Fibrinogen is converted to fibrin by the action of thrombin, and fibrin is the protein that actually forms a blood clot. An inability to properly form clots, such as is seen in hemophilia, could arise from a defect in fibrinogen, resulting in a problem with the clotting pathway.

Renin is an enzyme released by the kidney, which creates angiotensin to help raise blood pressure. Vasopressin is a posterior pituitary hormone that acts on the kidney to retain water. Albumin is a plasma protein that functions to maintain the oncotic pressure in the blood, as well as to transport fatty acids. Major histocompatibility complex (MHC) proteins are used to present antigens to the immune system.

The plasma portion of the blood is the extracellular matrix that suspends the erythrocytes and lymphocytes traveling through circulation. The plasma contains water, proteins (chiefly albumin), electrolytes, and clotting factors (such as thrombin). Whole blood contains the cells, as well as thx extracellular plasma. Blood serum refers to blood plasma that has had the clotting factors removed.

Mature erythrocytes (red blood cells) do not contain a nucleus and lack most organelles. While immature, the erythrocyte contains a nucleus, but this gets extruded in order to make more room for hemoglobin to transport oxygen. It is important to understand the implications of not having any additional organelles; the red blood cell cannot derive energy from the citric acid cycle and cannot produce, transport, or exocytose proteins. This may seem like a problem, but it actually prevents the red blood cell from consuming the oxygen that it is transporting. If erythrocytes contained nuclei or mitochondria, they would metabolize the oxygen attached to hemoglobin, failing to deliver it to other tissues.

The mature erythrocyte (red blood cell) contains no nucleus, mitochondria, Golgi apparatus, or endoplasmic reticulum. Consequently, the red blood cell can only derive energy from glycolysis, the anaerobic breakdown of glucose in the cytosol. Mitochondria would be required for any other metabolic process.

Thrombin, created from prothrombin after activation by coagulation factors from the liver, is responsible for turning fibrinogen to fibrin. Fibrin adheres to endothelial cells and platelets to allow clots to form in response to vessel damage. Complement helps attract immune cells to the site of injury. Platelet monoamine oxidase degrades amines, such as serotonin, in neuronal synapses.

Erythrocytes are chiefly responsible for delivering oxygen to cells throughout the body and do not possess nuclei in their mature form. The absence of a nucleus and mitochondria from mature erythrocytes prevents transported oxygen from being consumed by the carrier cell, instead of being delivered to the body.

Granulocytes are a division of white blood cells and frequently have lobed or multiple nuclei. Neurons contain nuclei in their somas and adipocytes contain peripheral nuclei, located close to the membrane.

Carbonic anhydrase inhibitors will decrease the blood's pH. Conditions that will shift the hemoglobin dissociation curve to the right are increased temperature, increased 2,3-DPG, and decreased pH.

When someone is cold they, will experience vasoconstriction in their hands, allowing less blood to flow to the hands. Their hands will feel colder, since less blood is reaching their hands. When the body is cold, it concentrates blood flow to the core. This maintains a constant, warm blood supply to the vital organs, as well as reduces the blood flow to extremities that have a greater surface area for heat loss to the environment.

During heavy exercise, the body temperature increases. Heat must leave as efficiently as possible, and thus the body's blood vessels will vasodilate. Vasodilation leads to the vessels being closer to the skin, thus allowing quicker heat transfer to the environment. This effect is accomplished, in part, by stimulation from the sympathetic nervous system.

On a cold day, it is vital to keep body at a steady temperature through homeostatic processes. The body will compensate for heat loss by having the blood vessels vasoconstrict. This leads to the vessels being farther from the skin, allowing less heat loss due to blood flow. This also explains why conditions like frostbite generally start at the extremities, where surface area is greatest. These regions have the greatest amount of vasoconstriction, and thus contain the smallest amount of warm blood, leaving them prone to the cold environment due to a lack of internal heating.