-
What is the main driving pressure for arterial blood flow?
- MAP
- maintain at a constant level of ~100mmHg
-
Cardiac Output
- the V of blood that is pumped by the heart over a period of time
- cardiac output = stroke V x heart rate
-
Stroke Volume
amount of blood that is ejected by each ventricular contraction
-
Stroke V is affected by what 3 factors
- 1. Preload (amount of blood returning to the heart)
- 2. Contractility (how hard the heart is contracting)
- 3. Afterload (how much resistance the heart has to pump against)
-
Stroke V factor: Contractility
Preload: for the left ventricle is measured by looking at "ejection fraction" - the amount of end-diastolic V that is pumped during one contraction (normally ~50%)
-
Stroke V factor: Preload
for the left ventricle is the end-diastolic V (venous return to the left side)
-
Stroke V factor: afterload
for the left ventricle is aortic P, since that is the force it has to pump against to eject blood
-
Cardiac Work
- the "work" the heart performs on each beat
- cardiac output represents "V work"
- aortic P (after load) represents "P work"
- For the left ventricle:
- Work = cardiac output x aortic P
-
What coordinates with cardiac work?
myocardial oxygen consumption
-
Which type of work is more costly: P or V work?
**P work is more costly than V work: it is harder for the heart to pump against a P, than it is to move a V of blood
L ventricle works harder than right one (that's why its bigger)
-
What does myocardial hypertrophy result from?
myocardial hypertrophy results when the ventricles need to pump against an increased force ie aortic stenosis causes left heart enlargement
-
Blood Pressure
- Blood P = resistance x cardiac output
- (BP = resistance x [heart rate x stroke V])
-
For BP to be maintained it needs:
vascular tone, heart rate, venous return, contractility
-
2 systems to monitor BP
- Neurally-mediated
- -eg baroreceptor reflex
- -restores BP values to normal within seconds
- Hormonally-mediated
- -eg renin-angiotensin-aldosterone system
- -regulates BP more slowly by affecting blood V
-
Neurally-mediated BP systems involves nervous system responses such as:
- 1. Baroreceptors
- 2. Chemoreceptors
- 3. Atrial reflexes
- 4. CNS ischmia reflexes
These respond almost immediately (seconds) to changes in systemic BP
-
Baroreceptor reflex *
pressure sensors
- BP neutrally-mediated system
-
- *Pressure sensors are located in the walls of the:
- --Carotid sinus (responds to increases and decreases in P, uses CN IX)
- --Aortic arch (responds mainly to increases in P, uses CN X)
Both systems relay info to the brain stem where responses are immediately initiated to correct the abnormality
-
What are baroreceptors sensitive to?
- Changes in MAP results in stretching/relaxation of baroreceptor nerve endings
- are sensitive not only to absolute P, but to changes in P and the rate of changes
Sends impulses to the vasomotor centre in the medulla and pons to elicit changes in output of the sympathetic and parasympathetic systems
-
Changes in the output of sympathetic and parasympathetic systems to the heart and blood vessels will alter:
- Heart rate
- Vascular tone
- Cardiac contractility
- *BP* ultimately
-
Function of sympathetic constriction of the veins
- Much of the body's blood V is in the venous system at any time
- Sympathetic stimulation that results in constriction of veins will "shrink" the dimensions of the CV system
- This mechanism can maintain near-normal CV function even when as much as 25% of the blood V has been lost
-
Chemoreceptor reflexes for neutrally mediated BP systems
-location
-activity
- Chemosensitive cells are located in carotid bodies and aortic body
- -role in responding to low arterial oxygen P
- Whenever BP and blood flow decrease below a critical level, chemoreceptors are stimulated by decreased availability of oxygen and accumulation of CO2 and H+
- stimulates activity in vasomotor center to increase sympathetic tone to return BP back to a normal level
- *are more important for regulating the respiratory system than the cardiovascular system
-
BP Neurally mediated systems: Atrial reflexes
Atria contain low-P stretch R's similar to baroreceptors in large arteries
Important for both short-term and long-term control of BP
-
BP neutrally mediated system: CNS ischemic reflex
- occurs when blood flow to the medullary vasomotor centre is decreased, causing ischemia or hypoxia
- when this occurs there is an intense outpouring of sympathetic NS activity, resulting in profound increases in BP
- becomes active when MAP <50mmHg, and reaches max activity when MAP is 15-20mmHg
- Last ditch effort to improve BP before death
-
Moderate-term control of BP
seconds - minutes
- relies on HORMONAL responses not neural
- -catecholamine induced vasoconstriction
- -renin-angiotensin induced vasoconstriction
- -ADH-induced vasoconstriction
All of these mechanisms attempt to increase BP by increasing vascular resistance
-
Long-term control of BP
Delay onset (hours to days) but has a sustained effect on BP
Kidneys regulate Na+ and water to adjust blood V
Changes in Blood V leads to alterations in cardiac output (venous return) and BP
Relies on renin-angiotensin-aldosterone system
-
Autoregulation
- Blood flow to a specific organ or system can increase or decrease depending on its metabolic needs
- *Occurs independent of the systemic arterial P
- Changes in blood flow to an organ are achieved by altering arteriolar resistance
- 1. Local control
- 2. Neural or hormonal control
-
How are changes in blood flow to an organ achieved?
- Changes in blood flow to an organ are achieved by altering arteriolar resistance
- 1. Local control
- 2. Neural or hormonal control
-
Local control of blood flow to an organ (autoregulation)
Primary mechanism for matching blood flow to the metabolic needs of a tissue
Based on the need for oxygen or other nutrients (such as glucose) by the tissue
- Exerted through the direct action of "vasodilator metabolites" (eg adenosine, lactate, CO2, K+)
- When these substances accumulate, they induce vasodilation of arterioles, decrease resistance, and increase flow to meet the increased O2 demands of the tissue
-
time fore local autoregulatory responses
Local autoregulatory responses to sudden changes in BP typically occur within 1-2 mins
allows an organ or tissue to maintain relatively constant blood flow over a wide range of systemic arterial blood pressures
-
Autoregulation: neural control of blood FLOW to organs
- characterized by rapid response time (1 sec)
- conveys the ability to regulate blood flow to certain tissues at the expense to others
- SYMPATHETIC nervous system is the most important component of the ANS for regulating blood flow
- parasympathetic has little to no role in BP
-
Autoregulation: Sympathetic NS role in neural control of blood flow to organs
- Sympathetic NS transmits impulses via the spinal cord to all blood vessels in the body -->produces a continuous, sustained state of partial vasoconstriction in the body
- Impulses are also sent to the adrenal gland to stimulate release of epinephrine and norepinephrine into circulation where they act directly on adrenergic R's in vascular smooth muscle
- NE stimulates alpha1 receptors to induce vasoconstriction of small arterioles, affecting resistance to blood flow through all tissues
-
Autoregulation of blood flow: Hormonal control
-types
- There are many hormones that may influence local tissue blood flow:
- -Constrictors: epinephrine, norepinephrine, angiotensin, ADH
- -Dilators: bradykinin, serotonin, histamine, prostaglandins, H+, K+, CO2
-
"SHOCK"
- Circulatory shock is characterized by inadequate oxygen delivery to cells, resulting in generalized deterioration of organ function
- Due to inadequate tissue blood flow for whatever reason -I.e. hemorrhage, hypovolemic, neurogenic, septic
- end result is the same- if left untreated the patient will die
-
Delivery of O2 formula
Delivery of O2 = CO x CaO2
-
Stages of Shock
- Physical examination findings will depend on how-long standing the process is and how the patient is able to respond--- don't get fooled if they look like they are doing OK
- Compensated: normal blood pressure
- uncompensated: low blood pressure
- terminal: patient is about to die
|
|