A single drop of blood typically contains about 250 million red blood cells, and about fifty thousand white blood cells. Platelets are also present in drops of blood, but their numbers can vary from 150,000-450,000.

Portal circulation is found in the gut and liver, it features vessels that carry nutrient rich blood from the gut to the liver for processing. Systemic circulation is between the heart and the tissues of the body. Pulmonary circulation is between the heart and the lungs. There is no special name for circulation of the mouth and head. The other portal circulation is found in the brain - between the hypothalamus and pituitary gland.

The lymphatic system drains excess tissue fluid as well as fights infections. The renal system is responsible for excretion and reabsorption. The circulatory system is responsible for the transportation of oxygen and nutrients throughout the body. 

The confluence of sinuses connects the superior sagittal sinus, straight sinus, and occipital sinus. Blood in the confluence of sinuses drains into the left and right transverse sinuses. From the transverse sinus blood drains to the sigmoid sinus, which ultimately drains into the internal jugular vein. 

Red blood cells, carry oxygen  and carbon dioxide  in the bloodstream. 

Red blood cells are produced in the bone marrow and circulate throughout the body to provide oxygen exchange. They have an average lifespan of 120 days (roughly three months). At the end of their lifespan they are destroyed by specific cells (macrophages) located in the spleen.

Hemoglobin is the protein found in red blood cells that contains an iron-containing organic molecule called a heme attached to a protein called a globin. Fibrinogen is a glycoprotein found in the blood that plays a role in the formation of blood clots. Platelets are a component of blood (fragments of larger cells, megakaroyctes) that also play an important role in clotting blood. Plasma is a component of whole blood. Whole blood consists of red and white cells and plasma suspended in a fluid called plasma. Complement is a protein that plays a role in the immune response. 

Neutrophils are the body's first line of defense against invading bacteria. They search out, kill, and phagocytize bacteria that are involved in infection. They are classified as a granulocyte. Basophils and Eosinophils are also both granuclocytes. Basophils are similar to mast cells and release histamine and heparin. Eosinophils play an important role in parasitic infections. Platelets are fragments of a larger cell called a megakarocyte. Platelets help form blood clots and prevent blood loss. Lymphocytes are involved in specific immune responses and include natural killer cells, b-lymphocytes and t-lymphocytes. 

Erythrocytes, or red blood cells, are responsible for transporting oxygen from the lungs to the tissues and are also involved in carbon dioxide transport. Monocytes are  a class of white blood cells which may become tissues macrophages, produce dendritic cells, or generate osteoclasts. Platelets are cell fragments which play an important role in blood clotting and plugging holes in damaged blood vessels. B-lymphocytes are a type of lymphocyte that produces antibodies. T-lymphocytes are a type of lymphocyte that regulate the activity of other cells and kill virus-infected cells. 

Hemostasis is the number of reactions that are initiated to stop bleeding. Hemostasis means prevention of blood loss. Hematopoiesis is the formation of blood cells. Coagulation is the process of blood clotting that results in a fibrin mass. The intrinsic pathway is one of two pathways that initiate blood clotting. Hemophilia is a X-linked disorder that is caused by the absence of a clotting factor.