Introduction to the Circulatory System

All organisms, except for the very small, must have an internal transport system (1). Circulatory system is a type of a transport system that allows the transport gasses, nutrients, metabolic wastes, hormones and antibodies. Small organisms are structured in a way that allows their cells to be in close proximity to an aqueous environment eliminating the need for the transport system. Large organisms such as vertebrates, have the most complex and highly evolved circulatory system. The circulatory system of such animals functions in conjunction with the kidneys, liver, and many other organs and responds to a variety of changing external conditions helping the organism maintain constant internal environment (homeostasis).

The Lymphatic System in Relation to a Cardiovascular System

Figure 1. Lymphatic vessels pick up fluid in the tissues and return it to the blood in vessels near the heart.

Image Credit: Encyclopedia.bg.com 

Circulatory system is basically a set of connective tubes that transport fluid (2). The vertebrate circulatory system has two components: blood and lymph vascular systems. The lymphatic system consists of lymphatic vessels and the lymph, the fluid they carry. The blood-vascular system of vertebrates  unlike that of invertebrates (except for annelids), is a continuum of ducts and thus it is a closed system (1). Yet, fluid constituents of the blood leak out of the capillaries into the tissue fluids.This fluid does not accumulate in tissue, however, because it is drained away by the second component of the circulatory system - the lymphatic system. Tissue fluids enter net-like lymphatic capillaries where they constitute lymph. Lymph, then travels through the progressively larger lymphatic vessels until it is drained into the venous system at several points in the body. The heart provides no pressure to drive the lymphatic fluid circulation, therefore it relies exclusively upon the pressures generated by the respiratory and other body motions, as well as on their own intrinsic musculature. Like the veins of the cardiovascular system, the lymphatic vessel have one-way valves to prevent retrograde flow of  lymph. Many lower vertebrates also have lymph sinuses, some of which beat weakly as lymphatic hearts.

Figure 1. Major Components of the Circulatory System

Cardiovascular System

The blood-vascular system consists of of the heart, blood vessels, and blood an is often referred to as the cardiovascular system (2).There blood vessels within the cardiovascular system are distinguished by the direction in which they carry blood (Figure 1). Arteries distribute blood from the heart to the tissues, while veins return blood from tissues to the heart. Very small arteries and veins are known as arterioles and venules respectively. Arterioles and venules are joined to each other by capillaries. Capillaries are only about 1 mm long but they are located only a fraction of a millimeter away from one another and form the vascular network within all tissues (1). Sets of capillaries serving one area of tissue constitutes the capillary bed (Figure 2).

Figure 2. SEMvcc of normal rat liver (orig. magn., 20×). A dense vascular network is arranged around the circumference of the common bile duct. A: transverse section of Extrahepatic peribiliary plexus. Observe thearterial and venous vessels on the outer layer (B) and a dense capillary network in the inner layer of the plexus (C). Image Credit: Biology Online

Structure of Arteries, Veins, and Capillaries

Arteries and veins have tubular walls and are organized into three distinct layers surrounding the central tube (2). The innermost layer, known as tunica intima, includes the lining of endothelial  cells (Fig.3). The outermost layer, tunica adventitia, is made up of fibrous connective tissue. The tunica media is the middle layer. This layer varies greatly between arteries and veins. In arteries it is composed predominantly of elastic fibers with some smooth muscle. In veins, tunica media consists mostly of smooth muscle, with almost no elastic fibers present at all. Veins usually have one-way valves within their walls, while arteries lack such valves.
Gasses, nutrients, water, dissolved ions and heat are transported across the walls of blood capillaries. Capillaries are small and have very thin walls (they lack tunica media and tunica adventitia), which allows efficient exchange of all dissolved solutes present in the blood.

Figure 3. Image Credit: Teacherweb

Function of Arteries 

The structure of the arteries varies with their size (5). The larger the artery, the more elastic fibers it has within its walls; small arteries have almost none. These structural differences correspond to the differences in function between large and small arteries. Since the primary function of arteries is to carry the blood away from the heart and out to body tissues, it is crucial that the arteries are able to absorb and distribute the sudden surge of blood through them when the heart contracts. Rhythmic contractions of the heart send spurts of blood into large arteries. Their elastic walls is what allows them to expand and accommodate the sudden injection of blood. The elasticity allows the arterial walls to recoil between contractions, which is what drives the volume of blood through progressively smaller arteries all the way to arterioles and capillaries.
 As blood flows from large arteries, such as aorta, to capillaries and veins, the initial pressure exerted by the force of the heart contraction declines due to (1) friction - as the blood encounters resistance from the luminal walls of vessels. (2) increase in total cross-sectional area of vessels - smaller vessels have larger cross-sectional area, with capillarity being particularly large in terms of total cross-sectional area. As a result blood teaching the veins retains very little pressure (Fig 4).

Function of Veins

The main function of veins is to return blood to the heart, which makes them the collecting tubes (2),(5). At any given moment, up to 70% of the circulating blood within the body, may reside in veins. Unlike the arterial circulation, blood reaching the venous side of the circulation, retains very little pressure (see above). In fact in some of the large veins, forces moving the blood, may fall to zero or even become negative, forcing the  blood to flow in reverse direction. The structure of veins allows them to effectively deal with this unfavorable pressure. For example, the one-way valves found within the walls of the veins prevent the retrograde flow and force the blood to move back to the heart (Fig. 2).

Blood Pressure and Blood Velocity as a Function of the 

Total Cross-Sectional Area of the Vessels

Figure 3.  Depicts hemodynamics of circulation. The cross-sectional area of the vessels is inversely proportional to blood pressure and blood velocity. 

 Image Credit: StudyBlue

Blood 

Blood is comprised of plasma and formed elements. The plasma is the fluid component of the blood in which  all formed elements are suspended. Formed elements are all cellular components of the blood produced by hemopoietic tissues containing hematopoietic stem cells. Red blood cells also known as erythrocytes, are one type of the cells that are suspended in the plasma.The other two types of cells are the white blood cells or leukocytes and the platelets.

Plasma

Plasma and the formed elements give blood a wide variety of functions, including respiration, protection from disease, nutrition, (blood carries carbohydrates, lipids and proteins), excretion (blood carries spent metabolites), and regulation of body temperature (blood carries and distributes heat), maintenance of water balance, and transport of hormones (2),(3). Besides the formed elements plasma plasma also contains dissolved salts and minerals like calcium, sodium, magnesium, and potassium.

Red Blood Cells

The red blood cells of all animals, except for mammals have nuclei. Mature erythrocytes in mammals lack nuclei. Red blood cells contain more than 300 mg/mL of hemoglobin to carry oxygen from the lungs to tissues and carbon dioxide from tissues to the lungs Thus, blood cells function as containers for hemoglobin - a major oxygen transport molecule. If hemoglobin is not associated with the red blood cells and is left free in plasma, it will get excreted by the kidneys. Red blood cells of different species can vary in size. For example in humans the red blood cells are 8 microns in diameter; elephants red blood cells are 9 microns in diameter; and salamander's red blood cells reach 80 microns in diameter. A resilient, spectrin-actin membrane cytoskeleton maintains the biconcave shape even after the cell is heavily distorted each time it passes through a small capillary. The elasticity of the membrane skeleton allows it to regain its shape. After circulating in blood for 120 days, erythrocytes abruptly become senescent, and phagocytes in the spleen, liver, and bone marrow remove them from the blood. The biochemical basis of this precise cellular aging and clearance process is still being investigated

The Membrane Cytoskeleton of the Red Blood Cells Plasma Membrane

Figure 4. A, Whole cell. B, Cut-away drawing. C, Detailed drawing. Nodes consisting of a short actin filament and associated proteins interact with multiple spectrin molecules, which, in turn, bind to two transmembrane proteins: glycophorin and (via ankyrin) Band 3. D, An electron micrograph of the actin-spectrin network. (D, Courtesy of R. Josephs, University of Chicago, Illinois.)

White Blood Cells and Platelets

White blood cells are the second type of the cellular blood components produced by hemopoietic tissues. Leukocytes defend the organism from infection and disease.The white blood cells are short-lived, lasting only a few days to a few weeks. A drop of blood can contain anywhere from 7,000 to 25,000 white blood cells at a time. The Platelets are third formed element in the blood. They are small anucleate cellular fragments derived from megakaryocytes, contribute to both blood clotting and the repair of minor defects in the sheet of endothelial cells that lines blood vessels (4). They accomplish this by releasing factors that produce a cascade of chemical events, which ultimately leads to the formation of a blood clot, or thrombus. Human platelets circulate in the blood for about seven days. During this time, some are utilized to repair vascular defects, while phagocytic cells in the liver and spleen remove others.

The Composition of Blood

Figure 5. A, Light micrograph of a dried blood smear prepared with Wright's stain. B, Family tree of blood cells showing the developmental relationships of the various lineages. Looping-back arrows indicate renewal of the cell type. Forward-oriented arrowsindicate differentiation and proliferation. RBC, red blood cell. (A, Courtesy of J.-P. Revel, California Institute of Technology, Pasadena.)