CH30 Circulation of Blood

Venous Pumps

Blood-pumping action of respirations and skeletal muscle contractions facilitates venous return by increasing the pressure gradient between the peripheral veins and the venae cavae (central veins) Respirations: Inspiration increases the pressure gradient between peripheral and central veins by decreasing central venous pressure and also by increasing peripheral venous pressure *Diaphragm creates pressure gradient promoting venous return to venae cavae Skeletal muscle contractions promote venous return by squeezing veins through a contracting muscle and milking the blood toward the heart -The value of skeletal muscle contractions in moving blood through veins is illustrated by a common experience: standing still for several minutes is more uncomfortable and tiring than walking.

Factors that Affect Heart Rate: Carotid Sinus and Aortic Reflexes

Carotid Sinus Reflex -Located at the beginning of the internal carotid artery -Sensory fibers from carotid sinus baroreceptors run through the carotid sinus nerve and the glossopharyngeal nerve to the cardiac control center ---The carotid sinus lies just under the sternocleidomastoid muscle at the level of the upper margin of the thyroid cartilage. -Parasympathetic impulses leave the cardiac control center and travel through the vagus nerve to reach the SA node ---If the integrators in the cardiac control center detect an increase in blood pressure above the setpoint, a correction signal is sent to the SA node by way of efferent parasympathetic fibers in the vagus (tenth cranial) nerve. Aortic Reflex Sensory fibers extend from baroreceptors in the wall of the aortic arch through the aortic nerve and through the vagus nerve to terminate in the cardiac control center Stimulation causes the cardiac control center to increase vagal inhibition, thus slowing the heart A sudden increase in blood pressure beyond the setpoint in the aorta or carotid sinus stimulates the aortic or carotid baroreceptors. A decrease in aortic or carotid blood pressure usually allows some acceleration of the heart by way of correction signals through the cardiac nerve.

Total Blood Volume

Changes in total blood volume change the amount of blood returned to the heart At the arterial end of a capillary, outward hydrostatic pressure is the strongest force; it moves fluid out of the plasma and into the interstitial fluid (IF) At the venous end of a capillary, inward osmotic pressure is the strongest force; it moves fluid into the plasma from the IF; 90% of fluid lost by plasma at the arterial end is recovered The lymphatic system recovers the fluid not recovered by the capillary and returns it to the venous blood before it is returned to the heart

Functions of Circulation

Circulation is the only means by which cells can receive materials necessary for survival and have wastes removed. -Circulation of different volumes of blood per minute is essential for healthy survival Circulation control mechanisms must accomplish two functions: -Maintain circulation -Vary the volume and distribution of the blood circulated More active cells need more blood per minute than less active cells. Because blood circulates, it can continually bring in more oxygen and nutrients to replace what is consumed.

Factors that Affect Stroke Volume: Chemical Factors

Contractility (strength of contraction) can also be influenced by chemical factors: -Neural: Norepinephrine -Endocrine: Epinephrine -Triggered by stress, exercise

Blood Pressure and Bleeding

During arterial bleeding, blood escapes from the artery in spurts because of the alternating increase and decrease of arterial blood pressure During venous bleeding, blood flows slowly and steadily as a result of low and practically constant pressure

Velocity of Blood Flow

Governed by this physical principle: When a liquid flows from an area of one cross-sectional size to an area of larger size, its velocity decreases in the area with the larger cross section Blood flows more slowly through arterioles than arteries because the total cross-sectional area of arterioles is greater than that of arteries, and capillary blood flow is slower than arteriole blood flow The venule cross-sectional area is smaller than the capillary cross-sectional area The most rapid blood flow takes place in the arteries, and the slowest takes place in the capillaries (which makes sense functionally so we have maximal exchange with tissues).

Arterial Blood Pressure

Measured with the aid of a sphygmomanometer and stethoscope; listen for Korotkoff sounds as the pressure in the cuff gradually decreases -The pressure cuff is pumped with air until the pressure inside the cuff exceeds the expected systolic pressure of the large arteries of the arm. At this point, no sound caused by the pulsing of blood in the arteries can be head with a stethoscope. -As the pressure inside the cuff is slowly released from a valve, the air pressure equals the maximum pressure of the pulse waves in the artery; therefore, the pulsing sounds now can be heard. -Systolic blood pressure: Force of the blood pushing against artery walls as ventricles contract -Diastolic blood pressure: Force of the blood pushing against artery walls when ventricles are relaxed and during isovolumetric ventricular contraction -Pulse pressure: Difference between the systolic and diastolic blood pressures The mean arterial pressure (MAP) is the average blood pressure in the arteries for the perfusion of tissues.

Where the pulse can be felt

Radial artery: At the wrist Temporal artery: In front of the ear or on the outer side of the eye Common carotid: Anterior edge of the sternocleidomastoid muscle Facial artery: Lower margin of the lower jawbone Brachial artery: Bend of the elbow, along inner margin of the biceps muscle Femoral artery: Middle of the groin Popliteal artery: Behind the knee Dorsal pedal: Midline or slightly medial on dorsum of foot Posterior tibial: medial aspect of foot, inferior to medial malleolus

Factors that Affect Heart Rate: Other Reflexes

Reflexes that involve important factors such as emotions, exercise, hormones, blood temperature, pain, and stimulation of various exteroceptors also influence the heart rate. Norepinephrine released as a result of sympathetic stimulation increases the heart rate and the strength of cardiac muscle contraction. Emotions produce changes in the heart rate through the influence of impulses from the cerebrum by way of the hypothalamus -Anxiety, fear, and anger often increase the heart rate -Grief tends to decrease the heart rate Exercise normally increases the heart rate Increased blood temperature or stimulation of skin heat receptors increases the heart rate Decreased blood temperature or stimulation of skin cold receptors decreases the heart rate

Peripheral Resistance

Resistance to blood flow imposed by the force of friction between blood and the walls of its vessels Arterial blood pressure tends to vary directly with peripheral resistance -Increased resistance and decreased arteriole runoff lead to higher arterial pressure. -This can occur locally (in one organ), or the total peripheral resistance (TPR) may increase, thereby generally raising the systemic arterial pressure. Factors that influence peripheral resistance: a. Blood viscosity: The thickness of blood as a fluid -Blood viscosity is determined mainly by the proportion of red blood cells (hematocrit), but also partly by the number of protein molecules. b. Diameter of arterioles -Bigger "straw" - lower pressure but easier to get through (less friction) -smaller "straw" - higher pressure but harder to get through (more friction) -Arterioles have a muscular coat that allows them to constrict or dilate and change the amount of resistance to blood flow

Factors that Affect Stroke Volume: Starling's Law of the Heart

Starling's law of the heart (Frank-Starling mechanism) Within limits, the longer, or more stretched, the heart fibers are at the beginning of contraction, the stronger the contraction The amount of blood in the heart at the end of diastole determines the amount of stretch or preload placed on the heart fibers Unlike mechanical pumps, the myocardium contracts with enough strength to match its pumping load (within certain limits) with each stroke According to Starling's law of the heart, the heart pumps out what it receives. In other words, within certain limits, the strength of myocardial contraction matches the pumping load, or preload (unlike mechanical pumps, which do not adjust themselves to their input with every stroke). Starling's law of the heart ensures that increased amounts of blood returned to the heart are pumped out of it; that is, under usual conditions, the heart automatically adjusts cardiac output to venous return.

Changes in Total Blood Volume: Renin-Angiotensin-Aldosterone System (RAAS)

The RAAS is another mechanism that changes blood plasma volume by reducing total water loss. Because angiotensin-converting enzyme (ACE) regulates the amount of available angiotensin II, drugs that act as ACE inhibitors can reduce angiotensin II and thus block vasoconstriction; this effect is useful for reducing abnormally high blood pressure.

Factors that Affect Stroke Volume: Ejection Fraction

The ejection fraction (EF) is the ratio of SV to end-diastolic volume (EDV) - when the ventricles have finished filling Normal EDV is 120 mL, end systolic volume is 50 mL, SV is 70mL Usually expressed as a percentage: EF = (SV ÷ EDV) × 100 In a healthy adult, the EF is at least 55% The EF decreases as the myocardium fails *Increasing the EDV (fill volume) and/or introducing a stimulant (norepinephrine) will increase the stroke volume.

Arterial Blood Pressure and Blood Volume

The primary determinant of arterial blood pressure is the volume of blood in the arteries; a direct relationship exists between arterial blood pressure and arterial blood volume -An increase in arterial blood volume tends to increase arterial pressure; conversely, a decrease in arterial volume tends to decrease arterial pressure. Many factors determine the arterial pressure through their influence on arterial volume.

Factors That Affect Heart Rate

The sinoatrial (SA) node normally initiates each heartbeat. However, various factors can and do change the rate of the heartbeat; these are often called chronotropic factors. One major modifier of SA node activity, and therefore of the heart rate, is the ratio of sympathetic and parasympathetic impulses conducted to the node per minute. -Cardiac Pressoreflexes -Carotid Sinus Reflex -Aortic Reflex -Other Reflexes: Emotions, Exercise, Pain, Temperature, etc.

Vasomotor Control Mechanism: Vasomotor Pressoreflexes

The vasomotor mechanism controls changes in the diameter of arterioles; it plays a role in maintenance of the general blood pressure and in distribution of blood to areas of special need Vasomotor pressoreflexes Sudden increase in arterial blood pressure stimulates aortic and carotid baroreceptors, resulting in dilation of the arterioles and venules of the blood reservoirs Decrease in arterial blood pressure results in stimulation of vasoconstrictor centers, causing vascular smooth muscle to constrict The carotid sinus and aortic baroreceptors detect changes in blood pressure and feed the information back to the cardiac control center and the vasomotor center in the medulla. In response, these control centers alter the ratio between sympathetic and parasympathetic output. -If the pressure is too high, increased parasympathetic impulses and reduced sympathetic impulses decrease the pressure by slowing the heart rate, reducing the stroke volume, and dilating blood reservoir vessels. -If the pressure is too low, an increase in sympathetic impulses increase it by increasing the heart rate and stroke volume and constricting arterioles and reservoir vessels. *Negative feedback loop

Vasomotor Control Mechanism: Vasomotor Chemoreflexes

Vasomotor chemoreflexes involve chemoreceptors in the aortic and carotid bodies; these are sensitive to hypercapnia, hypoxia, and decreased arterial blood pH The chemoreceptor reflex functions as an emergency mechanism when hypoxia or hypercapnia endangers the stability of the internal environment. The chemoreceptors in the carotid and aortic bodies, as well as the chemoreceptive neurons in the vasomotor center of the medulla itself, detect increases in carbon dioxide, decreases in blood oxygen, and/or decreases in pH (which is actually an increase in hydrogen ions). This information is fed back to the cardiac control center and the vasomotor control center of the medulla, which in turn alter the ratio of parasympathetic and sympathetic output.

Venous Return to the Heart

Venous return is the amount of blood returned to the heart by the veins The stress-relaxation effect occurs when a change in blood pressure causes a change in vessel diameter (because of elasticity) that accommodates the new pressure and thereby keeps blood flowing (works only within certain limits) The pull of gravity on venous blood while sitting or standing tends to cause a decrease in venous return (orthostatic effect) Note that at rest, most of the body's blood supply is in the systemic veins and venules.

Cardiac Output (CO)

is the volume of blood pumped out of the heart per unit of time (ml/min or L/min) influences the flow rate to the various organs of the body •The resting cardiac output from the left ventricle into the systemic arteries is roughly 5000 ml/min. determined by volume of blood pumped out of a ventricle by each beat (stroke volume, SV) and heart rate, HR SV (volume/beat) X HR (beat/min) = CO (volume/min) In practice, CO is computed by Fick's formula anything that makes the heart beat faster or stronger will tend to increase the cardiac output (*but not always) The cardiac reserve is the amount the CO can increase above the resting CO

Changes in Total Blood Volume: Antidiuretic Hormone (ADH)

mechanisms that change total blood volume most quickly are those that cause water to quickly move into or out of the plasma The antidiuretic hormone (ADH) mechanism reduces the amount of water lost by the body by increasing the amount of water the kidneys resorb from urine before the urine is excreted from the body; this mechanism is triggered by input from baroreceptors and osmoreceptors -The more ADH is secreted, the more water is reabsorbed into the blood from the urine, and the greater the blood plasma volume.

Local Control of Arterioles

several local mechanisms produce vasodilation in localized areas; referred to as reactive hyperemia

Hemodynamics

A collection of mechanisms that influence the dynamic (active and changing) circulation of blood

Factors that Affect Stroke Volume: Afterload

Afterload is the pumping work the heart must do to push blood into the arteries The harder it is to push blood out of the ventricles, the lower the stroke volume Abnormally high afterload from flow resistance in the arteries can cause heart failure •If pulmonary blood flow "backs up" in the pulmonary arteries because of obstructions in pulmonary blood flow, the stroke volume can be reduced below normal, thereby causing a type of heart failure.

Factors that Affect Heart Rate: Cardiac Pressoreflexes

Aortic baroreceptors and carotid baroreceptors are located in the aorta and carotid sinus They are extremely important because they affect the autonomic cardiac control center, and therefore parasympathetic and sympathetic outflow, to aid in control of blood pressure The pressoreflexes operate in a feedback loop that maintains the homeostasis of blood pressure by decreasing the heart rate when the blood pressure surpasses the setpoint.

Primary Principle of Circulation

Blood flows because a pressure gradient exists between different parts of its volume For example, blood circulates from the left ventricle to the right atrium of the heart because a blood pressure gradient exists between these two structures and a blood pressure gradient drives blood flow from the right ventricle to the left atrium. P1-P2 is the symbol used to represent a pressure gradient; P1 represents the higher pressure and P2 the lower pressure A perfusion pressure (PP) gradient is needed to maintain blood flow through a local tissue

Circulatory Shock

*not tested Cardiogenic shock: Results from any type of heart failure Hypovolemic shock: Results from loss of blood volume in the blood vessels Neurogenic shock: Caused by widespread dilation of blood vessels Anaphylactic shock: Results from anaphylaxis Septic shock: Results from septicemia complications