Sodium Concentration Controls Fluid Transport

Biological Strategy

Mammals

Eleanor Banwell

Image: Don Whitaker / Flickr / CC BY NC - Creative Commons Attribution + Noncommercial

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In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Regulate Reproduction or Growth

Reproduction and growth are two physiological processes that occur in all living systems. There are situations when conditions are right for both, and other situations when continuing either harms the living system because both have a very high energy cost. Reproduction and growth are unique in that both can stop until conditions improve, although stopping either for an extended time can cause problems. An example of regulating reproduction is a process called delayed implantation or embryonic diapause found in some mammals, such as otters. An otter’s embryos sometimes temporarily cease developing and won’t develop further until the female senses that conditions are suitable.

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Regulate Cellular Processes

Cells are the basic building blocks of all living systems, so cellular processes dictate how physiological processes occur within those systems. Cells (whether entire unicellular organisms or parts of multicellular living systems) grow, metabolize nutrients (that is, chemically transform them), produce proteins and enzymes, replicate, and move. Cells as part of multicellular systems rarely act alone, instead having ways to signal to start and complete simple to quite complex interactions. How skin heals is a good example of the role of cellular processes. Blood cells called platelets release clotting factors to stop the bleeding; white blood cells rid the area of foreign materials and release molecules to coordinate healing; cells called fibroblasts start rebuilding using proteins called collagen; new blood vessels form; and skin cells called keratinocytes create the new surface.

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Mammals

Class Mammalia (“breast”): Bats, cats, whales, horses, humans

Mammals make up less than 1% of all animals on earth, but they include some of the most well-known species. We know first-hand some of the characteristics that make mammals unique, like having hair, being able to sweat, and producing milk through mammary glands. Another critical shared feature is a set of highly-specialized teeth. Unlike sharks or alligators, for example, whose teeth are generally all the same size and shape, mammals have differently shaped teeth in different areas of the jaws to target specific foods or foraging strategies.

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Sodium channels in mammalian cells regulate fluid transport by cilia via changing fluid levels.

Introduction

Cilia are tiny hair-like tufts on the surface of cells. They are found in many eukaryotes, from single-celled organisms to large animals. In humans they are found on the cells that line the airways, in the oviduct that carries the egg to the uterus, and in the ventricles (cavities) in the brain. Cilia beat in a coordinated rhythm in order to transport materials in one direction. In the airways, they clear contamination from the lungs with the help of mucus, in the oviduct they transport the egg, and in brain ventricles they generate currents in cerebrospinal fluid.

The Strategy

Beating cilia have two distinct phases: a power stroke that moves liquid forward, and a return stroke that prepares them for the next power stroke. Cilia rely on the surrounding liquid to sweep particles along with the flow. Without the appropriate quantity or consistency of liquid, the cilia cannot generate currents. As a result, transport can be regulated by controlling the liquid. In the oviduct, for example, liquid is only present during ovulation.

In order to alter the quantity and consistency of the liquid bathing the cilia, water must pass through the membranes of the cells that line the tissue. Water passes passively through the cell membrane depending on the concentration of salts on each side. Salt transport is tightly controlled by the cell and these ions cannot easily pass through the membrane. Water will pass through a semi-permeable membrane like that of the cell until the concentration of salts is the same on each side. In this way, if there is a higher concentration of salt on one side, water will move towards that side until the concentrations are balanced. Cells take advantage of this to regulate the amount of liquid that is bathing cilia. By actively pumping sodium ions into the space, they draw water out of the cell and bathe the cilia, promoting their action and allowing transport. To switch off cilia, cells pump sodium ions back inside the cell, drawing the water out of the space and preventing their action.

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The Potential

Several elements are at work in this strategy which could be useful to guide human innovation. There is the use of fluctuating bristles to control the movement of fluid (or the movement of a body through a fluid). There is the release of water to change the quality of the surrounding liquid to improve effectiveness of the beating cilia. Finally, there is the active pumping of sodium which controls the release of that water molecularly, without squeezing or pumping the water itself. Such techniques at a small scale could be used to propel nanomachines through the body for medical procedures. At a large scale, they could be used in specific locations and patterns to alter the hydrodynamics around parts of water-going vessels.

Last Updated April 19, 2018

References

“Motile cilia appear as tufts on the surface of epithelial cells that line body cavities in three organs: oviduct that transfers the ova from the ovary to the uterus, respiratory airways in the lung and brain ventricles (Lyons et al. 2006; Shah et al. 2009). Rhythmic beating of these cilia propels the fluid around the cilia” (Enuka et al. 2012:339)

“The volume and the composition of the fluid bathing the cilia are crucial for motile cilia function (Widdicombe 2002; Marshall and Kintner 2008). A major determinant of fluid flow in biological fluids is the relative osmolarity of the extracellular and intracellular liquids (Bourque 2008). In vertebrates, sodium ion and its counter-ions are the major determinants of the osmolarity of the interstitial fluid (ISF) and blood (Takei 2000). Therefore, changes in Na+ concentration affect both the volume and the flow of water across membranes to maintain osmolarity, and the volume of interstitial fluid.” (Enuka et al. 2012:339)

“Although the physiological functions of tissues with motile cilia are vastly different, the motile cilia have the same function and that is muco-ciliary transport. To carry out their function, the cilia need to be in a fluid environment, with a tightly regulated volume and fluid depth” (Enuka et al. 2012:352)

Journal article

Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways

Histochemistry and Cell Biology, 137(3): 339–353 | 01/03/2012 | Enuka Y, Hanukoglu I, Edelheit O, Vaknine H, Hanukoglu A

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Reference

“As it is well known, 60–70% of the human body weight is water. About 2/3 of this water is within the cells (intracellular fluid, ICF) and the remaining 1/3 fills the extracellular spaces and the vascular bed in the circulatory system (extracellular fluid, ECF) (Ruth and Wassner, 2006). The cell membrane, as a semi-permeable barrier, is permeable to water molecules. Yet, the net movement of water between ECF and ICF depends on the relative osmolarity of these compartments and the permeability of the membranes (Fischbarg, 2010).” (Hanukoglu and Hanukoglu 2016:95)

Journal article

Epithelial sodium channel (ENaC) family: Phylogeny, structure–function, tissue distribution, and associated inherited diseases

Gene, 579(2): 95-132 | 01/04/2016 | Hanukoglu I, Hanukoglu A

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