Introduction[edit | edit source]
- Are the smallest blood cells, typically around 2μm in diameter and anucleated (thus do not have any nucleus), but are rich in nucleic acids like mitochondrial DNA and RNA.
- Have an average lifespan of 7 to 10 days in humans.
- Are the second most abundant cells in the blood circulation, with the concentration ranging between 150-400 × 10^9 /L.
- Form by the fragmentation of megakaryocytes within the bone marrow.
- Circulate in the blood in discoid form but undergo significant structural changes following activation, transforming from discoid forms into compact spheres with dendritic extensions (which allow platelet adhesion). See Image 1.
- Their most prominent function is to initiate blood coagulation. Through their clotting activity and activation of the coagulation cascade, they are crucial to maintaining adequate blood volume in those with vascular injury.
- immune system by interacting with pathogens like bacteria, viruses, fungi, and protozoans. Some platelets secrete effector molecules that act as chemoattractants for macrophages, lymphocytes, and other immune cells.
Platelets play a key role in hemostasis, and their dysfunction can result in a variety of inherited and acquired bleeding disorders. They also have involvement in the development of thrombi in the context of atherosclerosis, a pathological process associated with significant morbidity and mortality, primarily through myocardial infarction and stroke. 
Function[edit | edit source]
Platelets maintain hemostasis by adhering to the vascular endothelium, aggregating with other platelets, and initiating the coagulation cascade, leading to the production of a fibrin mesh, which effectively prevents significant blood loss. Platelets are also crucial in inflammation, tissue growth, and the immune response. These processes are under the mediation of the release of compounds from the alpha and dense granules which are inside the platelet. Alpha granules and dense granules contain specific compounds that are critical for a variety of functions.
- α granules (alpha granules) – contain P-selectin, platelet factor 4, transforming growth factor-β1, platelet-derived growth factor, fibronectin, B-thromboglobulin, vWF, fibrinogen, and coagulation factors V and XIII
- δ granules (delta or dense granules) – contain ADP or ATP, calcium, and serotonin
Mechanism[edit | edit source]
Platelets can become activated in response to exposed collagen, thrombin, ADP, or other compounds. In response to tissue injury, exposed collagen on the subendothelial surface can bind directly to either the platelet or to vWF (a specific compound in the alpha granules). This begin the complex cascade involved in platelet activation and clot formation or propagation.
Platelet Disorders[edit | edit source]
A disorder of platelet function is a thrombocytopathy. Platelet disorders include:
- An abnormal increase in platelets. Elevated platelet concentration is called thrombocytosis, and is either: congenital; reactive (to cytokines); or due to unregulated production eg one of the myeloproliferative neoplasms or certain other myeloid neoplasms
- A decrease in platelets. Low platelet concentration is called thrombocytopenia, and is due to either decreased production or increased destruction.
- Platelet dysfunction
Any of these conditions, even those in which platelets are increased, may cause defective formation of hemostatic plugs and bleeding.
Platelet Count and Exercise[edit | edit source]
A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. Having more than 450,000 platelets is a condition called thrombocytosis; having less than 150,000 is known as thrombocytopenia. A platelet number is obtained from a complete blood count (CBC). The forms of exercise a person can safely participate in with a low platelet count depends on how low the platelet count is.
Specific exercise recommendations for different levels of low platelets.
- Limit all activity when the count is less than 15,000.
- When platelets are 15 to 20,000, a person may do gentle exercising that does not involve resistance eg sitting or standing, gentle stretching or taking an easy walk.
- A platelet count of 20 to 40,000 allows for some light resistance, eg weights or latex bands and a fast walk and climb stairs.
- At platelet levels of 40 to 60,000, add exercises such as stationary cycling and golfing.
- Higher levels, that are still considered too low, allow for aerobic exercise such as biking and jogging, but require wearing proper gear and taking caution against injury.
See also Physical Activity in Cancer
Platelet Transfusions[edit | edit source]
Platelet transfusions came into medical use in the 1950s and 1960s. It is on the World Health Organization's List of Essential Medicines, the safest and most effective medicines needed in a health system.
People with hematological disorders or cancer receive the largest proportion of platelet transfusions. Most are given to prevent bleeding during treatment with chemotherapy or stem cell transplant. Much of the remainder are used in general medicine, cardiac surgery and in intensive care. ie Those patients who are bleeding or at high risk of bleeding with:
- Low platelet counts eg this occurs in people receiving cancer chemotherapy
- Platelet dysfunction
All platelet components are leucodepleted and irradiated prior to release to the hospital. Platelets can also be collected from a donor using an apheresis machine that allows the selective collection of platelets and some plasma. Platelet products must be stored at room temperature and can only be kept for a few days.
Platelets and Ageing[edit | edit source]
In the circulation, platelets have a lifespan of 7–10 days. Although circulating platelet counts decline with age, somewhat paradoxically, this decrease in the numbers of these cells is associated with acquisition of a pro-inflammatory phenotype. The age-related transition of platelets to a more reactive state is associated with increased eg. generation of reactive oxygen species (ROS) and mitochondrial dysfunction.
- These age-related alterations in the numbers and reactivity of circulating platelets are likely to reflect the chronic, low-grade, systemic inflammation that is associated with age-related co-morbidities.
- This type of sub-clinical, intravascular instability may predispose elderly adult humans for development of severe organ dysfunction, or even failure, during episodes of serious microbial or viral infection.
References[edit | edit source]
- Fountain JH, Lappin SL. Physiology, Platelet. StatPearls [Internet]. 2020 Sep 18.Available: https://www.statpearls.com/articlelibrary/viewarticle/27317/ (accessed 30.5.2021)
- Microbenotes Platelets Available from: https://microbenotes.com/platelets-thrombocytes/ (accessed 30.5.2021)
- Williams O, Sergent SR. Histolology, Platelets. StatPearls [Internet]. 2020 May 21.Available from: https://www.ncbi.nlm.nih.gov/books/NBK557800/(accessed 30.5.2021)
- MSD manual Platelet disorders Available:https://www.msdmanuals.com/en-au/professional/hematology-and-oncology/thrombocytopenia-and-platelet-dysfunction/overview-of-platelet-disorders (accessed 30.5.2021)
- Livestrong Exercise and low platelet count Available from:https://www.livestrong.com/article/339420-exercise-low-platelet-count/ (accessed 30.5.2021)
- WHO World Health Organization model list of essential medicines: 21st list 2019 Available:http://apps.who.int/iris/handle/10665/325771 (accessed 31.5.2021)
- RCH Platelet transfusion Available from:https://www.rch.org.au/bloodtrans/about_blood_products/Platelet_transfusion/ (accessed 31.5.2021)
- Feldman C, Anderson R. Platelets and their role in the pathogenesis of cardiovascular events in patients with community-acquired pneumonia. Frontiers in Immunology. 2020 Sep 17;11:2304.Available from: https://www.frontiersin.org/articles/10.3389/fimmu.2020.577303/full(accessed 30.5.2021)