Hemostasis - Platelets

Question 1

What is ‘hemostasis’?

Answer 1

Prevention of blood loss.

Question 2

State the mechanisms by which hemostasis is achieved.

Answer 2

(1) vascular constriction
(2) formation of a platelet plug
(3) formation of a blood clot (as a result of blood coagulation)
(4) eventual growth of fibrous tissue into the blood clot to close the hole in the vessel permanently

Question 3

When a blood vessel has been cut or ruptured, the trauma to the vessel wall causes the smooth muscle in the wall to contract.
What results in these contractions?
What is the instantaneous result?

Answer 3

Contraction results from :

(1) local myogenic spasm
(2) local autocoid factors from the traumatised tissues, vascular endothelium and blood platelets.
(3) nervous reflexes

Question 4

How does vasoconstriction in small vessels occur?

Answer 4

Platelets release a vasoconstrictor substance, thromboxane A2.

Question 5

Instead of a blood clot what is formed in small vessels?

Answer 5

Platelet plug

Question 6

What is the diameter of thrombocytes?

Answer 6

1 to 4 micrometers

Question 7

Where are platelets formed? From which cells do they originate?

Answer 7

in the bone marrow

from megakaryocytes

Question 8

What is the normal concentration of platelets in the blood?

Answer 8

150,000 and 450,000 / μL

Question 9

Platelets have many functional characteristics of whole cells. State some exceptions.

Answer 9

Do not have nuclei

Cannot reproduce

Question 10

State 4 things found in the platelet cytoplasm.

Answer 10

(1) actin and myosin molecules, thrombosthenin - contractile proteins

(2) residuals of endoplasmic reticulum and Golgi apparatus
- synthesise various enzymes
- store large quantities of calcium ions

(3) mitochondria and enzyme systems
- form ATP and ADP

(4) enzyme systems that synthesise prostaglandins
- local hormones that cause many vascular and other local tissue reactions

(5) fibrin-stabilising factor

(6) growth factor
- vascular endothelial cells, vascular smooth muscle cells and fibroblasts
multiply and grow, causing cellular growth that eventually helps in repairing damaged vascular walls

Question 11

What is the platelet cell membrane surface made of? What do the these constituents do?

Answer 11

Glycoproteins

• repulses adherence to to normal endothelium
• causes adherence to injured areas of vessel wall ( especially to endothelial cells and even more so to any exposed collagen from deep within the wall)

Phospholipids

• activate multiple stages in blood-clotting process

Question 12

What is the half-life of a platelet?

Answer 12

8 - 12 days

Question 13

By which system are platelets eliminated from the circulation?

Answer 13

By the tissue macrophage system and spleen

Question 14

During the formation of a platelet plug what is the protein that leaks into the traumatized tissue from the plasma?

Answer 14

von Willebrand factor (vWF)

Question 15

What do platelets secrete when their surface glycoproteins bind to vWF?

Answer 15

ADP
Platelet-activating factor (PAF)

(their enzymes form thromboxane A2)

Question 16

How long does it take for the clot formation to begin if the trauma to the wall is,

i) severe?
ii) minor?

Answer 16

i) 15 - 20 seconds

ii) 1 to 2 minutes

Question 17

Which structures secrete activator substances to initiate the clotting process?

Answer 17

Traumatized vascular wall
Platelets
Blood proteins (adhering to traumatized vascular wall)

Question 18

How long does it take for the entire opening or broken end of the vessel to be filled with the clot?

Answer 18

3 to 6 minutes if the opening is not too large

Question 19

After how long does the clot start retracting?

Answer 19

20 to 60 minutes

Question 20

What are the two courses that a formed blood clot has chances of following?

Answer 20

It can become,

(1) invaded by fibroblasts, which subsequently form connective tissue all through the clot
(2) dissolved

Question 21

What is the usual course for a clot that forms in a small hole of a vessel wall?

Answer 21

Invasion by fibroblasts, starting few hours after the clot is formed.
It is promoted at least partially by growth factor secreted by platelets.