While Dick and Jane were backpacking on a trail in Yellowstone Park, they encoun
ID: 67079 • Letter: W
Question
While Dick and Jane were backpacking on a trail in Yellowstone Park, they encountered a grizzly bear cub that seemed amazingly tame. However, while Dick tried to feed the cub, its mother appeared and attacked him. Jane escaped by climbing a tree, but Dick received several deep lacerations (cuts) and lost a lot of blood over the next several hours. Jane helped him reach medical aid, and he survived. Which of the following mechanism was (were) activated to help keep Dick alive? Explain your choice. 1. Baroreceptor mechanism 2. CNS ischemic response 3. Renin-angiotensin-aldosterone mechanism 4. Fluid shift mechanism 5. Antidiuretic hormone mechanism 6. Adrenal medullary responseA. 1,2,3,4,5,6 B. 1,3,4,5,6 C. 1,6 D. 1,4,6 E. 1 While Dick and Jane were backpacking on a trail in Yellowstone Park, they encountered a grizzly bear cub that seemed amazingly tame. However, while Dick tried to feed the cub, its mother appeared and attacked him. Jane escaped by climbing a tree, but Dick received several deep lacerations (cuts) and lost a lot of blood over the next several hours. Jane helped him reach medical aid, and he survived. Which of the following mechanism was (were) activated to help keep Dick alive? Explain your choice. 1. Baroreceptor mechanism 2. CNS ischemic response 3. Renin-angiotensin-aldosterone mechanism 4. Fluid shift mechanism 5. Antidiuretic hormone mechanism 6. Adrenal medullary response
A. 1,2,3,4,5,6 B. 1,3,4,5,6 C. 1,6 D. 1,4,6 E. 1 While Dick and Jane were backpacking on a trail in Yellowstone Park, they encountered a grizzly bear cub that seemed amazingly tame. However, while Dick tried to feed the cub, its mother appeared and attacked him. Jane escaped by climbing a tree, but Dick received several deep lacerations (cuts) and lost a lot of blood over the next several hours. Jane helped him reach medical aid, and he survived. Which of the following mechanism was (were) activated to help keep Dick alive? Explain your choice. 1. Baroreceptor mechanism 2. CNS ischemic response 3. Renin-angiotensin-aldosterone mechanism 4. Fluid shift mechanism 5. Antidiuretic hormone mechanism 6. Adrenal medullary response
A. 1,2,3,4,5,6 B. 1,3,4,5,6 C. 1,6 D. 1,4,6 E. 1
Explanation / Answer
Option A is the correct answer.
Heavy blood loss causes a clinical syndrome known as Hemorrhagic Shock which leads to reduced cardiac output and organ perfusion. The reduction in blood volume during acute blood loss causes a fall in central venous pressure and cardiac filling. This leads to reduced cardiac output and arterial pressure. The body has a number of compensatory mechanisms that become activated in an attempt to restore arterial pressure and blood volume back to normal.
All the mechanisms mentioned in option A help keep Dick alive. The baroreceptor reflex is the first mechanism to become activated in response to blood loss. The body can quickly sense a fall in blood pressure through its arterial and cardiopulmonary baroreceptors, and then activate the sympathetic system to stimulate the heart (increase heart rate and contractility), venoconstriction and vasoconstriction which helps to restore blood pressure. Cardiac output is redistributed from less important organs to the brain and myocardium, both of which are critical for survival. If BP falls below 50 mmHg, the CNS ischemic response is activated, with more powerful sympathetic activation than baroreceptors where the systemic arterial pressure often rises to a level as high as the heart can possibly pump. Activated sympathetic nerves release norepinephrine (noradrenaline) and adrenal medulla, which releases epinephrine (adrenaline) – results in the constriction of the peripheral blood vessels in the skin and increased heart contractility and heart rate.
Sympathetic vasoconstriction of renal artery plus reduced renal artery pressure stimulates renin-angiotensin--aldosterone system and the production of angiotensin II, a powerful vasoconstrictor. Renin is released by the juxtamedullary complex leading to increased aldosterone levels and eventually to sodium and water resorption. Vasopressin (Antidiuretic hormone) is stimulated by angiotensin II, reduced arterial pressure causing water retention at the distal tubules. All these hormones act together to increase blood volume through their renal actions (sodium and water retention) to elevate arterial pressure.
Hypotension, combined with constriction of precapillary resistance vessels (small arteries and arterioles), causes a fall in capillary hydrostatic pressure which helps in fluid shift mechanism. When capillary hydrostatic pressure is reduced, less fluid leaves the capillaries, and when the pressure falls sufficiently low as occurs following moderate-to-severe blood loss, net reabsorption of fluid can occur from the tissue interstitium back into the capillary plasma. It contain electrolytes and some protein, and therefore increases the plasma volume.
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