Blood pressure (BP) is the pressure or force of blood on or against the artery walls. It is dependent on ‘cardiac output‘ and is also influenced by any type of friction or ‘resistance’ between the blood and blood vessel walls.

Cardiac output or total blood flow is the quantity of blood that travels through systemic or pulmonary (respiratory) blood vessels each minute; and is affected by pressure fluctuations in the blood. Blood moves from areas of high pressure to low pressure; the larger the pressure difference, the greater the flow. Resistance is the opposing forces confronted by the blood in the vessels. The higher the resistance, the smaller the flow.

High blood pressure

When BP is continually high and arterial walls are subjected to surges of blood under high pressure, this is called hypertension. Hypertension is a risk factor for cardiovascular diseases (eg. atrial fibrillation and congestive heart failure), atherosclerosis and stroke; and as hypertension affects the arteries, it is a risk factor for arterial thrombosis.

In developed countries, hypertension is the most common condition affecting the heart and blood vessels. Data in 2023 from the World Health Organization (WHO) show that approximately 46% of adults do not know they have high BP. It is also known as the ‘silent killer’ as it does damage to blood vessels and other organs before any pain or symptoms present.

Hypertension is estimated to be directly or indirectly responsible for approximately nine million deaths around the world each year (WHO).

There are two types of hypertension – primary and secondary.

Primary – also called ‘essential hypertension’, occurring in 90%-95% of cases, is defined as having persistent high BP without a known or recognisable cause. Commonly develops over time with atherosclerosis (accumulation of plaque in the arteries) increasing the risk.

Secondary – this form of hypertension can appear suddenly and is usually caused from an underlying health condition. These include congenital heart defects, kidney disease, thyroid problems, obstructive sleep apnea, cold & flu medications, and oral contraceptives.

Effects of hypertension on the blood vessels

In 1884, Rudolph Virchow (a physician and pathologist) suggested that thrombosis was the result of [among others] the stasis of blood flow, which since then has been recognised as a risk factor.

It is important for the blood to flow unrestricted and without obstruction.

When blood flow is reduced or blocked in any way, a number of serious conditions and complications may result. Hypertension meets the three main points presented by Virchow – abnormalities to the vessel wall, to blood flow and to coagulation factors involved in haemostasis.

Arteries

Healthy arteries are strong, move, bend and are smooth so blood can flow easily through them, transporting essential nutrients and oxygen throughout the body. Eventually, hypertension can increase the pressure of blood flowing through the vessels, which can lead to damage to the arteries and restricted blood flow. Long term high BP is shown to be linked to abnormalities of the vascular walls, leading to thickening and less elasticity.

Heart

Hypertension can affect the arteries which supply blood to the heart. This can lead to:

• Carotid artery disease – high BP can cause weakened artery walls, increasing the chance of damage.
• Hardened arteries reducing blood and oxygen to the heart.
• Chest pain or angina from reduced blood flow.
• Irregular heart rhythms or arrhythmias.
• Heart failure – when the heart is unable to send or pump sufficient amounts of blood and oxygen to vital organs.
• Heart attack – blood supply to the heart is obstructed = oxygen deficiency = heart muscles die.
• Irregular heartbeat.

Brain

High BP can also have a serious impact on the brain.

• Transient ischaemic attack or mini stroke – can occur from blocked blood flow (or blood clots) to the brain or hardened arteries.
• Stroke – blood clots which block blood flow and insufficient oxygen and nutrients being delivered to the brain increases the risk of stroke. Hypertension can also cause arteries to burst causing a stroke.

Risk factors

Hypertension commonly occurs in adults, however children with heart or kidney problems can be at an increased risk. High BP is usually a result of lifestyle habits, in both growing children and adults. Here is a list of some general factors which may increase risk:

• Age – commonly increases with age, however prior to age 64 men are more at risk and after 65, hypertension is higher in women.
• Family – higher chance if a parent or sibling has hypertension.
• Overweight – being obese or overweight affects the blood vessels which increases BP.
• Inactivity – can contribute to excess weight, however also can lead to a higher heart rate.
• Tobacco or vaping – these can raise BP; and tobacco smoking can damage the walls of the blood vessels, accelerating the process which can harden arteries.
• Sodium – high salt intake can cause fluid retention which increases BP.
• High alcohol intake.
• Stress – high stress levels may cause short and temporary increases in BP.
• Race – more common in black people.
  • Research from 2018 of over 5,000 young adults (aged 18-30 years) with a follow-up period of almost 30 years, found approximately a 4-fold increased risk of stroke among the Black participants compared to the White.
  • The Black Women’s Health Study followed 59,000 participants in the US since 1999. The research found that the women who had hypertension before the age of 35 had a threefold increased risk of having a stroke during midlife. The research is yet to be published; however, the findings will be presented at the 2024 International Stroke Conference 2024.

Preventative measures

• Diet – high in fruits, vegetables, whole grains, nuts, legumes, lean protein, low refined sugar, low-fat dairy products.
• Sodium – minimise salt intake. The recommended daily intake is approximately 2.3g per day which is equal to about 1 teaspoon.
• Potassium – needed to balance salt in the cells of the body. Daily intake is recommended around 3,500mg. Examples include 1 cup cooked beet greens » 1309mg; 1 medium baked potato with skin » 926mg; 1 cup cooked spinach » 839mg; 1 cup canned tomatoes » 528mg; 1 medium banana »451mg.
• Physical activity – studies have shown that regular physical activity is a protective factor against hypertension and also helps to control blood pressure among individuals who have hypertension. Both aerobic and resistance training have shown benefits.

Low blood pressure

With respect to venous thromboembolism, a study from 2023 has shown that low BP can increase the risk. The reasoning for this is that low BP could lead to a reduction in blood flow, lowering the levels of circulating oxygenated blood. This could in turn affect the endothelium (internal lining of blood vessels) causing hypoxaemia (a reduction in the partial pressure of oxygen in the blood). Hypoxaemic endothelium may lead to inflammation which could activate the coagulation process.

Hypertension, pregnancy and VTE

During pregnancy, women are at a higher risk of venous thromboembolism. Research has found that pregnant women with hypertension are at an even greater risk of VTE, and it is highest among those who have preeclampsia.

---

References

• Brosolo, G., Da Porto, A., Bulfone, L., Vacca, A., Bertin, N., Vivarelli, C., Sechi, L.A., & Catena, C. (2023). Association of arterial stiffness with a prothrombotic state in uncomplicated nondiabetic hypertensive patients. Front Cardiovasc Med.;10:1119516. doi: 10.3389/fcvm.2023.1119516.
• Kitt, J., Fox, R., Tucker, K.L., McManus, R.J. (2019). New approaches in hypertension management: a review of current and developing technologies and their potential impact on hypertension care. Curr Hypertens Rep;21(6):44. doi: 10.1007/s11906-019-0949-4. 
• Tortora, G.J. & Grabowski, S.R. (2003). Principles of anatomy and physiology (10th Ed). John Wiley & Sons, Inc.
• Nazarzadeh, M., Bidel, Z., Mohseni, H., Canoy, D., Pinho-Gomes, A.C., Hassaine, A., Dehghan, A., Tregouet, D.A., Smith, N.L., & Rahimi, K. (2023). Blood pressure and risk of venous thromboembolism: a cohort analysis of 5.5 million UK adults and Mendelian randomization studies. Cardiovasc Res; 119(3):835-842. doi: 10.1093/cvr/cvac135.
• Pastori, D., Cormaci, V.M., Marucci, S., Franchino, G., Del Sole, F., Capozza, A., Fallarino, A., Corso, C., Valeriani, E., Menichelli, D., Pignatelli, P. (2023). A comprehensive review of risk factors for venous thromboembolism: From epidemiology to pathophysiology. Int J Mol Sci.;24(4): 3169. doi: 10.3390/ijms24043169.
• Varughese, G.I., & Lip, G.Y.H. (2005). Is hypertension a prothrombotic state? Current Hypertension Reports, 7(3), 168–173. https://doi.org/10.1007/s11906-005-0005-4
• Carotid artery disease - Symptoms and causes - Mayo Clinic
• Mechanisms of hypoxemia - PMC (nih.gov)
• Hypertension (who.int)
• High blood pressure dangers: Hypertension's effects on your body - Mayo Clinic
• High blood pressure (hypertension) - Symptoms & causes - Mayo Clinic
• https://www.medscape.com/viewarticle/hypertension-before-35-tied-triple-stroke-risk-midlife-2024a10002bn?ecd=mkm_ret_240225_mscpmrk_neuro_stroke_etid6333661&uac=423562PT&impID=6333661
• Sodium Intake and Health: What Should We Recommend Based on the Current Evidence? - PMC (nih.gov)
• Executive summary - Guideline: Potassium Intake for Adults and Children - NCBI Bookshelf (nih.gov)
• Food Sources of Potassium | Dietary Guidelines for Americans
• Prevention and Control of Hypertension: JACC Health Promotion Series - PMC (nih.gov)