Question

4. What are the major compensatory responses to acute blood loss? List the most important hormones involved. What is a major difference between chronic and acute blood loss? Give some examples.

Answer 1

Blood loss-Definition:

Blood loss (Hypovolemia) leads to loss of circulating blood volume which results in Shock. Shock may be classified into:

Hypovolemic shock: This condition arises due to decreased blood volume

Cardiogenic shock: It is due to poor heart function

Vascular shock: It is due improper vasodilation

Obstructive shock: This condition is due to obstruction of blood flow

Compensatory responses to acute blood loss: When the circulating blood volume is reduced to blood loss, homeostasis mechanisms comes into play by negative feedback mechanism which returns the cardiac output and blood pressure to normal levels even upto 10% blood loss

The compensatory mechanisms include hormones like Renin, Angiotensin-II, Aldosterone, Antidiuretic hormone, epinephrine, norepinephrine.

Mechanism how homeostasis is restored: Due to blood loss, there is decreased blood flow to the kidneys, which causes release of renin and triggers the activation of renin angiotensin system. Angiotensin II (produced from angiotensinogen with the help of angiotensin converting enzymes) is a potent vasoconstrictor and stimulates the adrenal cortex to secrete aldosterone which helps in reabsorption of Sodium ions and water in the kidneys that increases the blood pressure and volume.

Anti Diuretic hormone (ADH) is also released by posterior pituitary in response to reduction in blood pressure. ADH also causes water reabsorption and vasoconstriction. Aortic and Carotid baroreceptors initiate the sympathetic responses to counteract the blood loss which results in secretion of epinephrine and norepinephrine that act as vasoconstrictors and have positive inotropic and chronotropic effects on heart which make the heart rate and blood pressure normal.

Blood clotting mechanisms comes into action for local injuries or tissue damage to plug the damaged surface and prevent further blood loss.

Major differences between chronic and acute blood loss and EXAMPLES are listed in the following table:

Acute blood loss	Chronic blood loss
RBC are normocytic and normochromatic	RBC are microcytic and hypochromic
There is blood volume disruption	There is no blood volume disruption
There is no iron deficiency	It may be associated with iron deficiency either directly or indirectly
It is caused due to Trauma	It is caused due to malnutrition or malabsorption in the GIT tract, menstruation, urinary tract infections
Hematocrit is normal	Hematocrit is decreased
White blood cell count is increased	White blood cell count is normal
Platelet levels are increased which initiate the blood clotting mechanisms that further prevent blood loss	There is no change in platelet volume
Reticulocytes are normal	Reticulocytes are increased
Ex. Trauma, burns, accidents, vomiting, Diarrhoea, polyuria, hypotonic solution administration, Mismatched blood transfusion, snake venom containing hemotoxins, hemorrhagic small pox and measles, hemorrhoids	Ex. Myocardial infarction, Ischemia, heart valve disorders, arrythmias, inappropriate dilation of arterioles and venules, Pernincious Anemia, Pulmonary embolism, aneurysm etc