Now you're stepping onto my territory........
You gotta be careful, my territory is vast.
Kidneys. Approximately 20% of all cardiac output is in the kidneys during an average time. Times of stress or illness changes this number, but the kidneys are very highly vascularized organs.
Blood flowing into the kidney is still in the arterial side of the cardiovascular system. It flows into an arterial capillary structure, called a gloumerulus. The gloumerulus is fed by an afferent arteriole and blood exits via an efferent arteriole. The gloumerulus is a capillary, but not in the traditional sense of a zone of exchange where the blood flows from the arterial side of the system and enters the venous side. It has to be arterial because it is a very high pressure structure that literally filters the blood of water, ions, some proteins and nitrogenous waste products, along with some hormones and other metabolites. This filtration is so effective that both kidneys are able to filter 125ml of filtrate per min, or about 45 gallons a day from the blood. 45 gallons a day all day, every day for your life. No kidding.
The gloumerulus is surrounded by Bowman's capsule. Bowman's capsule is the beginning of the tubular component of the functional unit of the kidney, the nephron. We have already talked about the vascular component, the gloumerulus. Bowman's capsule drains into the proximal convoluted tubule, which feeds into the loop of Henle and finally into the distal convoluted tubule and empties into the collecting duct which then eventually ends up in your bladder and beyond. In the proximal tubule, the body is able to reabsorb larger molecules, like proteins and amino acids, glucose, vitamins and fatty acids. Once these molecules are actively transported out, water follows out because there is an osmotic gradient created. The efferent arteriole from the gloumerulus encircles the convoluted tubule to receive these substances and to also later deliver hormones to the kidney. The remaining filtrate continues through the tubular component. Water is absorbed in the loop of Henle, solutes (ions, mostly) are absorbed in the near distal tubule, more water is absorbed through the rest of the tubule and the collecting duct. Secretion happens in the late distal tubule. Luckily, only 1% of that filtrate makes it to urine. So, .5 gallons a day is about what you have to get rid of.
Now, it is not over. What controls this? Well, is physically controlled by blood flow and pressure in the gloumerulus. Lower pressure or lower flow means lower filtration rate. It is also somewhat controlled by the composition of the blood. Your body can filter and reabsorb only so much before it shows up in the urine. For example, both kidneys see about 600 mg of glucose per minute in the blood. Of that, about 25% is filtered out. So, about 150mg/min enters the kidney and none of enters the urine because it is all transported back into the blood stream. In diabetics, their glucose level is so high, the kidney is unable to force it all back into the blood. Way back when, diabetes mellitus (Type 1) was diagnosed by the sugar crystals on mens' shoes. Want to test your threshold? Eat about a pound of sugar and start tasting your urine. When it tastes sweet, you know your kidneys have been overwhelmed.
But back to control. Control is hormonal. Vasopressin is secreted by the hypothalamus and stored in the posterior pituitary. When released it has two main effects. It causes constriction of the afferent (in) arteriole and this slows down the filtration rate, conserving water. It also binds receptors in the tubular system to open aquaporins that allow for water reabsorption. Again, conserving water. Vasopressin is released when low blood pressure is sensed by baroreceptors in the carotid arteries and the right atrium of the heart. Low blood pressure can be indicative of dehydration or blood loss. Protein or solute concentration of the blood also controls the release of vasopressin. Low protein caused by liver insufficiency or malnutrition will cause the body to move water from the blood to the interstitial space outside of the cardiac system. In that case, the body needs to conserve water to maintain pressure, blood volume and blood composition. Diabetes insipidus is when there is a lack of vasopressin production from the endocrine system or an insufficiency of receptors. In that case, water is not reabsorbed, but it is forced out of the body in vast quantities. Polydipisa/Polyuria are signs of Type 2 diabetes. Large amounts of water are consumed and large amounts of urine are produced. Wanna test for it? Taste the urine. It is so dilute, it has no taste. Again, olde time diagnostics.
How’d I do, Sean? 
Michael
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