The Effects of Immersion on the Cardiovascular and the Respiratory System

[David Novo]

Following on a previous topic of the SSI Science of Diving course, I found the explanation on the effects of immersion on the cardiovascular and the respiratory system a bit confusing. Below is what is stated in the course:

Residual Volume. The amount of air that remains in the lungs after maximal expiration. Another way to define this term would be the volume of air from maximal expiration to total collapse.

Expiratory Reserve Volume. The amount of air which can be exhaled after the normal exhalation.

Functional Reserve Capacity. The amount of air remaining in the lungs after a normal exhalation.

1. The water pressure shifts blood to the thorax.
2. Abdominal compression displaces the diaphragm upwards. This limits lung expansion.
3. Almost a liter of blood shifts from the limbs to the thoracic region. This expands the vascular space in the chest area. The expansion of vascular space impinges on alveolar volume.
4. Residual volume and expiratory reserve volume are decreased.
5. The functional reserve capacity is decreased.
6. The volume at which the alveolus will close is reduced (closing volume.) Normally, with even exhalation, most of the alveoli remain open. Aging can increase the number of alveoli that remain closed, causing poor lung ventilation and impairing gas exchange. The diver can partially offset poor lung ventilation by full complete exhalations.
7. Vascular shifting increases blood return to the heart. This increased return will cause a larger cardiac output resulting in higher pulmonary arterial pressure.

My understanding/doubts from what I have been reading so far is:.

1. The blood shift to the thorax due to the mammalian reflex. However, for this, immersion is enough. How does pressure increase the amount of blood that flows to the thorax?
2. Abdominal area compresses due to the blood shifting to the thorax. Correct?
3. Directly follows from previous points.
4. My interpretation is that this happens due to 3 factors: 1) There is a lower alveolar volume; 2) The volume at which the alveolus will close is reduced; 3) The alveolus do not close all at the same volume. Is this correct?
5. Same as 4.
6. Why is the volume at which the alveolus will close reduced? This seems to be more a cause of points 4 and 5 than a consequence. The diver can partially offset poor lung ventilation by full complete exhalations -> my interpretation is that does this occurs by exhaling the air in the alveolus that do not close with normal exhalation and is partial because some alveolus will already be closed. Is it correct?
7. There is more blood at a given moment going through the lungs, so there is more arterial pressure. Why there is no increase in venous pressure? Blood is also getting to the lungs in larger quantities, no?

 

[Jcp2]

Diving to depth horizontally would increase blood into the chest cavity through the immersion reflex, with peripheral vasoconstriction, through compression of the abdomen from water pressure, and through the lack of blood pooling in areas of the body below the level of the heart.

 

[David Novo]

Diving to depth

What is the effect of depth / pressure in shifting the blood to thorax? The mammalian reflex occurs if the head is submersed, whether at depth or surface right?

horizontally

The effect is different if vertically? Why?

through compression of the abdomen from water pressure

Abdominal area compresses due to the blood shifting to the thorax or due to a different effect?

and through the lack of blood pooling in areas of the body below the level of the heart.

This is a result of vasoconstriction, which is a result of the mammalian reflex, right?

 

[Jcp2]

Gravity.

 

[David Novo]

Gravity.

This is the answer to which question?

 

[outofofficebrb]

In most cases, the water temperature contributes to blood being shifted to the thoracic region as a way to stay warmer. vs having it in the hands/arms/legs/feet.This happens even if you are wearing a wetsuit and are comfortable in the water.

 

[David Novo]

In most cases, the water temperature contributes to blood being shifted to the thoracic region as a way to stay warmer. vs having it in the hands/arms/legs/feet.This happens even if you are wearing a wetsuit and are comfortable in the water.

This makes sense but does not explain the effect of pressure mentioned by SSI.

 

[outofofficebrb]

Alert Diver had this to say, though it is an article about diuresis and mentions thoracic blood volume.

"When you lie down on land, blood pressure in your arms and legs is about equal. When you stand, leg blood pressure rises because of the weight of blood in the vessels above. You get some pooling in your legs because of gravity, and because your veins (more than arteries) expand to hold more blood. Less blood flows back to your heart. When you immerse in water, effects on your blood volume are mostly counteracted by buoyancy. This blood pressure equalization (not squeezing from water pressure) increases thorax blood volume and diuresis. In space the pull of gravity is weak, and like buoyancy underwater, counteracts downward blood pressure. In space, blood does not pool in your legs. Astronauts and mission control scientists use a technical term for the headward fluid volume shift during space flight. They call it the "Fat-Face-Chicken-Legs-Effect."

 

[Ukmc]

Here is my understanding.
Increased pressure works as compression stockings and pushes “excess” of blood stored in the venous system towards the center (thorax). If cold, vasoconstriction will also contribute to decreased peripheral vascular reservoir.
Once blood shifted into thorax, it physically dilates intrathoracic venous volume, increased volume of right ventricle and atrium and thus increases cardiac preload which results in increased cardiac output. This will result in increased pulmonary arterial pressure (still venous blood) which will result in pulmonary vascular crowding which will decrease ability of alveoli to stay open if they already functioning borderline (eg older people).
As far as Expiratory volumes , they all are decreased because the volume of rib cage (thorax) is fixed, but intrathoracic volume is decreased by extra blood in the intrathoracic vascularature (dilated) and by diaphragm which is now sits higher by compression of abdomen.

 

[John C. Ratliff]

Do you realize that there is a difference in physiological response depending upon whether the diver is on scuba or is breath-holding and swimming to depth? I think the two are mixed up in the above posts. We need to know which type of diver is being profiled here.

SraRat