Although the details of circulatory function are complex, three basic principles underlie all functions of the system. 1. Blood flow to most tissues is controlled according to the tissue need. When tissues are active, they need a greatly increased supply of nutrients and therefore much more blood flow than when at rest— occasionally as much as 20 to 30 times the resting level. Yet, the heart normally cannot increase its cardiac output more than four to seven times greater than resting levels. Therefore, it is not possible simply to increase blood flow everywhere in the body when a particular tissue demands increased flow. Instead, the microvessels of each tissue continuously monitor tissue needs, such as the availability of oxygen and other nutrients and the accumulation of carbon dioxide and other tissue waste products, and these microvessels in turn act directly on the local blood vessels, dilating or constricting them, to control local blood flow precisely to that level required for the tissue activity. Also, nervous control of the circulation from the central nervous system and hormones provide additional help in controlling tissue blood flow.

2. Cardiac output is the sum of all the local tissue f lows. When blood flows through a tissue, it immediately returns by way of the veins to the heart. The heart responds automatically to this increased inflow of blood by pumping it immediately back into the arteries. Thus, the heart acts as an automaton, responding to the demands of the tissues. The heart, however, often needs help in the form of special nerve signals to make it pump the required amounts of blood flow.

3. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control. The circulatory system is provided with an extensive system for controlling the arterial blood pressure. For instance, if at any time the pressure falls significantly below the normal level of about 100 mm Hg, within seconds a barrage of nervous reflexes elicits a series of circulatory changes to raise the pressure back toward normal. The nervous signals especially (a) increase the force of heart pumping, (b) cause contraction of the large venous reservoirs to provide more blood to the heart, and (c) cause generalized constriction of the arterioles in many tissues so that more blood accumulates in the large arteries to increase the arterial pressure. Then, over more prolonged periods— hours and days—the kidneys play an additional major role in pressure control both by secreting pressure-controlling hormones and by regulating the blood volume.

Thus, the needs of the individual tissues are served specifically by the circulation. In the remainder of this chapter, we begin to discuss the basic details of the management of tissue blood flow and control of cardiac output and arterial pressure.

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

Physical Characteristics of the Circulation

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