cardiac output will decrease.
• The mechanisms that aid venous return are:
  • Pocket valves
  • Skeletal muscle pump
  • Respiratory pump
  • Smooth muscle
  • Gravity
• These mechanisms help to maintain or increase venous return during exercise to make sure that cardiac output and stroke volume are suffient to supply the demand for oxygen during exercise.
• Blood pooling: If there is not enough force to push blood back to the heart it causes blood pooling.
• In order to prevent blood pooling, an active cool down should be done.

Exam style answer:

Venous return is the volume of blood returning to the heart via the veins per beat. The problem with this is that most of the blood has to travel against gravity and there is low blood pressure in the veins. In order to aid venous return, the skeletal muscle pump helps to squeeze blood back to the heart. Also, the pocket vavles prevent back flow of blood and the respiratory pump helps to pull blood back up to the heart during inspiraion. The smooth muscle around the vein causes them to venoconstrict. The venomotor decreases the lumen diameter which in turn increases the pressure in the veins. During exercise, venous return increases. Stroke volume and cardiac output also increases because stroke volume is dependent on venous return. This is known as Starling's law. The increase in venous return means that more blood enters the atria, this causes the atrial walls to stretch. This stimulates the SA node which in turn increases the heart rate.Ventricular walls also stretches as more blood enters; this increases the end diastolic volume and the sysolic volume decreases. Therefore stroke volume increases. An effective venous return will lead to a faster revcory rate and it can delay lactate threshold.