Blood Learning Objectives

How does blood work as part of the cardiovascular system?

There are three major constituents to the cardiovascular system:

Lymphatics are a related system of vessels and tissues that also exchange materials with the interstitial fluid. Ultimately, the contents of the lymphatic system are returned to major veins in the thorax.

Describe the functional relationship between blood and interstitial fluid.

Vasculature transports blood to and from metabolically active tissues where there is an exchange of materials (between blood, interstitial fluid, and tissues) at specialized vessels called capillaries. Blood is inside capillaries, while interstitial fluid is outside of capillaries, surrounding cells of metabolically active tissues.  Any fluid or materials not returned to the capillaries is typically picked up by lymphatics and ultimately transported to venous circulation in the thorax.

What are the important physicochemical properties of blood?

A person typically has between 4-6 liters of blood, accounting for roughly 8% of their total body weight. Blood is primarily water with a substantial amount of cells, cell products, biological macromolecules, nutrients, molecules, and other substances. Circulating blood is approximately 38°C (100.4°F), and physiological processes maintain a range of blood pH typically between 7.35-7.45.

Describe how the carbonic acid-bicarbonate ion buffer system works to maintain plasma pH homeostasis. 

Blood typically has a pH of 7.35-7.45, which is influenced by a variety of physiological processes involving the sequestering or release of various molecules. A pH of 7.35-7.45 is physiologically neutral, whereas a pH of less than 7.35 is considered acidic (in the blood = acidemia) and a pH of greater than 7.45 is alkaline (alkalemia). 

 The carbonic acid-bicarbonate ion buffer system (CA-BI) is among the principal ways in which the blood may resist drastic changes in pH. Carbonic acid (H2CO3) is formed when carbon dioxide (CO2) is released by metabolically active tissues as a waste product of glucose catabolism and reacts with water (H2O). Carbonic acid may also dissociate into bicarbonate ion (HCO3-) and hydrogen ion (H+). These two reactions are reversible and any changes to the chemical environment of the blood may influence the direction of the reactions.

Lungs & Respiration

Among their major functions, lungs expel carbon dioxide from the body. Faster breathing expels more carbon dioxide, whereas slower breathing expels less carbon dioxide. Rates of respiration are typically 12-20 breaths per minute (bpm). Whenever the blood becomes more acidic (acidemia: acid + -emia=blood), the equation shifts to the left (toward producing more carbon dioxide). The body can compensate for acidemia by increasing the rate of respiration to expel the increasing levels of carbon dioxide. Tachypnea (>20 bpm) may therefore be a symptom of an acidemia, whereas bradypnea (<12 bpm) may be a symptom of alkalemia (as slower breathing helps to retain carbon dioxide). 

Kidneys & Urination

Kidneys are dynamic filters of the blood. Among the many molecules and ions secreted into the urine or retained by the blood are bicarbonate and hydrogen ions. Under acidic conditions, the kidneys may secrete more hydrogen ions and retain more bicarbonate, whereas under alkaline conditions, the reverse is true.

Describe blood as a solution. How do the various solutes in blood vary?

Blood consists of two parts: plasma & formed elements. 

A solution has two components: a solute and a solvent. Blood plasma (55% of blood by volume) is a solution with water (91.5% of blood plasma by volume) as the solvent. Proteins (7% of plasma by volume) are the most abundant solute. There are several types of proteins commonly found in blood:

The remaining solutes consist of nutrients, electrolytes, blood gasses, hormones & cytokines, and waste products.

The addition of formed elements (45% of blood by volume), blood cells and cellular products, makes blood a mixture. There are three types of formed elements:

What are ‘formed elements’ of blood? What are the typical relative abundances of the formed elements? Trace the development of the formed elements.

There are three types of formed elements:

A blood smear is a simple clinical test using light microscopy to gauge the relative abundances and morphologies of the formed elements as well as to inspect for blood parasites. Blood is smeared on a slide and then stained to best visualize the numbers and shapes of cells.

Erythrocyte shapes may fall into one of three categories:

Formed elements arise from hematopoietic connective tissue. Hematopoiesis (or, hemopoiesis) refers to the process of creating new formed elements. A variety of tissues may participate in hematopoiesis, depending on the developmental state at the time.

Prenatal hematopoiesis occurs primarily in the yolk sac (1st trimester), liver & spleen (2nd trimester), and bone marrow (3rd trimester). Postnatal hematopoiesis primarily occurs in the bone marrow with some contributions from lymph nodes. 

Pluripotent stem cells give rise to either myeloid stem cells or lymphoid stem cells. Myeloid stem cells give rise to erythrocytes, thrombocytes, granular leukocytes, mast cells, & monocytes. Lymphoid stem cells give rise to agranular leukocytes (excepting monocytes).

Explain hematopoiesis as a process. How is hematopoiesis regulated?

Hematopoiesis is the development and specialization of formed elements through increasingly specialized cellular pathways. Various regulatory molecules encourage and increase rates of hematopoiesis. 

Cytokines

Cytokines are small signaling molecules that are either peptides or glycoproteins. They are secreted by most nucleated cells, act upon surface membrane receptors of cells, and may act upon the cell that secretes them (autocrine), the tissues around them (paracrine), or distant tissues (endocrine). Cytokines are also pleiotrophic, meaning they may have different effects on different targets. For instance, Interleukin 4 (IL4) effects these cells in the following ways:

Cytokines may also be pathologically dysregulated, e.g. certain conditions can lead to an overabundance of cytokines, or hypercytokinemia (also known as a cytokine storm). This can lead to systemic inflammatory responses and exacerbated immune responses.

Cytokines that stimulate the growth and/or development of formed elements are known as hematopoietic cytokines.

Hematopoietic cytokines include:


What are the major hematological malignancies? From which tissues do they arise?

A malignancy is a condition where abnormal cells divide uncontrollably and can spread to other parts of the body. We colloquially refer to malignancies as cancers. There are three categories of blood cancers, each of variable diversity and prognosis. These include:

The details of each of these cancers are well beyond the scope of this course, but an excellent resource for learning more about them is: http://seer.cancer.gov/


What are blood group systems?

Blood group systems are categorizations of blood (blood types) based on erythrocyte plasma membrane surface antigens (aka agglutinogens). Antigens are substances that can generate an immune response. RBC surface antigens are called agglutinogens because they can crosslink adjacent RBCs thus causing agglutination. There are well over 100 genetically identified RBC surface antigens that have been organized into 35 different recognized systems. The two most commonly used blood types include the ABO and Rh groups.  


How do the genetics of the ABO blood group work? What are the ramifications of the ABO blood group?

The ABO blood group is named for the presence of A, B, AB, or the absence (O) of antigens on the surface of RBCs. The A & B antigens are different glycosphingolipids (lipids containing sphingosine with a covalently bonded carbohydrate). There are various types of antibodies, each with different functions. A person’s blood plasma typically includes IgM (Immunoglobulin-M) antibodies (also known as agglutinins for their cross-linking abilities) for whichever A or B antigen they do not have on their RBCs. A person may have the following blood types:

When an antigen comes into contact with an IgM antibody, agglutination (cell clumping due to cross-linking) occurs. In the blood, agglutination can be life threatening.

As such, a person with Type A blood may only receive blood from a donor with Type A or Type O blood. A person with Type B blood may only receive blood from a donor with Type B or Type O blood. A person with Type AB blood may receive blood from any blood type (universal recipient). A person with Type O blood may only receive blood from another Type O donor, but can donate to any blood type (universal donor). 


What are the ramifications of the Rh blood group?

Blood types have an associated + or -. These indicate Rh factor type. Rh factor (antigen) is a type of integral plasma membrane protein found on RBCs. 

Rh factor (called Rh because it was discovered in Rhesus monkeys) is also known as antigen-D.

The associated antibodies found in the maternal blood plasma are typically both IgG and IgM. IgM antibodies are large and cause agglutination, whereas IgG antibodies are small and are involved in both agglutination and complement reactions (a system for destroying pathogens and surrounding cells). Due to their relative sizes, IgM antibodies cannot cross the placental barrier, whereas IgG antibodies can cross the placental barrier from mother to developing fetus.

Rh status is predominantly an issue for an Rh- mother carrying an Rh+ fetus and typically is most problematic for a second pregnancy. What may happen is that exposure to Rh factor during delivery causes the mother to produce anti-Rh antibodies, the IgG portion of which can cross the placenta in a second pregnancy and attack fetal RBCs, participating in both agglutination and hemolysis (lysing RBCs). This is known as hemolytic disease of the newborn (HDN). HDN is preventable through a simple series of injections with RhoGAM®, a substance that stops the mother from producing anti-Rh antibodies. This therapy is the standard of care for all Rh- pregnancies. 

What are erythrocytes? What are the physical properties of erythrocytes?

Erythrocytes (red blood cells; RBCs) are the most abundant formed element in the blood (4.8-5.4 million/µL). RBCs are without nuclei (anucleate) and lack other organelles (e.g. mitochondria). As a consequence, RBCs are shaped like biconcave discs that are 7-8 µm in diameter. The major constituents of an RBC are water and special heterotetramer (4-unit protein) hemoglobin. An RBC consists of 33% hemoglobin (or 96% hemoglobin if dehydrated). On average an RBC holds about 280 million hemoglobin molecules. As anucleate cells, RBCs have a short life-span, typically lasting only 120 days. RBCs are the principal agents in gas transportation in the blood. As such, the average person produces 2 million RBCs every second.

Describe the structure and functions of hemoglobin. What are the genetics of the globin proteins?

Hemoglobin is a heterotetrameric molecule of 4 globin proteins (polypeptides). There are 6 basic globin proteins that can form a hemoglobin molecule, but the most common 2 globin proteins are: Hbα (alpha) & Hbβ (beta). 

An α-like globin protein is Hbζ (zeta). α & ζ globins are produced by alleles on Chromosome 16.

β-like globin proteins are Hb𝛅 (delta), Hbϒ (gamma), & Hbε (sigma). β and β-like globins are produced by alleles on Chromosome 11. 

97-98% of adult hemoglobin is formed from 2 Hbα & 2 Hbβ = 2α2β. 2α2β is known as HbA (adult hemoglobin). A small percentage of hemoglobins also present in adult blood are 2α2𝛅=HbA2 (Adult 2) and 2α2ϒ=HbF (Fetal).

At the center of each globin is heme, a iron-containing pigment that reversibly bonds to oxygen and other gasses. 

The relative abundances of globin proteins changes throughout development, impacting the frequencies of different types of hemoglobins. α-globin is largely the most dominant globin protein.  ζ-globin is typically only expressed in the first trimester of fetal development, which is very similar to the trajectory of ε-globin. ϒ-globin is the most dominant β-like globin protein during fetal development, and β-globin becomes more frequent post-parturition (birth). 𝛅-globin is produced at low levels post-parturition. 

Describe the significance of hemoglobin’s affinity for: oxygen, carbon dioxide, and carbon monoxide.

Heme groups are sites of cooperative binding between hemoglobin and oxygen molecules. With cooperative binding the more oxygen bound to hemoglobin the more hemoglobin’s affinity for oxygen binding increases (and vice-versa). Oxygen is not the only molecule that can bind to the heme groups. Carbon monoxide can competitively bind to heme as well, to the exclusion of oxygen. In fact, hemoglobin’s affinity for carbon monoxide is much greater than it is for oxygen, which is why elevated concentrations of carbon monoxide are dangerous. 

Other factors can affect hemoglobin’s affinity for oxygen, including:

The relationships between hemoglobin’s affinity for oxygen can be graphically illustrated by plotting hemoglobin saturation vs. partial pressure of oxygen.

Factors increasing affinity will shift the curve left, factors decreasing affinity for oxygen shift the curve right. With increased affinity, hemoglobin is more likely to pick up oxygen from the blood and hold tightly to it. With decreased affinity, hemoglobin holds oxygen less tight and is more likely to dump oxygen into the blood. This arrangement is beneficial, as under certain conditions (e.g. the capillaries of the alveoli of the lungs) it is highly beneficial for hemoglobin to high high affinity and pick up oxygen, and under other conditions (e.g. the capillaries serving metabolically active tissues) it is highly beneficial for hemoglobin to release oxygen. 

Hemoglobin also has allosteric binding sites for other molecules (CO2 & H+, and nitric oxide).

Describe erythropoiesis. What is blood doping?

Erythropoiesis is the process of erythrocyte formation. In fetal development, erythropoiesis begins in the yolk sac, then the liver and spleen. Post-parturition, erythropoiesis occurs in red bone marrow. You would be correct to infer a trend between location of erythropoiesis and globin protein production.

As part of the suite of homeostatic mechanisms, the body works to maintain sufficient processes for oxygen transport and delivery. If levels of blood oxygen are low, specialized cells in the kidneys (fibroblasts called peritubular interstitial cells) secrete a substance called erythropoietin (EPO). In the fetus, EPO is secreted by perisinusoidal cells of the liver. EPO is the principal hormone driving erythropoiesis. With low levels of blood oxygen (hypoxia), levels of circulating EPO rise 1000-fold from 10 mU/ml to 10,000  mU/ml in the blood. EPO acts upon recruitment and development of erythrocyte precursor cells, starting with CFU-E, pro-erythroblasts, etc. 

Hypoxia-inducible factors (HIFs) are transcription factors that help cells respond to hypoxia in a number of ways, one of which is to initiate EPO production. 

Blood doping is a process to elevate the amount of erythrocytes, usually to gain an unfair athletic advantage. Blood doping may be accomplished through transfusions of RBCs either from one’s own stored supply (autologous) or from another person (homologous). Blood doping may also be accomplished through injections of exogenous EPO. Athletes may also choose to train under hypoxic conditions (e.g. high elevations) to increase their RBCs, although this generally isn’t viewed as cheating. A major risk of blood doping is polycythemia, or having too many RBCs. Whenever the total volume of RBCs exceeds 60% of blood volume, a person is at risk for spontaneous clotting.   

Define, and describe the significance of, hematocrit. What is anemia? What is hypoxia?

Hematocrit is the percentage of RBCs by blood volume. Typical hematocrit in an adult is 38-54%. Hematocrit below 38% is called anemia (a=without, -emia=blood). Hematocrit above 54% is called polycythemia. 

Critical thresholds for hematocrit include:

As hematocrit is a percentage of blood volume, it may be affected by either the number of RBCs or the amount of plasma. Decreased hematocrit is anemia, which is the state of reduced oxygen carrying capacity of the blood. There are many different types of anemia, including, but not limited to:

Anemia may also be the result of hemorrhage, menstruation, dehydration, etc. It is important to note that post-hemorrhage blood plasma may rebound quickly, but hematocrit is slow to rebound as the process of erythropoiesis takes about 10 days.

Hypoxia is the condition of having generally too little oxygen in the tissues. This can be the result of being in a hypoxic environment (insufficient environmental oxygen), or there being a pathological issue preventing the distribution of oxygen. Hypoxia is sometimes used interchangeably with hypoxemia, which is decreased oxygen levels in the blood. Anemia often causes hypoxia.  


What are leukocytes? What are the various types of leukocytes, and what are their functions? What information might a differential white blood count provide?

Leukocytes (leuko=white, cytes=cells)/white blood cells/WBCs are a diverse group of blood cells involved in immune protection from pathogens and foreign substances. WBCs are classified by their developmental lineage and their physical characteristics. Most WBCs exist as formed elements in the blood, whereas some may emigrate to interstitial tissues as part of their action, and some may also exist in tissues and also re-enter the blood.

When dealing with pathogens and foreign substances, there are two major approaches: innate (present at birth; no previous exposure to pathogens) & acquired (adaptive; requires experience with pathogens) immunity. Most WBCs function as part of innate immunity, whereas some (e.g. most lymphocytes) are the basis for acquired immunity.

There are three common actions of WBCs that enable them to be effective as part of innate immunity:

Chemotaxis & emigration: some WBCs can ‘detect’ the presence of a pathogen and exit the bloodstream to deal with it

To clearly see tissues and cells using light microscopy dyes are added to stain cells and enhance features for visualization. A common stain combination is hematoxylin and eosin (H & E). Hematoxylin is a basic, blue stain readily absorbed by nuclei and certain organelles. Eosin is an acidic, red stain readily absorbed by cytoplasm.        

Among the many derivatives from myeloid stem cells, there are three lineages of WBCs: granular leukocytes, monocytes, and mast cells.

Granular leukocytes get their name from inclusion granules within their cytoplasm. These cells include Basophils, Eosinophils, and Neutrophils (BEN), each named for their appearance after H&E staining.

Basophils:

Eosinophils:

Neutrophils:

Monocytes:

Mast cells:

Lymphoid stem cells produce lymphocytes: T cells (adaptive), B cells (adaptive), and Natural Killer cells (innate), the actions of which will be discussed in the Lymphatics Sessions. 

As a whole, leukocytes account for about 4,000-11,000 cells/μl blood. Elevated levels of leukocytes is known as leukocytosis, whereas low levels of leukocytes is known as leukopenia. A common blood test is a differential white blood count, which looks at the relative abundances of all WBCs and their morphologies to elucidate potential issues or infections.

What are thrombocytes? How do they work? What is TPO, and how does it work?

Thrombocytes (thrombo=clot, cyte=cell), better known as platelets, are pieces of cytoplasm that are a vital part of the coagulation (clotting) process. Platelets are small fragments without nuclei and appear as stained fragments under a microscope. Platelets are derived from megakaryocytes, which are very large, polyploid (up to 64N!) cells in bone marrow.  As megakaryocytes break down, they become pro-platelets and finally platelets.

Platelets:

Coagulation (clotting) is an important protective process for when there is vascular injury. The process of coagulation is very physiologically detailed, involving many factors and co-factors beyond the scope of an anatomy course. With vascular injury, soluble fibrin becomes insoluble, which forms a meshwork that works with activated platelets to form a plug to slow and stop bleeding.