Anatomy & Physiology · Chapter 18

The Cardiovascular System: Blood

Blood is the only tissue in the body you can pour into a test tube, and it is easy to underestimate — it looks like a plain red liquid. But that liquid is a connective tissue, a living fleet of cells drifting in a protein-rich sea, and it touches every other cell you own. In this chapter we take a tube of blood apart layer by layer: the straw-colored plasma on top, the packed red cells below, and the thin white line between them. You will learn what each part carries, where blood cells are born, how a red cell ferries oxygen, how a cut seals itself in seconds, and why matching a patient's blood type before a transfusion is a matter of life and death.

By the end of this chapter you can

List the major functions of blood — transport, regulation, and protection — and explain why blood counts as a connective tissue.
Describe what a spun tube of blood separates into, and define plasma, buffy coat, and hematocrit.
Name the three formed elements and the five kinds of white blood cell, and give the main job of each.
Trace hematopoiesis from a single stem cell down the myeloid and lymphoid lines, and name the hormone that drives red-cell production.
Explain how hemoglobin loads oxygen in the lungs and unloads it in the tissues, and how blood carries carbon dioxide.
Walk through the three steps of hemostasis and describe how the clotting cascade builds a fibrin mesh.
Predict ABO and Rh compatibility and identify the universal donor and universal recipient.

Section 1What blood is made of

Blood composition and hematocrit reference figure — **Reference.** A spun tube of blood — plasma, buffy coat, and hematocrit, with anemia and polycythemia compared — OpenStax, CC BY 3.0 (via Wikimedia Commons)

A tissue you can pour

It surprises most students to hear that blood is a connective tissue. Connective tissues are defined by scattered cells sitting in an extracellular matrix, and blood fits the definition perfectly — its "cells" are the red cells, white cells, and platelets, and its matrix is the liquid plasma they float in. What makes blood unusual is that the matrix is a fluid rather than something solid like bone or rubbery like cartilage. An average adult carries about 5 liters of it, a little less in women and a little more in men, and it makes up roughly 8 percent of body weight. Blood is also heavier and stickier than water — about five times as viscous — it runs warm at about 38 °C, and it is slightly alkaline, held in the narrow range of pH 7.35 to 7.45.

Three jobs: transport, regulation, protection

Everything blood does falls into three categories. First is transport: blood delivers O₂ and nutrients to cells, hauls CO₂ and metabolic wastes away, and carries hormones from glands to their distant targets. Second is regulation: by shifting where it flows and how much heat it carries to the skin, blood helps stabilize body temperature; its dissolved buffers steady the body's pH; and its proteins help control fluid balance between blood and tissues. Third is protection: the white cells and antibodies defend against invaders, and the clotting system seals leaks so you do not bleed out from a small wound.

Spin it down and it splits in three

Put a tube of blood in a centrifuge and the heavier parts sink. You end up with three visible layers, and their proportions tell a clinical story:

Layer	What it contains	Fraction of blood
Plasma (top)	Water, plasma proteins, nutrients, gases, hormones, wastes, ions	About 55%
Buffy coat (middle)	Leukocytes (white cells) and platelets	Less than 1%
Erythrocytes (bottom)	Red blood cells — this fraction is the hematocrit	About 45%

That bottom fraction has its own name: the hematocrit is the percentage of whole-blood volume made up of red cells. Normal values run roughly 37–47 percent in women and 42–52 percent in men. The plasma itself is about 92 percent water; most of the rest is plasma proteins — albumin (the most abundant, which holds water in the vessels and ferries fatty molecules), globulins (which include antibodies and transport proteins), and fibrinogen (the raw material for clots). One term worth pinning down now: if you let blood clot and remove the clot, the pale fluid left behind is serum — plasma minus its clotting proteins.

Hematocrit is a headline number. A low hematocrit means anemia — too few red cells to carry oxygen well, leaving a patient tired and short of breath. A high hematocrit (polycythemia) thickens the blood and strains the heart. One spun tube tells you a great deal about how well oxygen is being delivered.

Spin your own tube and pull apart the layers in the interactive below.

Interactive Spin a tube of blood into its three layers and tap each to see what it holds. Open full ↑

Key points

Blood is a fluid connective tissue — cells (formed elements) suspended in a liquid matrix (plasma) — about 5 L and 8% of body weight.
Its jobs sort into transport, regulation, and protection.
Spun blood gives plasma (~55%), a thin buffy coat of white cells and platelets, and red cells (hematocrit, ~45%).
Serum is plasma with the clotting proteins removed.

Section 2The formed elements

Red blood cell, platelet, and white blood cell reference figure — **Reference.** The three formed elements under the microscope — a red cell, a platelet, and a white cell — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Three kinds of passenger

The living cargo of blood — the formed elements — comes in three types: erythrocytes (red blood cells), leukocytes (white blood cells), and platelets (also called thrombocytes). They are wildly unequal in number. Red cells dominate, at roughly 5 million per microliter; platelets number a few hundred thousand; and white cells are the rarest, at only about 5,000 to 10,000 per microliter. That lopsidedness is exactly why the spun tube from Section 1 is almost all red on the bottom with only a whisper of buffy coat.

Red cells: oxygen with everything stripped away

An erythrocyte is a design study in doing one job well. It is a biconcave disc — a flattened circle pinched in on both faces — a shape that gives it lots of surface area for gas exchange and lets it fold to squeeze through capillaries narrower than itself. To make room for as much hemoglobin as possible, the mature red cell throws almost everything else overboard: it has no nucleus and no organelles. That sacrifice has a cost — with no nucleus it cannot repair itself, so it wears out in about 120 days and is recycled by the spleen and liver. We will follow its oxygen work in Section 4.

White cells: the mobile defense force

Unlike red cells, every leukocyte keeps its nucleus, and all of them are soldiers of the immune system. They split into two families by whether they carry visible granules. The granulocytes — neutrophils, eosinophils, and basophils — have grainy cytoplasm; the agranulocytes — lymphocytes and monocytes — do not. Learning them in order of abundance is easiest with a mnemonic: Never Let Monkeys Eat Bananas (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils).

White cell	Group	Rough share	Main job
Neutrophil	Granulocyte	50–70%	First responder; phagocytoses bacteria
Lymphocyte	Agranulocyte	20–30%	Adaptive immunity — T, B, and NK cells
Monocyte	Agranulocyte	2–8%	Becomes a macrophage; big-appetite phagocyte
Eosinophil	Granulocyte	1–4%	Attacks parasites; roles in allergy
Basophil	Granulocyte	<1%	Releases histamine and heparin; inflammation

Platelets: fragments that plug leaks

Platelets are not whole cells at all — they are cell fragments pinched off from giant marrow cells called megakaryocytes. They have no nucleus, live only about 10 days, and exist for one purpose: to rush to a wound, pile up, and start a clot. We will watch them do it in Section 5.

Read the "diff" like a story. A complete blood count reports a white-cell differential — the percentages above. A jump in neutrophils points to a bacterial infection; a rise in lymphocytes often means a viral one; and climbing eosinophils suggest a parasite or an allergy. The numbers narrow the diagnosis before any culture comes back.

Meet all three formed elements — and the five white cells — in the interactive.

Interactive Tap each formed element — red cell, five white cells, and platelet — to see its job. Open full ↑

Key points

The three formed elements are erythrocytes (oxygen), leukocytes (defense), and platelets (clotting); red cells vastly outnumber the rest.
The red cell is a biconcave disc with no nucleus, packed with hemoglobin, lasting ~120 days.
Five white cells, most to least: neutrophils, lymphocytes, monocytes, eosinophils, basophils (Never Let Monkeys Eat Bananas).
Platelets are megakaryocyte fragments that trigger clotting.

Section 3Where blood cells are born

Hematopoiesis lineage tree reference figure — **Reference.** The hematopoiesis family tree — one stem cell branching into the myeloid and lymphoid lines — OpenStax, CC BY 4.0 (via Wikimedia Commons)

One stem cell, every blood cell

Red cells wear out in four months and neutrophils in a matter of days, so the body must replace blood cells constantly — on the order of hundreds of billions each day. That factory is the red bone marrow, and the whole process of making blood is called hematopoiesis. Astonishingly, every one of those cells traces back to a single kind of ancestor: the hematopoietic stem cell (also called a hemocytoblast). It is multipotent, meaning it can become any blood cell — but only blood cells, not liver or nerve or muscle — and it renews itself so the supply never runs dry. (Reserve the word pluripotent for embryonic and induced pluripotent stem cells, which can give rise to tissues of all three germ layers.)

Two family lines: myeloid and lymphoid

Early on, the stem cell's descendants commit to one of two branches. The myeloid line is the larger one — it produces red cells, platelets (by way of megakaryocytes), and most white cells: neutrophils, eosinophils, basophils, and monocytes. The lymphoid line is dedicated to the lymphocytes (the T, B, and NK cells of adaptive immunity). Knowing which line a cell comes from is more than trivia — leukemias are classified as myeloid or lymphoid precisely by which branch of this tree went wrong.

Line	Cells it produces	Notice
Myeloid	Erythrocytes, platelets, neutrophils, eosinophils, basophils, monocytes	The broad branch — nearly everything
Lymphoid	Lymphocytes (T cells, B cells, NK cells)	The specialist branch — adaptive immunity

Hormones set the pace

Production is not left to chance; hormones and growth factors tune it to demand. The star is erythropoietin (EPO), released by the kidneys when they sense low oxygen — EPO tells the marrow to make more red cells. Thrombopoietin (from the liver) drives platelet production, and a family of colony-stimulating factors and interleukins ramps up white-cell output during infection. Building a red cell also needs raw materials: iron for hemoglobin, plus vitamin B₁₂ and folate for DNA synthesis. Run short on any of these and you become anemic, which is why the freshly released, not-quite-finished red cells — the reticulocytes — are counted as a gauge of how hard the marrow is working.

Sick kidneys make you anemic. Because the kidney makes EPO, people with chronic kidney disease often run low red counts — the oxygen sensor is broken, so the marrow never gets the memo. This is also the loophole endurance athletes abuse: injecting synthetic EPO forces extra red cells and illegally boosts oxygen delivery.

Climb the family tree node by node in the interactive.

Interactive Follow the tree from one stem cell down the myeloid and lymphoid branches. Open full ↑

Key points

Hematopoiesis happens in red bone marrow; every blood cell descends from a multipotent hematopoietic stem cell.
The stem cell branches into the myeloid line (red cells, platelets, most white cells) and the lymphoid line (lymphocytes).
Erythropoietin from the kidney drives red-cell production; iron, B₁₂, and folate are the raw materials.

Section 4How blood carries oxygen

Hemoglobin oxygen loading and unloading reference figure — **Reference.** Hemoglobin loading oxygen in the lungs and releasing it in the tissues — OpenStax, CC BY 3.0 (via Wikimedia Commons)

The molecule that does the carrying

Red cells matter because of one molecule: hemoglobin. Each hemoglobin is built from four protein chains (two alpha and two beta globins), and every chain cradles a heme group with an atom of iron at its center. Oxygen binds to that iron. Since one hemoglobin has four heme groups, it can carry four O₂ molecules at once — and because a single red cell is stuffed with roughly 270 million hemoglobins, one cell can shuttle over a billion oxygen molecules. Iron is the actual seat oxygen sits on, which is why iron deficiency so directly starves the tissues of oxygen.

Load in the lungs, unload in the tissues

Hemoglobin's genius is that it grabs oxygen where oxygen is plentiful and lets go where it is scarce. In the lungs, where the partial pressure of oxygen is high, hemoglobin binds O₂ to become oxyhemoglobin — and this bright, oxygen-rich blood is a vivid scarlet. Out in the tissues, where cells have burned through their oxygen, the partial pressure is low and hemoglobin releases its cargo, becoming deoxyhemoglobin, a darker brick red. That color swing is why arterial blood looks so much brighter than venous blood. The binding is also cooperative: once the first O₂ is on, the next bind more easily, so loading and unloading both happen fast. Warmth, extra CO₂, and a more acidic environment — exactly the conditions in a hardworking muscle — nudge hemoglobin to release more oxygen right where it is needed.

And the return trip: carbon dioxide

Blood does not just deliver oxygen; it also clears the CO₂ that cells produce. That waste rides home three ways, and the proportions are worth knowing:

Gas	How it travels	Share
O₂	Bound to hemoglobin as oxyhemoglobin	~98.5%
O₂	Dissolved in plasma	~1.5%
CO₂	As bicarbonate (HCO₃⁻) in plasma	~70%
CO₂	Bound to hemoglobin (carbaminohemoglobin)	~23%
CO₂	Dissolved in plasma	~7%

Carbon monoxide is a thief. CO binds hemoglobin more than 200 times more tightly than oxygen does, locking up the iron seats so O₂ cannot board. A poisoned patient may look deceptively healthy — cherry-red rather than blue — because the blood is saturated, just with the wrong gas. It is a vivid reminder that oxygen delivery depends entirely on hemoglobin being free to bind.

Watch a red cell load up and drop off, on a loop, in the interactive.

Interactive Follow one red cell picking up O₂ in the lungs and dropping it in the tissues. Open full ↑

Key points

Hemoglobin = four globin chains, each with an iron-containing heme; it carries up to four O₂ per molecule.
It forms oxyhemoglobin (bright red) in the lungs and releases O₂ as deoxyhemoglobin (darker) in the tissues.
CO₂ travels home mostly as bicarbonate (~70%), some on hemoglobin, a little dissolved.
Carbon monoxide outcompetes oxygen for hemoglobin, blocking delivery.

Section 5How bleeding stops

Hemostasis steps and clotting cascade reference figure — **Reference.** The steps of hemostasis and the fibrin-synthesis cascade — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Three fast steps

Cut a vessel and the body has to seal the leak before too much blood escapes — a process called hemostasis (literally "blood standstill"). It unfolds in three overlapping steps, each buying time for the next. The whole thing can begin within seconds.

Step	What happens	Result
1. Vascular spasm	Smooth muscle in the vessel wall contracts near the injury	The vessel narrows, slowing blood loss
2. Platelet plug	Platelets stick to exposed collagen, activate, and pile up	A soft plug fills the hole
3. Coagulation	A cascade of clotting factors builds a fibrin mesh	A firm clot locks the plug in place

The platelet plug snowballs

When the smooth inner lining of a vessel is torn, the tough collagen underneath is exposed. Platelets grab onto it (with the help of a bridging protein, von Willebrand factor), and contact flips them from smooth discs into spiky, sticky activated platelets. Activated platelets then release chemicals — ADP, thromboxane A₂, and serotonin — that summon and activate still more platelets. This is a textbook positive feedback loop: each recruited platelet recruits others, so the plug grows quickly until the breach is covered. This is also exactly the step that aspirin blunts — it blocks thromboxane, which is why low-dose aspirin makes platelets less sticky.

Coagulation: turning liquid into gel

A platelet plug alone is fragile; coagulation reinforces it. A relay of plasma proteins called clotting factors triggers one another in a cascade, along an intrinsic and an extrinsic pathway that merge into a common pathway. The finale has two moves: the enzyme thrombin is switched on, and thrombin converts the soluble plasma protein fibrinogen into threads of insoluble fibrin. Those threads weave a mesh that traps red cells and platelets — the actual clot. Two behind-the-scenes players make it possible: calcium (clotting factor IV) is required at several steps, and vitamin K is needed by the liver to build several of the factors in the first place. Once the wound heals, the enzyme plasmin dissolves the clot (fibrinolysis).

Vitamin K is the reason warfarin works. The anticoagulant warfarin blocks vitamin K, so the liver cannot make its clotting factors and the blood thins — useful for preventing dangerous clots. On the other end, hemophilia is the failure of this cascade: a missing clotting factor (VIII or IX) means even small injuries bleed for a long time.

Step through the seal-and-reinforce sequence in the interactive.

Interactive Step through vascular spasm, the platelet plug, and the fibrin mesh that stops bleeding. Open full ↑

Key points

Hemostasis has three steps: vascular spasm → platelet plug → coagulation.
The platelet plug grows by positive feedback as activated platelets recruit more; aspirin blunts this.
Coagulation ends with thrombin converting fibrinogen to a fibrin mesh; it needs calcium and vitamin K.

Section 6Blood types and transfusion

ABO blood types antigens and antibodies reference figure — **Reference.** The ABO blood types — the antigens on each red cell and the antibodies in each plasma — OpenStax, CC BY 3.0 (via Wikimedia Commons)

Antigens on the cell, antibodies in the plasma

Your blood type is decided by molecular flags called antigens (agglutinogens) that stud the surface of your red cells. In the ABO system there are two possible flags, A and B, and your genes decide which you display. Here is the crucial rule: your plasma carries antibodies (agglutinins) against whichever antigen you lack. So a type A person has A antigens and anti-B antibodies; a type B person has B antigens and anti-A; a type AB person has both antigens and no ABO antibodies; and a type O person has neither antigen but both anti-A and anti-B.

Why a mismatch is dangerous

If a patient receives red cells carrying an antigen their plasma has antibodies against, those antibodies clamp onto the donor cells and clump them together — agglutination — and then rupture them (hemolysis). The debris can clog small vessels and overwhelm the kidneys; a serious transfusion reaction can be fatal. This is why blood is always typed and cross-matched before it is given.

Type	Antigen on red cells	Antibody in plasma	Can receive from
A	A	Anti-B	A, O
B	B	Anti-A	B, O
AB	A and B	None	A, B, AB, O — universal recipient
O	None	Anti-A and Anti-B	O only — universal donor of red cells

The Rh factor

Layered on top of ABO is the Rh system, named for the rhesus monkey used in the experiments that uncovered it. It carries several antigens — C, c, D, E, and e — but one of them, D, is by far the most immunogenic and the only one routinely typed. If you carry D you are Rh-positive; if not, Rh-negative. Unlike the ABO antibodies, which are present from early life, an Rh-negative person only makes anti-D antibodies after being exposed to Rh-positive blood. That detail drives one of the most important scenarios in obstetrics: hemolytic disease of the newborn. If an Rh-negative mother carries an Rh-positive baby, she can become sensitized to the D antigen, and in a later Rh-positive pregnancy her antibodies may cross the placenta and attack the fetus's red cells. The fix is elegant — a shot of Rh immune globulin (RhoGAM) clears the fetal cells before the mother's immune system can react.

Give O, take AB. Type O-negative red cells carry no A, B, or D antigen, so they set off no antibodies — the universal donor for emergencies. Type AB-positive plasma carries no ABO antibodies, so it can receive any red cells — the universal recipient. Note the flip: for plasma donation the rule reverses, because plasma is all about the antibodies.

Pick a blood type and see exactly who it can give to and take from in the interactive.

Interactive Choose a blood type and reveal its antigens, antibodies, and who it is compatible with. Open full ↑

Key points

Blood type = antigens on your red cells; your plasma holds antibodies against the antigens you lack.
Mismatched transfusion causes agglutination and hemolysis, so blood is typed and cross-matched.
O-negative is the universal red-cell donor; AB-positive is the universal recipient.
Rh-negative mothers with Rh-positive babies risk hemolytic disease of the newborn, prevented by RhoGAM.

The chapter in one breath

Blood is a fluid connective tissue — formed elements suspended in plasma — that handles transport, regulation, and protection.
Spun blood separates into plasma (~55%), a thin buffy coat, and red cells (hematocrit, ~45%); plasma is mostly water plus albumin, globulins, and fibrinogen.
The formed elements are erythrocytes (oxygen), five kinds of leukocyte (defense), and platelets (clotting).
Hematopoiesis in red marrow starts from one stem cell that branches into the myeloid and lymphoid lines; the kidney's erythropoietin drives red-cell output.
Hemoglobin carries four O₂ on iron-heme seats, loading in the lungs and unloading in the tissues; CO₂ rides home mostly as bicarbonate.
Hemostasis seals a wound in three steps — vascular spasm, platelet plug, and coagulation — ending in a fibrin mesh.
Blood type is set by red-cell antigens (A, B, Rh); plasma holds antibodies against the ones you lack, making O-negative the universal donor and AB-positive the universal recipient.

ReferenceWords worth knowing

A quick reference for the terms in this chapter — skim it now, or circle back whenever a word trips you up.

Agglutination: The clumping of red blood cells when antibodies bind matching antigens, as in a mismatched transfusion.
Albumin: The most abundant plasma protein; it holds water in the vessels (osmotic pressure) and transports fatty molecules and drugs.
Antibody (agglutinin): A plasma protein that recognizes and binds a specific antigen; ABO antibodies target the A or B antigen a person lacks.
Antigen (agglutinogen): A surface molecule on red cells (A, B, or Rh D) that defines blood type and can trigger an immune response.
Buffy coat: The thin layer of white blood cells and platelets sitting between plasma and red cells in a spun sample.
Coagulation: The clotting cascade in which factors activate one another to convert fibrinogen into a fibrin mesh.
Erythrocyte: A red blood cell — a biconcave, anucleate cell packed with hemoglobin to carry oxygen.
Erythropoietin (EPO): A hormone from the kidney that stimulates red bone marrow to produce more red blood cells when oxygen is low.
Fibrin: The insoluble protein thread, formed from fibrinogen by thrombin, that meshes to trap cells and form a clot.
Formed elements: The cellular components of blood: erythrocytes, leukocytes, and platelets.
Hematocrit: The percentage of whole-blood volume made up of red blood cells (about 37–52%, varying by sex).
Hematopoiesis: The production of blood cells in red bone marrow, all descending from a multipotent hematopoietic stem cell.
Hemoglobin: The iron-containing protein in red cells (four globin chains, four heme groups) that binds and transports oxygen.
Hemostasis: The stopping of bleeding through vascular spasm, platelet plug formation, and coagulation.
Leukocyte: A white blood cell; the nucleated defenders of the body, divided into granulocytes and agranulocytes.
Plasma: The liquid matrix of blood — about 92% water plus proteins, nutrients, gases, hormones, and wastes.
Platelet (thrombocyte): A cell fragment from a megakaryocyte that adheres to wounds and initiates clotting.
Rh factor: The D antigen of the Rh blood-group system; present in Rh-positive blood, absent in Rh-negative.
Serum: Plasma with its clotting proteins removed; the fluid left after blood clots.
Thrombin: The enzyme that converts fibrinogen into fibrin, the final step in building a clot.

Check yourself

1. Why is blood classified as a connective tissue, and what serves as its matrix?

Because it is scattered cells (formed elements) suspended in an extracellular matrix — here a liquid, the plasma. Connective tissues are defined by cells in a matrix; blood's matrix simply happens to be fluid rather than solid or gel-like.

2. A spun tube of blood shows three layers. Name them from top to bottom and give the approximate fraction of each.

Plasma (~55%) on top, a thin buffy coat (<1%), and red cells (~45%) on the bottom. The buffy coat holds the white cells and platelets; the red-cell fraction is the hematocrit.

3. What is the hematocrit, and what does an abnormally low value suggest?

The percentage of blood volume that is red cells; a low value suggests anemia. Too few red cells means reduced oxygen-carrying capacity, causing fatigue and shortness of breath. A high value (polycythemia) thickens the blood.

4. Name the three formed elements and the one job each is specialized for.

Erythrocytes carry oxygen, leukocytes defend against invaders, and platelets start clots. Red cells vastly outnumber the other two, which is why spun blood is mostly red on the bottom.

5. A mature red blood cell has thrown out its nucleus and organelles. What does it gain, and what does it lose?

It gains room for more hemoglobin (and a flexible biconcave shape); it loses the ability to repair itself. With no nucleus it cannot make new proteins, so it wears out in about 120 days and is recycled.

6. List the five white blood cells from most to least abundant. What mnemonic helps?

Neutrophils, lymphocytes, monocytes, eosinophils, basophils — "Never Let Monkeys Eat Bananas." Neutrophils are first responders to bacteria; lymphocytes run adaptive immunity; monocytes become macrophages.

7. During hematopoiesis, the stem cell commits to one of two lines. Name them and say which cells each produces.

The myeloid line makes red cells, platelets, and most white cells; the lymphoid line makes lymphocytes. Both descend from a single multipotent hematopoietic stem cell in red bone marrow.

8. Which hormone drives red-cell production, where is it made, and why are kidney patients often anemic?

Erythropoietin (EPO), made by the kidneys; damaged kidneys make too little of it. Without the EPO signal, the marrow is never told to ramp up red-cell production, so red counts fall.

9. Describe how a single hemoglobin molecule carries oxygen, and where the oxygen actually binds.

Hemoglobin has four globin chains, each with a heme group; oxygen binds the iron atom at the center of each heme, so one molecule carries up to four O₂. Iron is literally the seat oxygen sits on, which is why iron deficiency impairs oxygen transport.

10. In what form does most carbon dioxide travel back to the lungs?

As bicarbonate (HCO₃⁻) dissolved in plasma — roughly 70%. The rest rides on hemoglobin as carbaminohemoglobin (~23%) or is dissolved in plasma (~7%).

11. List the three steps of hemostasis in order, and name the protein mesh that forms in the final step.

Vascular spasm → platelet plug → coagulation; the final mesh is fibrin. Thrombin converts soluble fibrinogen into fibrin threads that trap cells and lock the plug in place. Calcium and vitamin K are required.

12. A patient with type O-negative blood is called the universal donor, while AB-positive is the universal recipient. Explain both, using antigens and antibodies.

O-negative red cells carry no A, B, or D antigen, so no recipient's antibodies attack them; AB-positive plasma has no ABO antibodies, so it accepts any red cells. A reaction occurs only when donor antigens meet matching recipient antibodies — O-negative supplies none, and AB-positive brings none to the fight.

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