Fenestrated Capillaries

What are fenestrated capillaries?

Fenestrated capillaries are capillaries that have tiny openings, or pores. In Latin, the word “fenestrae” means windows. The “windows” in fenestrated capillaries allow larger molecules and proteins to move from your blood into organs and glands.

You have fenestrated capillaries in your kidneys, intestines, pancreas and endocrine glands. These organs need a continuous exchange of nutrients, waste and substances between the capillaries and tissues. The pores in the capillaries make it easier for this exchange to happen.

What is the function of fenestrated capillaries?

These capillaries move blood, fluids and waste throughout your body. The windows, or openings, in fenestrated capillaries allow larger molecules to pass through them.

Fenestrated capillaries are especially important in your kidneys. They exist in and near the nephrons (the filtering units). You have about a million nephrons in each kidney. Inside each one is a bundle of fenestrated capillaries called a glomerulus. This serves as the filter.

Other fenestrated capillaries surround tubes inside the nephrons. Together, these capillaries play an essential role in helping your kidneys filter waste from blood, create urine and reabsorb nutrients your body needs.

Where are fenestrated capillaries located?

You have fenestrated capillaries in organs and glands that require a continuous exchange of blood. These organs and glands include:

• Kidneys, which filter waste from blood and change the waste into urine. Your kidneys are part of your urinary system.
• Small intestines, which absorb nutrients from the food you eat. Your intestines are part of your digestive system.
• Pancreas, which helps your body digest food by breaking down sugar and fat.
• Endocrine glands, which are part of your endocrine system. They send hormones throughout your body that regulate your mood, development and growth.

What do fenestrated capillaries look like?

Most capillaries are about 3 or 4 micrometers (about the size of a thread in a spider web). They have two layers of cells. The cells on the inside layer of the capillary are endothelial cells. The outside layer has epithelial cells. Providers also call this layer the basement membrane.

Fenestrated capillaries have tiny pores in their inner layer. Each pore (or “window”) has a membrane that acts like a diaphragm (a thin partition that opens and closes). The membrane allows larger molecules (such as minerals) to pass through easily.

What conditions and disorders affect the fenestrated capillaries?

People with chronic kidney disease (CKD) often have problems with a type of fenestrated capillary called a peritubular capillary. These capillaries filter waste from the kidneys. Vitamin D deficiency, inflammation and disease can damage the peritubular capillaries and cause them to lose their density (the amount of mass compared to volume). Providers call this condition capillary rarefaction. When this happens, your capillaries don’t work as they should. This leads to hypertension (high blood pressure) and other health problems.

Other types of kidney disease, such as glomerulonephritis, also affect the kidney’s entire filtration. Kidney disease is very common. Around 15% of people in the United States have some type of chronic kidney disease. Untreated, it can lead to kidney failure.

A very rare, life-threatening condition called systemic capillary leak syndrome (SCLS) damages the walls of capillaries. It can lead to sudden low blood pressure.

How can I keep my fenestrated capillaries healthy?

You can keep your capillaries working properly by focusing on staying healthy overall. Capillaries help many systems in your body work properly. They are part of your circulatory system, which also includes other blood vessels, arteries and veins.

Various types of vascular disease can affect how all types of capillaries work. You can prevent vascular disease and other health problems by:

• Getting plenty of exercise and staying active.
• Eating a balanced diet, limiting alcohol and making heart-healthy food choices.
• Maintaining a healthy weight and losing weight if you are overweight or obese.
• Managing diabetes, high blood pressure and other chronic health conditions.
• Quitting smoking.

When should I call my doctor about my capillaries?

Call your provider if you have signs of kidney disease, including:

• Changes in urine, including blood (hematuria) or protein (proteinuria) in your urine, or the need to go more often.
• Fatigue, difficulty concentrating or trouble sleeping.
• Loss of appetite.
• Swelling or puffiness around your eyes that lasts for more than a few days.
• Muscle cramps.

A sudden drop in blood pressure may be a sign of an extremely rare condition called systemic capillary leak syndrome. This condition requires immediate medical help. Call your provider right away if your blood pressure drops suddenly, especially if you also have:

• Abdominal pain and nausea.
• Cough and congestion.
• Edema (swelling), especially in your legs and arms.
• Fatigue.
• Headaches.
• Lightheadedness, dizziness and fainting.

A note from Cleveland Clinic

Fenestrated capillaries work with the rest of your circulatory system to move nutrients, waste and oxygen through your body. These little blood vessels play an essential role in helping your kidneys filter waste and create urine. They allow your small intestines to absorb vitamins and minerals from foods you eat. They also help hormones regulate your mood, development and growth. To keep your capillaries working properly, you should focus on staying healthy overall. Eat a balanced diet, maintain a healthy weight, stay active and see your provider for regular checkups.