Your kidneys are small but essential organs that filter about 50 gallons of fluid every day to clear out waste products and toxins. They also play a key role in controlling your blood pressure and red blood cell (RBC) production. Many other organs and hormones work together with the kidneys to keep your body’s functions stable and balanced.

This article will cover how the kidneys work by breaking down their anatomy and main functions. We’ll also discuss what happens when they’re not working properly.

What Are the Kidneys?

The kidneys are bean-shaped organs that sit just below your rib cage. You have two kidneys, one on each side of your spine along the backside of your body. Each kidney is roughly the size of a fist. The term “renal” is often used to describe parts, conditions, or functions of the kidneys. It comes from the Latin word “rēnēs,” meaning “kidneys.”

When you think of your kidneys, you probably think of them as filters. One of their main jobs is to filter and remove extra fluid and waste products from your blood. However, the kidneys also play important roles in:

• Making new RBCs
• Controlling your blood pressure levels
• Balancing calcium and other mineral levels
• Keeping your bones healthy
• Removing extra fluid and toxins from the body

How Do the Kidneys Filter Blood?

The kidneys are part of the urinary system, which includes your ureters, bladder, and urethra. To best understand how the kidneys work, it helps to explain the entire system.

Blood with waste products flows into the kidneys through the renal artery, splits into smaller blood vessels, and reaches millions of tiny filtering units called nephrons. Every kidney contains about a million of these units. According to the National Institute for Diabetes and Digestive and Kidney Diseases, nephrons have two parts:

• The glomerulus — Acts like a coffee filter. It lets water and waste products pass into the tubule while keeping red blood cells and proteins in the bloodstream.
• The tubule — This part reabsorbs most water, nutrients, and minerals back into the body while the remaining fluid forms urine.

Blood vessels run alongside the tubules to take water, nutrients, and minerals back to the rest of your body. Blood leaves your kidneys through the renal vein, clean and free of waste.

Any water that isn’t reabsorbed from the tubule goes on to make urine. This water mixes with urea and other waste products. Urine then leaves the kidneys and flows into a tube called the ureter. The ureter leads into the bladder — a triangle-shaped organ that stores urine.

How Do the Kidneys Maintain Homeostasis?

The kidneys also help your body maintain homeostasis — a state of balance your body needs to work properly. Your body constantly responds to changes in blood pressure, electrolytes, acid levels, and temperature. The kidneys work alongside other organs to keep this balance.

Electrolyte Levels

Electrolytes are electrically charged molecules that dissolve in water. Examples include calcium, potassium, and sodium. Your body needs the right balance of electrolytes to keep its normal functions. These molecules play a role in:

• Supporting muscle and nerve function
• Keeping a healthy acid-base balance in your blood
• Controlling the amount of water your body holds on to

Your kidneys help control your electrolyte levels by raising or lowering your fluid levels. When you have more fluid in your body, it dilutes electrolytes, lowering their concentrations. On the other hand, less fluid means higher electrolyte concentrations. The kidneys get rid of extra fluid through your urine.

Blood vessels in the kidneys also absorb needed electrolytes back into the bloodstream. The electrolytes travel back into the tubule along with water. If you have too many electrolytes, your kidneys will get rid of the extra minerals in your urine, and you’ll pee them out.

People whose kidneys don’t work well may have extra electrolytes in their bloodstream. Electrolyte imbalances can be harmful to your heart and bones. For example, high sodium levels can lead to high blood pressure.

Acid-Base Balance

Part of maintaining homeostasis is to have a proper balance of acids and bases in your bloodstream. Part of the kidney’s job is to break down and filter out organic acids into urine. During this process, they get rid of a bicarbonate base, which makes your blood slightly acidic. The kidneys can also make extra bicarbonate to balance your blood chemistry. People with kidney damage don’t have enough bicarbonate to balance their acid-base levels.

Blood Pressure

Your kidneys play a key role in controlling your blood pressure levels. This is because your blood pressure is affected by how much fluid is in your body. The kidneys belong to the renin-angiotensin-aldosterone system (RAAS). This complex system involves many organs making different hormones and proteins.

The RAAS raises blood pressure by helping your body hold on to more salt and water while making your blood vessels narrower. Hormones and enzymes (proteins) work together to make your kidneys hold on to sodium. These substances also tell your kidneys to release potassium into your urine. The more sodium in your bloodstream, the more water stays in your body. This increase in fluid eventually raises your blood pressure levels.

If your blood pressure is too high, your doctor may prescribe diuretics. Also known as water pills, these medications encourage your kidneys to get rid of extra salt and water. The extra salt and fluid leave your body as urine. As your fluid levels fall, so do your blood pressure levels.

New Blood Cell Production

RBCs carry oxygen to your organs and tissues. The kidneys make a hormone called erythropoietin. When your oxygen levels drop too low, the kidneys release erythropoietin. This substance tells the spongy tissue in your bones (bone marrow) to make new RBCs. If your RBC levels drop too low, you may develop anemia.

What Happens When Your Kidneys Don’t Work Like They Should?

Some people have certain diseases or health conditions that affect their kidney health. The most common is chronic kidney disease (CKD). According to Cleveland Clinic, the most common causes of CKD include high blood pressure and diabetes.

Rare kidney diseases affect less than 200,000 people in the United States. These diseases usually develop in people with gene changes (mutations) or immune system problems. Examples of rare kidney diseases include:

• Complement 3 glomerulopathy
• Immune complex membranoproliferative glomerulonephritis
• Alport syndrome
• Polycystic kidney disease

If you have kidney damage, you’re at risk of developing health problems. It’s important to work closely with your doctor to protect your kidney function and prevent more damage. A nephrologist specializes in diagnosing and treating rare kidney diseases. They’ll make medication and lifestyle suggestions for managing your condition.

If your kidneys don’t work as well as they should, you may develop kidney failure. Treatment options include dialysis or a kidney transplant.

Talk to Your Doctor

The kidneys are important organs that do more than just filter waste. They help keep your body in balance by controlling blood pressure, making red blood cells, and managing fluid and mineral levels. Keeping your kidneys healthy is key to your overall well-being, so if you have any concerns, talk to your doctor.

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