6.6 Circulatory routes and regional circulation

The circulatory system is a complex network of blood vessels that deliver oxygen and nutrients to tissues throughout the body. This section explores the major routes of blood flow, including the pulmonary and systemic circuits, as well as specialized pathways like the coronary and cerebral circulations.

Understanding how blood flow is regulated in different organs is crucial for grasping the cardiovascular system's function. We'll look at factors that influence regional blood flow, such as local metabolic demands and systemic controls, and examine how organs maintain consistent perfusion through autoregulation.

Major circulatory routes

Pulmonary and systemic circuits

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• The pulmonary circuit carries deoxygenated blood from the right ventricle to the lungs for gas exchange and returns oxygenated blood to the left atrium
• The systemic circuit carries oxygenated blood from the left ventricle to all tissues of the body, except the lungs, and returns deoxygenated blood to the right atrium
• The pulmonary and systemic circuits work together to ensure that oxygen-rich blood is delivered to the body's tissues while carbon dioxide-rich blood is sent to the lungs for gas exchange

Specialized circulatory routes

• The coronary circulation supplies oxygenated blood to the myocardium via the left and right coronary arteries, which branch off the ascending aorta just above the aortic valve
  • The coronary arteries deliver oxygen and nutrients to the heart muscle, enabling it to function properly
• The hepatic portal system carries nutrient-rich, deoxygenated blood from the gastrointestinal tract and spleen to the liver for processing before returning to the systemic circulation
  • This unique circulation allows the liver to filter and process nutrients, toxins, and other substances absorbed from the digestive system (intestines) before they enter the general circulation
• The fetal circulation includes unique structures such as the foramen ovale, ductus arteriosus, and ductus venosus, which allow the fetus to obtain oxygenated blood from the placenta and bypass the lungs
  • These structures enable the fetus to receive oxygen and nutrients from the mother's circulation while the lungs are not yet functional
  • The foramen ovale (heart), ductus arteriosus (between pulmonary artery and aorta), and ductus venosus (liver) close shortly after birth as the newborn transitions to independent circulation and respiration

Coronary, cerebral, and pulmonary circulations

Unique features of coronary circulation

• Coronary circulation is unique because it is driven by both aortic pressure during diastole and the squeezing effect of the contracting myocardium during systole
  • During systole, the contracting heart muscle compresses the coronary arteries, reducing blood flow to the myocardium
  • Most coronary blood flow occurs during diastole when the heart muscle relaxes and the coronary arteries are no longer compressed
• The coronary arteries are more susceptible to atherosclerosis due to the high pressure and pulsatile nature of blood flow
  • Atherosclerotic plaques can narrow the coronary arteries, reducing blood flow to the myocardium and potentially causing ischemia or infarction (heart attack)
• Coronary artery disease is a major cause of morbidity and mortality worldwide, often resulting from lifestyle factors such as diet, physical inactivity, and smoking

Characteristics of cerebral circulation

• Cerebral circulation is characterized by the presence of the blood-brain barrier, which selectively permits the passage of substances between the blood and brain tissue
  • The blood-brain barrier is formed by tight junctions between endothelial cells, which restrict the movement of large or hydrophilic molecules while allowing the passage of small, lipophilic substances (oxygen, carbon dioxide)
• Cerebral autoregulation maintains relatively constant blood flow to the brain despite changes in systemic blood pressure
  • This mechanism ensures that the brain receives a consistent supply of oxygen and nutrients even if blood pressure fluctuates
• The circle of Willis provides collateral circulation to the brain, allowing for the maintenance of blood flow if one of the main arteries becomes occluded
  • The circle of Willis is an anastomotic ring of arteries at the base of the brain that interconnects the anterior and posterior cerebral circulations
  • If one artery is blocked, blood can still reach the brain tissue through the collateral vessels of the circle of Willis

Distinguishing features of pulmonary circulation

• Pulmonary circulation is a low-pressure, low-resistance system compared to the systemic circulation
  • The lower pressure in the pulmonary circulation reduces the workload on the right ventricle and prevents fluid accumulation in the lungs
• The pulmonary arteries carry deoxygenated blood, while the pulmonary veins carry oxygenated blood, which is the opposite of the systemic circulation
  • This unique arrangement allows for efficient gas exchange between the alveoli and the pulmonary capillaries
• Hypoxic pulmonary vasoconstriction is a unique response in which pulmonary arterioles constrict in response to low oxygen levels, diverting blood flow to better-ventilated areas of the lungs
  • This mechanism optimizes the matching of ventilation and perfusion, ensuring that blood flow is directed to lung regions with adequate oxygenation
  • Hypoxic pulmonary vasoconstriction helps to maintain arterial oxygen saturation in the face of regional lung pathology (pneumonia, atelectasis)

Factors influencing regional blood flow

Local factors

• Metabolic factors, such as increased oxygen demand or the accumulation of metabolic wastes (CO2 and H+), can cause local vasodilation and increase regional blood flow
  • When tissues are more metabolically active, they release vasodilator substances (adenosine, nitric oxide) that relax smooth muscle cells in the arterioles, increasing blood flow to meet the higher metabolic demands
• Autoregulation is the intrinsic ability of organs and tissues to maintain relatively constant blood flow despite changes in perfusion pressure
  • Autoregulation ensures that critical organs (brain, heart, kidneys) receive a consistent supply of blood even if systemic blood pressure fluctuates
  • Myogenic and metabolic mechanisms contribute to autoregulation in different organs and tissues

Systemic factors

• Neural factors, such as the sympathetic nervous system, can cause vasoconstriction or vasodilation in different organs and tissues to redistribute blood flow according to the body's needs
  • Sympathetic activation during exercise or stress causes vasoconstriction in the splanchnic and cutaneous circulations, diverting blood flow to the skeletal muscles and brain
  • Sympathetic tone maintains basal vascular resistance and helps to regulate systemic blood pressure
• Hormonal factors, such as epinephrine, norepinephrine, and angiotensin II, can influence regional blood flow by causing vasoconstriction or vasodilation in specific vascular beds
  • Epinephrine and norepinephrine, released from the adrenal medulla, cause vasoconstriction in most systemic arteries while dilating coronary and skeletal muscle vessels
  • Angiotensin II, produced by the renin-angiotensin-aldosterone system, is a potent vasoconstrictor that helps to maintain blood pressure and fluid balance
• Extrinsic factors, such as temperature, gravity, and physical activity, can also influence regional blood flow by altering vascular tone or the distribution of blood volume
  • Cold temperatures cause cutaneous vasoconstriction to reduce heat loss, while warm temperatures lead to vasodilation and increased skin blood flow
  • Gravity causes pooling of blood in the lower extremities when standing, which is counteracted by the skeletal muscle pump and venoconstriction
  • Physical activity increases blood flow to active skeletal muscles through local metabolic vasodilation and sympathetic redistribution of flow

Autoregulation in organs and tissues

Myogenic and metabolic mechanisms

• Myogenic autoregulation occurs when smooth muscle cells in the arterioles respond to changes in transmural pressure by contracting or relaxing to maintain constant blood flow
  • When transmural pressure increases, smooth muscle cells stretch and contract, narrowing the arteriolar lumen and increasing resistance to flow
  • When transmural pressure decreases, smooth muscle cells relax, widening the arteriolar lumen and decreasing resistance to flow
  • This mechanism is particularly important in the kidneys, brain, and coronary circulation
• Metabolic autoregulation involves the local release of vasodilator substances, such as adenosine, nitric oxide, and hydrogen ions, in response to increased metabolic demand or decreased oxygen supply
  • When tissues are hypoxic or acidotic, they release these vasodilator substances, which relax smooth muscle cells in the arterioles and increase blood flow
  • This mechanism is prominent in the heart, skeletal muscles, and brain
  • Metabolic autoregulation ensures that blood flow matches the metabolic needs of the tissue

Organ-specific autoregulatory mechanisms

• Tubuloglomerular feedback is a specific form of autoregulation in the kidneys that adjusts afferent arteriolar resistance in response to changes in the composition of tubular fluid at the macula densa
  • When the macula densa detects increased sodium chloride concentration in the tubular fluid, it signals the afferent arteriole to constrict, reducing glomerular filtration rate and tubular flow
  • This negative feedback loop helps to maintain constant renal blood flow and glomerular filtration rate despite changes in systemic blood pressure
• Cerebral autoregulation involves the complex interplay of myogenic, metabolic, and neurogenic factors to maintain constant cerebral blood flow across a wide range of systemic blood pressures
  • The cerebral circulation is able to maintain relatively constant flow between mean arterial pressures of 60-150 mmHg
  • Myogenic and metabolic mechanisms are the primary drivers of cerebral autoregulation, with neural factors playing a modulatory role
• Coronary autoregulation is mediated by both myogenic and metabolic factors, with metabolic factors being the dominant mechanism during periods of increased cardiac workload
  • When myocardial oxygen demand increases, such as during exercise, local metabolic vasodilation ensures that coronary blood flow increases to meet the higher metabolic needs of the heart muscle
  • Myogenic autoregulation helps to maintain constant coronary blood flow during changes in aortic pressure, particularly during diastole when most coronary perfusion occurs