Understanding how kidneys work requires you to embrace complexity, which is at the heart of science. We’ll take a simplified approach and leave the full story for Anatomy & Physiology classes. However, this will give us a chance to use all of the types of membrane transport we have learned so far – facilitated diffusion, osmosis, primary active transport and secondary active transport.

Why this is important…Membrane transport is vital to the function of every cell in the body and therefore every organ. Studying this in a complex system like the kidneys also gives you practice at another important skill – analysis. This involves breaking complex processes down into smaller pieces so you can understand each of them. Then you will put them back together to understand the larger process. To understand something as complex as the human body, this is a skill you will rely on constantly.

Foundations

When substances move across cell membranes (membrane transport) there are two major factors at work:

What is the chemical character of the substance being transported?
Is it being moved with or against its gradient? (or you can also say, “up” or “down” its gradient)

Nonpolar substances diffuse across the membrane without any help from transport proteins. We have seen this with simple diffusion of oxygen, carbon dioxide and lipids. Any other substance sees the membrane as a barrier and needs a transport protein to get through.

Polar and ionic substances need transport proteins. How they move depends on which type of proteins are used. These movements can be categorized as follows:

Active transport pushes substances from low to high concentration (against the gradient). This includes:
- Primary active transport – protein pumps use ATP to power the movement of ions
- Secondary active transport – cotransport proteins allow movement of one substance with its concentration gradient, which powers the movement of another substance against its gradient
Passive transport allows substances to move from high to low concentration (with the gradient). This includes:
- Facilitated diffusion – movement of ions or polar molecules through carrier proteins or protein channels
- Osmosis – movement of water by a mix of simple diffusion and facilitated diffusion (through aquaporins)

Details

The kidneys filter your blood by pumping and diffusing solutes from the blood in a series of twisted tubes. Unlike filters in your house or car, however, kidneys never need to be replaced. Remarkable!

The nephron is the functional unit of the kidney (Figure 7-3). There are over 1 million of them in each kidney waiting to take in blood plasma and create urine.

Here is the basic overview…The blood plasma is filtered out in the glomerulus, creating fluid that moves into a different set of tubes, which ultimately drain into the urinary bladder. Before the fluid can reach the bladder there are a series of exchanges between the kidney tubes and the blood vessels that move most of the fluids and solutes back into the bloodstream. About 180 liters of fluid are filtered every day, but only 1-2 liters leave as urine. By the time the fluid drains into the urinary bladder it mainly contains water, nitrogenous wastes and some electrolytes.

Diagram of nephron of the kidney, showing the arrangement in the kidney. The nephron is a series of tubes snaking around one another.

FIGURE 7-3. Nephron of the kidneys. ("Nephron" by OpenStax College is licensed under CC BY 4.0)

For a more detailed view of how this amazing filtration system works we need to break it into stages. These roughly correspond to the arrangement of the tubes in the nephron (Figure 7-3) but we will use a simplified diagram (Figure 7-4) to help us pick up the key steps. Each of the 4 major exchanges is lettered in Figure 7-4 and in the descriptions below. This is just an overview of the process, so don’t think of this as a series of ordered steps.

Simplified diagram of nephron with letters, A through D, indicating processes that occur there. Collecting duct shown beside blood vessel.

FIGURE 7-4. Simplified nephron. This illustrates the filtration process of the kidneys. Letters of stages correspond to descriptions below. (“Simplified nephron” by J. Buegge is in the Public Domain )

Plasma is filtered out of the bloodstream. Blood pressure in the arteries entering the nephron is high, which helps squeeze fluid through the small pores in the cells surrounding the artery. The openings are too small for blood cells or larger proteins to leak out, so these remain in the bloodstream. The fluid that enters the kidney tubules contains water, electrolytes, glucose, amino acids and nitrogenous wastes. The goal is to get most of this back into the bloodstream, except for nitrogenous wastes.
Kidney tubules actively pump out sodium and glucose, which reenter the bloodstream. Cells lining the kidney tubules extract sodium and glucose from the filtered plasma using secondary active transport. The other side of the kidney tubule cells use a sodium-potassium pump (primary active transport) to move sodium out of the cells. Sodium returns to the bloodstream and pulls along lots of water by osmosis. Glucose also exits the kidney tubule cells, but using facilitated diffusion. Once it is out of the kidney tubules, it reenters the bloodstream.
Capillaries around the kidney tubules secrete urea and adjust pH. Not all urea was emptied from the blood by filtration, so this gets rid of more. The pH of blood is also adjusted here by secreting either H⁺ ions or bicarbonate out of the blood.
The last part of the kidney tubules recovers most of the water and return it to the bloodstream. This part of the kidney tubules is very permeable to water but not to solutes. Since the fluid surrounding the tubules is high in salt, water is drawn out by osmosis before it drains into the bladder. This is how the kidneys are able to concentrate wastes in urine and minimize water loss.

Stop & Think #1: Dialysis is an artificial blood filtration system used for patients whose kidneys are not working properly. Blood is passed through dialysis tubing which is suspended in fluid. The tubing has small pores which allow small molecules in and out, but not blood cells or large proteins. The goals are: (1) remove nitrogenous wastes by allowing them to diffuse out of the tubing; and (2) to keep the rest of the solutes in the blood. What chemicals should be present and absent in the dialysis fluid to accomplish this? (Tip: first, draw a diagram to help make sense of the setup.)

Active transport requires a lot of energy. Kidneys use about 10% of your body’s daily energy, but not at a constant rate. During exercise, for example, blood is diverted away from the kidneys, which decreases urine production. This not only distributes energy to where it is needed, it is also convenient so that you won’t have to pee while you are out on a jog.

Kidneys regulate our water balance. The last stage discussed above (D) is fairly adjustable. Hormones secreted by the brain in response to low water levels (or salty blood) cause more aquaporins to be put in the kidney tubule membranes. This allows more water to be recovered. If we have too much water, the number of aquaporins is decreased and we lose more water in urine (and need to pee more often).

Kidneys regulate our pH levels. Although they work more slowly than the lungs at adjusting blood pH, the kidneys have a larger influence on blood pH. The amount of hydrogen ions or bicarbonate ions lost to urine has a large effect on blood pH. Not surprisingly, the pH of urine varies quite a bit as well. While usually a pH of about 6, it can range from about 4 to 8, which is a 10,000-fold difference in H⁺ concentration.

Learning objectives

Describe the differences between passive and active membrane transport.
Identify the type of membrane transport used under different scenarios.
Explain the basic function of the kidneys including the types of membrane transport used and why they require so much energy.

Stop & Think Questions Explained

Dialysis fluid should contain components that should remain in the blood so that they don’t diffuse out. This includes water, glucose, amino acids, and electrolytes. Dialysis fluid should be free of urea so that a concentration gradient exists for urea to leave. This is not capable of removing all urea, but if the dialysis fluid volume is fairly large then concentrations are not likely to equal one another. Often dialysis fluid has a low concentration of potassium ions to remove excess potassium from the blood, which is often happens when kidneys fail.