Chapter 14: Cardiac Output, Blood Flow, & Blood Pressure
Cardiac Rate
Beats per minute, usually 70 in a resting adult but wide variations occur
Unique Blood Flow Requirements of the Heart
- 2,500 - 4,000 capillaries / cubic mm of heart, each cell within 10µm of capillary - Myocardium is highly aerobic, many mitochondria, high in respiratory enzymes - Contraction of myocardium compresses blood vessels during systole, creates lack of oxygen - Heart cells compensate with high myoglobin content= holds extra O2 for use during systole - Heart has high aerobic demand
Venous pressure is only about
- 2 mmHg - This facilities movement of fluid from the interstitial space - Need help to move venous fluid to heart
Aldosterone and Blood Volume Regulation in the Kidney
Dietary salt is required for maintenance of blood volume and pressure - Na+ and Cl- are normally excreted by kidneys - Aldosterone is secreted when salt content is too low - Promotes resorption of NaCL and water by equivalent amounts.
Venous return is assisted by
smooth muscle contraction in vein walls, skeletal muscle pumps and pressure difference between thoracic and abdominal cavities from the diaphragm.
If Venous return is decreased then...
EDV also decreases = Total blood volume and venous pressure - Veins are capacitance vessels, while muscular arteries are resistance vessels.
Contractility
Strength of ventricular contraction - directly related to stroke volume. Increase of contractility, increase of stroke volume. - can be affected by the ANS - Epinephrine and norepinephrine increase strength of contraction = increases cardiac rate and stroke volume.
Cardiac Output (ml/min) =
Stroke volume (ml/beat) x Cardiac Rate (beats/min)
ACE inhibitors
block enzyme that converts angiotensin I to angiotensin II - captopril, enalapril
ADH and Kidney blood volume regulation
- AKA Arginine = vasopressin - Produced by neurons of hypothalamus, transported to posterior pituitary, released in response to hypothalamic stimulation. - Osmoreceptor of hypothalamus detect changes in osmotic pressure (dehydration, excessive salt intake) - ADH causes kidneys to secrete less water in urine
Mechanisms of Blood Volume Regulation in the Kidney
- Antidiuretic Hormone (ADH) - Aldosterone - Renin - Angiotensin - Atrial Natriuretic Factor ( ANF)
Congestive Heart Failure
- Cardiac output is insufficient to main blood flow to the system - causes may be myocardial infarction, congenital defect, hypertension, disturbance in blood electrolyte balance - TOO much K+ decreases RMP of cardiocytes and low blood CA2+ makes contraction inefficient. Heart stops at diastole. - Too little plasma K+ and too much Ca2+ arrests heart in systole
Frank-Starling Law of the Heart
- EDV rises and the myocardium is stretched - Stretch makes contraction more forceful - During diastole (rest) sarcomeres are very short - Stretching of the ventricle positions actin and myosin filaments at optimum distances so stretching increases the force of heart muscle contraction - Strength of contraction is intrinsically adjusted by EDV
Stroke Volume is dependent on 3 variables
- End Diastolic Volume (EDV) - Total Peripheral resistance - Contractility
Paracrine Regulation of Blood Flow
- Endothelium teases factors which relax smooth muscle of arterioles - Endothelium-derived relaxing factors - NO, bradykinin, prostacyclin - Endothelium is important factor in regulation of resting tone
Total Body water
- Extracellular water = 1/3 ( 80% interstitial fluid, 20% blood) - Intracellular water = 2/3
"Congestive"?
- Failure of left ventricle raises pressure in left atrium, producing pulmonary congestion & edema. Results in shortness of breath & fatigue -Failure of right ventricle raises pressure in right atrium, causing congestion in the systemic circulation -In congestive heart failure, the body still tries to compensate: * Activation of sympathoadrenal axis to increase cardiac rate and induce vasoconstriction * Responds as if in hypovolemic shock * Elevated blood volume and dilation and hypertrophy of ventricles
Regulation of Cardiac Rate: Sympathetic System
- Faster pace - Norepinephrine (sympathetic nerve fibers) - Epinephrine (adrenal medulla)
Kidneys Regulate Blood Volume
- Formation of urine by the kidneys is similar to movement of water from blood to interstitial fluid - filtration. - Kidneys produce 180 L/day of blood filtrate - Only 5.5 L of blood in body - most of kidney filtrate is returned to the system and recycled - only 1.5 L urine is excreted daily
Hypertension
- High BP - Adults: hypertension is greater than 140/90 - Primary/Essential: majority of hypertension - Secondary: secondary to a known disease process such as adrenal tumor, kidney disease (5% of hypertension) - 20-30% of adults in the us have it
Intrinsic regulation of Blood Flow
- Intrinsic - built in - within the organ - Called autoregulation - Products of metabolism: Decreased o2 consumption, increased CO2, decreased pH, release of adenosine or K+
Factors that Change Blood Pressure
- Kidney Function - controls volume, stroke volume; Renin-Angiotensin - Sympathoadrenal System - raise BP by vasoconstriction (raises total peripheral resistance); increases cardiac output - Endocrine System: ADH, Aldosterone, paracrine mediators. - Heart- ANF - Intrinsic mechanisms - Baroreceptors - Atrial stretch reflex
Cardiac Control Center
- Medulla Oblongata of brain stem - Coordinates autonomic effects on the heart - Current Controversy: Cardiaoaccelerators and cardioinhibitors
Regulation of Cardiac Rate: Vagus Nerve
- Parasympathetic, slower pace - AcH promotes opening of K+ channels and hyperpolarizes the pacemaker cells
Baroreceptors
- Pressure sensors in the aorta and carotid arteries - Fall in blood pressure produces increase in heart rate = Baroreceptor reflex
Vagus Nerve and the Heart
- Provides main parasympathetic input - Cardiac rate increases during exercise due to decreased vagus nerve inhibition of the SA node - Resting Bradycardia of athletes is due to high vagal activity.
Hypertension and the Renin-Angiotensin System
- Renin - Angiotensin system should work in reverse - high salt intake and high blood volume should inhibit renin secretion and promote salt elimination in urine - Many individuals with hypertension have malfunction in renin-angiotensin system. - Angiotensin Converting Enzyme (ACE) Inhibitors (captopril) - Angiotensin II Receptor Antagonists (Cozaar) - prevent left ventricular hypertrophy associated with congestive heart failure
Regulation of Cardiac Rate
- Sinoatrial node, the pacemaker - Autonomic nervous system can regulate the pacemaker - Sympathetic system and the vagus nerve both act on the pacemaker potential
Contractility
- Strength of ventricular contraction - Stroke volume is also directly proportional to contractility - Greater the contraction strength, the greater the stroke volume - Contractility may be affected by the ANS (epi and norepinephrine increase strength of contraction) = positive inotropic effect
Total Peripheral Resistance (2)
- Sum of all the vascular resistances in the circulation - Vasodilation in one large organ MAY decrease the total peripheral resistance, decreasing MAP - Normally prevented by increase in cardiac output and vasoconstriction in other areas
Extrinsic Mechanism for control of coronary blood flow
- Sympathetic and adrenal control - alpha adrenergic receptors promote vasoconstriction - beta adrenergics promote vasodilation
Factors that Control Cardiac Rate
- Sympathetic nervous system - Parasympathetic nervous system (vagus) - Cardiac control center - Baroreceptors (don't have a direct effect on stroke volume)
Read This : Resistance and blood flow
- The longer the tube in which a fluid flows, and the smaller the tube is, the greater the resistance and the harder it is to move the fluid (greater pressure required) - viscosity also increases resistance (polycythemia) - Resistance in our blood vessels correlates to total peripheral resistance and mean arterial pressure
Veins are highly distensible
- They will stretch - they have higher compliance than arteries. - A given amount of pressure will cause more dissension of the veins than the arteries.
Intrinsic controls of coronary blood flow
- Vasodilation during exercise - Metabolism of myocardium increases - local accumulation of CO2, K+, and adenosine and lack of oxygen promotes cardiac muscle relaxation and vasodilatio
EDV
- Volume of blood in ventricles at end of diastole - Amount of blood in ventricles when AV and semilunar valves are shut, the ventricles are filled, just prior to contraction - Stroke Volume is directly proportional to preload - an increase in EDV causes an increase in stroke volume - AKA Preload - PAYCHECK
Water = Diuretic
- dilutes blood and decreases plasma osmolality and inhibits release of ADH - larger volume of urine is excreted
Atrial Natriuretic factor
- hormone from the heart - natriuresis - Na+ excretion - ANF - small polypeptide hormone produced by the atria - Promotoes salt and water excretion in response to rise in blood volume and pressure. - Antagonistic to aldosterone and angiotensin II; PROMOTES vasodilation -
Essential Hypertension
- increased peripheral resistance - cardiac rate & cardiac output elevated in many - defect in renin-angiotensin system - hypertension should inhibit renin secretion totally - high salt intake combined with metabolic defect - sustained high stress - genetics
Control of Cerebral Circulation
- involves CO2 sensor = rise in brain CO2 causes vasodilation - CO2 sensor may be an arachidonic acid metabolite. -May involve astrocyte - endothelial cell interactions - Astrocytes may sense many aspects of neuronal signaling and metabolic needs and transmit this information to the cerebral microvasculature
Treatment of Hypertension
- lifestyle changes - exercise - increase dietary k+ and ca2+ - Pharmacological therapy
Atrial Stretch Receptors
- located in atria of heart - activated by increase in venous return: - Stimulates reflex tachycardia through increased sympathetic nerve activity - inhibit ADH release causing larger urine volume and decline in blood volume - Increase ANF secretion
Anaphylactic Shock
- low BP in result of severe allergic reaction - widespread release of histamine (vasodilator)
Septic Shock
- low BP resulting from infection - due to endotoxin - 50-70% mortality - NOS activation in macrophages - Recent treatment involves NOS inhibitors.
Dangers of Hypertension
- organ damage due to high perfusion pressure - thickening of vessel walls, vascular damage, atherosclerosis - increase in "after load" makes ventricle work harder to pump blood - left ventricular hypertrophy - damage to cerebral vasculature = stroke - usually asymptomatic
Digitalis
-Used to treat congestive heart failure -Inhibits Na-K+ ATPase -Elevates intracellular Na+ -Stimulates Na+/Ca2+ exchanger - Increases cardiac [Ca2+]i and improves strength of contraction
Veins are 1) ________, while muscular arteries are 2)__________
1) capacitance vessels 2) resistance vessels
Increased BP or Venous Obstruction
Increases capillary filtration pressure so that fluid moves into tissues
Colloid Osmotic Pressure
Osmotic pressure exerted by plasma proteins - Plasma osmotic pressure is greater than osmotic pressure of the interstitial fluid
Starling Forces
P(c) = hydrostatic pressure in the capillary π(i) = colloid osmotic pressure of interstitial fluid. P(i) = hydrostatic pressure of interstitial fluid π(p) = colloid osmotic pressure of blood plasma (Pc + πi) - (Pi + πp) (Fluid out) - (fluid in) - At the arteriole end of capillary, equation is (+), so fluid moves out of capillary. - At venue end of capillary, equation is (-), so fluid moves into capillary
EDV is controlled by...
Venous Return: - Blood volume, urine volume, tissue fluid volume - Negative intrathoracic pressure = breathing = helps suction venous circulation back up the heart. Higher EDV and Higher stroke volume = higher cardiac output - Venous pressure: venoconstriction (smooth muscle in veins constrict) and skeletal muscle pump = positively correlated to venous return = increase of pressure = increase of venous return = increased EDV = higher stroke volume = higher cardiac function - Blood volume is inversely related to urine volume and tissue fluid volume. - If Venous return decreases, the EDV decreases, stroke volume decreases, cardiac output. - Decrease of urine volume increase EDV - Increase of urine volume decreases blood volume - ADH and Water are diuretics so they DECREASE urine volume which in turn INCREASES blood volume.
Total Blood Volume
Volume of all the blood in the system, about 5.5 L
Stroke Volume
Volume of blood pumped per beat by each ventricle, usually 70-80 ml per beat
Cardiac Output
Volume of blood pumped per minute by each ventricle
Ventricle pumps into the
arteries
Ventricle pumps into the...
arteries
Increased interstitial fluid protein
decreased osmosis of water into venous end of capillary - can be caused by leakage of plasma protein through capillaries in inflammation
Hypovolemic Shock
caused by low blood volume as in dehydration, hemorrhage, or burns - decreased BP and cardiac output, activation of sympathoadrnal axis
Intrinsic regulation of blood flow
control by metabolic factors
Extrinsic Regulation of Blood Flow
controlled by the autonomic, endocrine (paracrine and autocrine) systems. - usually the first responder
Decreased Plasma Protein
decreases osmosis of water into venous end of capillaries and it causes and increase of liver disease, kidney disease and protein malnutrition
Myxedema
excessive production of glycoproteins (mucin) or extracellular matrix proteins, as occurs in hypothyroidism
Lymphatic obstruction
filaria infection
Total Peripheral Resistance
frictional resistance; impedance to flow from arteries - Inverse relation to stroke volume. - greater the peripheral resistance, the less the stroke volume.
Total peripheral resistance
impedes blood flow or imposes an after load on the ventricle - In order for the heart to eject blood, the pressure in the ventricle must be greater than the arterial pressure= Blood flows from higher pressure to lower pressure - After load is inversely proportional to stroke volume and directly proportional to peripheral resistance
Read this
in liver cirrosis stroke volume decreases bc it results in lower blood volume that results in EDEMA and results in lowered EDV
Diuretics
increase urine volume, HCTZ, furosemide
Venous obstruction
mechanical compression or blocage of vein ; phlebitis
Stroke volume is inversely proportional to
peripheral resistance
Oncotic Pressure
the difference between the plasma osmotic pressure and the osmotic pressure of the interstitial fluid
Frank Starling Law of the Heart
the greater the volume, the more forcefully the heart will contract
The greater the peripheral resistance...
the less the stroke volume but a healthy normal heart can accommodate for increased peripheral resistance
In order for the heart to eject blood...
the pressure in the ventricle must be greater than arterial pressure. - Blood flows from higher pressure to lower pressure
