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BIOS255 – Anatomy & Physiology IIIProf Haneen Salhieh, M.S.
Week 1 - The Cardiovascular System: Blood
Introduction to the Cardiovascular System
The cardiovascular system consists of only the heart and blood vessels. When we also include blood, this
is known as the circulatory system. The study of blood is known as hematology. The fundamental
objective of the circulatory system is to transport substances from one place in the body to another, using
blood as the liquid medium for this purpose. Key functions of the circulatory system include:
Transportation: Blood carries oxygen, carbon dioxide, nutrients (from digestive tract), hormones,
heat, and metabolic wastes.
Regulation: Components of this system regulate homeostasis of all body fluids (managing the volume
and distribution), maintain the pH of extracellular fluids, and help regulate body temperature and
metabolic heat.
Protection: plays several roles in inflammation to limit the spread of infection, contains cells and
proteins that destroy pathogens or remove tissue debris, and initiates blood clotting to minimize blood
loss and maintain blood vessel structure.
General Characteristics of Blood
Adults have about 4 to 6 liters of blood, with normal blood volume ranges of 5-6L for males and 4-5L for
females. Blood is more viscous than water as it is filled with solutes, nutrients, gases, cells, and dissolved
proteins. General characteristics of blood include:
Composition: plasma and formed elements (discussed in next slides).
Temperature: blood is slightly warmer than the body’s core temperature, with a temp of 38°C
(100.4°F).
Color: ranges from bright when oxygen is bound to red blood cells, to dark red when oxygen is absent.
pH Level: normal pH range between 7.35–7.45 that is crucial for proper bodily function. Different
buffer systems are present in blood that help regulate pH levels.
Density: Blood is denser (heavier) than water.
Recall that blood is a liquid connective tissue. Like all other connective tissues, it consists of an
extracellular matrix and cells.
The matrix component of blood is known as plasma. Plasma is a clear, light-yellow fluid that
makes up 55% of the blood volume.
The cellular components of blood are referred to as formed elements, constituting 45% of the
blood volume. They consist of red blood cells (erythrocytes or RBCs), white blood cells
(leukocytes or WBCs), and cell fragments called platelets (thrombocytes). Formed elements are
suspended in the blood plasma and are so named due to their definitive membranous structure,
observable under a microscope.
Blood Centrifugation
Think of plasma as the liquid portion of blood and formed elements as the solid. These can be separated
through a process called blood fractionation. A blood sample is collected in a tube, which is then placed in
a centrifuge machine to separate its components based on density. Heavier components settle at the
bottom of the tube, while lighter components remain on top.
Upon centrifugation, blood components settle in this order from top to bottom:
Plasma. 55%
Buffy Coat (WBCs and platelets) 1%
Erythrocytes (RBCs) 45%
RBCs settle at the bottom of the tube as they are the densest, constituting 37-52% of the total volume.
This value is known as the hematocrit or packed cell volume (measurement of % total blood occupied by
erythrocytes).
, BIOS255 – Anatomy & Physiology III
Prof Haneen Salhieh, M.S.
Platelets and WBCs are lighter than RBCs but heavier than plasma, so they form a narrow cream-colored
zone in the middle, known as the buffy coat. This constitutes approximately 1% of the blood volume.
Plasma rests on top as the lightest component, comprising about 47-63% of the total blood volume. Blood
serum essentially consists of plasma with clotting proteins removed after undergoing centrifugation again.
Blood Plasma
Plasma makes up over half of blood volume. It is a complex mixture of water, nutrients, hormones,
electrolytes, gases, nitrogenous wastes, and proteins. Plasma proteins are mainly produced by liver and
exert the greatest effect on the physical characteristics of whole blood. The major plasma proteins in
blood include albumin, globulins, and fibrinogen. Main job is transport and buffer.
Albumin constitutes 60% of proteins in plasma and has two main functions: (1) transport different
solutes such as lipids, hormones, and electrolytes, and (2) buffer blood pH. As the most
abundant plasma protein, albumin significantly influences the viscosity and osmolarity of blood.
Let’s examine these two physical properties of blood.
o Viscosity: Excessive albumin concentration, known as hyperalbuminemia, results in
thicker blood that moves sluggishly. Conversely, a deficiency in plasma albumin
concentration, termed hypoalbuminemia, leads to thinner blood that flows too easily.
o Osmolarity: This refers to the concentration of dissolved particles in a fluid, which
directly affects the movement of water. Remember osmosis: wherever solutes are, water
follows. In cases of hyperalbuminemia, blood osmolarity becomes too high, and excess
water enters the bloodstream. This increases blood volume and blood pressure. On the
other hand, hypoalbuminemia reduces blood osmolarity, causing excess water to remain
in the tissues rather than enter the bloodstream. This decreases blood volume and blood
pressure and can result in tissue edema (swelling).
Globulins make up 36% of plasma proteins and are divided into three subclasses: alpha, beta,
and gamma globulins. Alpha and beta globulins play roles in transporting different substances
such as lipids, minerals, hormones, and hemoglobin from dead RBCs. Gamma globulins are
antibodies secreted to combat pathogens in specialized immune responses.
Fibrinogen constitutes about 4% of all plasma proteins. It is a soluble precursor of another
protein called fibrin. Fibrin is a sticky protein that forms the framework of a blood clot during
hemostasis, or the cessation of bleeding (discussed in the next concept). Fibrinogen can impact
the viscosity of blood and may lead to increased blood pressure if elevated. When fibrinogen is
removed from blood plasma, this is known as blood serum.
Hematopoiesis (Production of Blood)
It is important to note that just as we lose blood components daily, we also replace them. The process of
producing blood, especially its formed elements, is called hematopoiesis.
An adult typically produces 400 billion platelets, 200 billion RBCs, and 10 billion WBCs each day via
hematopoiesis. The tissues that produce these new blood cells are called hematopoietic tissues. During
fetal development, hematopoietic tissues were the yolk sac, liver, spleen, thymus, and bone marrow. After
birth, most tissues stop producing blood cells except red bone marrow. Red bone serves as the main
hematopoietic tissue from infancy onward. In children, red bone marrow is distributed throughout the
skeletal system but gradually becomes concentrated in specific bones upon aging. These bones include
the sternum, ribs, skull, pelvis, vertebrae, and epiphyses of long bones like the femur, tibia, and humerus.
All formed elements originate from a common cell in hematopoietic tissues known as hematopoietic
stem cells (HSC), or pluripotent stem cells (PPSCs). Pluripotency means the ability to differentiate into
various cell types. Some HSCs specialize in forming myeloid stem cells which mature into platelets, RBC,
and certain WBCs (neutrophils, eosinophils, basophils, monocytes). Other HSCs form lymphoid stem
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