You can lose quite a bit of blood without experiencing any side effects or complications. The exact amount depends on your size, age, and general health.

It helps to think of loss in percentages instead of total amounts. Adult men, on average, have more blood than most adult women. This means they can typically lose a little more before experiencing adverse effects. Children, on the other hand, have much less blood than adults, so even small blood losses could affect a child negatively.

Typical causes of blood loss — giving a blood sample for testing at the doctor’s office, menstruation, a nosebleed — won’t usually cause complications. But sustaining an injury or undergoing surgery may cause severe bleeding and require a red blood cell transfusion.

Read on to learn how much blood is lost in situations like these and how much you can lose before nausea, fainting, or other complications occur.

Most adults can lose up to 14 percent of their blood without experiencing any major side effects or changes in vital signs. Some, however, may feel lightheaded or dizzy if this amount is lost quickly.

You’ll start to feel mild side effects, such as nausea, when blood loss reaches 15 to 30 percent of total blood volume. This amount of loss increases your heart and respiratory rates. Your urine output and blood pressure will be decreased. You may feel anxious or uneasy.

Your body starts to compensate for blood loss by constricting the blood vessels in your limbs and extremities. This is your body’s attempt to maintain your blood pressure and blood flow. This subsequently lowers the amount of blood your heart pumps outside the center of your body. Your skin may become cooler and pale.

When blood loss nears 30 to 40 percent of total blood volume, your body will have a traumatic reaction. Your blood pressure will drop down even further, and your heart rate will further increase.

You may show signs of obvious confusion or disorientation. Your breathing will be more rapid and shallow.

As the volume loss climbs, your body may not be able to maintain circulation and adequate blood pressure. At this point, you may pass out. You’ll need help quickly to prevent additional blood loss and greater side effects.

Hemorrhagic, or hypovolemic, shock occurs when you’ve lost 20 percent or more of your total blood volume. Your symptoms will become more severe as the blood loss increases.

You may experience:

  • rapid breathing
  • weakness or fatigue
  • confusion
  • cool, pale skin
  • sweaty, moist skin
  • anxiety or unease
  • low urine output
  • drowsiness
  • unconsciousness

Your body can’t compensate for much longer on its own in a blood volume loss over 40 percent. At this stage, your heart can’t properly maintain blood pressure, pumping, or circulation. Your organs may begin to fail without adequate blood and fluid. You’ll likely pass out and slip into a coma.

Without treatment measures, your body will completely lose its ability to pump blood and maintain oxygen delivery once you’ve lost about 50 percent of your blood volume.

Your heart will stop pumping, other organs will shut down, and you’ll likely be in a coma. Death is likely if aggressive life-saving measures haven’t been taken.

Your body can compensate for a good deal of blood loss. However, at a certain point, it shuts down unnecessary components in order to protect your heart.

You’ll likely feel very fatigued in the moments before entering into a coma. If close to death, these feelings may not even be noticed.

The average hemoglobin level is between 13.5 to 17.5 grams per deciliter for men and 12 to 15.5 grams per deciliter for women. Most doctors won’t consider a transfusion until the hemoglobin levels in your blood reach 7 or 8 grams per deciliter.

This isn’t the only parameter involved in the approach to treating blood volume loss if you’re actively bleeding. However, hemoglobin level is important for making a red blood cell transfusion decision. Your doctor and care team will use these and other factors to decide if a transfusion is necessary and if it’ll be effective for your situation.

Volume blood loss greater than 40 percent may be difficult for doctors to correct with a transfusion. That’s especially true if the bleeding’s poorly controlled.

Your doctor will take several factors into account when deciding if a transfusion is right for you. This includes:

  • your additional injuries
  • the rate of blood loss
  • the site of blood loss
  • your overall health

Minor blood loss isn’t inherently harmful or even dangerous. The average adult can lose a fair amount of blood without experiencing any symptoms.

Here’s how much blood is lost and what to expect from:

Blood donation

The average person loses one pint of blood when donating. Your body has about 10 pints of blood, so you only lose about 10 percent of your total blood volume when you give blood.

A nosebleed

Nosebleeds may feel bloodier than they are because of the exposure to blood coming from your nose. The amount of blood you typically lose isn’t enough to cause complications. However, if you soak through gauze or tissue several times in a five-minute span, you may need to seek medical treatment to end your nosebleed.

A bleeding hemorrhoid

Bright red blood on toilet paper or in underwear may look alarming, but it’s rarely serious. Most people lose small amounts of blood with a bleeding hemorrhoid. This level of blood loss isn’t typically a cause for concern.

Menstruation

The average person loses 60 milliliters of blood during their period. People with heavier periods lose about 80 milliliters. If you believe you’re losing more than that, see your doctor. Explaining how quickly you go through pads or tampons will help your doctor determine whether the bleeding is severe.

A miscarriage

Bleeding from a miscarriage that happens very early in a pregnancy is similar to bleeding during menstruation. However, the later in a pregnancy a miscarriage occurs, the greater the blood loss will be. It may come on very suddenly and be quite heavy. Other signs of a miscarriage include severe abdominal pain, back pain, and contractions.

Childbirth

The average person loses 500 milliliters of blood during vaginal childbirth. That’s just half a quart. Those who have a cesarean delivery typically lose 1000 milliliters. You may lose more if complications arise, but your doctor and delivery team can usually manage the bleeding.

Lab testing

The average blood vial holds a scant 8.5 milliliters. You’d have to have about 88 of these vials of your blood taken before you begin to experience side effects.

Surgery

Doctors and surgical staff work diligently to lower blood loss during a surgery. However, some surgeries produce major blood loss, or it occurs as a complication of the procedure. Your doctor can give you an idea of how much you might lose during your surgery and what can be done if you lose more than expected.

Your body can handle blood loss, but how it happens and how much you lose determines a lot about the outcome.

In some cases, blood loss can happen all at once. It isn’t unusual to lose significant amounts of blood as a result of an injury or accident. It can also happen slowly over a longer period of time, which can make recognizing the symptoms trickier.

If you suspect you may have a slow, internal bleed, see your doctor. They can assess your symptoms and diagnose any underlying condition.

If you’re losing a lot of blood rapidly, seek out emergency medical care.