The circulation, shown in Figure 1, is divided into the systemic circulation and the pulmonary circulation. Because the systemic circulation supplies blood flow to all the tissues of the body except the lungs, it is also called the greater circulation or peripheral circulation.

Fig1. Distribution of blood (in percentage of total blood) in  the different parts of the circulatory system. 

Functional Parts of the Circulation. Before discussing the details of circulatory function, it is important to understand the role of each part of the circulation.

The function of the arteries is to transport blood under high pressure to the tissues. For this reason, the arteries have strong vascular walls, and blood flows at a high velocity in the arteries.

The arterioles are the last small branches of the arterial system; they act as control conduits through which blood is released into the capillaries. Arterioles have strong muscular walls that can close the arterioles completely or can, by relaxing, dilate the vessels severalfold, thus having the capability of vastly altering blood flow in each tissue in response to its needs.

The function of the capillaries is to exchange fluid, nutrients, electrolytes, hormones, and other substances between the blood and the interstitial fluid. To serve this role, the capillary walls are thin and have numerous minute capillary pores permeable to water and other small molecular substances.

The venules collect blood from the capillaries and gradually coalesce into progressively larger veins.

The veins function as conduits for transport of blood from the venules back to the heart; equally important, they serve as a major reservoir of extra blood. Because the pressure in the venous system is very low, the venous walls are thin. Even so, they are muscular enough to contract or expand and thereby serve as a controllable reservoir for the extra blood, either a small or a large amount, depending on the needs of the circulation.

Volumes of Blood in the Different Parts of the Circulation. Figure 1 gives an overview of the circulation and lists the percentage of the total blood volume in major segments of the circulation. For instance, about 84 percent of the entire blood volume of the body is in the systemic circulation and 16 percent is in the heart and lungs. Of the 84 percent in the systemic circulation, approximately 64 percent is in the veins, 13 percent is in the arteries, and 7 percent is in the systemic arterioles and capillaries. The heart contains 7 percent of the blood, and the pulmonary vessels, 9 percent.

Most surprising is the low blood volume in the capillaries. It is here, however, that the most important function of the circulation occurs—diffusion of substances back and forth between the blood and the tissues. This function is discussed in detail in Chapter 16.

Cross-Sectional Areas and Velocities of Blood Flow. If all the systemic vessels of each type were put side by side, their approximate total cross-sectional areas for the average human being would be as follows:

Note particularly that the cross-sectional areas of the veins are much larger than those of the arteries, averaging about four times those of the corresponding arteries. This difference explains the large blood storage capacity of the venous system in comparison with the arterial system.

Because the same volume of blood flow (F) must pass through each segment of the circulation each minute, the velocity of blood flow (v) is inversely proportional to vascular cross-sectional area (A):

V= F/A

Thus, under resting conditions, the velocity averages about 33 cm/sec in the aorta but is only 1/1000 as rapid in the capillaries—about 0.3 mm/sec. However, because the capillaries have a typical length of only 0.3 to 1 millimeter, the blood remains in the capillaries for only 1 to 3 seconds, which is surprising because all diffusion of nutrient food substances and electrolytes that occurs through the capillary walls must be performed in this short time.

Pressures in the Various Portions of the Circulation. Because the heart pumps blood continually into the aorta, the mean pressure in the aorta is high, averaging about 100 mm Hg. Also, because heart pumping is pulsatile, the arterial pressure alternates between a systolic pressure level of 120 mm Hg and a diastolic pressure level of 80 mm Hg, as shown on the left side of Figure 2.

Fig2. Normal blood pressures in the different portions of the circulatory system when a person is lying in the horizontal position. 

As the blood flows through the systemic circulation, its mean pressure falls progressively to about 0 mm Hg by the time it reaches the termination of the superior and inferior venae cavae where they empty into the right atrium of the heart.

The pressure in the systemic capillaries varies from as high as 35 mm Hg near the arteriolar ends to as low as 10 mm Hg near the venous ends, but their average “functional” pressure in most vascular beds is about 17 mm Hg, a pressure low enough that little of the plasma leaks through the minute pores of the capillary walls, even though nutrients can diffuse easily through these same pores to the outlying tissue cells.

Note at the far right side of Figure 2 the respective pressures in the different parts of the pulmonary circulation. In the pulmonary arteries, the pressure is pulsatile, just as in the aorta, but the pressure is far less: pulmonary artery systolic pressure averages about 25 mm Hg and diastolic pressure averages about 8 mm Hg, with a mean pulmonary arterial pressure of only 16 mm Hg. The mean pulmonary capillary pressure averages only 7 mm Hg. Yet, the total blood flow through the lungs each minute is the same as through the systemic circulation. The low pressures of the pulmonary system are in accord with the needs of the lungs because all that is required is to expose the blood in the pulmonary capillaries to oxygen and other gases in the pulmonary alveoli.

اخر الاخبار

اخبار العتبة العباسية المقدسة

أكاديمية التطوير الإداري تنظّم دورة في اللغة الفارسية لعدد من ملاكات العتبة العباسية المقدسة

استعدادًا لافتتاحه.. المشرف على قسم الشؤون الطبية يجري جولة في مركز السيد جعفر الحلي للخدمات الطبية.

الهيأة العليا لإحياء التراث تقيم الدورة العلمية التخصصية لكتاب في رحاب العقيدة

سماحة السيد الصافي يؤكد ضرورة تنمية الموارد المالية لدائرة التقاعد الوطنية وتعظيمها

المزيد

الآخبار الصحية

العلاج بالماء البارد.. باحثون يكشفون فوائده الصحية

من دون صالة رياضية.. عادة بسيطة لتحسين الصحة وخسارة الوزن

لصحة القلب وتفادي السرطان.. استهلك هذا النوع من الطعام

دراسة تحذر من خطر شرب القهوة في أكواب البلاستيك

المزيد

الآخبار التكنلوجية

إنجاز طبي تاريخي.. جراحة قلب بلا شق في الصدر

اليابان.. مراحيض توفر فحص للحالة الصحية عبر "تحليل البراز" !

اختراق واسع لمنصة "إنستغرام".. ونشر بيانات على "الدارك ويب"

علماء يحذرون: هكذا سيفنى كوكب الأرض

المزيد

مواضيع ذات صلة

Basic Principles of Circulatory Function

Abnormal Rhythms That Result From Block of Heart Signals Within the Intracardiac Conduction Pathways Sinoatrial Block

Abnormal Sinus Rhythms Tachycardia

Flow of Current Around the Heart During the Cardiac Cycle

Voltage and Time Calibration of The Electrocardiogram

Characteristics of the Normal Electrocardiogram: Depolarization Waves versus Repolarization Waves

Control of Excitation and Conduction in The Heart

Automatic Electrical Rhythmicity of the Sinus Fibers

Regulation of Heart Pumping

Diastole and Systole

ACTION POTENTIALS IN CARDIAC MUSCLE

Fibrinolysis