introduction to the blood system

blood

a liquid that fills the vascular compartment and serves to transport dissolved materials and blood cells throughout the body

the average human has 4-6L of blood

what are the constituent parts of blood

plasma (makes up approx 55% of total blood volume) - is a straw coloured liquid

WBC and platelets - <1% of the total volume - is called the buffy coat

Red blood cells ( approx 45% of whole blood volume)

Red blood cells and WBC and platelets together is called the haematocrit - but it often only refers to RBCs

Blood consists of plasma and cells - plasma 55%, cellular elements 45%

functions of blood

• respiration - RBCs supply oxygen to tissues and remove CO2 from tissues and cells

• transport of (in the plasma) -

  • nutrients to the tissues and cells

  • waste products from cells

  • messages such as hormones around the body

• protection from infection (WBCs) - the immune system

• repair (platelets) - repairing tissue damage

• thermoregulation - plasma

thermoregulation

the maintenance of physiological core body temperature

extreme heat -

• circulation diverts blood to the surface to cool the body

• sweating

extreme cold -

• circulation diverts blood to deep/core to maintain body heat

• hair stands up

• shivering

plasma

most water but also:

• ions (such as Na+ etc)

• plasma proteins (such as albumin etc.)

• substances transported by blood (i.e. nutrients, waste products, hormones etc.)

Haematopoiesis

formation of blood

occurs in bone marrow (spongy tissue inside bones)

Red marrow in flat bones (such as pelvis, sternum or ribs) produce most blood cells#

yellow marrow in long bones produces some WBCs

Bone marrow consists of blood cells at various stages of development as well as supporting tissue called stroma

stroma is made up of fibroblasts macrophages, adipocytes, osteoblasts, osteoclasts and epithelial cells etc.

stroma produces growth factors that tell the stem cells what to become

haematopoiesis definition

production of blood cells

megakaryopoiesis or thrombopoiesis

production of platelets from megakaryocytes

erythropoiesis

production of red blood cells

granulopoiesis

production of granulocytes

monocytopoiesis

production of monocytes

lymphopoiesis

production of lymphocytes

totipotent

stem cells with the ability to become any cell in the body (e.g. cells in a fertilized ovum)

pluripotent

stem cells with the ability to become nearly all cells in the body (e.g. cells from the 3 germ layers)

multipotent

stem cells with the ability to become a number of different cells of the same family e.g multipotent haematopoietic stem cells in the bone marrow are able to become any of the different blood cells

oligopotent

stem cells with the ability to differentiate into a few different cell types e.g. common myeloid precursor can become any of the myeloid cells (platelets, RBCs

destruction of blood cells

spleen (an ovoid organ found in the upper left abdominal cavity) destroys red blood cells

the spleen is the largest ‘filter’ of blood in the body removing old or damaged blood cells (engulfed by phagocytes)

4 million blood cells are destroyed every second, 2.5 of these are RBCs

Red blood cells (erythrocytes)

biconcave disks

7-12um in diameter

hold approximately 90fl

doesn’t have a nucleus (is anucleate) - so can carry more oxygen

no mitochondria - as don’t want mitochondria using up oxygen to produce energy

RBC have a different energy

approx 4-6×10^12 RBCs/L

about 25 trillion total in the average human but it varies particularly between male and females

have a long lifespan of approx 120 days during which cells travel approx 300 miles

functions:

• transport of oxygen and carbon dioxide around the body

• critical role in respiration

why are mammalian RBCs anucleate

Humans are warm-blooded so have higher metabolic rates

By ejecting the nucleus, the RBCs have a higher O2 carrying efficiency

However this also means bone marrow has to work continuously

Haemoglobin (Hb)

responsible for red appearance of RBCs

about 250 million Hb molecules per RBC

There are 4 polypeptide chains each with a cofactor called a haem group that has an iron atom at the centre

Each iron atom binds one molecule of O2

co-operative binding - once one molecule of O2 has bound, it is easier to bind the 2nd and 3rd molecules but then it is harder to bind the 4th molecule. Likewise it is easier to offload the 2nd and 3rd molecules but harder to offload the 4th molecule

Hb binds to O2, CO2 and NO (can also bind CO) and transports these molecules around the body

Pulmonary circulation

blood vessels surrounding the lungs enable gas exchange (O2 and CO2)
pulmonary artery → heart → lungs (deoxygenated)

pulmonary vein → heart → body (oxygenated)

capillary bed surrounds the alveoli

Transport of oxygen

once in plasma, O2 is taken into RBCs and reversibly binds to haem group of Hb

oxyhaemoglobin (oxygenated state) - is bright red

deoxyhaemoglobin (deoxygenated state) - is dark red

Transport of CO2

CO2 follows the concentration gradient from the tissues into the interstitial fluid and then plasma

once in blood stream some CO2 binds to Hb but most reacts with the water in RBCs to become carbonic acid

HCO3- travels in the plasma and the H+ binds to Hb so the pH of the blood isn’t altered

In the lungs the reverse happens:

the HCO3- and the H+ recombine to carbonic acid and dissociates into CO2 and H2O

The CO2 follows the conc gradient out of the blood into the alveolar sacks and is inhaled

Anaemia

reduced haemoglobin conc in blood

there are several classes

it often causes low haematocrit and small RBCs

most common causes:

• poor diet - iron deficiency

• chronic blood loss i.e. internal bleeding, heavy periods etc

• malabsorption of iron

• pregnancy

signs and symptoms:

• fatigue

• pallor

• tachycardia

• shortness of breath

White blood cells

can be broken down into phagocytes (granulocytes and monocytes) and immunocytes (T-cells, B-cells, NK cells)

Neutrophil

approx twice the size of RBCs

multi-lobed nucleus

has neutral staining granules (myeloperoxidase)

2.5-7.5×10^9/L

life span - 6 hours to a few days

function:

• vital role in protection from bacterial infections

• phagocytosis of bacteria

Eosinophils

approx same size as neutrophil

bi-lobed nucleus

large pink-staining granules (histamine, plasminogen, DNase, RNase) - stain with eosin

0.04-0.44×10^9/L

life span - 8-12 days

Functions:

• immune-protection

• phagocytosis of antibody-coated pathogens

• attacks parasites

• allergic responses

Basophils

slightly smaller than neutrophil

bi-lobed nucleus

large purple-staining granules (histamine, heparin, proteolytic enzymes) - stain with methylene blue

0.01-0.1×10^9/L

life span - a few hours to a few days

functions:

• release of histamine during inflammation

• are not phagocytes

mast cells are nor a type of basophil

monocytes (blood)/macrophages (tissue)

large kidney shaped nucleus

larger than neutrophil

0.2-0.8×10^9/L

life span - lasts many months

function:

• vital role in protection from infections

• ingest bacteria, dead cells and cellular debris

• phagocytosis

Lymphocytes

large and relatively round nucleus that fills the cytoplasm

slightly smaller than neutrophil

1.5-3.5×10^9/L

life span - can persist for many years

divided into two types of cell due to function: T cells and B cells

function:

• central role in the immune system - protecting from infections especially viral infections

• some lymphocytes attack pathogens directly (T-cells) and some produce antibodies (B-cells)

Natural killer cells

look similar to neutrophils

large round nucleus

purple-staining granules (perforin, proteolytic enzyme)

secrete cytokines (IFN gamma and TNF alpha)

very rare, not typically counted (elevated with physical exercise, massage etc.)

life span - 1-2 weeks

functions:

• immunological surveillance

• kill virus infected cells and some tumour cells non-specifically i.e are part of innate immunity

Platelets

small anucleate fragments of large precursor cells called megakaryocytes

2-4um diameter

appear in blood films as dark-staining granules

15—400×10^9/L

life span - 5-10 days

functions:

• roles in blood clotting and prevention of blood loss (haemostasis)

activated platelets change from a smooth discoid shape to sending out filopodia and forming lamellipodia