Blood pressure
Blood pressure is the force needed to pump blood around the body. Blood pressure measurements are divided into systolic pressure, which is when the heart contracts forcing blood out into the arteries, and diastolic when the heart rests and fills with blood. Blood pressure measurement is expressed as systolic pressure over diastolic pressure.
Hypertension:
is high blood pressure at rest and causes reduced blood flow, increases the force needed to pump blood around the body, increases workload on the heart and increases oxygen demand. This can cause damage to blood vessels as well as end organ tissue damage to the kidneys, eyes and nerves. Hypertension also increases risk of cardiovascular disease, including stroke and heart attack.
Hypotension:
is low blood pressure resulting in insufficient blood flow to the brain and vital organs which causes symptoms including dizziness, fainting, nausea, headache, blurred vision, numbness and tingling. Low blood pressure can be due to several conditions including dehydration, loss of blood or as a result of a neurological condition, or secondary to a condition like diabetes. Orthostatic or postural hypotension is a sudden drop in blood pressure when changing position from lying or sitting to standing, caused by pooling of blood in the extremities, thereby reducing the volume of blood available to be pumped by the heart to the brain and vital organs.
Adrenergic receptors and blood pressure
Adrenergic receptors bind to specific chemicals
that cause contraction of blood vessel walls or vasoconstriction, like the
neurotransmitters adrenaline and noradrenaline.
Vasoconstriction increases resistance to blood flow, which increases
blood pressure. Vasodilation or
relaxation of blood vessels reduces resistance to blood flow, which lowers
blood pressure.
Adrenergic receptors exist as alpha and beta
forms and alpha receptors have the subtypes alpha1 and alpha 2. Both alpha subtypes are found in smooth
muscle such as in blood vessels and promote vasoconstriction in response to
binding of adrenaline and noradrenaline to the receptor. Alpha 2 receptors are also found in the
central nervous system, where they inhibit the release of adrenaline and
noradrenaline when activated.
Drugs that target adrenergic receptors to reduce blood pressure
Several drugs are used to treat high blood pressure by interacting with alpha adrenergic receptors:
- Doxazosin is an alpha-1 adrenergic antagonist that blocks the direct action of vasoconstrictors like adrenaline and noradrenaline on smooth muscle of the blood vessels
- Clonidine is an alpha-2 adrenergic agonist that acts on the central nervous system and stimulates receptors in the brain that inhibits the release of adrenaline and noradrenaline, which reduces vasoconstriction. It also acts to slow heart rate due to stimulation by the vagus nerve .
Drugs that target adrenergic receptors to increase blood pressure
Midodrine is a produg for desglymidodrine, a potent alpha adrenergic agonist that binds to alpha 1 adrenergic receptor in smooth muscle of blood vessels and mimics the action of neurotransmitters like noradrenaline that cause vasoconstriction. This action increases blood pressure.
Drugs that target cardiac muscle
Cardiac muscle is a type of muscle only found in the heart and contraction of cardiac muscle propels blood through the chambers of the heart and around the body. Heart failure is a condition in which the heart muscle no longer pumps efficiently and results in symptoms including shortness of breath and oedema (water retention in the tissues) causing swelling. Arrhythmias like atrial fibrillation result in irregular heart beat and are caused by abnormal electrical activity in the heart.
Drugs known as cardiac glycosides, like digoxin act directly on the heart muscle to slow down the rate but increase the force of contraction to improve pumping efficiency, which helps reduce symptoms of heart failure.
Digoxin also corrects abnormal electrical activity in the heart by acting on the ion pump (Na+/K+ pump) in the membrane of heart cells. This action helps restore normal heart beat in the treatment of arrhythmias.