What are the main elements of blood

ЛЕКЦИЯ № 15. Blood

ЛЕКЦИЯ № 15. Blood

Blood is considered a modified type of connective tissue. Mesoder-mal in origin, it is composed of cells and cell frag ments (erythrocytes, leukocytes, platelets), fibrous proteins (fibrinogen – fibrin during clotting), and an extracellular amorphous ground substance of fluid and proteins (plasma). Blood carries oxygen and nutrients to all cells of the body and waste materials away from cells to the kidney and lungs. It also contains cellular elements of the immune system as well as humoral factors. This chapter will discuss the differ ent elements of blood and the processes by which they are formed.

Formed elements of the blood

The formed elements of the blood include erythrocytes, leukocytes, and platelets.

Erythrocytes, or red blood cells, are important in trans porting oxygen from the lungs to tissues and in returning carbon dioxide to the lungs. Oxygen and carbon dioxide carried in the RBC combine with hemoglobin to form oxyhemoglobin and carbaminohemoglobin, respectively.

Mature erythrocytes are denucleated, biconcave disks with a diameter of 7-8 mm. The biconcave shape results in a 20-30% increase in sur face area compared to a sphere.

Erythrocytes have a very large surface area: volume ratio that allows for efficient gas transfer. Erythrocyte membranes are remarkably pliable, enabling the cells to squeeze through the narrowest capillaries. In sickle cell anemia, this plasticity is lost, and the subsequent clogging of capillaries leads to sickle crisis. The normal concentration of erythrocytes in blood is 3,5-5,5 million/mm 3 in women and 4,3– 5,9 million/mm 3 in men. Higher counts in men are attributed to the erythrogenic androgens. The packed volume of blood cells per total volume of known as the hematocrit. Normal hematocrit values are 46% for women and 41-53% for men.

When aging RBCs develop subtle changes, macrophages in the bone marrow, spleen, and liver engulf and digest them. The iron is carried by transferring in the blood to certain tissues, where it combines with apoferritin to form ferritin. The heme is catabolized into biliver-din, which is converted to bilirubin. The latter is secreted with bile salts.

Leukocytes, or white blood cells, are primarily with the cellular and humoral defense of the organism foreign materials. Leukocytes are classified as granulocytes (neutrophils, eosinophils, basophils) and agranulocytes (lympmonocytes).

Granulocytes are named according to the staining properties of their specific granules. Neutrophils sare 10-16 mm in diameter.

They have 3-5 nuclear lobes and contain azurophilic granules (ly-sosomes), which contain hydrolytic enzymes for bacterial destruction, in their cytoplasm. Specific granules contain bactericidal enzymes (e. g., lysozyme). Neutrophils are phagocytes that are drawn (chemo-taxis) to bacterial chemoattractants. They are the primary cells involved in the acute inflammatory response and represent 54-62% of leukocytes.

Eosinophils: they have a bilobed nucleus and possess acid granulations in their cytoplasm. These granules contain hydrolytic enzymes and peroxidase, which a discharged into phagocytic vacuoles.

Eosinophils are more numerous in the blood durii asitic infections and allergic diseases; they norma asent onlyi – 3% of leukocytes.

Basophils: they possess large spheroid granules, which are basophi-lic and metachromatic, due to heparin, a glycosaminoglycan. Their granules also contain histamine.

Basophils degranulate in certain immune reaction, releasing hepa-rin and histamine into their surroundings. They also release additional vasoactive amines and slow reacting substance of anaphylaxis (SRS-A) consisting of leukotrienes LTC4, LTD4, and LTE4. They represent less than 1% – of leukocytes.

Agranulocytes are named according to their lack of specific granules. Lymphocytes are generally small cells measuring 7-10 mm in diameter and constitute 25-33% of leukocytes. They con tain circular dark-stained nuclei and scanty clear blue cyto plasm. Circulating lymphocytes enter the blood from the lymphatic tissues. Two principal types of immunocompetent lymphocytes can be identified using im-munologic and bio chemical techniques: T lymphocytes and В lymphocytes.

T cells differentiate in the thymus and then circulate in the peripheral blood, where they are the principal effec tors of cell-mediated immunity. They also function as helper and suppressor cells, by modulating the immune response through their effect on В cells, plasma cells, macrophages, and other T Cells.

В cells differentiate in bone marrow and possibly in the gut-associated lymphatic tissues (GALT). They are the principal mediators of humoral immunity through their production of antibodies. Once activated by contact with an antigen, they differentiate into plasma cells, which synthesize antibodies that are secreted into the blood, intercellular fluid, and lymph. В lymphocytes also give rise to memory cells, which differentiate into plas ma cells only after the second exposure to the antigen. They are responsible for the secondary, or amnestic response that occurs when the body is exposed to an antigen for a second time. Monocytes vary in diameter from 15-18 mm and are the largest of the peripheral blood cells. They constitute 3-7% of leukocytes.

Monocytes possess an eccentric U-shaped or kidney-shaped nucleus. The cytoplasm has a ground-glass appearance and fine azurophi-lic granules.

Their nuclei stain lighter than lymphocyte nuclei because of their loosely arranged chromatin.

Monocytes are the precursors for members of the mononuclear phagocyte system, including tissue macrophages (histiocytes), osteoclasts, alveolar macrophages, and Kupffer cells of the liver.

Platelets (thromboplastids) are 2-3 mm in diameter.

They are a nuclear, membrane-bound cellular fragments derived by cytoplasmic fragmentation of giant cells, called megakaryocytes, in the bone marrow.

They have a short life span of approximately 10 days.

There are normally 150 000-400 000 platelets per mm 3 of blood. Ultrastructurally, platelets contain two portions: a peripheral, light-staining hyalomere that sends out fine cytoplasmic processes, and a central, dark-staining granulomere that con tains mitochondria, va-cuoles, glycogen granules, and granules. Platelets seal minute breaks in blood vessels and maintain endothelial integrity by adhering to the damaged vessel in a process known as platelet aggregation. Platelets are able to form a plug at the rupture site of a vessel because their mem brane permits them to agglutinate and adhere to surfaces.

Platelets aggregate to set up the cascade of enzymatic reac tions that convert fibrinogen into the fibrin fibers that make up the clot.

Blood is connective tissue, flowing through capillaries, veins, and arteries of all vertebrates. The red color is characteristic due to the presence of hemoglobin in the red blood cells.

It is a type of connective tissue specialized with a matrix colloidal liquid and a complex constitution. It is a solid phase (formed elements), which includes the erythrocytes (or red blood cells), the leukocytes (white blood cells) and platelets, and a liquid phase, represented by blood plasma.

These phases are also called blood components, which are divided into the serum components (liquid phase) and cellular component (solid phase).

Its main function is the logistics of distribution and systemic integration, whose containment in the blood vessels (vascular space) supports its distribution (blood circulation) to almost the entire body.

Table of Contents

Functions of Blood:

Blood composition(or) Blood Components

Like any tissue, blood consists of cells and extracellular components (the extracellular matrix). These two tissue fractions (or) blood components are represented by:

The formed elements constitute about 45% of the blood. Such percentage magnitude is known of hematocrit (cell fraction), ascribable almost entirely to the red cell mass. The other 55% is represented by the blood plasma (acellular fraction).

Cellular Fraction:

Formed elements of the blood are varied in size, structure, and function, and are grouped into:

Red Blood Cells (RBC):

Red blood cells, red blood cells or red blood cells constitute about 96% of figurative elements. Its normal value (count) average is around 4,800,000 in women, and about 5,400,000 in the male, red blood cells per mm ³ (or microliter).

These corpuscles lack nucleus and organelles (only in mammals). Their cytoplasm is composed almost entirely by hemoglobin a protein responsible for transporting oxygen and also contains some enzymes. The carbon dioxide is transported in the blood (free dissolved 8%, and 27% carbamino compounds, and as bicarbonate, the latter regulating the pH in the blood). In the plasma membrane of the erythrocytes are glycoprotein (CDs) that define the different blood groups and other cell identifiers.

Erythrocytes have biconcave disc shape depressed in the center. This particular shape increases the effective membrane surface. Mature red blood cells lack a nucleus because it expelled in the bone marrow before entering the bloodstream (this does not occur in birds, amphibians and certain other animals). Adult human erythrocytes are formed in the bone marrow.

Hemoglobin:

Normal hemoglobin levels are between 12 and 18 g / dl of blood, and this amount is proportional to the quantity and quality of red blood cells (red cell mass). Hemoglobin makes up 90% of erythrocytes and, as a pigment, gives its characteristic red color, although this only occurs when the red blood cell is loaded with oxygen.

After a half-life of 120 days, erythrocytes are destroyed and removed from the blood by the spleen, the liver and the bone marrow, where hemoglobin is degraded in bilirubin and iron is recycled to form new hemoglobin.

White Blood Cells (WBC):

The white blood cells are part of cellular players’ immune system and are cells with migratory capacity using the blood as a vehicle to access different parts of the body. Leukocytes are responsible for destroying infectious agents and infected cells and secrete protective substances such as antibodies, which fight infections.

Normal leukocyte count is within a range of 4,500 to 11,500 cells permm³ (or microliter) blood, varies according to the physiological conditions (pregnancy, stress, sport, age, etc.) and pathological (infection, cancer, immune suppression, aplasia, etc.). The percentage counts of different types of leukocytes are called “differential count” (see CBC, below).

According to the microscopic characteristics of their cytoplasm (staining) and core (morphology), they are divided into:

Granulocytes or Polymorphonuclear cells:

Agranulocytes or Monomorphonuclear cells:

B-Lymphocytes

B-lymphocytes are responsible for humoral immunity, i.e. antibody secretion (substances that recognize and bind to bacteria and allow them their phagocytoses and destruction). Granulocytes and monocytes can better recognize and destroy bacteria when antibodies are attached to them (opsonization). Cells are also responsible for the production of some components of blood serum, called immunoglobulin.

T-Lymphocytes

T-lymphocytes recognize the infected cells and destroy the virus using macrophages. These cells amplify or suppress the overall immune response by regulating the other components of the immune system, and secrete many cytokines. They constitute 70% of all lymphocytes.

Both the T lymphocytes and B have the ability to “remember” previous exposure to a specific antigen, and when there is a new exposure to it, the action of the immune system will be more effective.

Platelets (or) Thrombocytes:

Platelets (thrombocytes) are small cell fragments (2-3 mm in diameter), oval and coreless. They are produced in the bone marrow from the cytoplasm fragmentation of megakaryocytes being free in the bloodstream. The normal quantitative value is between 250,000 and 450,000 platelets per mm³.

Blood plasma Fraction:

Blood plasma is the liquid portion of blood in which the formed elements are embedded. It is the major component of blood, representing 55% of total blood volume, with about 40-50 mL / kg weight. It is salty and yellowish translucent. Besides transporting blood cells, it carries nutrients and waste substances contained in the cells.

Blood plasma is essentially a solution aqueous, slightly denser than water, with 91% water, 8% protein and trace amounts of other materials. Plasma is a mixture of many vital proteins, amino acids, carbohydrates, lipids, salts, hormones, enzymes, antibodies, urea, dissolved gases and inorganic substances such as sodium, potassium, calcium chloride, carbonate, and bicarbonate. Among these proteins are fibrinogen (for coagulation), globulins (regulate the water content in the cell form antibodies against infectious disease), albumin (exert osmotic pressure to deliver water between plasma and body fluids) and lipoproteins (buffer the pH changes in the blood cells and makes the blood more viscous than water).

Other major plasma proteins act as carriers to tissues of essential nutrients such as copper, iron, other metals, and various hormones. Plasma components are formed in the liver (albumin and fibrinogen), endocrine glands (hormones), and others in the intestine.

When the blood clots and clotting factors are consumed, the remaining fluid fraction is called serum blood.

Blood Basics

Blood is a specialized body fluid. It has four main components: plasma, red blood cells, white blood cells, and platelets. Blood has many different functions, including:

The blood that runs through the veins, arteries, and capillaries is known as whole blood, a mixture of about 55 percent plasma and 45 percent blood cells. About 7 to 8 percent of your total body weight is blood. An average-sized man has about 12 pints of blood in his body, and an average-sized woman has about nine pints.

The Components of Blood and Their Importance

If you or someone you care about is diagnosed with a blood disorder, your primary care physician may refer you to a hematologist for further testing and treatment.

Plasma

Red Blood Cells (also called erythrocytes or RBCs)

Production of red blood cells is controlled by erythropoietin, a hormone produced primarily by the kidneys. Red blood cells start as immature cells in the bone marrow and after approximately seven days of maturation are released into the bloodstream. Unlike many other cells, red blood cells have no nucleus and can easily change shape, helping them fit through the various blood vessels in your body. However, while the lack of a nucleus makes a red blood cell more flexible, it also limits the life of the cell as it travels through the smallest blood vessels, damaging the cell’s membranes and depleting its energy supplies. The red blood cell survives on average only 120 days.

Red cells contain a special protein called hemoglobin, which helps carry oxygen from the lungs to the rest of the body and then returns carbon dioxide from the body to the lungs so it can be exhaled. Blood appears red because of the large number of red blood cells, which get their color from the hemoglobin. The percentage of whole blood volume that is made up of red blood cells is called the hematocrit and is a common measure of red blood cell levels.

White Blood Cells (also called leukocytes)

White blood cells protect the body from infection. They are much fewer in number than red blood cells, accounting for about 1 percent of your blood.

The most common type of white blood cell is the neutrophil, which is the «immediate response» cell and accounts for 55 to 70 percent of the total white blood cell count. Each neutrophil lives less than a day, so your bone marrow must constantly make new neutrophils to maintain protection against infection. Transfusion of neutrophils is generally not effective since they do not remain in the body for very long.

The other major type of white blood cell is a lymphocyte. There are two main populations of these cells. T lymphocytes help regulate the function of other immune cells and directly attack various infected cells and tumors. B lymphocytes make antibodies, which are proteins that specifically target bacteria, viruses, and other foreign materials.

Platelets (also called thrombocytes)

Unlike red and white blood cells, platelets are not actually cells but rather small fragments of cells. Platelets help the blood clotting process (or coagulation) by gathering at the site of an injury, sticking to the lining of the injured blood vessel, and forming a platform on which blood coagulation can occur. This results in the formation of a fibrin clot, which covers the wound and prevents blood from leaking out. Fibrin also forms the initial scaffolding upon which new tissue forms, thus promoting healing.

A higher than normal number of platelets can cause unnecessary clotting, which can lead to strokes and heart attacks; however, thanks to advances made in antiplatelet therapies, there are treatments available to help prevent these potentially fatal events. Conversely, lower than normal counts can lead to extensive bleeding.

Complete Blood Count (CBC)

A complete blood count (CBC) test gives your doctor important information about the types and numbers of cells in your blood, especially the red blood cells and their percentage (hematocrit) or protein content (hemoglobin), white blood cells, and platelets. The results of a CBC may diagnose conditions like anemia, infection, and other disorders. The platelet count and plasma clotting tests (prothombin time, partial thromboplastin time, and thrombin time) may be used to evaluate bleeding and clotting disorders.

Your doctor may also perform a blood smear, which is a way of looking at your blood cells under the microscope. In a normal blood smear, red blood cells will appear as regular, round cells with a pale center. Variations in the size or shape of these cells may suggest a blood disorder.

Where Do Blood Cells Come From?

Where Can I Find More Information?

If you are interested in learning more about blood diseases and disorders, here are a few other resources that may be of some help:

The American Society of Hematology (ASH) Education Book, updated yearly by experts in the field, is a collection of articles about the current treatment options available to patients. The articles are categorized here by disease type. If you are interested in learning more about a particular blood disease, we encourage you to share and discuss these articles with your doctor.

Search Blood, the official journal of ASH, for the results of the latest blood research. While recent articles generally require a subscriber login, patients interested in viewing an access-controlled article in Blood may obtain a copy by e-mailing a request to the Blood Publishing Office.

This section includes a list of Web links to patient groups and other organizations that provide information.

Blood function and composition

Blood facts

Functions of blood

Blood has three main functions: transport, protection and regulation.

Transport

Blood transports the following substances:

Protection

Blood has several roles in inflammation:

Regulation

Blood helps regulate:

Composition of blood

Blood is classified as a connective tissue and consists of two main components:

The formed elements are so named because they are enclosed in a plasma membrane and have a definite structure and shape. All formed elements are cells except for the platelets, which are tiny fragments of bone marrow cells.

Formed elements are:

Leukocytes are further classified into two subcategories called granulocytes which consist of neutrophils, eosinophils and basophils; and agranulocytes which consist of lymphocytes and monocytes.

The formed elements can be separated from plasma by centrifuge, where a blood sample is spun for a few minutes in a tube to separate its components according to their densities. RBCs are denser than plasma, and so become packed into the bottom of the tube to make up 45% of total volume. This volume is known as the haematocrit. WBCs and platelets form a narrow cream-coloured coat known as the buffy coat immediately above the RBCs. Finally, the plasma makes up the top of the tube, which is a pale yellow colour and contains just under 55% of the total volume.

Blood plasma

Blood plasma is a mixture of proteins, enzymes, nutrients, wastes, hormones and gases. The specific composition and function of its components are as follows:

Proteins

These are the most abundant substance in plasma by weight and play a part in a variety of roles including clotting, defence and transport. Collectively, they serve several functions:

There are three major categories of plasma proteins, and each individual type of proteins has its own specific properties and functions in addition to their overall collective role:

Amino acids

These are formed from the break down of tissue proteins or from the digestion of digested proteins.

Nitrogenous waste

Being toxic end products of the break down of substances in the body, these are usually cleared from the bloodstream and are excreted by the kidneys at a rate that balances their production.

Nutrients

Those absorbed by the digestive tract are transported in the blood plasma. These include glucose, amino acids, fats, cholesterol, phospholipids, vitamins and minerals.

Gases

Some oxygen and carbon dioxide are transported by plasma. Plasma also contains a substantial amount of dissolved nitrogen.

Electrolytes

The most abundant of these are sodium ions, which account for more of the blood’s osmolarity than any other solute.

Red blood cells

Red blood cells (RBCs), also known as erythrocytes, have two main functions:

An erythrocyte is a disc-shaped cell with a thick rim and a thin sunken centre. The plasma membrane of a mature RBC has glycoproteins and glycolipids that determine a person’s blood type. On its inner surface are two proteins called spectrin and actin that give the membrane resilience and durability. This allows the RBCs to stretch, bend and fold as they squeeze through small blood vessels, and to spring back to their original shape as they pass through larger vessels.

RBCs are incapable of aerobic respiration, preventing them from consuming the oxygen they transport because they lose nearly all their inner cellular components during maturation. The inner cellular components lost include their mitochondria, which normally provide energy to a cell, and their nucleus, which contains the genetic material of the cell and enable it to repair itself. The lack of a nucleus means that RBCs are unable to repair themselves. However, the resulting biconcave shape is that the cell has a greater ratio of surface area to volume, enabling O₂ and CO₂ to diffuse quickly to and from Hb.

The cytoplasm of a RBC consists mainly of a 33% solution of haemoglobin (Hb), which gives RBCs their red colour. Haemoglobin carries most of the oxygen and some of the carbon dioxide transported by the blood.

Circulating erythrocytes live for about 120 days. As a RBC ages, its membrane grows increasingly fragile. Without key organelles such as a nucleus or ribosomes, RBCs cannot repair themselves. Many RBCs die in the spleen, where they become trapped in narrow channels, broken up and destroyed. Haemolysis refers to the rupture of RBCs, where haemoglobin is released leaving empty plasma membranes which are easily digested by cells known as macrophages in the liver and spleen. The Hb is then further broken down into its different components and either recycled in the body for further use or disposed of.

White blood cells

White blood cells (WBCs) are also known as leukocytes. They can be divided into granulocytes and agranulocytes. The former have cytoplasms that contain organelles that appear as coloured granules through light microscopy, hence their name. Granulocytes consist of neutrophils, eosinophils and basophils. In contrast, agranulocytes do not contain granules. They consist of lymphocytes and monocytes.

Granulocytes

Agranulocytes

Platelets

Platelets are small fragments of bone marrow cells and are therefore not really classified as cells themselves.

Platelets have the following functions:

The first three functions listed above refer to important haemostatic mechanisms in which platelets play a role in during bleeding: vascular spasms, platelet plug formation and blood clotting (coagulation).

Vascular spasm

This is a prompt constriction of the broken blood vessel and is the most immediate protection against blood loss. Injury stimulates pain receptors. Some of these receptors directly innervate nearby blood vessels and cause them to constrict. After a few minutes, other mechanisms take over. Injury to the smooth muscle of the blood vessel itself causes a longer-lasting vasoconstriction where platelets release a chemical vasoconstrictor called serotonin. This maintains vascular spasm long enough for the other haemostatic mechanisms to come into play.

Platelet plug formation

Under normal conditions, platelets do not usually adhere to the wall of undamaged blood vessels, since the vessel lining tends to be smooth and coated with a platelet repellent. When a vessel is broken, platelets put out long spiny extensions to adhere to the vessel wall as well as to other platelets. These extensions then contract and draw the walls of the vessel together. The mass of platelets formed is known as a platelet plug, and can reduce or stop minor bleeding.

Coagulation

This is the last and most effective defence against bleeding. During bleeding, it is important for the blood to clot quickly to minimise blood loss, but it is equally important for blood not to clot in undamaged vessels. Coagulation is a very complex process aimed at clotting the blood at appropriate amounts. The objective of coagulation is to convert plasma protein fibrinogen into fibrin, which is a sticky protein that adheres to the walls of a vessel. Blood cells and platelets become stuck to fibrin, and the resulting mass helps to seal the break in the blood vessel. The forming of fibrin is what makes coagulation so complicated, as it involved numerous chemicals reactions and many coagulation factors.

Production of blood

Haemopoiesis

Haemopoiesis is the production of the formed elements of blood. Haemopoietic tissues refer to the tissues that produce blood. The earliest haemopoietic tissue to develop is the yolk sac, which also functions in the transfer of yolk nutrients of the embryo. In the foetus, blood cells are produced by the bone marrow, liver, spleen and thymus. This changes during and after birth. The liver stops producing blood cells around the time of birth, while the spleen stops producing them soon after birth but continues to produce lymphocytes for life. From infancy onwards, all formed elements are produced in the red bone marrow. Lymphocytes are additionally produced in lymphoid tissues and organs widely distributed in the body, including the thymus, tonsils, lymph nodes, spleen and patches of lymphoid tissues in the intestine.

Erythropoesis

Erythropoiesis refers specifically to the production of erythrocytes or red blood cells (RBCs). These are formed through the following sequence of cell transformations:

The proerythroblast has receptors for the hormone erythropoietin (EPO). Once EPO receptors are in place, the cell is committed to exclusively producing RBCs. The erythroblasts then multiply and synthesise haemoglobin (Hb), which is a red oxygen transport protein. The nucleus from the erythroblasts is then discarded, giving rise to cells named reticulocytes. The overall transformation from haemocytoblast to reticulocytes involves a reduction in cell size, an increase in cell number, the synthesis of haemoglobin, and the loss of the cell nucleus. These reticulocytes leave the bone marrow and enter the bloodstream where they mature into erythrocytes when their endoplasmic reticulum disappears.

Leukopoiesis

Leukopoiesis refers to the production of leukocytes (WBCs). It begins when some types of haemocytoblasts differentiate into three types of committed cells:

These cells have receptors for colony-stimulating factors (CSFs). Each CSF stimulates a different WBC type to develop in response to specific needs. Mature lymphocytes and macrophages secrete several types of CSFs in response to infections and other immune challenges. The red bone marrow stores granulocytes and monocytes until they are needed in the bloodstream. However, circulating leukocytes do not stay in the blood for very long. Granulocytes circulate for 4-8 hours and then migrate into the tissues where they live for another 4-5 days. Monocytes travel in the blood for 10-20 hours, then migrate into the tissues and transform into a variety of macrophages which can live as long as a few years. Lymphocytes are responsible for long-tern immunity and can survive from a few weeks to decades. They are continually recycled from blood to tissue fluid to lymph and finally back to the blood.

Thrombopoiesis

Thrombopoiesis refers to the production of platelets in the blood, because platelets used to be called thrombocytes. This starts when a haemocytoblast develops receptors for the hormone thrombopoietin which is produced by the liver and kidneys. When these receptors are in place, the haemocytoblast becomes a committed cell called a megakaryoblast. This replicates its DNA, producing a large cell called a megakaryocyte, which breaks up into tiny fragments that enter the bloodstream. About 25-40% of the platelets are stored in the spleen and released as needed. The remainder circulate freely in the blood are live for about 10 days.

Ageing changes in the blood

The properties of blood change as we grow older. It is thought that these changes might contribute to the increased incident of clot formation and atherosclerosis in older people. Some of the most prominent findings on these changes include:

The increased level of plasma fibrinogen is thought to be due to either its rapid production or slower degradation. As age progresses, fibrinogen and plasma viscosity tend to be positively correlated, with the rise in plasma viscosity being largely attributed to the rise in fibrinogen.

The viscosity of blood depends on factors such as shear rate, haemocrit, red cell deformability, plasma viscosity and red cell aggregation. Although there are many factors involved, hyperviscosity syndrome can be generated by a rise in only one factor. A state of hyperviscosity causes sluggish blood flow and reduced oxygen supply to the tissue.

An age-dependent increase in various coagulation factors, a positive correlation with fibrinogen and a negative correlation with plasma albumin has also been found. Both platelet and red cell aggregation increase with age, with red cell aggregation appearing to be the primary factor responsible for a rise in blood viscosity at low shear rates.

The decrease in red cell deformability (increase in rigidity) refers to its ability to deform under flow forces. Less deformable cells offer more resistance to flow in the microcirculation, which influences the delivery of oxygen to the tissues. Studies have found that older people have less fluid membranes in their red cells.

Blood H+ has also been found to be positively correlated with age, making the blood slightly more acidic as we age. This results in a swelling of the cell, making the red cells less deformable. This sets up a cycle for further increase in blood viscosity and worsening of blood flow parameters.

Since ageing causes a reduction in total body water, blood volume decreases due to less fluid being present in the bloodstream. The number of red blood cells, and the corresponding haemoglobin and haemocrit levels, are reduced which contributes to fatigue in the individual. Most of the white blood cells stay at their original levels, although there is a decrease in lymphocyte number and ability to fight off bacteria, leading to a reduced ability to resist infection.

Overall, the rise in fibrinogen is the most common and significant change in blood during ageing because it contributes to a rise in plasma viscosity, red blood cell aggregation and a rise in blood viscosity at low shear rates. Increased age is associated with a state of hypercoagulation of blood, making older people more susceptible to clot formation and atherosclerosis.

More information

For more information on blood, blood types, blood tests, and blood donation and transfusion, see Blood.

References

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Blood

Author: Dr. Christopher A. Becker • Reviewer: Dimitrios Mytilinaios MD, PhD
Last reviewed: March 28, 2022
Reading time: 8 minutes

Blood makes up about 8% of the human body weight. It contains erythrocytes, leucocytes, thrombocytes (platelets) and plasma.

The volume percentage of all blood cells in the whole blood is about 45% of adults (hematocrit). The rest consists of liquid plasma (e.g. water, plasma proteins, electrolytes etc.).

The blood is composed of:

Function

Messenger and waste removal

Blood is the most important transport medium in the human body. It transports gases (oxygen, carbon dioxide, nitrogen etc.) as well as nutrients (metabolism) and end products of cell metabolism. Hence the blood has the task of assuring the exchange of substances. It provides the tissues with blood gases and nutrients and in exchange transports end products (e.g. carbon dioxide, urea, uric acid, creatinine etc.) to the eliminating organs (lung, liver, kidney). Furthermore, it carries chemical messengers (hormones) to their target organs.

Acid-Base Balance

The acid-base homeostasis is regulated in the blood through the diffusion of gases between alveoli and blood in the lung (alveolar diffusion) oxygen diffuses from the alveoli into the blood due to the concentration gradient. It is taken up by the carrying protein hemoglobin (hem = iron-containing, globin = protein). Contrariwise carbon dioxide diffuses from the blood into the alveoli due to its higher blood concentration where it is breathed out.

Start with the structure and function of blood with our study unit.

Oxygen supply and carbon dioxide removal

The blood transports the oxygen from the alveoli to the remotest cells of the body. Because of the higher gas pressure in the plasma (relative to the cells), it diffuses to the tissues.

Carbon dioxide diffuses from the cells into the blood due to the higher gas pressure in the tissue. Here it undergoes a chemical reaction and forms carbonic acid (CO2 + H2O → H2CO3) which dissociates into a hydrogen ion (H+) and bicarbonate (HCO3-). Thus the metabolism end product carbon dioxide is transported in the form of carbonic acid (or rather hydrogen ion and bicarbonate). In the lung, the above mentioned chemical reaction reverses and carbon dioxide is exhaled.

To sum it up the blood regulates the acid-base homeostasis by the gas exchange. The blood is also responsible for the homeostasis, e.g. balancing the water between the blood capillaries on the one hand and intracellular and extracellular space on the other hand. It also maintains constant body temperature.

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Coagulation

Coagulation factors (proteins) are solved in the blood and stop bleeding after a complex (cascade-like) activation of coagulation factors through damage to blood vessels finally leading to the building of thrombus (thrombogenesis). Simultaneously, fibrinogen/fibrin prevents the pathological development of blood clots in the blood vessels. Blood coagulation and fibrinolysis influence each other and maintain a sensitive equilibrium.

Blood cellular components

Erythrocytes

The function of the erythrocytes is the transport of oxygen from the lung to the tissue by bonding oxygen to the iron-containing heme group of the hemoglobin. Erythrocytes are round and have a biconcave shape as they have no nucleus. An erythrocyte has a diameter of 8 to 10 µm. A healthy adult has about 5 million/µl erythrocytes. Also, the blood group antigens are expressed on the surface membrane of the erythrocytes.

Leukocytes

Unlike mature erythrocytes, leucocytes have a nucleus. Different types of leucocytes can be found in the blood:

The normal concentration of leucocytes ranges from 4,000 to 10,000 per µl, depending on age and health status. Both leucocytes and erythrocytes are descendants of pluripotent hematopoietic stem cells from the bone marrow.