How to increase blood volume

Poor blood circulation can put you at a risk of several health problems. But did you ever know that your diet can help you fight this condition with ease? If you have a doubt that it wouldn’t, there is certain evidence that show that there are certain foods that improve blood circulation in legs and lower body.

How Is Diet And Blood Circulation Linked?

The food we take plays an important role in maintaining our health and there is no doubt when we say that it is also vital to improve the blood circulation in our body. Poor circulation can lead to various health issues and hence it becomes important to keep a tab on what you are eating every day.

Our human blood circulation system is driven by the heart and you need to have healthy, nutrient rich food that keeps your heart healthy. You need to focus on at least five portions of fruits and vegetables every day to boost circulation.

Super Foods You Can Have To Improve Blood Circulation in Legs

Blackberries – Blackberries help in the formation of the new blood cells in our body. They also help in blood purification. These are very high in their nutrient content and have phenolic compounds that work as antioxidants. You can add fresh blackberries to your breakfast mix them in cereals or even add a few in the yoghurt to make a very tasty and healthy snack.

Cayenne Pepper – Cayenne pepper is one great ingredient that can be used to improve the blood circulation and your metabolic rate. These peppers help strength the arteries and also the blood vessels. These can be used if you experience toe numbness frequently which occurs due to low blood circulation in the feet.

This also helps get rid of inflammation, thanks to the rich contents of vitamin A and vitamin C in them. This is why there are known to be very effective when it comes to blood circulation. These can be added to juices or even salads, but remember that they are a little spicy too.

Fish – We all are aware of the fact the fish is good for the heart as it has high contents of omega 3 fats. This is one nutrient that is very good for blood circulation. If you are suffering from poor blood circulation you can have fish like salmon or even try the supplements that are available in your local medical store. This is one of the best blood circulation in legs remedies available.

Ginger – Ginger can be termed as a wonder herb. It helps treat various health conditions, and low blood circulation is one among them. Regular intake of ginger and help stimulate the blood flow to all your organs. It also boosts the immune system, and thus clears congestion as well. Ginger is also available in the form of tincture or as a herbal tea. If you have fresh ginger available, you can add it to your recipes or just make a cup of tea using fresh grated ginger and honey.

Garlic – Garlic like ginger provides a lot of health benefits. It also works great as a blood thinner. If you wish to increase the blood flow to your legs or hands, it is time you add garlic to your every day diet. It also helps clear the clogged arteries and can be had in its raw form every day in moderate amounts. If you are not good with having it raw, you can add it to your cooking. You can even take garlic supplements to improve circulation .

Gingko Biloba – For those who are not aware what Gingko Biloba, it is a herb that is available as a tincture. This is made using the leaves of the ginkgo tree which also goes by the name of a memory tree. This can be used to stabilize the blood vessels and hence allows the proper flow of the blood. This can also be taken if you have a poor circulation of blood in your legs or the lower body. This also increases the flow of blood to your brain.

Orange – Orange is filled with bioflavonoids and vitamin C, both of which increase the blood flow. This happens as the capillary walls are strengthened, thus allows the free flow of blood.

Pumpkin Seeds – Pumpkin seeds are rich sources of vitamins, especially vitamin E. This helps the body to maintain the free flow of blood and also prevents any blood clots in the body. As vitamin C and vitamin E are important vitamins to improve blood circulation in legs, pumpkin seeds can thus be used in your every diet.

Nuts – Who doesn’t like nuts? Most of the nuts have good contents of niacin and vitamin B3. These provide the required boost to our blood can thus prevent any blood circulation problems.

Watermelons – Watermelons are not only filled with fiber and water. They are also rich in a compound called Lycopene. This can prevent the build-up of plaque and thus promotes good blood circulation.

Celery – This is one of the best foods you can add to your daily diet. You can add it to your salads or add to any other dish. Celery is filled with nutrients like Vitamin K that help in proper blood circulation.

Oats – Oats too have a good content of vitamins and minerals, both of which are very essential for proper blood flow. You can grab a cup of oats every day in the morning to keep you full and energetic throughout the day. Regular intake of oats can also prevent numbness in the legs and feet.

Whole Grains – Whole grains are filled with fiber and this fiber is needed to keep your heart healthy. Fiber also helps to maintain low cholesterol levels and this will in turn help in proper circulation of blood.

The above are some super foods that can be included in your every day diet in some or the other from. These are those foods that improve blood circulation in legs and help maintain the health of your heart.

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How to increase blood volume

For most people, low blood pressure (around 90/60 mmHg) is considered a positive indicator of good heart health. When blood pressure persistently dips below 90/60, or if you experience frequent side effects of low blood pressure (hypotension), such as fainting, consult your primary healthcare provider for an evaluation. Your doctor can diagnose any underlying medical condition that might be causing your hypotension and guide you regarding whether or not your low blood pressure requires treatment. You also can discuss the following natural remedies for low blood pressure to find out if they’re appropriate and safe for you to try.

Caveats to Treating Hypotension at Home

Most of the time, doctors decline to treat mild or moderate hypotension because low blood pressure benefits your heart health. In severe cases, doctors may prescribe medications to boost low blood pressure that is causing frequent falling or other undesired effects. Doctors also may treat an underlying condition causing hypotension.

People who have been diagnosed with heart or kidney disease—including anyone who takes medication for high blood pressure or heart failure—should never take steps to raise their blood pressure on their own, as this can have serious negative health consequences. If you’re in this group and experience symptoms of hypotension, contact your prescribing physician or primary care provider for guidance regarding how to proceed.

Otherwise healthy adults who occasionally experience symptoms of hypotension (like lightheadedness after heavy physical activity or sun exposure) can try a home remedy for low blood pressure.

How to Raise Low Blood Pressure

Without prescription medicine, there are two primary approaches to raising low blood pressure: increasing blood volume or promoting vasoconstriction. To simplify, adding fluids and electrolytes to your body can increase your blood volume, while taking certain herbs or other substances can cause your blood vessels to constrict, which raises blood pressure. It’s very unwise to use both approaches at the same time, as doing so can cause your blood pressure to spike to dangerous levels.

How to Increase Blood Volume

Dehydration can cause hypotension by literally shrinking the volume of blood circulating throughout your body. Even losing a small percentage of your blood volume to dehydration can cause blood pressure to dip temporarily. Sweating due to physical activity, excessive sun exposure, vomiting, and diarrhea all can cause dehydration that provokes a hypotensive episode.

You can address this type of low blood pressure by:

Drinking extra water. Make a conscious effort to consume more water until your hypotensive symptoms subside. Be careful not to drink large quantities of water within a short period of time, as this can induce a life-threatening medical condition called ‘water intoxication.’ The recommended amount of water for an average person aged 8 years and older is 64 ounces a day, but you should drink water as necessary, preferably before you are thirsty.

Drinking an electrolyte replacement beverage (‘sports drink’). These beverages contain vital minerals (electrolytes) that enable your cells to better take in and retain the water you need to rehydrate.

Eating a few salty meals or snacks. Salt (sodium) helps your body retain fluid, so consuming some salty snacks or adding a little salt to a few meals in succession should correct any blood volume deficiency causing you to experience hypotensive symptoms. Eat a small handful of chips after exercising or a long hike to replenish your sodium.

Herbs for Low Blood Pressure

Many herbal supplements and commercial beverages can cause vasoconstriction that elevates blood pressure substantially, which is why people who take medicine for high blood pressure must use caution when taking over-the-counter decongestants and supplements. Even if you are otherwise healthy, you should stop taking any supplement that produces symptoms of hypertension, such as a headache.

To treat symptoms of hypotension with herbs and other substances, try:

Energy drinks, which often contain caffeine, sugar, and other vasoconstrictors

Ephedra (ma huang), an herb used for its decongestant properties

Licorice—but only true licorice, not licorice-flavored products

Keep in mind these substances can have a potent effect on blood pressure, even to the point of inducing a stroke due to acute hypertension. If your low blood pressure causes symptoms, it’s better to see a doctor than to try treating it yourself.

In some cases, options other than blood product transfusions may be used.

Volume expanders

When a patient has lost a lot of fluids, the body can go into a type of shock. This may be treated or prevented by giving solutions to expand fluid volume in order to keep blood circulating through vital organs. The solutions are put right into the bloodstream through a vein. They boost fluid volume and help with circulation, but don’t carry oxygen or raise the number of blood cells.

Growth factors

The body naturally makes hormone-like substances called hematopoietic growth factors that cause the bone marrow to make more blood cells. Man-made versions of some of these growth factors are available to help people with low blood cell counts. Growth factors can be used to boost red blood cell, white blood cell, or platelet counts.

Growth factors may help patients who would otherwise need transfusions. But they have some drawbacks that may limit their use in some cases:

  • Unlike transfusions, growth factors often take many days or weeks to raise blood counts, so they may not be useful in people who need their blood cell levels raised quickly, such as those who are actively bleeding.
  • People who have severe bone marrow disease may not respond to the growth factors because they don’t have enough blood-producing cells in their bone marrow.
  • Some growth factors might cause certain types of cancer cells (such as lymphocytic leukemia, multiple myeloma, head and neck cancer, breast cancer, cervical cancer, and some kinds of lung cancer cells) to grow more quickly.
  • Growth factors generally cost a lot more than transfusions.

Because of these drawbacks, certain growth factors are not used in people whose treatment is expected to cure their cancer. And when they are used, they are given for as short a time as possible.

Intra-operative or post-operative blood salvage

Patients getting surgery sometimes need transfusions to replace blood lost during or after the operation. Sometimes this lost blood can be “salvaged” or saved by collecting it with a special machine and giving it back into the patient.

Giving a person back his or her own blood is called an autologous transfusion. It cuts down on the need for transfusions from other donors. But some studies have found tumor cells in blood salvaged during cancer operations, and this isn’t something that can be done for all patients. (Another type of autologous transfusion is described in Donating Blood.)

Blood substitutes

So far, there is no real substitute for human blood. But researchers are working to develop a liquid that can carry oxygen and replace blood, at least for a short time, in certain situations.

Some products being tested can do some of the work of red blood cells, such as carrying oxygen to tissues, but cannot replace the many other functions of human blood.

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Jacki Arevalo chasing the sun on the Big Water Trail, Wasatch Mountains, Utah. Photo by Louis Arevalo

In the movie, This Is Spinal Tap, there’s a scene where the lead guitarist of the band is showing off his amplifiers, when the camera zooms in on the knobs controlling volume. Only, there’s a twist. The knobs go to 11, instead of just 10. As the rocker says, “It’s one louder, isn’t it?”

Similarly, turning your blood volume up to 11 can give your performance a boost on the trails. Smart training adds an extra setting to your blood volume knob (called hypervolemia), and you might not even know it. So let’s break it down.

A 2000 review article in “Medicine & Science in Sport & Exercise” deconstructed some of the blood volume basics. According to the article, blood volume is the sum of erythrocyte volume (red blood cells) and plasma volume (mostly water plus some dissolved proteins, glucose, clotting factors, electrolytes, hormones, carbon dioxide and oxygen). This paragraph sounds like a vampire reading a nutrition label.

Red blood cells transport oxygen, while more plasma lets your heart pump more volume per beat per minute (greater cardiac output). Red blood cell increases usually occurs over weeks and months, while plasma volume responds more rapidly (days or even hours). They can change independently of one another to change total blood volume. This one sounds like a vampire reading a cookbook.

So blood volume is a big determinant in performance, and it can change over time. How can we turn it up to 11 with training? There are three main principles to think about.

Train consistently, mixing intensities

The main avenue to increasing blood volume is training consistently. In untrained individuals, within just 24 hours of training, it can increase by around 10 percent due to plasma volume expansion. After two to three weeks, many studies measure red blood cell increases, increasing progressively thereafter. In studies, the amount of blood volume change is most influenced by initial fitness level (higher fitness means less room for blood volume to grow), training intensity (mixing intensities and including some hard workouts is better) and type of activity studied. (Interesting note: being in space reduces blood volume a ton, so try to avoid space travel during important training blocks).

The main takeaway? Consistent, long-term training is what matters most, with no shortcuts to improvement over time. For example, a 2013 study in the “International Journal of Sports Physiology & Performance” found significant increases in blood volume after seven weeks of endurance training with intensity mixed in, with similar findings echoed across many studies.

What accounts for the blood volume change? The review study mentioned in the intro broke down a bunch of studies into two cohorts, one that had training durations of less than 12 days, and one with more than 21 days. In the short duration studies, participants had an 11 percent increase in plasma volume and no increase in red blood cells. In the longer duration studies, participants had similar plasma volume change and a 9 percent increase in red blood cells. Put it all together, and the longer-term group improved VO2 max by about twice as much. In other words, fitness increases from blood volume expansion can compound on each other over time (to a point).

So blood volume can rise relatively rapidly with smart training, and the longer the training period the better. But it can fall quickly too, as explained in this 2007 review from the “American Journal of the Medical Sciences.” In fact, a seminal study from the 1980s in the “Journal of Applied Physiology” found decreases of 5 to 12 percent after just a few days of inactivity. While that is mostly plasma volume that will bounce back quickly, too much stop-and-start training will eventually put a stop to running progression and make all your runs harder. That’s why the first run after a few days off might make you feel like a constipated water buffalo, even though you won’t lose any fitness in that time.

In your training, strive for consistency over epicness. Build up to at least five days of running a week if you can, mixing in a day or two of higher intensity efforts. Cross training that supports your running is great too. Then, you might get the benefit of the blood volume positive feedback loop: increases lead to more enjoyable and faster training, which makes blood volume increase a little more, making you even faster. Eventually, it all will likely stabilize at some higher set point, but that’s a good problem to have.

Use altitude if you can

The hormone erythropoietin (EPO) probably rings a bell, especially if you once had to sadly remove a Livestrong bracelet sometime around 2010. In addition to its sinister uses in pro cycling and other endurance sports, EPO is a natural hormone that responds to training and altitude. Due to natural EPO production, people that have lived at high altitude their whole lives have higher blood volume than sea-level dwellers, mostly due to increased red blood cells. Tons of studies show how training at altitude can increase red blood cells after a couple weeks exposure. Most elite training groups spend time at altitude for that reason.

In your training, if you live at low altitude, try to schedule family vacations at high elevations (or even better, move to Flagstaff). A couple-week stint at elevation can make a big difference in red blood cell volume. Some pro athletes even use altitude-generation systems, like Hypoxico, which let you simulate altitude with a mask or a tent. Just make sure your significant other is on board.

Heat is your friend

You may have heard people talking about the magical performance benefits of the sauna. The reason is not that being around a bunch of sweaty, hairy, naked people makes you run extra fast out of the sauna. It’s that heat adaptation increases blood volume.

At rest, plasma volume expands by around 5 percent in the hottest months, and contracts by 3 percent in the coldest months (the technical term should be “shrinkage”). When heat sources like the sauna are introduced, plasma volume can increase by as much as 20 percent in some people for a short period. For example, a 2015 study in the “European Journal of Applied Physiology” found big increases in blood volume in highly trained cyclists after just four exposures to post-exercise sauna. As this 2016 review outlines, heat stress could stimulate EPO production too.

For your training, as summer comes around, don’t be shy about running in the heat. It’s your friend, giving your blood volume an extra boost. On top of that, some of the athletes I coach will do 20 to 30 minutes in a dry sauna after exercise, followed by 45 minutes without cooling off or rehydrating (enhancing the blood volume stress, particularly before hot-weather races like the Western States 100). Just be sure that any routine is approved by your doctor first.

On top of training influences, it’s important to have enough iron to build red blood cells and to avoid chronic dehydration, which reduces plasma volume. For most athletes, the big takeaway of the literature on blood volume is just to train consistently, not shying away from heat or mountains, while eating enough iron-rich foods (or supplementing) to get the maximum bang for your training buck.

—David Roche partners with runners of all abilities through his coaching service, Some Work, All Play.

More Articles

  1. Initial Responses of the Cardiovascular System to Exercise
  2. Difference Between O Positive & O Negative Blood
  3. Pros & Cons of Water & Gatorade
  4. What Are the Three Buffer Systems in Body Fluid?
  5. Does Exercise Cause an Adrenaline Rush?

Endurance exercise, such as jogging, swimming and cycling, increases your blood volume over time. Blood-volume adaptations begin after a single bout of exercise and amplify within weeks of training. Enhanced blood volume enhances your exercise performance and cardiovascular fitness.

Blood Volume Defined

Blood volume is composed of red blood cells and blood plasma circulating throughout your body. While red blood cells transport oxygen, blood plasma is a yellowish liquid that carries your red blood cells. Composed mostly of water, blood plasma contains dissolved proteins, hormones and minerals. A typical adult male maintains a blood volume of 5 to 6 L, while females typically have 4 to 5 L of blood. Increased blood volume provides greater amounts of blood to your heart and increases how much blood your heart pumps per beat and per minute.

  • Blood volume is composed of red blood cells and blood plasma circulating throughout your body.
  • Increased blood volume provides greater amounts of blood to your heart and increases how much blood your heart pumps per beat and per minute.

Blood Plasma and Hormonal Changes

Initial Responses of the Cardiovascular System to Exercise

Three factors influence increased blood volume from exercise. For example, cardiovascular exercise boosts the release of anti-diuretic hormone and aldosterone — two hormones that cause your kidneys to retain or reabsorb water. Increased water retention within your kidneys increases blood plasma levels and produces a greater blood volume overall. Generally, plasma volume increases within two to three weeks of endurance training.

  • Three factors influence increased blood volume from exercise.
  • Generally, plasma volume increases within two to three weeks of endurance training.

Blood Plasma and Changes in Blood Protein

Exercise raises the amount of proteins within your blood plasma. According to the book “Exercise Physiology: Human Bioenergetics and Its Applications” by George A. Brooks, Thomas D. Fahey and Kenneth M. Baldwin, levels of a common blood protein known as albumin increase within your blood plasma within one hour of your first bout of exercise. Greater levels of plasma protein cause water retention within your blood plasma, which results in elevated blood volume.

Red Blood Cell Changes

Difference Between O Positive & O Negative Blood

For some individuals, exercise increases the total number of red blood cells within the body. A typical athlete has a greater amount of red blood cells than an individual who does not exercise regularly. Additional red blood cells enhance the transportation of oxygen to working muscles during exercise and increase your blood volume.

Detraining and Blood Volume

Detraining is a loss of fitness from quitting exercise. Like most exercise-induced health benefits, your blood volume reverts to a pre-training level if you stop exercising. You may experience reduced exercise performance after a few weeks without working out. However, you regain an increased blood volume within a few weeks of restarting your workout routine. Consult a doctor before beginning an exercise program.

Why is water important for blood pressure?

FUN FACT: water makes up 60% of our body weight! About two-thirds of that water weight comes from the water inside healthy cells. The remaining third of water either sits between your cells, or else travels around your body as part of blood. While its the oxygen, nutrients, antibodies, and other essential molecules that make blood as essential as it is, blood would be nothing without water. Water makes up the volume of blood, and allows for the circulation of essential nutrients to organs and tissues [1].

The volume of the blood in your body dictates the pressure generated by your heart to move the blood through your veins and arteries. If the volume of blood is off—or, if there is an incorrect volume of water in your body—your heart will have difficulty keeping its normal, healthy rhythms.

How are dehydration and blood pressure related?

How to increase blood volume

This can be tricky to understand, so let’s break this down: Imagine y our heart is a pump that works to circulate your blood around your body. When you’re healthy and properly hydrated, your heart pumps with a certain amount of force to move blood around your body. If you’re dehydrated, the volume of blood decreases, so the pump has to work harder (apply more force) to get the same amount of blood to organs and tissues as it is used to. If you’re over-hydrated, and your body has a larger volume of blood than normal, your heart will begin to pump with less force. So, if you’re dehydrated (low blood volume), your heart rate increase. If you’re over-hydrated (higher blood volume), your heart rate decreases.

For the first litre of fluid you lose as you become dehydrated, blood vessels can constrict and become narrower to counter the change in volume and maintain blood pressure [2]. If more than 1L is lost, your blood pressure starts to fall directly in proportion to the degree of your dehydration [3]. You may feel your heart beating fast to try to stop this, but even though your heart is beating faster, less blood is pushed out per heartbeat, and your blood pressure decreases [4].

What role do electrolytes play in dehydration and blood pressure?

We’ve discussed the role of water when it’s part of the blood, but what about the water that’s between and in your cells? How does water even get in and out of cells?

How to increase blood volume

Electrolytes help the movement of water in and out of your cells, which require very specific concentrations of electrolytes inside and outside to function normally. The difference in electrolyte concentrations inside and outside of a cell creates what is known as a concentration gradient. Just like gravity forces things from a high point to a low point (like a river down a waterfall), salts and sugars in your body move from areas of high concentration to areas of low concentration. For cells, the outer cell membrane acts as the boundary between areas of high and low concentration, or, put another way, between the inside and outside of the cell. Sometimes, instead of the salt or sugar passing through the cell membrane, it’s easier for water to move in or out of the cell: out of the cell to dilute a higher concentration of electrolytes outside of the cell, or into the cell to increase the concentration of electrolytes outside the cell.

Normally, Potassium is kept at a much higher concentration in the cells than outside, whereas Sodium is kept at much higher concentration outside than inside. Water moves with and in response to these concentrations. This means that electrolytes, by affecting the volume of water in and in between your cells, can have an effect on blood pressure.

How does dehydration have different effects on blood pressure?

How to increase blood volume

When you’re dehydrated, you can lose water, salt, or a mixture of the two. It’s most common for humans to lose both salt and water at the same time. If you’ve lost too much fluid, water found surrounding your cells can move into the bloodstream to help maintain blood pressure and keep your organs and tissues well supplied. But, when your salt level is also reduced, water can’t move as effectively into the bloodstream to counter low blood pressure. Dehydration with salt depletion is known to have a bigger effect on blood pressure than water loss on its own [5].

Why are changes in blood pressure a problem?

A fall in blood pressure decreases your cardiac output, the amount of blood pumped around the body by the heart per minute [6]. A decreased blood pressure also makes it harder for the body to cool down, as decreased blood flow to the skin can decrease sweat rates [7]. In extreme cases, a decreased sweat rate can increase your body temperature to the point that it causes heat stroke. If allowed to drop far enough, decrease in blood pressure can even lead to shock, and sometimes organ damage.

How can I tell if my blood pressure has fallen?

When you stand up, gravity pulls your blood towards your feet. Normally, the blood vessels in your lower body will constrict to stop all the blood from pooling in your legs. But with dehydration, there usually isn’t enough fluid to maintain pressure throughout your body when you stand; you’ll likely feel faint as blood rushes from your head. Your skin will also feel cold as less blood is pumped to the surface, in an attempt to maintain the central circulation above all else. You may also notice an increase in heart rate, as we’ve discussed [2].

To avoid these side effects, you should sip fluids to rehydrate regularly. Mixing a pack of Hydrant into a glass of water makes it easy to make sure your electrolyte balance is in check too!

Lack of Blood Foods

Blood deficiency is a condition which is associated with the inability of the liver to aid in the process of blood formation. This as per Chinese medicine is referred to as ‘qi stagnation’ and can result in abnormal functioning of the liver and damage to the spleen. The primary causes of blood deficiency are poor diet and inappropriate life style patterns which include consumption of excessive amount of alcohol.

Blood Deficiency Symptoms

There are a wide range of symptoms of blood deficiency associated, as the condition tends to affect the entire body.

  • Forgetfulness, poor memory functions
  • Shortness of breath and palpitations
  • Dizziness and fatigue with excessive tiredness on slightest excretion
  • Pale complexion with dry and cracked lips
  • Dryness of the mouth
  • Headache associated with lightheadedness
  • Increased anxiety and apprehension associated with insomnia
  • Menstrual irregularities in females
  • Constipation
  • Tingling and numbness in the limbs
  • Tired feeling on getting up in the morning

Diet to Increase Blood Levels

An array of foods help cope up with blood deficiency.

In addition to the foods, there are certain herbs that work to stimulate the liver functions and enhance the production of blood by the liver. These herbs include fleeceflower root, Chinese foxglove root, donkey hide glue, Chinese angelica root and white peony root.


  • 1 University of North Carolina-Chapel Hill, Chapel Hill, North Carolina, USA. [email protected]
  • PMID: 16432852
  • DOI: 10.1002/ajh.20517

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  • 1 University of North Carolina-Chapel Hill, Chapel Hill, North Carolina, USA. [email protected]
  • PMID: 16432852
  • DOI: 10.1002/ajh.20517


While it has generally been accepted that a 3% change in hematocrit is equivalent to a 1-“unit” loss of blood, few studies have been published to actually document this. A more complete understanding of this correlation would be helpful in estimating blood loss from changes in hematocrit in patients sustaining hemorrhage as well as in those receiving transfusions. In this retrospective study, we analyzed the post-transfusion alterations in hematocrit from 61 independent transfusions in 48 different pelvic fracture patients (age range, 16-62 years). Transfusion volumes were correlated with changes in hematocrit over 24-hr periods between the 5th and 12th hospital days, a time when there was no ongoing hemorrhage in these patients. The average increase in hematocrit per liter of packed red blood cells transfused was 6.4% +/- 4.1%. If 1 “unit” of packed red blood cells is approximately 300 mL, this becomes a change of hematocrit of 1.9% +/- 1.2% per “unit” of blood. The accepted correlation of about 1 “unit” of blood loss per 3% change in hematocrit would be valid for a 500-cc unit, but a typical unit of packed red blood cells is typically 300 cc. In addition, the variability is substantial as indicated by our standard deviation. Limitations of this study include hematocrit changes due to fluid resuscitation, dehydration, age, persistent hemorrhage, and the retrospective nature of the evaluation.