The Composition of Blood

Blood is a fluid connective tissue. It circulates around the body through the blood vessels by the pumping action of the heart.
Blood accounts for about 7% of the human body weight. The average adult has a blood volume of roughly 5 liters, composed of:

1. A fluid called plasma where are the blood cells and other substances.
2. Several kinds of blood cells. Within the blood plasma, are:

• Erythrocytes (red blood cells)
  
• Leukocytes (white blood cells)
  
• Thrombocytes (platelets)

The cells that make up the blood can be seen in Figure

Plasma
Plasma is the golden-yellow liquid part of the blood. Plasma is 90% water and 10% dissolved materials including proteins, glucose, ions, hormones, and gases.
Red Blood Cells
Red blood cells, also known as erythrocytes, are flattened, doubly concave cells that carry oxygen. There are about 4 to 6 million cells per cubic millimeter of blood. Red blood cells are continuously made in the red bone marrow of long bones, ribs, skull, and vertebrae. Each red blood cell lives for only 120 days, after which they are destroyed in liver and spleen.
Mature red blood cells do not have a nucleus or other organelles. They contain the protein hemoglobin which iron and this protein gives blood its red color
White Blood Cells
White blood cells, also known as leukocytes, are generally larger than red blood cells,. They have a nucleus. White blood cells make up less than one percent of the blood's volume. They are made from stem cells in bone marrow. They take part in the immune response. There are five types of white blood cells but the most important are:
Monocytes enter the tissue fluid by squeezing through capillary walls and transform into Macrophages that phagocytose (swallow) cell debris and. bacteria or viruses
Neutrophils also swallow foreign bodies.
Lymphocytes fight infection. They produce antibodies, proteins that travel in blood to identify and neutralize foreign objects, such as bacteria and viruses
Macrophage showing cytoplasmic extensions that allow it to swallow particles or pathogens. In the image here, a mouse macrophage stretches its arms to engulf two particles at once.
Platelets
Platelets, also known as thrombocytes, are small, regularly-shaped clear cell fragments. They are important in blood clotting. Platelets carry chemicals essential to blood clotting. This clot stops more blood from leaving the body when damage to wall of blood vessels occurs

Cells of the blood. From left to right: Red blood cell, platelet, white blood cell.

The Components of Blood

Blood Vessels

The blood vessels are part of the cardiovascular system and function to transport blood throughout the body. The two most important types are arteries and veins. Arteries carry blood away from the heart, while veins return blood to the heart. Capillaries are also importan.
Arteries are the large, muscular vessels that carry blood away from the heart toward the organs.. Arteries have thicker walls than veins. The elastic qualities of artery walls allow them to carry pressurized blood from the heart while maintaining blood pressure.
Arteries branch into smaller arteries and finally the capillaries that carry nutrients to the body’s cells and tissues.

Veins are vessels that carry blood from the organs toward the heart. The walls of veins have one-way valves that prevent blood from flowing backward and pooling in the legs, feet, arms or hands due to the pull of gravity. The location of veins can vary from person to person.
Capillaries are the smallest of the body's blood vessels, measuring 5-10 μm in diameter. They connect smoller arteries (arterioles) and smoller veins (venules), and they are important for the exchange of oxygen, carbon dioxide, and other substances between blood and body cells. The walls of capillaries are made of only a single layer of endothelial cells. This layer is so thin that molecules such as oxygen, water and lipids can pass through them by diffusion and enter the body tissues.
In general the term arterial blood is used to describe blood high in oxygen and venous blood to describe blood low in oxygen, although the pulmonary arteries carry deoxygenated blood and blood flowing in the pulmonary vein is rich in oxygen. (...)
Blood Vessels and Blood Pressure
Blood pressure refers to the force exerted by circulating blood on the walls of blood vessels. The pressure of the circulating blood gradually decreases as blood moves from the arteries, capillaries, and veins. The term "blood pressure" generally refers to arterial pressure, which is the pressure in the larger arteries that take blood away from the heart. Arterial pressure results from the force that is applied to blood by the contracting heart, where the blood “presses” against the walls of the arteries. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure.
The systolic arterial pressure is defined as the peak pressure in the arteries, which occurs systole( contraction); the diastolic arterial pressure is the lowest pressure when the heart is in diastole (relaxation).
the healthy ranges for arterial pressure are:
• Systolic: less than 120 mm Hg
• Diastolic: less than 80 mm Hg
Factors such as age, gender and race influence blood pressure values. Pressure also varies with exercise, emotional reactions, sleep, stress, nutritional factors, drugs, or disease. Hypertension is a condition in which a person’s blood pressure is chronically high.

This link to Discovery Health you have a video with a good explanation of blood pressure and this video is also interesting