Blood Pressure, Blood Flow, and Resistance Explained (Hemodynamics) | Physiology

Hypertension is defined as sustained blood pressure greater than 140/90 mmHg, classified as primary (95% of cases, no underlying cause) or secondary (5%, caused by underlying conditions like renal disease or endocrine disorders). The pathophysiology involves increased peripheral resistance, sodium retention, and genetic factors affecting catecholamine receptor sensitivity. Atherosclerosis, the most common cause of cardiovascular disease, develops through endothelial injury, lipid accumulation, and inflammatory response, with risk factors including hypertension, hyperlipidemia, smoking, diabetes, and aging. The progression involves five key cellular events: endothelial injury, LDL accumulation, monocyte/macrophage infiltration, smooth muscle cell proliferation, and plaque formation. Major complications include hypertensive nephrosclerosis, coronary artery disease, stroke, and retinopathy.

In series arrangement, arteriolar constriction causing hypertension increases total peripheral resistance (afterload), requiring the heart to pump under more pressure. Even smaller arteriolar narrowing causes large blood pressure rise because resistance is inversely proportional to radius to the fourth power. Atherosclerosis causes local resistance increase at single artery narrowing, affecting the whole circulatory system and causing tissue ischemia. In parallel arrangement, amputation of a limb removes one parallel pathway, increasing total resistance and causing slight blood pressure rise. Similarly, removing one kidney increases total peripheral resistance. Conversely, an arteriovenous fistula (for dialysis or congenital) creates an extra parallel pathway, reducing total resistance and increasing venous return and cardiac output. In septic shock, widespread vasodilation causes severe decrease in parallel resistance, leading to severe hypertension.

Atherosclerosis develops through progressive arterial damage caused by poor diet and sedentary lifestyle. Fatty substances, macrophages, and cellular debris accumulate within arterial walls, forming plaques that narrow vessel lumens. This narrowing reduces blood vessel radius, creating physical obstructions that impede blood flow. The relationship between vessel radius and resistance follows Poiseuille's law, where resistance increases dramatically as radius decreases. Over decades, multiple plaques develop, further reducing vessel radii and increasing total vascular resistance. This pathophysiological process demonstrates how lifestyle factors indirectly elevate blood pressure by mechanically obstructing blood flow pathways, requiring greater pressure to maintain adequate perfusion.

Hypertension and atherosclerosis form a central physiological axis responsible for over 75% of serious cardiovascular events. Hypertension caused 10.8 million deaths in 2023, while atherosclerosis explains 85% of heart attacks and 65% of cerebrovascular events. Endothelial dysfunction is the first event before clinical manifestation, where loss of nitric oxide, increased oxidative stress, and immunological activation create a chronic inflammatory process. The atherothrombotic cycle involves hypertension creating inflammation, pressure, and lesion damage that generates progressive silent destruction. Modern diagnosis incorporates biomarkers: ProBNP above 300 indicates up to 20% future cardiovascular events, while high-sensitivity C-reactive protein identifies vascular inflammation. The ankle-brachial index predicts cardiovascular events in patients over 65, diabetics, smokers, or hypertensives. Coronary calcium scoring above 100 indicates subclinical atherosclerosis requiring intensive treatment. Treatment must be early, stratified, and personalized, with fixed combination therapy preferred over monotherapy. Blood pressure targets are 140/80 mmHg for patients under 75, with 130/135 mmHg indicating alarm. LDL targets are less than 70 mg/dL for high-risk patients and less than 55 mg/dL for very high-risk patients. Anti-inflammatory approaches including colchicine and SGLT2 inhibitors are emerging options. The new medicine does not wait for symptoms but acts on risk, requiring active anticipation rather than late reaction.

Hypertension (high blood pressure) results from increased vascular resistance, cardiac output, blood volume, or blood viscosity. Atherosclerosis is a progressive condition where fatty deposits (plaque) accumulate on arterial inner walls, causing narrowing and stiffening of blood vessels. This narrowing increases vascular resistance, forcing the heart to pump harder to maintain adequate blood flow. Over time, this increased cardiac workload can lead to heart failure. Additionally, turbulent blood flow in narrowed arteries promotes blood clot formation (thrombosis), which can break loose and block smaller vessels downstream, potentially causing heart attacks or strokes. Renal denervation is a minimally invasive procedure for treating resistant hypertension, using radiofrequency energy or ultrasound to destroy sympathetic nerves connecting the kidneys to the brain.

The Starling forces describe the balance of pressures that govern fluid exchange between capillaries and interstitial fluid: capillary hydrostatic pressure (pushing water out) and interstitial hydrostatic pressure (pulling water in) work against capillary oncotic pressure (pulling water back in) and interstitial oncotic pressure (pushing water out). In most tissues, the net filtration pressure is positive at the beginning of capillaries (favoring filtration) and negative at the end (favoring reabsorption), meaning slightly more water is filtered than reabsorbed, with excess water normally returned to circulation via the lymphatic system. However, when net filtration pressure increases excessively (due to factors like vasodilation in hot weather), the lymphatic system cannot compensate, leading to fluid accumulation in interstitial spaces called edema.

Microcirculation involves the exchange of nutrients, oxygen, and waste between blood capillaries and tissues through the interstitial space, which is composed of collagen and proteoglycans bathed in interstitial fluid. This exchange is governed by Starling forces: capillary hydrostatic pressure pushes fluid out of capillaries, plasma colloid osmotic pressure pulls fluid back in, while interstitial hydrostatic and colloid osmotic pressures also influence fluid movement. Edema occurs when these forces are imbalanced, such as increased capillary hydrostatic pressure or decreased plasma colloid osmotic pressure. Excess interstitial fluid returns to circulation through the lymphatic system via the thoracic duct and right lymphatic duct.

Capillary fluid exchange is governed by hydrostatic pressure (blood pressure) and osmotic pressure (from plasma proteins). Hydrostatic pressure decreases from ~40 mmHg at arterial ends to ~15 mmHg at venous ends, while osmotic pressure remains constant at ~25 mmHg. At arterial ends, hydrostatic pressure exceeds osmotic pressure, pushing fluid out into interstitial spaces. At venous ends, osmotic pressure exceeds hydrostatic pressure, drawing fluid back. Small molecules (O2, CO2, glucose, salts) cross freely, but blood cells and proteins cannot. Starling's hypothesis states that fluid leaving capillaries exceeds fluid returning, but the lymphatic system collects excess fluid to prevent edema.

Starling forces are four fundamental forces governing fluid movement across capillary walls in microcirculation. These forces determine whether fluid moves from capillaries to interstitium (filtration) or vice versa (reabsorption). The forces include capillary hydrostatic pressure (pushing fluid out), plasma colloid osmotic pressure (pulling fluid in), interstitial fluid pressure (pushing fluid in), and interstitial fluid colloid osmotic pressure (pulling fluid out). This framework is essential for understanding how nutrients are delivered to tissues and waste products are removed from the body.

Local blood flow to tissues is regulated by metabolic demand, meaning tissues receive more blood when their metabolic needs increase. Since local blood flow affects venous return, and venous return affects cardiac output, higher metabolic demand in tissues leads to increased cardiac output to meet those needs.

Tissue blood flow is prioritized according to metabolic demand, with cardiac output distributed as: 14% brain, 4% heart, 22% kidneys, 27% liver, and remaining to other organs. Acute control occurs through vasoconstriction/vasodilation of arterioles and precapillary sphincters. Long-term control involves angiogenesis (new vessel formation) in response to increased metabolic demand. Autoregulation maintains constant blood flow despite pressure changes through myogenic theory (smooth muscle contraction in response to stretch) and metabolic theory (metabolites like adenosine, CO2, lactate, and histamine cause vasodilation). Nitric oxide, produced by endothelial cells in response to shear stress, is a key vasodilator. Active hyperemia is increased blood flow due to increased metabolism (exercise), caused by metabolite accumulation. Reactive hyperemia follows temporary occlusion, with accumulated metabolites causing vasodilation when flow resumes. The longer the occlusion, the greater the reactive hyperemia response. Coronary blood flow is regulated by both metabolic (adenosine production from constant oxygen consumption) and myogenic mechanisms.

Mean arterial pressure is determined by cardiac output and systemic vascular resistance, with maintenance of values over 65-70 mmHg being an evolutionary condition for adequate local tissue flow regulation. Cardiac output distribution among organs is highly variable, with kidneys receiving very high flow relative to their size due to excretory function needs rather than metabolic demand. For most organs, blood flow is fundamentally explained by metabolic activity. Autoregulation is the capacity of tissues to maintain constant blood flow independent of arterial pressure changes over a wide range (50-60 to 120-140 mmHg). Organs have tight control of resistance arterioles, dilating when pressure decreases and increasing tone when pressure increases. There is a critical arterial pressure below which autoregulation cannot compensate, and above which flow increases linearly with pressure.

Most tissues regulate their own blood flow in response to metabolic demands. When tissue metabolism increases, blood flow increases to deliver more oxygen and nutrients. This local control operates independently of systemic blood pressure changes. Mechanisms include accumulation of metabolic byproducts (adenosine, CO2, H+, K+) that cause vasodilation, and decreased oxygen tension that causes vasodilation. Reactive hyperemia is increased blood flow after ischemia, caused by accumulated metabolic byproducts. During exercise, metabolic hyperemia dramatically increases blood flow to active muscles. Autoregulation maintains relatively constant blood flow despite changes in perfusion pressure through myogenic (stretch-induced contraction) and metabolic (metabolite-induced) mechanisms. Long-term changes in blood flow requirements lead to vascular remodeling including angiogenesis (formation of new vessels) and changes in vessel structure. Vascularization is determined by maximum metabolic demand, not average demand.