3.5 - Blood Flow Through Body Tissues Tissue Perfusion

Tissue Perfusion (Module 3.5 Definition)

The process of blood flow through the capillaries, delivering oxygen and nutrients to body tissues and removing waste products; maintained by arterial (mean arterial) blood pressure

Mean Arterial Pressure Formula (Perfusion Context)

Mean arterial pressure = cardiac output (CO) x systemic vascular resistance (SVR)

Hypoperfusion (Ischemia)

Inadequate tissue perfusion, which can lead to tissue damage, dysfunction, and, if prolonged, serious health conditions such as organ failure

Signs of Compromised Tissue Perfusion

Pale skin, cool extremities, altered mental status, and delayed capillary refill time

Cyanosis

A bluish discoloration of the skin and mucous membranes due to insufficient oxygenation of the blood, which can result from reduced tissue perfusion limiting oxygen delivery

Astrocytes (Brain Circulation)

Specialized cells that wrap around capillary vessels in the brain to prevent the passage of unwanted substances, contributing to the blood-brain barrier

Cerebral Autoregulation

A mechanism that maintains constant blood flow to the brain despite fluctuations in systemic blood pressure, ensuring a stable oxygen and nutrient supply for normal neurological function

Cerebral Arterial Circle (Circle of Willis)

An anastomosis formed by branches of the carotid and vertebral arteries after entering the cranial cavity, providing alternate routes of blood flow to the brain in case of blockage

Main Components of the Cerebral Arterial Circle

The anterior and posterior communicating arteries and the anterior and posterior cerebral arteries

Blood Supply Distribution in the Brain

Branches to the anterior portion of the cerebrum are normally fed by the internal carotid arteries, while the remainder of the brain receives blood flow from branches of the vertebral arteries

Skin (Cutaneous) Circulation

Crucial for thermoregulation and nutrient exchange; cutaneous vasodilation promotes heat loss in response to elevated body temperature, while cutaneous vasoconstriction conserves heat in cold conditions by moving blood to deeper tissues and organs

Fetal Circulation Shunts

Fetal circulation is unique due to three major shunts: the foramen ovale, the ductus arteriosus, and the ductus venosus, which allow alternate paths for blood flow found only in the fetus

Foramen Ovale

An opening in the interatrial septum allowing blood to flow from the right atrium to the left atrium, bypassing the non-functioning fetal lungs; closes after birth as atrial pressure increases, leaving the fossa ovalis

Fossa Ovalis

The remnant that marks the location of the former foramen ovale in the interatrial septum after birth

Ductus Arteriosus

A short, muscular vessel connecting the pulmonary trunk to the aorta, diverting most blood from the right ventricle into the aorta so it bypasses the fetal lungs; constricts and seals off after birth as oxygen levels rise, eventually leaving only connective tissue

Ductus Venosus

A temporary blood vessel branching from the umbilical vein that allows freshly oxygenated blood from the placenta to bypass the fetal liver and go directly to the fetal heart; closes slowly during early infancy and degenerates after the umbilical cord falls off

Placenta

A temporary organ that allows gas exchange between the mother and fetus

Effect of Exercise on Cardiac Output

Cardiac output (heart rate x stroke volume) increases with exercise; well-trained aerobic athletes can raise cardiac output from about 5 L/min at rest to over 30 L/min during maximal exercise

Cardiovascular Adaptations to Regular Exercise

Regular exercise increases heart size and mass, enhancing both stroke volume and cardiac output, making the heart more efficient at pumping blood

Tissue Perfusion Changes During Exercise

Selective vasodilation occurs in skeletal muscles, heart, lungs, liver, and integument during exercise, while vasoconstriction occurs in vessels leading to the kidneys and digestive/reproductive organs; brain blood flow remains largely unchanged because cerebral vessels are less responsive to regulatory stimuli

Brain Blood Flow During Exercise

Remains constant at approximately 750 mL/min at rest, light exercise, and strenuous exercise, unlike other tissues

Skeletal Muscle Blood Flow: Rest vs Strenuous Exercise

Approximately 1,200 mL/min at rest, rising to approximately 12,500 mL/min during strenuous exercise

Frank-Starling Principle (Exercise Context)

Increased preload due to enhanced venous return results in more forceful contractions of the cardiac muscle

Cardiovascular Benefits of Regular Exercise

Promotes cardiovascular efficiency, increases blood delivery efficiency, lowers cholesterol levels, decreases cardiovascular disease risk by reducing plaque formation, and lowers blood pressure; even about 30 minutes of non-continuous exercise per day shows significant cardiovascular benefit

Normal Blood Pressure (AHA Guidelines)

Less than 120/80 mmHg

Elevated Blood Pressure (AHA Guidelines)

Systolic between 120-129 mmHg and diastolic less than 80 mmHg

Hypertension (AHA Guidelines)

Blood pressure of 130/80 mmHg or higher; often called the "silent killer" because it is typically a silent disorder and patients often fail to recognize its seriousness or follow treatment plans

Aneurysm (Hypertension Complication)

A bulging in a blood vessel caused by weakening of the vessel wall from prolonged high pressure; can rupture, causing internal bleeding and tissue damage; a stroke occurs if the rupture is in a brain vessel

Peripheral Arterial Disease (PAD)

Obstruction of vessels in the peripheral regions of the body, reducing blood flow to the limbs and potentially causing pain, mobility issues, or tissue death/amputation from prolonged ischemia

Chronic Kidney Disease (Hypertension Complication)

Results from prolonged high blood pressure damaging blood vessels in the kidneys, reducing their ability to filter waste and regulate fluid balance

Heart Failure (Hypertension Complication)

Results from prolonged high blood pressure increasing strain on the heart, leading to structural changes such as left ventricular hypertrophy and impairing the heart's pumping ability

Hemorrhage

A loss of blood that cannot be controlled by hemostatic mechanisms, distinct from minor blood loss managed by hemostasis and repair

Short-Term Response to Hemorrhage

Baroreceptor stimuli trigger cardiovascular centers to increase cardiac output and vasoconstriction via sympathetic responses; heart rate may rise to about 180-200 bpm; arteriolar vasoconstriction increases vascular resistance while venous constriction increases venous return, both raising blood pressure

Hormonal Response to Hemorrhage

Sympathetic stimulation triggers epinephrine and norepinephrine release (enhancing cardiac output and vasoconstriction); the RAA system stimulates thirst and increases renal reabsorption of sodium and water; the kidneys also increase EPO production, stimulating erythrocyte formation to restore blood volume

Blood Loss Threshold for Compensation

If blood loss is less than 20 percent of total blood volume, compensatory responses usually return blood pressure to normal; if compensatory mechanisms fail, the patient enters circulatory shock

Circulatory Shock

A life-threatening condition in which the circulatory system is unable to maintain blood flow to sustain cellular metabolism, causing lack of oxygen to cells and tissues, cellular death, and impaired organ function; may lead to multiple organ failure (MOF) and death if untreated

Typical Signs of Circulatory Shock

Increased heart rate but decreased blood pressure (though BP may temporarily remain normal in some cases), dramatically falling urine output, confusion, or loss of consciousness

Concerning Urine Output in Shock

Urine output less than 1 mL/kg body weight/hour is cause for concern

Hypovolemic Shock

Shock caused by severe blood or fluid loss; decreased blood volume leads to decreased preload, decreased cardiac output, and decreased blood pressure, impairing tissue perfusion; typically caused by hemorrhage in adults or severe vomiting/diarrhea in children, and can also result from extreme burns

Hypovolemic Shock Symptoms

Rapid heart rate, low body temperature, low blood pressure, weak pulse, cool/clammy skin (especially extremities), rapid shallow breathing, hypothermia, and decreased or no urine output

Hypovolemic Shock Treatment

Stopping fluid or blood loss, intravenous blood or fluid replacement, and drugs such as dopamine, epinephrine, and norepinephrine to raise blood pressure and cardiac output

Cardiogenic Shock

Shock resulting from the heart's inability to maintain cardiac output, most often due to myocardial infarction, but also arrhythmias, valve disorders, cardiomyopathies, cardiac failure, or insufficient coronary blood flow; treated by repairing the underlying cardiac damage

Distributive Shock

Shock occurring when arterioles lose normal muscular tone and systemic vasodilation occurs, causing blood to leak into tissues, decreased preload, decreased cardiac output and blood pressure, and inadequate tissue perfusion

Septic Shock

A form of distributive shock caused by widespread infection (sepsis) resulting in an inflammatory response whose chemical release causes vasodilation

Anaphylactic Shock

A form of distributive shock caused by a severe allergic response involving widespread histamine release, triggering vasodilation throughout the body

Neurogenic Shock

A form of distributive shock occurring when damage to the nervous system's vasomotor center causes loss of vasoconstriction needed to manage blood pressure

Distributive Shock Treatment

Fluid replacement and inotropic agents to restore vascular muscle tone, plus treatment of the underlying cause (e.g., antibiotics for sepsis, antihistamines for anaphylaxis)

Obstructive Shock

Shock occurring when a significant portion of the vascular system is blocked, resulting in decreased preload, cardiac output, and blood pressure, and inadequate tissue perfusion

Causes of Obstructive Shock

Pulmonary embolism (most common cause), aortic stenosis (narrowing of the aortic valve), cardiac tamponade (excess pericardial fluid interfering with heart filling), and pneumothorax (excess air in the thoracic cavity interfering with venous return)

Obstructive Shock Treatment

Depends on underlying cause; typically includes fluid administration, anticoagulants, removal of fluid from the pericardial cavity or air from the thoracic cavity, and often surgery