Circulation

Primary Principle of Circulation The primary fluid flow principle derives from Newton’s first and second laws of motion: A fluid does not flow when the pressure is the same in all parts of it A fluid flows only when its pressure is higher in one area than in another, and it flows always from its higher pressure area toward its lower pressure area

Arterial Blood Pressure High pressure must be maintained in the arteries to keep blood flowing through the cardiovascular system. The chief determinant of arterial blood pressure is the volume of blood in the arteries. Arterial blood volume is directly proportional to arterial pressure. An increase in arterial blood volume causes an increase in arterial pressure.

SV(volume/beat) x HR(beat/min) = CO(volume/min) Cardiac Output Cardiac output (CO) is determined by the volume of blood pumped out of the ventricles by each beat (stroke volume or SV) and by heart rate (HR) SV(volume/beat) x HR(beat/min) = CO(volume/min)

Peripheral Resistance Peripheral resistance is the resistance to blood flow caused by friction of blood passing through blood vessels. Friction develops because of a characteristic of blood—its viscosity, or stickiness—and partly from the small diameter of arterioles and cappilaries. Viscosity stems mainly from the red blood cells, but also from protein molecules present in the blood.

Aortic and carotid sinus pressoreflexes These pressoreflexes operate in a feedback loop that maintains the homeostasis of blood pressure by decreasing the heart rate when the blood pressure surpasses the set point. 

Relative Blood Volumes

Blood Pressure Control Mechanisms There are many factors that control the cardiovascular system. An area in the medulla called the vasomotor center or vasoconstrictor center, will, when stimulated, initiate an impulse outflow via sympathetic fibers that ends in the smooth muscle surrounding resistance vessels, arterioles, venules, and veins, causing their constriction.

Changes in Local Blood Flow During Exercise

Venous Return to the Heart Venous Pumps – the blood pumping action of respirations and skeletal muscle contraction Total Blood Volume – the greater the total volume of blood, the greater the volume of blood returned to the heart.

Venous Return

Semilunar Valves Local blood pressure pushes flaps open When pressure below the valve drops, blood flows backwards and the valve closes

Capillary Exchange and Total Blood Volume Capillary exchange – the exchange of materials between plasma in the capillaries and the surrounding interstitial fluid of the systemic tissues.

Measuring Blood Pressure The sphygmomanometer measures the amount of air pressure equal to the blood pressure in an artery. The sounds that one hears when taking a blood pressure are called Korotkoff sounds. The first sound indicates the systolic blood pressure and the last sound is an indication of the diastolic pressure. Normal blood pressure is about 120 over 80 (120/80)

Mechanism of Pulse Intermittent injections of blood from the heart into the aorta, which alternately increases and decreases the pressure in that vessel. If blood steadily poured out of the heart into the aorta, there would be no pulse. The elasticity of the arterial walls, which allows them to expand with each injection of blood and then recoil. If the vessels were fashioned from rigid material, you would not be able to feel a pulse even tough there would still be a pressure change.

Where the Pulse Can Be Felt The pulse can be felt whenever an artery lies near the surface and over a bone or other firm background.

Hypertension (High Blood Pressure) Defined as a blood pressure of 140/90 and above Risk factors include: genetics, sex, race, age, stress, high alcohol and caffeine intake, obesity, smoking and lack of exercise