Small in stature, the kidneys work tirelessly. Pound for pound, they are by far the most bloodthirsty of organs.
At 2.5% of body weight, the liver utilises an astounding 25-30% of cardiac output at rest. The kidneys also receive about 25% of the cardiac output but, combined, weigh only a fifth that of the liver. When it comes to blood flow per gram of tissue, the image below contends, nothing comes close to the kidneys.
Indeed, apart from the elimination of water-soluble wastes, in times of need (i.e. prolonged starvation) the kidneys can also match the liver at gluconeogenesis (GN). Furthermore, Stumvoll et al (1998) confirmed substrate selectivity — the kidneys preferring to make glucose out of glutamine rather than alanine, which predominates in the liver.
In 2010, Gerich demonstrated that the renal component to gluconeogenesis is about one-fifth to a quarter of all glucose production:
And the kidneys have the capacity to take up a fraction more of cardiac output still (image below).
Why so much of the blood volume directed to the kidneys?
Because the kidney’s are designed for bulk flow — the mass filtration of water and solutes together due to a pressure gradient. (Contrast this to solute diffusion down a concentration gradient.)
How does the kidney turn plasma into urine?
Four more-or-less sequential steps are involved in renal elimination of waste products from blood. But a certain minimum volume of water is required to achieve this.
The average adult needs to excrete a minimum solute load of 600 mOsm/day. The maximum urine-concentrating capacity of the kidneys is 1200 mOsm/L, meaning an obligatory urine output of about 500 mL.
Just quickly … if the average blood volume is 70 ml/kg, a 70 kg adult will have nigh on 5 L blood volume. And 5 L/min is the cardiac output at rest. In other words, the entire blood volume (on average) is circulated in a minute. The kidneys take their 25%, which is 1.25L in a minute (or 1800 L a day) of blood flowing through the renal arteries. The glomerulus filters, by bulk flow, about 20% of the plasma volume that passes through it. Plasma makes up about 55% of blood volume.
1800 L blood x 55% = 990 L plasma flow through the renal arteries each day
Of that 990 L, 20% is filtered.
990 L x 20% ≅ 200 L ultrafiltrate per day
The kidneys create 200 L of ultrafiltrate per day but can manage, when required, to reduce this to half a litre of urine. That’s a water conservation of 400 times.
How do two organs of combined weight 300 g accomodate 1800 L of blood — or 990 L of plasma — in one day and potentially turn that into only half a litre of urine for elimination?
The key lies in understanding the counter-current mechanism set up along the nephron, predominantly about its Loop of Henle.
The superficial answer, however, will suffice for now: first the plasma is filtered, then solutes are reabsorbed from this filtrate, before wastes and toxins are then secreted into this filtrate. This fluid is excreted (eliminated) as urine.
Ok. One step at a time.
Filtration is the mass movement of water and solutes from plasma to the renal tubule that occurs in the renal corpuscle.
Filtration is pushed through a tuft of leaky capillaries, the glomerulus, which receives its blood from a bounding afferent arteriole and drains via a more conservative efferent arteriole.
Schematically, we can represent the glomerulus, Bowman’s capsule, and the forces involved in filtration like this:
This sharp differential in blood vessel calibre, either end of the glomerulus, creates a significant pressure differential that forces plasma through the leaky wall of the glomerulus and into the blind receptacle that serves as the beginning of what is ultimately a urine collecting duct, the Bowman’s capsule.
In reabsorption, water and solutes are recovered back from the tubule into the plasma.
The tubules continue to secrete additional substances, like excess potassium and hydrogen ions, into the filtrate.
This is the easy part, as long as there are no impediments to urinary flow such as stones, a tumour, or a large prostate.
Stumvoll M, Meyer C, Perriello G, Kreider M, Welle S, Gerich J. Human kidney and liver gluconeogenesis: evidence for organ substrate selectivity. Am J Physiol-Endoc and Metab. Volume 274 Issue 5 May 1998 Pages E817-E826. https://doi.org/10.1152/ajpendo.1998.274.5.E817