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E. Animals: Structure and Function · Animals: Structure and Function

Blood: Components, Clotting and Blood Types

E3.3 E2.1 A1.12 A1.13 A2.1 A2.2About 65 minutes
Learning goals
  • I can describe the components of blood and the structure and function of each.
  • I can explain how a blood clot forms.
  • I can explain the ABO and Rh blood groups and predict which transfusions are safe.
  • I can describe the many substances blood transports and some careers that work with blood.

Words to know

plasma
The liquid part of blood (about 55%), mostly water with dissolved proteins, nutrients, salts, hormones and wastes.
erythrocyte (red blood cell)
A biconcave, disc-shaped cell with no nucleus that carries oxygen using hemoglobin.
leukocyte (white blood cell)
A nucleated blood cell that defends the body against infection.
platelet
A cell fragment that helps blood clot.
hematocrit
The percentage of blood volume made up of red blood cells (about 40 to 50%).
fibrin
Insoluble protein threads that form the mesh of a blood clot, made from fibrinogen.
antigen
A molecule (often on a cell surface) that the immune system can recognize.
antibody
A protein made by the immune system that binds to a specific antigen.

A liquid tissue

Blood is a connective tissue with a liquid matrix. An adult has about 5 L of it (roughly 7 to 8% of body mass). In this unit, blood is the link between everything: it carries oxygen from the lungs, nutrients from the gut, hormones from glands, and CO₂ and urea to the organs that remove them. It also spreads heat, helps control pH, seals its own leaks and fights infection. Today: what blood is made of, how it clots, and why blood type matters for a transfusion.

Spin it and see

If blood (kept from clotting) is spun in a centrifuge, it separates into layers:

  • Plasma (about 55%): about 92% water, plus plasma proteins (albumin for osmotic pressure; globulins, including antibodies; fibrinogen for clotting), glucose, amino acids, fats, salts (ions such as Na⁺, K⁺, Ca²⁺, HCO₃⁻), hormones, and wastes such as urea.
  • A thin buffy coat (under 1%): white blood cells and platelets.
  • Red blood cells (about 45%). This percentage is the hematocrit (about 40 to 50% in healthy adults).
plasma (about 55%)water, proteins, nutrients, salts, hormones, wastesbuffy coat (under 1%)white blood cells and plateletsred blood cells (about 45%)this percentage is the hematocritspun blood sample
Blood spun in a centrifuge: plasma on top, a thin buffy coat of white blood cells and platelets, and red blood cells at the bottom. (Typical proportions.)

The formed elements

Red blood cells (erythrocytes)

  • About 5 million in each microlitre (µL, the same as a cubic millimetre) of blood.
  • Biconcave discs about 7.5 µm across: the dimpled shape gives a large surface area for gas exchange and lets them fold to squeeze through capillaries.
  • In mammals, mature red blood cells have no nucleus and no mitochondria. This leaves room for hemoglobin, and means they don't use up the oxygen they carry.
  • Made in red bone marrow; they live about 120 days. Old ones are broken down in the spleen and liver: iron is recycled, and the rest of the heme becomes bilirubin, a yellow pigment in bile.

White blood cells (leukocytes)

  • About 5000 to 10 000 per µL; they have nuclei and can squeeze out of capillaries into tissues.
  • Neutrophils (the most common) and monocytes (which become macrophages) engulf microbes by phagocytosis.
  • Lymphocytes: B cells make antibodies; T cells attack infected cells and coordinate the immune response.
  • Eosinophils and basophils are involved in fighting parasites and in allergic reactions.

Platelets

  • About 150 000 to 400 000 per µL; tiny fragments of large bone-marrow cells, living about a week to 10 days.
  • They plug leaks and start clotting.
Worked example

Example 1: How many red blood cells?

An adult has 5.0 L of blood with 5.0 million red blood cells per µL. (1 L = 1 000 000 µL.) (a) How many red blood cells are there? (b) If each lives 120 days, about how many must be made each second to keep the number steady?

  1. (a) 5.0 L = 5.0 × 10⁶ µL. Number = (5.0 × 10⁶ µL) × (5.0 × 10⁶ cells/µL) = 2.5 × 10¹³ cells (25 trillion).
  2. (b) Seconds in 120 days = 120 × 24 × 60 × 60 = 1.04 × 10⁷ s.
  3. Cells replaced per second = 2.5 × 10¹³ ÷ 1.04 × 10⁷ ≈ 2.4 × 10⁶ (about 2 million every second).

(a) About 2.5 × 10¹³ red blood cells. (b) About 2 million new red blood cells every second.

Stopping a leak: blood clotting

When a blood vessel is cut:

  1. Vasoconstriction: the vessel's smooth muscle narrows it, slowing blood loss.
  2. Platelet plug: platelets stick to the exposed collagen of the damaged wall and to each other.
  3. Coagulation: platelets and damaged tissue release chemicals that start a chain of reactions among clotting factors (proteins made mostly by the liver, several of which need vitamin K). With calcium ions, this chain converts the plasma protein prothrombin into the enzyme thrombin.
  4. Thrombin converts soluble fibrinogen into insoluble fibrin threads, which form a mesh that traps red blood cells and platelets: a clot. It dries into a scab while the tissue repairs underneath.

Hemophilia is an inherited disorder in which one clotting factor is missing or low, so bleeding lasts much longer. The most common forms are X-linked (you'll see why that matters in the Genetic Processes unit). Unwanted clots inside vessels are dangerous too: a clot in a coronary artery or in the brain can cause a heart attack or stroke. Medicines called anticoagulants reduce clotting. One of them, heparin, was purified in the 1930s by a University of Toronto team that included Charles Best, and Toronto surgeon Gordon Murray pioneered its use to prevent clots during and after surgery.

vessel wall damagedplatelets stick and form a plugplatelets and damaged tissue release clotting factorsprothrombin(plasma protein)Ca²⁺, factorsthrombin (enzyme)fibrinogen(soluble)thrombinfibrin threads(insoluble) trap cells: clot
A simplified clotting chain. The final steps turn soluble fibrinogen into a mesh of insoluble fibrin threads that traps blood cells. (Many intermediate clotting factors are left out.)

Blood types

Red blood cells carry antigens (marker molecules) on their surface. In the ABO system:

  • Type A cells have A antigens; the plasma has anti-B antibodies.
  • Type B cells have B antigens; the plasma has anti-A.
  • Type AB cells have both; the plasma has neither antibody.
  • Type O cells have neither antigen; the plasma has both antibodies.

If red blood cells meet antibodies that match their antigens, the antibodies bind and the cells clump (agglutinate) and burst, which can be life-threatening. So for a red-cell transfusion, the donor's antigens must not match the recipient's antibodies.

The Rh system adds another antigen (the D antigen). People who have it are Rh-positive (+); people who don't are Rh-negative (−). Rh− people make anti-Rh antibodies only after being exposed to Rh+ blood.

  • O− red cells have no A, B or Rh antigens, so they can be given to almost anyone in an emergency: the universal donor.
  • AB+ people can receive red cells of any ABO and Rh type: the universal recipient.

O-positive is the most common blood type in Canada. Blood donors in Canada must be at least 17. Canadian Blood Services collects blood in every province and territory except Québec, where Héma-Québec does.

type AA antigensplasma antibodies:anti-Btype BB antigensplasma antibodies:anti-Atype ABA and B antigensplasma antibodies:neithertype Ono A or B antigensplasma antibodies:anti-A and anti-B= A antigen= B antigen
ABO blood types. The antigens on the red blood cells decide which antibodies a person's plasma carries: never the ones that would attack their own cells. (Antigens drawn much larger than real.)
Worked example

Example 2: Safe transfusion?

A patient with type B+ blood needs red blood cells. Which donor types are safe: A+, O−, AB−, B−, O+?

  1. The patient's plasma has anti-A (type B) antibodies; being Rh+, the patient makes no anti-Rh.
  2. The donor's red cells must not carry A antigens. Rh antigen is fine for an Rh+ recipient.
  3. A+: has A antigen → not safe.
  4. O−: no A, B or Rh → safe.
  5. AB−: has A antigen → not safe.
  6. B−: B antigen only → safe.
  7. O+: no A or B; Rh+ is fine → safe.

Safe: O−, B− and O+. Not safe: A+ and AB− (both carry the A antigen).

Careers that work with blood

  • Medical laboratory technologists analyse blood samples (cell counts, blood typing, clotting tests) in hospital labs; in Ontario this is a regulated profession that requires college or university training.
  • Phlebotomists and laboratory assistants collect blood samples.
  • Hematologists are physicians who specialize in blood disorders.
  • Transfusion medicine specialists and blood-service staff match, test and supply blood for patients.
Hands-on activity

Make a model of blood

You need: a tall clear glass or jar, a measuring cup, water, yellow food colouring, red lentils (or red candies), white rice, and sesame seeds (or poppy seeds).

  1. Mark the glass at the 100 mL level. Mix 55 mL of water with 1 drop of yellow food colouring: this is plasma.
  2. Add red lentils until the total reaches 100 mL (about 45 mL of lentils): red blood cells. Let them settle to the bottom, like spun blood.
  3. Sprinkle a thin layer of white rice and a few sesame seeds on the lentils: the buffy coat of white blood cells and platelets.
  4. Do the math: in real blood there are about 5 000 000 red cells, 7000 white cells and 250 000 platelets per µL. For every one white blood cell, how many red cells and platelets are there? Count how many lentils fit in 1 teaspoon, then figure out how many teaspoons of lentils you would need per single grain of rice to make the model accurate.

What to look for: about 700 red cells and about 35 platelets for every 1 white cell (5 000 000 ÷ 7000 ≈ 714; 250 000 ÷ 7000 ≈ 36). A teaspoon holds very roughly 100 to 150 red lentils, so you'd need several teaspoons of lentils for each grain of rice: white blood cells are rare, which is why they form such a thin layer. Wash up and don't eat the lentils raw.

Just for fun

Your bone marrow makes about 2 million red blood cells every second. By the time you finish reading this sentence, it has made several million more. Talk about a busy factory.

Your turn

1

List the four main components of blood and the percentage of blood volume for plasma and red cells.

2

Give two ways a red blood cell's structure suits its function.

3

Name two kinds of white blood cells and their jobs.

4

Put in order: fibrin forms, platelet plug, thrombin forms, vessel constricts.

5

What antigens and antibodies does a type A person have?

6

Why is O− called the universal donor?

7

Name three substances carried in plasma.

More practice: warm-up

1

What is the hematocrit?

2

Where are blood cells made?

3

How long does a red blood cell live?

4

What vitamin is needed to make several clotting factors?

5

What is agglutination?

More practice: on your own

1

A person's blood sample is 2.4 mL red blood cells out of a total of 5.0 mL after spinning. Find the hematocrit and say whether it is in the normal range.

2

Why does a person with severe liver disease bruise and bleed easily?

3

A type AB− patient needs red cells. List all safe ABO/Rh donor types.

4

Explain why a type O person cannot receive type A red cells.

5

Calculate the ratio of red blood cells to white blood cells if there are 4.8 million red cells and 6000 white cells per µL.

Challenge

1

Explain why thrombin is an enzyme, while fibrinogen is not, and why this design lets a clot form very quickly.

2

A person with type O+ blood has a baby whose blood type has not been tested yet. Explain why the blood bank can give this baby O− red cells in an emergency, but would not give it type A+ red cells.

3

Explain how red blood cells, plasma and platelets together help keep the internal environment stable (homeostasis). Give one example each.

Check your understanding

Quiz

1.

About what fraction of blood volume is plasma?

2.

Mature human red blood cells lack:

3.

Which protein forms the threads of a blood clot?

4.

A type B person has which antibodies in their plasma?

5.

Which white blood cells make antibodies?

6.

Which donor's red cells are safe for an A− patient?

For parents and teachers

Big idea: E3.3 includes blood components and the transport of substances vital to health. This lesson covers the cells and plasma of blood, clotting, and blood types, with calculations in scientific notation (A1.12, A1.13), careers (A2.1) and Canadian contributions (A2.2).

How to help: If you know your own blood type, share it and ask your teen which types you could receive. The model-blood activity uses only kitchen items.

Common difficulties:

  • Mixing up antigens and antibodies. Antigens are on the cells; antibodies are in the plasma and are the opposite letter.
  • Universal donor vs recipient. O− gives to all; AB+ receives from all.
  • Fibrin vs fibrinogen. Fibrinogen is the soluble protein; fibrin is the clot.

Note: blood donation is a common community activity in Canada; donors must be at least 17. Students with a bleeding disorder may find the clotting section personal; keep the conversation factual and supportive.