Erythrocytes

CRIT vs. RBC count

Before asking what your erythrocytes(RBCs): Red blood cells responsible for oxygen transport. look like, you might ask how many you have.  Good question.  On a CBC, the RBC count is a fixed number. It represents red blood cellsThe basic structural and functional units of life. in some volume of blood (deciliter, picoliter, etc…).  That is unhelpful to me without a reference range because I don’t know how many should be in a deciliter.  That’s really small.  But, the CRIT can give you an idea of what percentage of your whole blood is your erythrocytes.  CRIT is actually a measurement of all your formed elementsThe cellular components of blood, including red blood cells, white blood cells, and platelets.: erythrocytes, leukocytes, and platelets.  The buffy coatThe thin layer of white blood cells and platelets between plasma and red blood cells in centrifuged is so small. The layer of erythrocytes is so large. Therefore, we can use CRIT as an indicator.

Know that CRIT is like a percentage of red blood cells in your whole blood. You might immediately think of anemiaA condition characterized by a deficiency of red blood cells or hemoglobin, leading to reduced oxyge and low CRIT. That is true if your patient has had anemia for a short period of time. However, if your patient has been suffering from anemia for months, the bone marrow has enough time. It tries to compensate by producing more red blood cells. Of course, this doesn’t help, because each red blood cell is deficient. Making more of them just creates more non functional red blood cells.

Bioconcave shape

Red blood cells have a biconcave shape. 

This shape does two things: maximizes volume and maximizes their surface area to volume ratio.  Of course, maximizing volume is good.  More volume, more oxygen.  On a CBC, volume is measured as the MCV or mean corpuscular volume.  That fancy name just means average volume of a red blood cell.  You might think that MCV should be really high. However, there is an upper limit to the optimal volume of this shape. 

Red blood cells also have no organellestructures within a cell that perform specialized functions..  They exocytozed them long ago so that they could fit in more oxygen-binding hemoglobinThe oxygen-carrying protein in red blood cells that gives blood its red color..  With no nucleusThe control center of the cell that contains DNA and directs cellular activities. and no way to make proteinsLarge molecules made of amino acids with various functions in the body., erythrocytes have no way to repair themselves.  As they circulate, they suffer abrasion. They bend to fit through capillariesThe smallest blood vessels where gas, nutrient, and waste exchange occurs between blood and tissues.. They also bump against every other thing in your whole blood.  After about 120 days of this torture, a erythrocyte says, “I’m out.” It then goes to the spleen or liverA large organ that produces bile, detoxifies blood, and stores nutrients.. There, it can be broken down and recycled.

Importance of RBC Morphology

The volume is limited by the available amount of surface area. This maximizes the ratio of surface area to volume. The bioconcave shape allows for fast pickup of oxygen. It also enables fast drop off of oxygen by maximizing the surface area to volume ratio.  This kitty can provide an analogy.  Let’s make fractions for hot kitty and cold kitty.  Let’s assume that this change in temperature from hot to cold is happening in one day.  So, it’s been a hot afternoon, but the evening and night are significantly colder.  My cat doesn’t gain or lose volume in one day.  Right?  She’s the same volume from afternoon to night.  But, when she’s all stretched out she has a really high surface area.  When she’s all curled up, she has a really low surface area.  These ratios here determine her ability to exchange heat with her surroundings. They are like the surface area to volume ratio of a red blood cell. This ratio influences its ability to drop off or pick up oxygen.

Blood Smear

We’re really interested in how many red blood cells you have in circulation. We’re also focused on the quality of those red blood cells. Are they biconcave or macrocytic?  Are they clumping abnormally?  Which of the items from this chart are present in your patient’s blood?  Just as you would take blood pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. and heart rate, you need to look at red blood cells too.