Every glass of water you drink, every salty snack, every run in the sun, all throw off your body’s internal balance, and yet, your excretory system constantly keeps the whole system running smoothly. It filters your blood, balances water and salt, controls pH, manages blood pressure, regulates red blood cell production, generates new glucose during long fasts, and much more! Let’s take a closer look.

Regulation of Blood Pressure

Let’s start with how the system helps regulate blood pressure. Two major players here are ADH (antidiuretic hormone) and the renin-angiotensin-aldosterone system (RAAS).

ADH is released by the posterior pituitary when your brain senses that your blood is too concentrated, meaning there’s not enough water compared to solutes. This can happen even with mild dehydration, like after sweating a lot or not drinking enough. It’s more about the saltiness of the blood than the total volume. Your hypothalamus detects the increased osmolarity (basically, too many solutes in too little water) and tells the pituitary to release ADH. This hormone travels to the kidneys and makes the collecting ducts more permeable to water by triggering the insertion of special channels called aquaporins. When aquaporins open, water can move out of the filtrate (the forming urine) and back into the bloodstream. As a result, you keep more water in your body, blood volume rises, and urine becomes more concentrated.

RAAS, on the other hand, responds more to actual drops in blood pressure and blood volume. So while ADH is sensitive to how concentrated your blood is, RAAS is triggered when there simply isn’t enough fluid circulating, like from significant dehydration, bleeding, or even heart failure. The kidneys release an enzyme called renin, which starts a hormonal cascade.

Renin converts a protein from the liver (angiotensinogen) into angiotensin I. Then, an enzyme in the lungs (ACE) turns angiotensin I into angiotensin II, a powerful molecule that constricts blood vessels and raises blood pressure. Angiotensin II also tells the adrenal glands to release aldosterone, which acts on the distal tubule and collecting duct to increase sodium reabsorption. Since water follows sodium, this leads to more water being reabsorbed too, which helps raise blood volume and pressure.

Osmoregulation

Osmoregulation is the regulation of solute concentration in the blood. If blood becomes too salty or too dilute, cells can either shrink or swell, which disrupts their function. ADH and aldosterone both help reduce blood osmolarity by increasing water reabsorption, but the kidney’s internal structure is also important in this; especially the loop of Henle and urea recycling.

The loop of Henle acts as a countercurrent multiplier. The descending limb is permeable to water but not solutes, so as filtrate travels deeper into the salty medulla, water leaves the tubule and enters the blood, which makes the filtrate more concentrated. The ascending limb, in contrast, is impermeable to water but actively pumps out sodium and chloride into the interstitial fluid. This keeps the medulla salty and sets up a strong osmotic gradient. It’s called “countercurrent” because the limbs run in opposite directions, and “multiplier” because this setup exaggerates the difference in concentration step by step as filtrate moves along.

Urea helps too. Some of it gets reabsorbed from the collecting duct into the medulla, which adds even more solutes to the environment outside the tubules. This further deepens the gradient, which allows even more water to be reabsorbed from the filtrate.

The collecting duct is where the final decision is made: how concentrated will the urine be? When ADH is present, aquaporins open and water leaves the filtrate to follow the salty medulla, which concentrates the urine. If ADH is absent, water stays in the tubule, and you end up with more dilute urine. This lets your body adjust fluid retention based on hydration, salt intake, or even stress, since ADH is also released under pain and anxiety.

Acid-Base Balance

The kidneys are also involved in regulating pH. If the blood becomes too acidic, the kidneys respond in two main ways. First, they secrete excess hydrogen ions (H⁺) into the urine, especially in the distal tubule. Second, they reabsorb bicarbonate ions (HCO₃⁻) into the bloodstream; this is useful for preventing the equilibrium from shifting after removing H⁺. If you remove H⁺, it will make the equilibrium shift right to produce more of it, but if you add HCO₃⁻, it cancels out this effect so the blood won’t become more acidic.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

Removal of Nitrogenous Wastes

Another job of the kidneys is to remove nitrogenous waste, byproducts of metabolism that can build up quickly and become toxic.

The most important of these is urea, which starts out as ammonia. When your body breaks down amino acids from proteins, it releases ammonia (NH₃), a small molecule that’s highly toxic even at low concentrations. The liver detoxifies it by converting it into urea through the urea cycle. Urea is still a waste product, but it’s much less reactive and far easier for the kidneys to handle. Once in the bloodstream, urea passes freely into the glomerular filtrate, and the nephron fine-tunes how much is reabsorbed. Some of it is reabsorbed in the collecting duct to help concentrate urine (as part of the osmotic gradient in the medulla mentioned earlier), but the majority is eventually excreted.

Creatinine comes from the breakdown of creatine phosphate in muscles. Unlike urea, creatinine is not reabsorbed, but filtered at the glomerulus and almost entirely excreted in the urine. That makes it especially useful for assessing kidney function: if creatinine levels in the blood are rising, it usually means the kidneys aren’t filtering properly.

Uric acid, produced from breaking down purines in DNA and RNA, is also filtered out of the blood at the glomerulus. After that, the nephron reabsorbs a significant portion of it in the proximal tubule. But later in that same segment, some uric acid is secreted back into the filtrate. The final amount that appears in the urine depends on the balance between how much is reabsorbed and how much is secreted. If this balance shifts, such as from kidney problems, certain drugs, or a high-purine diet, uric acid can build up in the blood and form sharp crystals in joints, which could lead to gout.

Kidney Anatomy

Zooming out, the kidney has three main regions: the cortex, medulla, and pelvis.

The cortex is the outer layer, where filtration begins. It contains the glomeruli, along with most of the proximal and distal convoluted tubules. These are the nephron segments responsible for much of the reabsorption and secretion you’ve already seen in action; everything from glucose and bicarbonate reclamation to acid-base adjustments and hormone responses.

Beneath the cortex lies the medulla, which has a distinct striped appearance from the parallel arrangement of loops of Henle and collecting ducts. This region is very important for building and maintaining the osmotic gradient that drives water reabsorption, which we talked about earlier in these notes.

At the center is the renal pelvis, a funnel-shaped cavity where urine from all the nephrons collects. From here, it drains into the ureters, which transport it to the bladder using rhythmic contractions (peristalsis). One-way valves at the bladder entrance prevent any backflow.

Nephron Structure

Each kidney contains about a million nephrons, the microscopic units that actually do the filtering and adjusting. The nephron begins with a glomerulus, a knot of capillaries inside Bowman’s capsule, where blood plasma is filtered under pressure. The filtrate then moves through a series of tubules: the proximal tubule, loop of Henle, distal tubule, and collecting duct. We already discussed how each of these regions plays a part in adjusting the fluid’s composition, regulating water, pH, ions, and waste. But in terms of the actual anatomy, the structure of the nephron is tightly adapted to its function: microvilli for surface area, mitochondria for active transport, and long loops that descend into the medulla to interact with the gradient system.

Urine exits the nephron through the collecting duct, which feeds into the renal pelvis. From there, the ureters carry it to the bladder, a hollow organ that stretches as it fills. Stretch receptors signal when it’s time to go. The urethra manages the release, controlled by two sphincters: an internal one made of smooth muscle (involuntary) and an external one made of skeletal muscle (voluntary). When you decide to urinate, relaxing the external sphincter signals the internal one to open, and the bladder contracts to expel the urine.

The blood supply is just as carefully organized. Blood enters the nephron via the afferent arteriole, gets filtered in the glomerulus, and exits through the narrower efferent arteriole, which helps maintain the pressure needed for filtration. From there, it moves into the peritubular capillaries, which wrap around the tubules and handle reabsorption and secretion. In the medulla, a specialized extension of these capillaries called the vasa recta runs alongside the loop of Henle, to preserve the medullary gradient through countercurrent exchange, as we discussed earlier.