Hypertension
Blood pressure (BP) is the force exerted by the blood against the walls of the blood vessel. It must be adequate to maintain tissue perfusion during activity and rest. The maintenance of normal BP and tissue perfusion requires the integration of both?
systemic factors and local peripheral vascular effects. BP is primarily a function of cardiac output (CO) and systemic vascular resistance (SVR)
CO is?
total blood flow through the systemic or pulmonary circulation per minute. It is described as the stroke volume (SV) or the amount of blood pumped out of the left ventricle per beat (approximately 70 mL) multiplied by the heart rate (HR).
Factors influencing BP: Sympathetic nervous system (SNS)
Systemic vascular resistance (SVR) is the force opposing the movement of blood within the blood vessels. The radius of the small arteries and arterioles is the principal factor determining SVR. As arteries narrow, resistance to blood flow increases. As arteries dilate, resistance to blood flow decreases. A small change in the radius of the arterioles creates a major change in the SVR. If SVR is increased and CO remains constant or increases, what happens?
arterial BP will increase
The mechanisms that regulate BP can affect either CO or SVR, or both. Regulation of BP is a complex process involving both short-term (seconds to hours) and long-term (days to weeks) mechanisms.
1) Short-term mechanisms, including?
2) Long-term mechanisms include?
1) sympathetic nervous system (SNS) and vascular endothelium, are active within a few seconds.
2) renal and hormonal processes that regulate arteriolar resistance and blood volume.
- In a healthy person these regulatory mechanisms function in response to the body’s demands.
Sympathetic Nervous System:
The nervous system, which reacts within seconds after a drop in BP, increases BP primarily by activating the SNS. Increased SNS activity increases?
HR and cardiac contractility, produces widespread vasoconstriction in the peripheral arterioles, and promotes the release of renin from the kidneys. The net effect of SNS activation is to increase BP by increasing both CO and SVR.
Sympathetic Nervous System: Specialized nerve cells called baroreceptors (pressoreceptors) are located in the carotid arteries and arch of the aorta.
1) These cells sense?
2) SNS efferent nerves innervate cardiac and vascular smooth muscle cells. Under normal conditions, a low level of continuous?
1) changes in BP and transmit this information to the vasomotor centers in the brainstem. The brainstem sends this information through complex networks of neurons that excite or inhibit efferent nerves.
2) SNS activity maintains vascular tone. BP may be reduced by withdrawal of SNS activity or by stimulation of the parasympathetic nervous system (PNS). The PNS decreases the HR (via the vagus nerve) and thereby decreases CO.
Sympathetic nervous system: The neurotransmitter norepinephrine (NE) is released from SNS nerve endings. 1) NE activates receptors located in the?
2) The response to NE depends on the type of receptors present. SNS receptors are classified as α1, α2, β1, and β2. The smooth muscle of the blood vessels has?
3) α-Adrenergic receptors located in the peripheral vasculature cause what when stimulated by NE?
4) β1-Adrenergic receptors in the heart respond to NE and epinephrine with?
5) β2-Adrenergic receptors are activated primarily by?
1) sinoatrial node, myocardium, and vascular smooth muscle.
2) α-adrenergic and β2-adrenergic receptors.
3) vasoconstriction when stimulated by NE.
4) increased HR (chronotropic), increased force of contraction (inotropic), and increased speed of conduction (dromotropic).
5) epinephrine released from the adrenal medulla and cause vasodilation
Sympathetic Nervous System Receptors Affecting BP
α1 Receptor:
-Location
-Response when activated
1)
- Location: Vascular smooth muscle
- Response when activated: Vasoconstriction
2)
- Location: Heart
- Response when activated: Increased contractility (positive inotropic effect)
Sympathetic Nervous System Receptors Affecting BP
α2 Receptor:
-Location
-Response when activated
1)
- Location: Presynaptic nerve terminals
- Response when activated: Inhibition of norepinephrine release
2)
- Location: Vascular smooth muscle
- Response when activated: Vasoconstriction
Sympathetic Nervous System Receptors Affecting BP
β1 Receptor:
-Location
-Response when activated
1)
- Location: Heart
- Response when activated:
* Increased contractility (positive inotropic effect)
* Increased heart rate (positive chronotropic effect)
* Increased conduction (positive dromotropic effect)
2)
- Location: Juxtaglomerular cells of the kidney
- Response when activated: Increased renin secretion
Sympathetic Nervous System Receptors Affecting BP
β2 Receptor:
-Location
-Response when activated
Sympathetic Nervous System Receptors Affecting BP
Dopamine Receptors:
-Location
-Response when activated
- Response when activated: Vasodilation
The sympathetic vasomotor center interacts with many areas of the brain to maintain normal BP under various conditions. It is activated during times of pain, stress, and exercise. The SNS response causes an appropriate increase in CO and BP to adjust to the body’s increased O2 demands.
- During postural change from lying to standing, there is a transient?
decrease in BP. The vasomotor center is stimulated, and the SNS response causes peripheral vasoconstriction and increased venous return to the heart. If this response did not occur, blood flow to the brain would be inadequate, resulting in dizziness or syncope.
Baroreceptors have an important role in the maintenance of BP stability during normal activities.
1) They are sensitive to stretching and, when stimulated by an increase in BP, they?
2) When a fall in BP is sensed by the baroreceptors, the?
1) send inhibitory impulses to the sympathetic vasomotor center. Inhibition of the SNS results in decreased HR, decreased force of contraction, and vasodilation in peripheral arterioles.
2) SNS is activated. The result is constriction of the peripheral arterioles, increased HR, and increased contractility of the heart. In long-standing hypertension, the baroreceptors become adjusted to elevated BP levels and recognize this level as their new “normal.”
- Send impulses to sympathetic vasomotor center
Baroreceptors
The vascular endothelium is a single-cell layer that lines the blood vessels.
- Functions:
Produces vasoactive substances to maintain low arterial tone
Vascular endothelium
Renal system
Renal System
- Control sodium excretion and ECF volume
The kidneys contribute to BP regulation by controlling sodium excretion and extracellular fluid (ECF) volume. Sodium retention results in water retention, which causes an increase in ECF volume. This increases the venous return to the heart and stroke volume. Together these increase CO and BP.
Renal System
- RAAS system
plays an important role in BP regulation.
Renal system
- Prostaglandins
Prostaglandins (PGE2 and PGI2) secreted by the renal medulla have a vasodilator effect on the systemic circulation. This results in decreased SVR and lowering of BP. The natriuretic peptides (atrial natriuretic peptide [ANP] and b-type natriuretic peptide [BNP]) are secreted by heart cells. They antagonize the effects of antidiuretic hormone (ADH) and aldosterone. This results in natriuresis (excretion of sodium in urine) and diuresis, resulting in reduced blood volume and BP.
Factors influencing BP: Endocrine system main points