Just couldn’t help myself..you know how it is. Magically Yours, Forgotten Physiology 🙂

Surprising isn’t it, that we’ve never talked about the kidneys until now. Well sure there were a few times that I might have mentioned them in other articles, but kidneys should never play supporting roles. We’re not talking Mary Kate and Ashley. No sir, the kidneys are Siegfried and Roy, center stage in Vegas with white tigers jumping through rings of fire.They deserve the full, Maxim magazine centerfold treatment.

The Low Down

CRAZY SIMPLIFIED – aorta branches to feed kidneys blood which do their filter thing, make the pee which drains through ureters and empties into bladder. Nothin but net!

Let me level with you for a minute. Urine…it’s nothing to be afraid of. It is the ultra filtrate of blood plasma. “Pee” is the result of a highly specialized filtration system, the kidneys, receiving a rich blood supply from the abdominal aorta. They selflessly separate the good stuff from the bad and send the filtered blood, squeaky clean to the vital organs. What waste products are left over leave the body as urine. A large component of urine is urea which is a substance representing the end of nitrogen metabolism. It’s actually odorless but when the stuff comes into contact with water or water vapor we get ammonia from the reaction which definitely has its own pungent, sweaty gym sock charm. So If I were ever tempted to give you the cliff notes summary of what the kidneys do I would say that they remove soluble waste or excess metabolic byproducts from the blood by concentrating them into urine.

No Dude, Seriously….

Friends don’t let friends forget about renal blood flow.

When serious people talk seriously about kidney function they DARN well better know that they need to mention filtration, reabsorption, and secretion. Otherwise they’re just wasting time, air, and parking space. So let’s do this thing right.

Filtration

The show kicks off with the renal artery which, branching off of the abdominal aorta, supplies blood to the whole operation (the kidneys). Like all fashionable arteries it branches into smaller vessels. STOP

[This is where we enter the microscopic neighborhood of the nephrons, the thousands of tiny functional units that make up the plumbing and wiring of the kidneys. Surprisingly, what does the work of each kidney is the collaborative effort of thousands of tiny nephrons. Your key words are: Tiny and Thousands. Please, please keep this in mind because these itty bitty nephrons perform some major league physiology. Ok back to the show.]

Those arteries having branched until they can’t branch any more now make up the afferent and efferent arterioles. These form a kind of yarn ball of capillaries called the glomerulus. The glomerulus forms such a fine mesh of blood vessels that not only are blood cells separated out of the blood but just about any substance exceeding a particular molecular weight to include most plasma proteins. Only things like water, sodium, the occasional glucose, and something called urea are all that slip through the gate and slide down the drain. Now that’s IF everything is working properly. This mesh of capillaries sits in a receptacle formed of cells called the Bowman’s capsule, which is kind of like the bowl basin of the sink.

The afferent arteriole sends blood to the glomerulus while the efferent arteriole carries blood away from the glomerulus and articulates into the peritubular capillaries that are going to surround the tubular portion of the nephron where they will play a super fun role in tubular reabsorption.

nice diagram right…took some serious google image surfing for this baby. Oh how the muscles burned!

Tubular Reabsorption

Ok so blood has passed, Brita filter like, through the gates of the glomerulus, but that was a nonspecific filter. Now we’ve entered the world of reabsorption which is way selective. To put this into perspective we’ve gone from the freckled, learners permit sporting, ticket stub tearer at the movie theater to the veteran TSA agent with white gloves, only 4hours of sleep, and an itchy trigger finger. When it comes to letting substances back into the blood post filtration, the body takes ONLY what the body needs (sodium, potassium, chloride, bicarbonate, glucose) any access gets the royal flush (secretion). When it comes to things like sodium there’s a combined effort between crazy natural forces like passive diffusion via osmosis and whatever mood our adrenal cortex is in. Let’s say it’s Monday afternoon and the body says “man, I sure could go for some more sodium.” So your adrenal cortex will secrete aldosterone which is like a chemical text message that directs the cells of the proximal convoluted tubule to absorb more sodium.

[Substances reabsorbed by PCT(proximal convoluted tubule) – sodium, potassium, water, urea, glucose, chloride]

The reabsorption of water is completely dependent on sodium reabsorption. This is where the whole osmosis thing comes into play because water is naturally going follow the flow of sodium ions, moving in the direction of higher concentrations, Tai Chi like to restore osmotic balance. Now of course, increasing water in the vessels (fluid volume) increases blood pressure. So if A+B=C then increasing sodium reabsorption increases blood pressure.

A change in blood pressure is sensed by the cells of the juxtaglomerular apparatus (another portion of the nephron) which secretes renin. Renin converts the protein angiotensinogen into…wait for it…angiotensin. This is the substance that signals the adrenal cortex to do its aldo thing in the first place. Sounds like one of them crazy feedback loops to me (another article I promise).

Thirst

Yes, your mind does play tricks on you. There is this clever thing that your hypothalamus does in response to high sodium concentrations in the blood. It tells you to drink more water. Meanwhile, it signals your pituitary to secrete ADH (antidiuretic hormone) which tells your distal convoluted tubule to reabsorb mas agua as well (my hypothalamus is bilingual BTW) rather than just dump it down the drain (excrete in urine).

the juxtaglomerular-whatchamacallit is a collection of cells in the neighborhood of the glomerulus

Loop of Henle

Best summer ever!

No, it’s not a 700ft water slide where they take your picture as you plummet half naked on a large rubber inner tube. It is the section of the nephron leading from the proximal convoluted tubule to the distal convoluted tubule and it represents some pretty badass architecture so show some respect! It is the sinking, looping construction of the Loop of Henle that allows urine to concentrate sodium, potassium, and chloride ions via a concentration gradient which allows water and valuable ions to be reabsorbed. This however, should be an article all to itself so..you may want to give this the Wiki treatment.

Secretion – Last Call

Ok so we came, we partied, we danced all night, and now the kidneys are saying “everybody out!” This is where the body rids itself of all those high and low molecular weight substances that got the reabsorption shaft (excess H2O, sodium, and urea/nitrogenous wastes).

Now you are probably saying “whoa now, there are way more substances that end up in urine after reabsorption.” Ok, there is a threshold that the kidneys reach. For example they reach a maximum capacity for how much glucose they can filter at one time and yep, the stuff slips through the glomerulus and if that concentration is high enough then the loop of Henle just doesn’t have the molecular horse power (energy) to pump glucose back into the blood.

And the moral of the story is….

Blood enters the kidneys to be maticulously filtered by thousands of microscopic, fancy pants structures called nephrons which filter soluble and insoluble substances from the blood, reabsorb vital substances like sodium, water, and glucose depending on the body’s need for it, and secrete excess waste by concentrating it into urine which flows through the ureters and empties into the bladder for the royal flush.

What’s that…you say there’s more to it than that? You bet your efferent arterioles there’s more, but for now stay classy, stay curious, and never stop learning my friends 🙂

“Yup, I’m old and mysterious and I could tell you stories that’ll make you sleep with the lights on.”

Oh yes, this is one sketchy organ. The spleen is the epitome of the mysterious old man the townsfolk all whisper about. No one really knows his story accept that he’s always lived in this town (our bodies). As far as organs go, the spleen hardly ever shows up on the 6 o’clock news. Most of us don’t know what it looks like, where it’s located, if it votes republican, democrat, or green party.

The spleen is one of those organs that helped us survive in the womb. When we are born and just fresh out of the wrapper like a snappy, hard pretzel our bodies were still adjusting to an oxygen rich environment. While we were still hitching a ride in Mommy Dearest our bones were still developing and not quite ready for the full time responsibility of being the primary producers of our blood cells (whites and reds). In the early stages of development our liver and spleen jump start us with all the oxygen supplying cells we need. As you develop, your organs pass the baton of blood cell production, or hematopoeisis, until this process is effectively carried out by the bone marrow. What a lot of folks don’t realize is that the spleen never stops looking after those red cells, in fact the spleen receives it’s own blood supply to act as blood filter extraordinaire. The spleen is an incredibly attentive, highly adapted filter to be more exact. Just calling it a filter is like saying the Iron Chef makes a mean peanut butter & jelly sandwich or that Yanni plays a mean chop sticks (which I’m sure he does). The liver receives a much larger blood supply and carries out more of the blood cleanup but the spleen is considerably more meticulous.

Ah, now why can’t anatomy be this cut and dry all the time?

The spleen can selectively remove the misshaped or malformed cells from circulation. To a red cell, shape and form is everything. They have to be firm and flexible to squeeze through vessels without leaking their precious hemoglobin payload. The spleen acts as a final physical exam for new reds entering the bloodstream. It squeezes them through tight fitting vessels and has it’s very own elite team of white cells to pick off trash along the red cell membranes(precipitated proteins or remnants of DNA), changing their shape, roughing them up. Cells that make it to the other side have essentially proven themselves worthy to deliver oxygen another day.

The spleen also acts as a kind of lymph node whereby a population of B and T cells reside to take part in the immune response. It has so many macrophages scouting the blood for trash that we typically classify the spleen as a lymphoid organ. It also houses a backup supply of blood for the body to compensate for blood loss.

So ok… yes, when someone has to have an enlarged or infected spleen removed (splenectomy) they can live a perfectly normal, healthy life without it. They are susceptible to a few more infections (especially the very young), but considering that they are an organ light they do pretty well in most cases.

This is my problem..I really don’t like referring to the spleen as an accessory organ or dispensable based only on the fact that we don’t die when we get them removed. I mean if science only considered something important based on whether or not something bad happened to us if it was gone then we would get nowhere as a civilization. So let me just say that perhaps the full functionality of the spleen is yet to be realized. Once upon a time someone said “there must be something in the air too tiny to see that can cause disease.” Science has always been wide open baby. Stay classy my friends and never stop learning.

I think it’s safe to say that the average American knows way more about the dating habits of Snookie than the basic functions of their own adrenal glands. My fellow Americans let’s beat the statistics shall we.

Jessica Biel – personal favorite tight leather sporting supporting actress

You know all this talk about adrenaline bores me to tears. Anytime anyone talks about the adrenal glands all you hear about is the “fight or flight” response. It’s like the preview to a summer blockbuster where all they show are the explosions and 15 seconds of car chases with tight leather wearing supporting actresses in the passenger seat (which I don’t mind so much). If anyone asked you what the movie is actually about, good luck. There is however, way more to this E-hollywood bad boy than running from a fight. He does the work. So right here and now we’re going to attempt to answer the question “Adrenal Cortex, what have you done for me lately?”

Anatomy 101

Your adrenal glands rest atop the kidneys like really ugly shoulder pads, secreting hormones into the bloodstream in response to stress. Anatomically speaking we’ve designated the adrenal gland into two distinct regions, the cortex and the medulla. The cortex (forming the exterior portion of the gland) is largely known for producing cortisol and aldosterone. The medulla (forming the center or guts of the gland) specializes in producing epinephrine and norepinephrine (one of the most notoriosly misleading names in physiology).

Fun fact – both epinephrine and norepinephrine contribute to the fight or flight response with a few subtle differences. So if you’ve been thinking this whole time that they were highschool rivals or that norepinephrine was Latin for “not friends with epinephrine” I promise that I won’t tell anyone…fingers crossed.

Also, why is it that the term cortex, which sounds a lot like “core” actually refers to the outside of the organ? Sorry, that just irritates me to no end.

Pop Culture

it happens all the time….one day a band makes it big, working their way to the top of the charts, but the next week it’s the lead guitarist who’s on the cover of Rolling Stone’s magazine. Whenever people talk about the human stress response to danger or exhiliration we tend to focus on the adrenal medulla, the site of epinephrine production A.K.A adrenaline. However, this is merely responsible for a short term response to stress…the increased heart rate, breathing rate, and rapid conversion of nutrients for the sudden burst of muscle action. We’ve forgotten all about the long term response to stress largely coordinated by the adrenal cortex, the electric bass player who never stopped setting the tempo.

Adrenal Cortex, boy you so crazy…

We like to talk about the big picture but the truth is that so many of our physiological responses begin with baby steps, minutia, small subtle changes that lead up to bigger, longer lasting events. If you want to affect an organ there is no secret doorbell with a note that says “push ringer to call the kidney.” Hormones like aldosterone act on the cellular level, and “talk” to the tiny functional units of the kidney.
One such hormone, aldosterone, produced by the zona glomerulosa of the adrenal cortex works on the distal convoluted tubule and collecting duct of the nephrons (tiny fuctional units of the kidneys) to increase the reabsorption of sodium from the plasma which in turn affects the water balance. Why do we care? This ultimately effects blood pressure. Your blood is more than 50% water (i.e. plasma) so since water makes up such a significant volume of the blood, influencing the salt/water concentration effects overall blood pressure. This again helps us to maintain metabolic balance.
Aldosterone production is triggered both by ACTH (adrenocorticortropic hormone) from our good friend the pituitary and angiotensin triggered by the juxtaglomerular cells (in the kidneys) when renal blood pressure drops (blood filtered through the kidneys).
Another fun trick the adrenal cortex has up its sleave is the production of cortisol. In response to prolonged stress. Cortisol is chiefly responsible for increasing blood glucose over time through glyconeogenesis, which is basically a means of utilizing non-carbohydrate sources such as lipids and amino acids to form glucose. This is just another way our bodies maintain homeostasis, that amazing zen-like balance of our metabolic needs. Cortisol also suppresses the immune system. This makes it quite useful in medicine as an effective anti-inflammatory agent.

It’s human nature…we like to compare things. That’s just how we roll and every now and then it helps us learn so….

Adrenal Medulla V.s. Adrenal Cortex

medulla – the production of epinephrine & norepinephrine – “Fight or flight” car chases, explosions, tight leather pants

cortex – the production of aldosterone & cortisol – homeostasis, zen-like balance, long term response to stress