Thursday, October 25, 2012

you're so vein

Arteries and veins are the major vessels that carry blood to your body parts. Arteries carry blood away from the heart, while veins carry blood back to your heart. Although these vessels seem to be identical mirror images of each other, they are certainly not. There are profound differences between arteries and veins.

Arteries are more highly pressurized the veins, since they are at the beginning of the circuit and carry blood being pumped right out of the heart. Like a garden hose with one of the spray triggers on the end: the water pressure is greatest right at the opening of the trigger, and less the farther away from the opening you get. To account for this pressure, the walls of the arteries have a layer of muscle that contract in rhythm with your heart to absorb the pressure waves (so when you feel your pulse in your wrist, it is this muscular contraction you are feeling rather than the pressure wave from your heart).

Veins are not muscularized and so do not assist in pumping blood back to the heart. On top of not having any muscular getty-up, the blood they carry is under little pressure on its way back to the heart. And furthermore, blood has to fight gravity on its way back to the heart, seeing as most of the body is below the position of the heart. So what makes blood "go" in the veins?

Skeletal muscle contraction squeezes it back through the veins. Walking, running, playing hackysack (that’s what the kids are doing these days, right?), any motor activity squeezes the blood like toothpaste through your veins. But there is still the low pressure issue, so there must be a mechanism in place to keep backflow from occurring- especially in the legs, where gravity is pulling it back down towards the feet. Once again, nature has got it covered.
Left: valve open, blood moves forward
Right: valve closed, blood is stationary

Your veins have little one-way valves in them that open when blood is squeezed forward, and shut when pressure drops and gravity starts to pull it back down. These valves are passive, meaning they require no expenditure of energy and no innervation. They run off of gravity and blood pressure generated by skeletal muscle contraction. An elegant solution. Unless, these valves fail. 

If the valves become worn out and do not close completely, backflow of blood occurs. This pooling of blood generates little out-pockets along the veins. Fairly benign, but unsightly. We call these "varicose veins." 

Two questions for you:

1. When you sleep and are not moving, how is blood squeezed back through your veins? Hint: Latin word for "partition"

2. Say you were in a severe car accident. The powers that be give you the option of having either an artery or a vein severed. Which would you choose and why?

Friday, October 19, 2012

stop it, your bib is turning me on

Male house sparrow (

We all have those certain “turn-ons” that get us going. For me, a solid set of shoulders and manly forearms catch my eye. And a little stubble never hurt anyone. And dark hair. Oh, and strong hands.  And a defined jaw. But I digress. Unlike human males, male house sparrows have black patches on their chin and upper chest, known as badges. It looks like they’re wearing little bibs. Some males have large ones, some have smaller ones (he he). Now, when one sex has a characteristic that the other doesn’t, one must always question if it plays a role in sexual attraction.

My taste for square shoulders and strong forearms is clearly linked to fitness. The latent cavewoman in me sees a potential mate who can build shelter and fight off threatening people or animals. Therefore, it is in my and my future offsprings’ best interest for me to be attracted to a male who is physically strong. But how is a black bib linked to fitness? Does it function in camouflage? Make the bird healthier in some way? What’s the deal?

A study by behavioral zoologist Anders Moller took a closer look at house sparrow badges. Come to find out, males with larger badges occupied prime real estate with more nesting sites. Territory defended by males with big badges had safer nests and fewer hatchling fatalities. So this showed that the bigger the badge, the more successful the offspring. But on top of that, Moller found that the big-badged males were… well, pimps. He pumped some females full of estradiol to get their lebidos up, and they were all over those big-badged males like a cheap suit. Poor little small-badged males were just sitting there, dejected and alone. Like me at my eighth grade dance.

So, back to the question. How does a badge function in fitness? Moller’s study reveals that the badge is a signal of fitness, but not a direct determinant. Which is really neat, I think. These types of characteristics- ones that signal but don’t directly function in fitness- show the nuances of evolution that go deeper than “survival of the fittest.” That black bib didn’t make ancient sparrows more likely to survive. Somewhere along the line, by chance, ancestral sparrow males with bigger badges happened to have more fit offspring frequently enough to make it significant. Then genetic drift began to waltz with selective pressure and chance, and before you know it, having a big black throat patch makes you an irresistible house sparrow.

Human sexual attraction seems to be so much more complicated than house sparrows. Do we have fitness signals? If so, what might they be? How do they compare to badges of the house sparrow?

Here's the Moller paper. Ya' know, for fun.

Volume 22, Number 5 (1988), 373-378

Monday, October 8, 2012

i'm feeling a little gassy

Fancy me this.

Let’s say you were to fill a Ziploc baggie partly with air and a little bit of solid mass. What would happen if you put it in a tank of water? Depending on the air:solid ratio, it would settle at one particular depth and stay there. Try to force it down, it will float back up. Pull it up, it will sink back down. To make it rest at a different depth, you have to alter the air:solid ratio. But in a closed system like a Ziploc- or a fish- how can buoyancy be adjusted to change depth?

In most fish, there exist buoyancy-control organs called the swim bladder. In some fish, the swim bladder is connected to the digestive tract and the fish can gulp air at the surface to expand the bladder, expel it to contract the bladder. So in that case, the fish is not exactly a “closed system”. The more air the fish gulps, the higher it settles in the water column. If it wants to descend, it burps some out.

In other fish, the swim bladder is unconnected to any other system. No amount of burping or gulping air will affect the fish’s buoyancy. Instead, a gland moderates how much gas fills the swim bladder. Fittingly, it is called the gas gland and excretes both lactic acid and carbon dioxide. The resulting reactions cause hemoglobin to release its oxygen from the blood stream, which then inflates the swim bladder. Up the fishy goes.

Oxygen diffuses back into the bloodstream and goes home to its hemoglobin at another little structure called the oval window. Down the fishy goes. And then the cycle is free to start again.

Although the sea kind of freaks me out, I do think it’s pretty neat that its residents must locomote in one more dimension than us. Our physiology does not need to accommodate “floatability.” Otherwise, we would have to evolve some sort of regulator like a swim bladder.