The cardiovascular system is an interesting biological system of the human body.
As the heart pumps, it pushes oxygenated blood throughout the body via arteries and arterioles. The contraction that pumps the blood creates a subsequent vacuum within the heart.
When the heart relaxes, this vacuum literally sucks deoxygenated blood back to the heart via capillaries and veins. This blood is then pumped from the heart to the lungs in order to release waste products from the body through the breath.
As you can imagine, a system that relies upon suction to draw waste products from the outer reaches of the physical body is not 100 per cent efficient. Some waste products are left behind in the extremities of the body, namely the legs and feet.
The system continues, and without periodic intervention, this residual waste buildup can often lead to less and less oxygen gas exchange to these extremity muscles.
This results in a weakening of the muscle endurance in these areas. This is precisely why yogis intersperse inversion poses throughout their asana practice.
Inversion poses, such as shoulder stands, headstands and plough pose allow a periodic intervention within the cardiovascular system.
In these poses, the legs and feet are now positioned above the heart.
In this inverted position, considerably less suction is required to draw the toxins out of the legs and feet as gravity helps to drain them.
When the yogi comes out of a held inversion pose, fresh oxygenated blood flows easily into the drained areas.
The tissue there, now cleansed of waste products, is better able to uptake the nutrients within this new blood flow.
Inversion poses also activate the lymphatic system which increases the body's ability to drain larger, non-gaseous waste products out. An increased lymphatic system goes a long way to reducing inflammation in the body.
As beneficial as inversion poses are, however, they are not for everyone. Anyone suffering from glaucoma or detached retinas, vertigo, severe structural asymmetries, previous spinal injuries (such as slipped discs), high blood pressure, and/or recent facial surgery should avoid inversion poses.
Pregnant students who do not suffer from any of the above ailments and who have received their medical practitioner's permission, may practise inversion poses but should limit the length of time they spend in any inverted pose.
Pregnant students may also require modifications for these poses, such as more basic versions supported against a wall or chair. As the pregnancy advances, your body and its conditions will change.
As a result, it is important a pregnant student continue to check in with your medical practitioner along the way to determine whether or not you should continue practising inversion poses at each stage.