Whether an animal has blood or not depends on its biological classification. In general, most complex multicellular animals have some form of blood or blood-like tissue. However, there are exceptions even among the vertebrates and invertebrates that make up the animal kingdom.
Vertebrates
Most vertebrate animals (animals with backbones) have blood. This includes:
- Mammals
- Birds
- Reptiles
- Amphibians
- Fish
In vertebrates, blood is essential for distributing nutrients and oxygen throughout the body and removing waste products. Blood is composed of plasma (a liquid medium) and different types of blood cells, including red blood cells, white blood cells, and platelets.
Exceptions in Vertebrates
There are a few vertebrate animals that do not have blood or the same blood components as other vertebrates:
- Some fish in very cold waters do not have red blood cells.
- Some salamanders only have plasma, not formed blood cells.
- Embryos of some vertebrate animals temporarily use hemolymph instead of blood early in development.
Invertebrates
Among invertebrate animals (animals without backbones), the presence of blood and blood-like tissue is more variable:
- Most mollusks and annelid worms have blood.
- Arthropods like insects and crustaceans have hemolymph instead of blood.
- Echinoderms like starfish have a hemal system with fluid.
- Some simpler invertebrates like sponges and cnidarians do not have blood or any circulatory system.
Invertebrate Blood and Hemolymph
In invertebrates with blood or hemolymph, these fluids serve similar functions as they do in vertebrates:
- Transport oxygen, nutrients, and waste
- Distribute immune cells
- Maintain hydrostatic pressure
However, hemolymph and invertebrate blood have some differences from vertebrate blood:
- Hemolymph does not contain red blood cells or hemoglobin.
- Invertebrate blood cells are less specialized than vertebrate blood cells.
- The plasma is watery rather than protein-rich.
Blood and Blood-Like Tissues
In summary, here are some key points on blood and blood-like fluids in the animal kingdom:
| Animal Group | Blood or Blood-Like Tissue? |
|---|---|
| Mammals | Yes, blood |
| Birds | Yes, blood |
| Reptiles | Yes, blood |
| Amphibians | Yes, blood |
| Fish | Yes, some exceptions |
| Insects and crustaceans | Hemolymph instead of blood |
| Mollusks and annelids | Yes, blood |
| Echinoderms | Hemal fluid |
| Sponges, cnidarians | No blood or circulatory system |
As this table shows, most complex animals have blood or some analog of it, but there are exceptions even within the vertebrate and invertebrate phyla. The exact components and functions can also vary across animal groups. Overall, some form of circulatory fluid is essential for delivering oxygen and nutrients to tissues and removing waste in all but the most basic animal body plans.
Mammalian Blood
In mammals like humans, blood is a specialized circulatory fluid that contains plasma, red blood cells, white blood cells, and cell fragments called platelets. Here is a more in-depth look at the components and functions of human blood:
Plasma
Plasma is the liquid portion of blood. It makes up about 55% of total blood volume. Plasma contains:
- Water – about 92%
- Proteins – including albumin, globulins, and fibrinogen
- Nutrients – glucose, lipids, amino acids, vitamins
- Wastes – urea, uric acid
- Electrolytes – sodium, potassium, calcium
- Clotting factors – proteins that help blood clot
- Antibodies and other immunologic molecules
- Hormones
- Gases – oxygen and carbon dioxide
Plasma transports gases, nutrients, wastes, and hormones through the circulatory system. Proteins help maintain osmotic balance. Electrolytes have many essential functions. Antibodies and clotting factors are vital for immune responses and preventing blood loss.
Red Blood Cells
Red blood cells, also called erythrocytes, are specialized cells that deliver oxygen throughout the body. They make up about 40-45% of blood volume. Red blood cells contain a protein called hemoglobin that binds oxygen. Each cell has millions of hemoglobin molecules. Features of red blood cells include:
- Biconcave disc shape provides a large surface area.
- No nucleus allows more space for hemoglobin.
- Flexibility helps them travel through narrow blood vessels.
- Lifespan of about 120 days before new cells replace them.
Without red blood cells distributing oxygen, tissues and organs could not function properly. Anemia results from reduced red blood cells or hemoglobin.
White Blood Cells
White blood cells, also called leukocytes, are cells of the immune system that help defend the body against infecting agents and foreign substances. They make up 1% of blood. The types include:
- Neutrophils – fight bacterial infections
- Eosinophils – respond to parasites and allergens
- Basophils – involve in inflammatory responses
- Lymphocytes – include B cells and T cells that adapt specifically to pathogens
- Monocytes – develop into macrophages that engulf cell debris, bacteria, and viruses
White blood cells identify and eliminate pathogens, damaged tissue, and abnormal cells. They also produce antibodies and communicate with other immune cells. A high white blood cell count often signals an infection.
Platelets
Platelets, also called thrombocytes, are cell fragments produced by megakaryocytes (a bone marrow cell). They make up a very small percentage of blood. Platelets circulate for about 10 days before spleen macrophages destroy them. Platelets have essential roles in blood clotting and wound repair. When a blood vessel is damaged, platelets change shape, release chemicals, and bind together to plug the wound. This initiates the clotting cascade that produces fibrin threads sealing the wound.
Bird Blood
Birds and mammals are both warm-blooded vertebrates, but some properties of bird blood differ from mammalian blood. Here are a few key differences:
- More red blood cells – allows more efficient oxygen delivery for flight.
- Nucleated red blood cells – mammalian red blood cells eject nuclei when mature.
- Hemoglobin more efficient at binding oxygen.
- White blood cells are lymphocytes and heterophils instead of lymphocytes and neutrophils.
- Lower baseline platelet levels.
- Plasma has higher uric acid levels.
Despite some differences in blood components, bird blood still fulfills the same essential functions – delivering oxygen, aiding immunity, and promoting hemostasis. The variations reflect evolutionary adaptations to meet high energy demands.
Reptile Blood
Reptiles include snakes, lizards, turtles, crocodilians, and tuataras. As vertebrates, they have blood with the same basic components as human blood, but some unique properties include:
- Lower metabolic rate means lower oxygen demands.
- Lower red blood cell counts; nuclei may be retained.
- Larger red blood cells in some species.
- Marked variation in size and structure of white blood cells.
- Thrombocytes involved in clotting instead of platelets.
- Plasma protein composition different from mammals.
The differences reflect adaptations to changing oxygen needs with reptile biology and environments. For example, lower red cell counts in reptiles match reduced metabolic rates. Larger red cells improve oxygen transport at colder temperatures.
Amphibian Blood
Frogs, toads, salamanders, and caecilians make up the amphibians. They have blood adapted to meet challenges of transitioning between water and land environments:
- Nucleated elliptical red blood cells like other non-mammalian vertebrates.
- Mature red blood cells larger than mammalian red cells.
- Thrombocytes involved in blood clotting.
- Lower blood cell counts correlated with lower metabolic rates.
- Differences in hemoglobin affect oxygen binding affinity.
For example, lower red blood cell and hemoglobin levels accommodate amphibians’ lower metabolic rates compared to birds and mammals. Larger red cell size improves oxygen transport in water. Differences in hemoglobin binding help gas exchange across moist skin.
Fish Blood
Fish have blood that transports gases, nutrients, wastes, and immune cells as in other vertebrates. Some key properties of fish blood include:
- Elliptical red blood cells, often with nucleus.
- Lower red blood cell counts compared to mammals.
- Kidneys produce erythropoietin to stimulate red blood cell production.
- Variation in hemoglobin structure and oxygen binding affinity.
- Leukocytes involved in immune responses.
- Thrombocytes participate in clotting cascade.
Colder water fish may lack distinct red blood cells. Instead, oxygen binds to plasma proteins. The differences in hemoglobin and RBC counts compared to mammals support respiration through gills and skin.
Insect Hemolymph
Insects have an open circulatory system with hemolymph instead of blood. Hemolymph has some similar roles as blood, but many differences as well:
- No red blood cells or hemoglobin – oxygen transported dissolved in plasma.
- Immune cells called hemocytes circulate in hemolymph.
- Nutrients, wastes, and hormones transported.
- Watery plasma with little protein compared to vertebrate blood.
- No clotting factors – loss of hemolymph generally fatal.
Without specialized oxygen carriers like hemoglobin, insects rely on diffusion across tissues. Hemolymph flow depends on heart pulsations and body movements. Open circulatory systems support small, thin insect bodies but cannot sustain larger, active organisms.
Mollusk Blood
Mollusks like clams, oysters, and octopuses have an open circulatory system with blood. Mollusk blood shares some features with vertebrate blood but also has important differences:
- Plasma, hemocyanin molecules, and hemocytes (blood cells).
- Hemocyanin carries oxygen instead of hemoglobin.
- No specialized red or white blood cells.
- Blood cells called hemocytes are less specialized than vertebrate cells.
- No platelets – hemocytes help form clots.
- Proteins and hormones transported in blood.
Lacking the complex blood components found in vertebrates, mollusk blood is still sufficient to deliver oxygen without high metabolic demands. Open circulatory systems work well for mollusks but cannot provide enough pressure for vigorous vertebrate lifestyles.
Annelid Worm Blood
Segmented worms like earthworms and leeches also have blood circulating in an open system:
- Plasma, hemoglobin, and amebocytes (blood cells).
- Hemoglobin dissolved in plasma carries oxygen.
- Amebocytes phagocytize bacteria and help clot blood.
- Five amebocyte types with some similar functions as vertebrate blood cells.
- Proteins, wastes, and nutrients transported.
Annelid hemoglobin is unusual in binding oxygen in blood plasma instead of within cells. Amebocytes patrol blood but with less complexity than human white blood cells. Like mollusks,annelids get by with simple open circulatory systems unlike vertebrates.
Echinoderm Hemal System
Echinoderms like starfish and sea cucumbers have a hemal system instead of true blood vessels. This system involves:
- Hemal fluid similar to blood plasma.
- No specialized cells like blood cells.
- Hemal fluid circulates between digestive system and hemal vessels.
- Responsible for nutrient transport and gas exchange.
- Connects hydraulically driven tube feet and appendages.
Lacking cellular elements like hemocytes, the echinoderm hemal system is simpler than mollusk or annelid blood. But it effectively facilitates respiration and movement within constraints of the echinoderm body plan.
Conclusion
In summary, most complex multicellular animals have some form of blood or blood-like tissue circulating nutrients, gases, wastes, and immune cells throughout the body. Vertebrates like mammals, birds, reptiles, amphibians and most fish have true blood, with variations in cell properties and plasma composition suiting different environments and physiologies.
Among invertebrates, arthropods have hemolymph instead of blood, while mollusks, annelids, and echinoderms have blood or hemal fluid with less complexity and specialization than in vertebrates. Simple animals like sponges lack specialized circulatory fluid altogether. While the presence and specific properties of blood or hemolymph vary across the animal kingdom, some type of circulating tissue supports multicellular life by delivering oxygen and nutrients that cells need.