Composition of the Blood

Onto another topic!

Define TBW, ECF, ICF, IF, Lymph, Plasma and Blood
Calculate TBW, ECF, IF and blood volume from sex and body weight
Define and know typical values for PCV 

• TBW: Total Body Water. It's... the total of all of the water in your body, I guess. This is usually around 60% of your weight for males and 50% for females (females have more fat so we get nicer curves :) ). Expanding on that a bit, overweight people will have a lower TBW for their weight, and vice versa.
• ECF: Extracellular Fluid. This is all of the fluid that is outside of the cells. It is roughly 1/3 of your TBW.
• ICF: Intracellular Fluid. This is all of the fluid that is inside the cells. It is roughly 2/3 of your TBW.
• IF: Interstitial Fluid. This is a subset of ECF- it's the ECF in solid organs. This is roughly 1/5 of your ECW, or 1/15 of your TBW.
• Lymph: Lymph is basically your ECF plus a few extra proteins. Essentially, extra water from your tissues is drained through the lymph system back into the venous system in order to be recirculated throughout the body.
• Plasma: Plasma is pretty much everything in blood except for the cells. It is straw-coloured and includes water, proteins, salts, nutrients, wastes and gases. Roughly 20% of ECF is plasma.
• Serum: Plasma without clotting factors.
• Blood: The red stuff that comes out when you get wounded. Need I say more?
• PCV (Packed Cell Volume): The percentage of blood volume that is made up of cells.

Know typical values for blood volume and PCV

I could've done this in conjunction with the list above, but I didn't want the list to be even more cluttered. Here's a separate list. Note that these are only a rough guide: these values vary from person to person, depending on gender, weight, fat content etc.

• TBW: roughly 42L
• ECF: roughly 14L
• ICF: roughly 28L
• IF: roughly 3L
• Blood volume: 5L
• PCV: 42% (female), 45% (male)

List the cells present in blood

More lists!

• Erythrocytes: red blood cells
• Leukocytes: white blood cells. Can be subdivided further into monocytes, eosinophils, basophils, neutrophils or lymphocytes, as I'll explain later.
• Platelets

Describe the role of erythrocytes in the transport of O₂ and CO₂
List the components of haemoglobin and where O₂ binds

As I'm sure you know, erythrocytes, or red blood cells, are biconcave discs that look red (hence the name). They do not have a nucleus or organelles, but they have shitloads of haemoglobin, which is good, because haemoglobin is useful for carrying around O₂ and, to a lesser extent, CO₂.

So what is haemoglobin exactly? It's a protein, just like many other important things in the body. It is made up of four polypeptide chains (a.k.a. "globins"): two α and two β. Each chain is associated with a haem unit (hence the haem unit plus polypeptide chain = haemoglobin!). The main feature of the haem unit is that it contains Fe²⁺, which oxygen can reversibly bind to in order to be transported around the blood. This is how most oxygen travels around the blood- only a small amount (~1.5%) is actually dissolved in the blood.

Carbon dioxide is transported around the blood in several different ways. It is more soluble than oxygen, so around 5% of CO₂ is transported this way. Carbonic anhydrase in the red blood cells can also catalyse the reaction between CO₂ and water to form H⁺ and HCO₃^-, both of which can travel around in solution. A third way that CO₂ can travel around is by binding to haemoglobin or to other proteins in the blood in order to form carbamino compounds (e.g. carbaminohaemoglobin). It is important to note that CO₂ does not bind to Fe²⁺, but to amino groups in haemoglobin.

Now for a note on red blood cell production! Erythrocyte production is regulated by erythropoietin, or Epo for short. It's usually released from the kidneys in response to hypoxia (low oxygen conditions), but can also be stimulated by growth hormone, thyroid hormone or testosterone (I guess you need more blood as you grow and develop...).

List the leukocytes present in blood
Describe the main function of each class of leukocyte

I've already listed the leukocytes present in blood: monocytes, eosinophils, basophils, neutrophils and lymphocytes. Now it's time to talk about them in more detail!

Eosinophils, basophils and neutrophils are collectively known as granulocytes as they contain granules, which contain important chemicals. They are also known as polymorphonuclear leukocytes (poly = many, morpho = shape, nuclear = nucleus, so these leukocytes' nuclei have all kinds of weird and wonderful shapes), but that's a bit of a mouthful to say. Anyway, the three granulocytes are named after the dyes that stain them: eosinophils are stained by the dye eosin, basophils are stained by basic dyes, and neutrophils don't get stained by either.

That's all well and good, you might say, but what about the function of these cells? Neutrophils, the most abundant leukocytes, are normally the first to arrive at the site of an infection, and they are phagocytic (they eat cells), especially towards bacterial cells. Eosinophils are also phagocytic (though only weakly) and are able to destroy parasites, and also play roles in allergies and inflammation. Basophils also play roles in allergies and inflammation. (I assume we'd go into much more detail when we study immunology next semester.)

Now for the non-granular cells, or agranulocytes! These are the monocytes and the lymphocytes. Monocytes are cells that can become macrophages, which basically go around eating everything up. They can also release messenger molecules called cytokines, which enhance the immune response. Lymphocytes come in two main varieties (yup, subcategories of subcategories): B-lymphocytes and T-lymphocytes. B-lymphocytes (which make up roughly 20% of all lymphocytes) are the ones that produce antibodies- this is known as humoral immunity. T-lymphocytes, which are responsible for cell-mediated immunity, come in two varieties (yup, even more subcategorisation): cytotoxic T-cells (kill stuff) and helper T-cells (release cytokines).

Phew!

Describe the function and origin of platelets

Platelets are fragments of megakaryocytes, which are large cells found in the bone marrow. As they are only fragments of the cytoplasm of megakaryocytes, they contain some organelles, but have no nuclei. They adhere to damaged blood vessel walls, and clump together to plug up the gap. They also release growth factors that help repair, as well as help coagulation enzymes form insoluble strands of fibrin to hold the clot together.

Wow, that was long... and we haven't even gotten into the nitty gritty of how everything works yet!