I’ve never understood how the kidneys work, or the salt glands in the noses of seabirds and marine iguanas.

The principle as usually described is that blood plasma is filtered to be ejected into the glands of the kidneys. And then the water is absorbed back into the body to concentrate the waste materials in the glands. The filtration is the easy bit, the difficulty I have is that absorbing the water back requires reverse osmosis. Reverse osmosis is very energy intensive, how is the energy applied?

I can’t believe that the water is absorbed back by active transport through cell walls. Many large chemicals are transferred through cell walls using active transport. But water?

The only other alternative I can see is how reverse osmosis works in industrial plants, by the application of high pressure. But where does this pressure come from in the kidneys, or in salt glands of marine vertebrates? The high pressure would have to be generated by muscles, or the intracellular equivalent of muscles – microtubules. Seawater has a natural osmotic pressure of 27 bar, so the salt glands of seabirds and marine iguanas would have to produce a higher pressure than that. Urine is 97% water. Seawater is 96.5% water.

The movement required for producing that pressure got me wondering about other little-known things that move in humans. The heart as a pump and skeletal muscles immediately come to mind. Peristaltic movement of the intestines. Roundworms and other nasties propel themselves through our digestive system. On a smaller scale the muscles of goosebumps. On a cellular scale we hare the cilia. Non-motile cilia are found in the inner ear where they sense sound waves (pressure waves).

Motile cilia are found in the lungs and trachea, where they help to clear the lungs of foreign substances by transporting them up the trachea.

From wikipedia:

“Most vertebrate cell types possess a single non-motile primary cilium, which functions as a cellular antenna. Olfactory neurons possess a great many non-motile cilia.

“Motile cilia are found on respiratory epithelial cells – around 200 cilia per cell, where they function in mucociliary clearance, and also have mechanosensory and chemosensory functions. Motile cilia on ependymal cells move the cerebrospinal fluid through the ventricular system of the brain. Motile cilia are also present in the fallopian tubes of female mammals where they function in moving the egg cell from the ovary to the uterus. Motile cilia … are responsible for the left-right asymmetry in bilateral animals.”

Flagella are most common in sperm.

There is also the movement of stem cells through the body, for example replacing overused olfactory cells in rats, and replacing overused brain cells in humans.

What else moves inside human beings?

cheers, has me a read of that, I will, master moll, half interesting and more

https://en.wikipedia.org/wiki/Cilium

“….The cilium (PL: cilia; from Latin ‘eyelash’), is a membrane-bound organelle found on most types of eukaryotic cell. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projection that extends from the surface of the much larger cell body. Eukaryotic flagella found on sperm cells and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion.

There are two major classes of cilia: motile and non-motile cilia, each with a subtype, giving four types in all. A cell will typically have one primary cilium or many motile cilia. The structure of the cilium core called the axoneme determines the cilium class. Most motile cilia have a central pair of single microtubules surrounded by nine pairs of double microtubules called a 9+2 axoneme. Most non-motile cilia have a 9+0 axoneme that lacks the central pair of microtubules. Also lacking are the associated components that enable motility including the outer and inner dynein arms, and radial spokes. Some motile cilia lack the central pair, and some non-motile cilia have the central pair, hence the four types.

Most non-motile cilia are termed primary cilia or sensory cilia and serve solely as sensory organelles. Most vertebrate cell types possess a single non-motile primary cilium, which functions as a cellular antenna….”

Reverse osmosis is very energy intensive, how is the energy applied?

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The sodium–potassium pump is apparently key to the active transport.

“The Na⁺/K⁺-ATPase enzyme is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported. Thus, there is a net export of a single positive charge per pump cycle. The net effect is an extracellular concentration of sodium ions which is 5 times the intracellular concentration, and an intracellular concentration of potassium ions which is 30 times the extracellular concentration.”