I. The cardiovascular stress response
A. Get the heart to beat faster: Increase sympathetic tone and decrease parasympathetic tone.
1. Background: Heart has 4 chambers: Blood enters the right atrium from the body and then goes to right ventricle. From there it’s pumped to the lungs to become oxygenated. It then goes back to the heart, specifically the left atrium, then down to the left ventricle, which pumps it out to the body. The 2 atria contract simultaneously, sending blood to the ventricles. Then both ventricles contract together, sending the blood to the lungs or body. The ventricular contraction is referred to as systole. Heart is supplied with blood for its own use only during diastole (the time between contractions). Normal human heart beats about 72/min.
2. How does it work? Sino-atrial node (at back of right atrium) generates an impulse, which causes the atria to contract. This is carried to the atrioventricular node, which delays the impulse before passing it to the AV bundle. The delay allows the atria to empty before the ventricles contract. The AV bundle fibers are very fast and innervate the bottom of the heart first. Therefore, the contraction begins at the bottom of the ventricles and squirts the blood up through the appropriate arteries.
3. The heart fibers are all interconnected (they are referred to as a syncytium). Therefore when one fiber becomes excited, the action potential spreads to all of them. Muscle is electrically similar to neurons, in that inflow of sodium (Na+) and calcium (Ca++) depolarizes it.
4. Heart tissue is self-excitatory because of slow inward leaks of Na+ and Ca++. The sino-atrial node is the natural pacemaker, because its rate of re-excitation is fastest. SA node discharges rest of heart; then before rest of heart can re-excite itself, SA node strikes again! If there is a block of the SA node or of conduction from SA to AV node, the heart fibers will excite themselves to beat about 15-40 times per minute.
5. Sympathetic NS: norepinephrine (NE) and epinephrine (Epi) increase rate of re-excitation in SA node by increasing inward slow flow of NA+ and Ca++. This Ca++ also enhances contractility. Sympathetic terminals also innervate the AV node and the whole myocardium: enhances excitability everywhere.
6. Parasympathetic NS: Vagus nerve (via acetylcholine, ACh) decreases rate by decreasing the inflow of Na+ and Ca++ and by increasing the subsequent outflow of potassium (K+). Acts at SA and AV nodes. Vagus can actually block some action potentials. (DeBakey notes that some people can ward off a sympathetically driven heart attack by either gagging or external massage of the carotid arteries, both of which activate vagal reflexes, which would counteract the overexcitation of the sympathetic NS.)
B. Increase blood pressure: Sapolsky’s hose analogy: if you want to squirt water—or blood—a long way, you choose a small diameter, relatively rigid hose, not a 3-foot diameter one with the consistency of a marshmallow. Arteries carry blood from the heart to the body, veins return the blood to the heart. Each artery is wrapped with circular muscle, and sympathetic innervation causes them to tighten, thereby increasing pressure. Capillaries: fine meshwork of vessels, connecting arterial and venous systems. All exchanges of fluid, nutrients, and wastes between blood and tissues occur across capillary walls. The lymphatic system: Fluid that passes through the capillary walls into the surrounding tissues is called interstitial fluid. Some of this fluid is returned directly to the capillaries in the venous part of the capillary meshwork; the rest accumulates in lymph vessels. Lymph nodes along these vessels cleanse the lymph before returning it to the veins. The force causing the fluid to return to the capillaries or lymph vessels is the increased pressure in tissues compared to the reduced pressure in the venous system, since so much fluid has left the arterial side of the capillaries.
1. Functions of the circulatory system: The blood transports oxygen and nutrients to tissues and removes wastes from tissues. Adequate blood pressure is needed for 2 reasons:
a. Plasma must be extruded through leaky capillary walls in order to get to the tissues to deliver nutrients and oxygen and to pick up wastes.
b. Blood must be pumped against gravity to the upper parts of the body.
2. How does sympathetic NS direct blood to active muscles, heart, and brain, and away from digestive organs? It's a matter of relative distribution of alpha receptors (activated primarily by NE), which constrict blood vessels, vs. beta receptors (activated primarily by Epi).
C. Final trick—conserve water by secreting more vasopressin (antidiuretic hormone). Kidney actively retains water. Fluid is forced out of capillaries in the kidney into collecting tubules. That fluid—all 180 liters per day!—would be lost as urine if most of it were not actively retained by the kidney. Kidney pumps Na+ across wall of collecting tubule; chloride (Cl-) follows passively, due to electrical attraction. Water follows passively, due to osmotic pressure. Therefore, much of the salt and water end up “inside the body” instead of as part of the urine that would be excreted. Vasopressin increases the ability of water to get across the collecting tubule wall. It accounts for about 25 liters of the conserved fluid. Only 1 to 2 liters are excreted as urine.
1. However, under truly urgent stress, there may be reflexive emptying of the bladder, which would help by getting rid of dead weight.
II. Chronic stress and cardiovascular disease
A. Repeated overuse can damage heart and blood vessels, like a pump and hoses you buy from Sears.
B. Branching (bifurcation) of blood vessels results in turbulence at the branch points. Damage to the smooth lining results. Fatty acids and glucose work their way under the smooth layer, thickening it. Epi. From adrenal medulla makes platelets stickier. (This is potentially adaptive, in that it would tend to stop bleeding from a wound.) However, clumps of platelets can also stick to the injured area. Finally, fatty foam cells join in the fray. These are all components of plaques that form under the lining of the blood vessels: atherosclerosis.
C. Purely social stress can cause atherosclerosis in mice and monkeys. Jay Kaplan: subordinate monkeys got atherosclerosis. However, so did dominant males in unstable social groups, where they had to fight a lot to maintain their status. This occurs even with a low fat diet. “Hot reactors”—monkeys with the greatest symp. NS activity—got the most atherosclerosis. But the risk was blocked by beta-blocker drugs.
1. Claudication: blockage of flow to lower half of body.
2. Coronary heart disease: decreases blood supply to heart.
3. Stroke: blockage of blood supply to brain (usually due to clot, but sometimes to bleeding)
4. (I don’t understand why stress results in vasoconstriction of blood supply to an atherosclerotic heart, unless there is an altered balance of alpha receptors, which constrict the vessels, and beta receptors, which dilate them. The other possibility is that there is more norepinephrine to activate the alpha receptors. Sapolsky doesn’t explain, and I couldn’t find any information.)
III. Sudden cardiac death
A. Ventricular arrhythmia: Ventricles contract too early or too late, compared to normal rhythm.
B. Ventricular fibrillation: All the little interconnected muscle fibers contract randomly, rather than all working together. Therefore, there is no blood pumped out from the ventricles. This results in ischemia (poor or no blood supply) of the heart. Muscle of a diseased heart becomes more excitable, and therefore it may not wait to be activated by the SA and AV nodes.
IV. Fatal pleasures
A. Even very good news or activities can activate the symp. NS enough to produce a heart attack. “Rage and ecstacy, grief and triumph all represent challenges to allostatic equilibrium.” P. 50
B. Elie Wiesel: “The opposite of love is not hate. The opposite of love is indifference.”
V. Voodoo death
A. Psychological death: occurs when one gives up hope.
B. Some disagreement about whether it’s due to massive parasympathetic activity or massive sympathetic activity.
VI. Personality and cardiac disease: Individual differences in vulnerability to cardiac disease may be due to genetic factors, life styles, and/or personality factors.