Venous (darker) and arterial (brighter) blood
Hemoglobin, a globular protein
green = haem (or heme) groups
red & blue = protein subunits
A scanning electron microscope (SEM) image of a normal red blood cell (left), a platelet (middle), and a white blood cell (right)
Vertebrate red blood cell types, measurements in micrometers
Frog red blood cells magnified 1000 times
Turtle red blood cells magnified 1000 times
Chicken red blood cells magnified 1000 times
Human red blood cells magnified 1000 times
Circulation of blood through the human heart
Basic hemoglobin saturation curve. It is moved to the right in higher acidity (more dissolved carbon dioxide) and to the left in lower acidity (less dissolved carbon dioxide)
Capillary blood from a bleeding finger
Venous blood collected during blood donation
Jan Janský is credited with the first classification of blood into four types (A, B, AB, and O)
Human blood fractioned by centrifugation: Plasma (upper, yellow layer), buffy coat (middle, thin white layer) and erythrocyte layer (bottom, red layer) can be seen.
Blood circulation: Red = oxygenated, blue = deoxygenated
Illustration depicting formed elements of blood
Two tubes of EDTA-anticoagulated blood.

Body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.

- Blood

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Image from a light microscope (500 ×) from a Giemsa-stained peripheral blood smear showing platelets (purple dots) surrounded by red blood cells (pink circular structures)
The ligands, denoted by letter L, signal for platelets (P) to migrate towards the wound (Site A). As more platelets gather around the opening, they produce more ligands to amplify the response. The platelets congregate around the wound in order to create a cap to stop blood flow out of the tissue.
Platelets derive from totipotent marrow stem cells.
Platelets extruded from megakaryocytes
3D rendering of four inactivated and three activated platelets
Scanning electron micrograph of blood cells. From left to right: human erythrocyte, activated platelet, leukocyte.
Diagram of the structure of a platelet showing the granules
Platelet clumps in a blood smear
On for example optical densitometry, a first and second wave of platelet aggregation is seen, in this case for an ADP-initiated aggregation. Data suggest that ADP activates the PI3K/Akt pathway during the first wave of aggregation, leading to thrombin generation and PAR‐1 activation, which evokes the second wave of aggregation.
Platelet concentrate
Platelets collected by using apheresis at an American Red Cross donation center

Platelets, also called thrombocytes (from Greek θρόμβος, "clot" and κύτος, "cell"), are a component of blood whose function (along with the coagulation factors) is to react to bleeding from blood vessel injury by clumping, thereby initiating a blood clot.

White blood cell

White blood cells, also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders.

A scanning electron microscope image of normal circulating human blood. In addition to the irregularly shaped leukocytes, both red blood cells and many small disc-shaped platelets are visible.
3D rendering of various types of white blood cells
Neutrophil engulfing anthrax bacteria

Leukocytes are found throughout the body, including the blood and lymphatic system.


A grasshopper has an open circulatory system, where hemolymph moves through interconnected sinuses or hemocoels, spaces surrounding the organs.
Above is a diagram of an open circulatory system. An open circulatory system is made up of a heart, vessels, and hemolymph. This diagram shows how the hemolymph is circulated throughout the body of a grasshopper. The hemolymph is first pumped through the heart, into the aorta, dispersed into the head and throughout the hemocoel, then back through the ostia that are located in the heart, where the process is repeated.

Hemolymph, or haemolymph, is a fluid, analogous to the blood in vertebrates, that circulates in the interior of the arthropod (invertebrate) body, remaining in direct contact with the animal's tissues.


Iron-containing oxygen-transport metalloprotein in red blood cells (erythrocytes) of almost all vertebrates (the exception being the fish family Channichthyidae) as well as the tissues of some invertebrates.

Max Perutz won the Nobel Prize for chemistry for his work determining the molecular structure of hemoglobin and myoglobin
Heme b group
A schematic visual model of oxygen-binding process, showing all four monomers and hemes, and protein chains only as diagrammatic coils, to facilitate visualization into the molecule. Oxygen is not shown in this model, but, for each of the iron atoms, it binds to the iron (red sphere) in the flat heme. For example, in the upper-left of the four hemes shown, oxygen binds at the left of the iron atom shown in the upper-left of diagram. This causes the iron atom to move backward into the heme that holds it (the iron moves upward as it binds oxygen, in this illustration), tugging the histidine residue (modeled as a red pentagon on the right of the iron) closer, as it does. This, in turn, pulls on the protein chain holding the histidine.
The sigmoidal shape of hemoglobin's oxygen-dissociation curve results from cooperative binding of oxygen to hemoglobin.
Gene expression of hemoglobin before and after birth. Also identifies the types of cells and organs in which the gene expression (data on Wood W.G., (1976). Br. Med. Bull. 32, 282.)
A hemoglobin concentration measurement being administered before a blood donation at the American Red Cross Boston Blood Donation Center.
The giant tube worm Riftia pachyptila showing red hemoglobin-containing plumes
Heart of Steel (Hemoglobin) (2005) by Julian Voss-Andreae. The images show the 5-foot (1.50 m) tall sculpture right after installation, after 10 days, and after several months of exposure to the elements.

Hemoglobin in blood carries oxygen from the respiratory organs (e.g. lungs or gills) to the rest of the body (i.e. tissues).

Body fluid

Body fluids, bodily fluids, or biofluids are liquids within the human body.

Intracellular and extracellular fluid compartments. The extracellular fluid compartment is further subdivided into the interstitial fluid and the intravascular fluid compartments.

Clinical samples are generally defined as non-infectious human or animal materials including blood, saliva, excreta, body tissue and tissue fluids, and also FDA-approved pharmaceuticals that are blood products.


Blood coagulation pathways in vivo showing the central role played by thrombin
Basic diagram showing blood clot formation, conversion of fibrin to thrombin, and scab formation
The interaction of vWF and GP1b alpha. The GP1b receptor on the surface of platelets allows the platelet to bind to vWF, which is exposed upon damage to vasculature. The vWF A1 domain (yellow) interacts with the extracellular domain of GP1ba (blue).
The classical blood coagulation pathway
Modern coagulation pathway. Hand-drawn composite from similar drawings presented by Professor Dzung Le, MD, PhD, at UCSD Clinical Chemistry conferences on 14 and 21 October 2014. Original schema from Introduction to Hematology by Samuel I. Rapaport. 2nd edition;Lippencott:1987. Dr Le added the factor XI portion based on a paper from about year 2000. Dr. Le's similar drawings presented the development of this cascade over 6 frames, like a comic.
Coagulation with arrows for negative and positive feedback.
The GP1b-IX receptor complex. This protein receptor complex is found on the surface of platelets, and in conjunction with GPV allows for platelets to adhere to the site of injury. Mutations in the genes associated with the glycoprotein Ib-IX-V complex are characteristic of Bernard–Soulier syndrome
Rivaroxaban drug bound to the coagulation factor Xa. The drug prevents this protein from activating the coagulation pathway by inhibiting its enzymatic activity.

Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot.

Blood vessel

Simple diagram of the human circulatory system
Transmission electron micrograph of a blood vessel displaying an erythrocyte (red blood cell, E) within its lumen, endothelial cells forming its tunica intima (inner layer), and pericytes forming its tunica adventitia (outer layer).
Diagram of blood vessel structures
Constricted blood vessel.

The blood vessels are the components of the circulatory system that transport blood throughout the human body.

Circulatory system

Several terms redirect here.

The human circulatory system (simplified). Red indicates oxygenated blood carried in arteries. Blue indicates deoxygenated blood carried in veins. Capillaries join the arteries and veins.
Blood flow in the pulmonary and systemic circulations showing capillary networks in the torso sections
Diagram of the human heart viewed from the front
The pulmonary circulation as it passes from the heart. Showing both the pulmonary and bronchial arteries.
Capillary bed
Diagram of capillary network joining the arterial system with the venous system.
Depiction of the heart, major veins and arteries constructed from body scans
Animation of a typical human red blood cell cycle in the circulatory system. This animation occurs at a faster rate (~20 seconds of the average 60-second cycle) and shows the red blood cell deforming as it enters capillaries, as well as the bars changing color as the cell alternates in states of oxygenation along the circulatory system.
Magnetic resonance angiography of aberrant subclavian artery
The open circulatory system of the grasshopper – made up of a heart, vessels and hemolymph. The hemolymph is pumped through the heart, into the aorta, dispersed into the head and throughout the hemocoel, then back through the ostia in the heart and the process repeated.
Flatworms, such as this Pseudoceros bifurcus, lack specialized circulatory organs.
Two-chambered heart of a fish
Human anatomical chart of blood vessels, with heart, lungs, liver and kidneys included. Other organs are numbered and arranged around it. Before cutting out the figures on this page, Vesalius suggests that readers glue the page onto parchment and gives instructions on how to assemble the pieces and paste the multilayered figure onto a base "muscle man" illustration. "Epitome", fol.14a. HMD Collection, WZ 240 V575dhZ 1543.
Image of veins from William Harvey's Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, 1628

The blood circulatory system, is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate.


Human heart during an autopsy
Computer-generated animation of a beating human heart
The human heart is in the middle of the thorax, with its apex pointing to the left.
Heart being dissected showing right and left ventricles, from above
Frontal section showing papillary muscles attached to the tricuspid valve on the right and to the mitral valve on the left via chordae tendineae.
Layers of the heart wall, including visceral and parietal pericardium
The swirling pattern of myocardium helps the heart pump effectively
Arterial supply to the heart (red), with other areas labelled (blue).
Autonomic innervation of the heart
Development of the human heart during the first eight weeks (top) and the formation of the heart chambers (bottom). In this figure, the blue and red colors represent blood inflow and outflow (not venous and arterial blood). Initially, all venous blood flows from the tail/atria to the ventricles/head, a very different pattern from that of an adult.
Blood flow through the valves
The cardiac cycle as correlated to the ECG
The x-axis reflects time with a recording of the heart sounds. The y-axis represents pressure.
Transmission of a cardiac action potential through the heart's conduction system
Conduction system of the heart
The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell; there is no resting potential.
3D echocardiogram showing the mitral valve (right), tricuspid and mitral valves (top left) and aortic valve (top right).
The closure of the heart valves causes the heart sounds.
Cardiac cycle shown against ECG
Heart and its blood vessels, by Leonardo da Vinci, 15th century
Animated heart
Elize Ryd making a heart sign at a concert in 2018
The tube-like heart (green) of the mosquito Anopheles gambiae extends horizontally across the body, interlinked with the diamond-shaped wing muscles (also green) and surrounded by pericardial cells (red). Blue depicts cell nuclei.
Basic arthropod body structure – heart shown in red
The human heart viewed from the front
The human heart viewed from behind
The coronary circulation
The human heart viewed from the front and from behind
Frontal section of the human heart
An anatomical specimen of the heart
Heart illustration with circulatory system
Animated Heart 3d Model Rendered in Computer

The heart is a muscular organ in most animals that pumps blood through the blood vessels of the circulatory system.


Diagram of an artery
Microscopic anatomy of an artery.
Cross-section of a human artery
Arteries form part of the human circulatory system
Diagram showing the effects of atherosclerosis on an artery.

An artery (plural arteries) is a blood vessel in humans and most other animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.).