The Cardiovascular System: Blood Vessels and Circulation

Anatomy and Organization of Blood Vessels

The cardiovascular system relies on a complex network of vessels to transport blood throughout the body. Arteries and arterioles carry blood away from the heart, typically transporting oxygenated blood in the systemic circuit. Conversely, venules and veins return blood to the heart, typically carrying deoxygenated blood. Between these two systems lie the capillaries, the smallest and most abundant vessels, which perfuse tissues to facilitate exchange. The pulmonary circuit transports oxygen-poor blood to the lungs and oxygen-rich blood back to the heart, while the systemic circuit delivers oxygen-rich blood to the tissues and returns oxygen-poor blood to the heart.

All blood vessels feature a lumen, which is the hollow central passageway. Arteries generally possess smaller, circular lumens, whereas veins have larger, more compressed lumens. Vessels are supported by the vasa vasorum, or "vessels of vessels," which nourish the vessel walls and remove waste. In arteries, the vasa vasorum is located in the outer layers, while in veins it is more internal; the high pressure in arteries makes arterial disease more common. The nervi vasorum, or "nerves of vessels," control the constriction and dilation of these pathways.

The Three Tunics of Blood Vessels

Blood vessel walls are composed of three distinct tissue layers known as tunics. The tunica intima, or tunica interna, is the innermost layer consisting of simple squamous epithelium called endothelium. This layer is continuous with the endocardium of the heart and releases endothelins, which are local vasoconstrictors used to increase blood pressure. It also includes a basement membrane made of areolar connective tissue with collagen and elastin fibers. In larger arteries, an internal elastic membrane is present in this layer, and in veins, thickened portions of the intima form valves.

The tunica media is the middle layer, which is characteristically thicker in arteries than in veins. It is composed of smooth muscle and elastin fibers. Its activity is controlled by the nervi vasorum and chemical signals to facilitate vasoconstriction and vasodilation. Larger arteries also feature an external elastic membrane in this layer.

The tunica externa, also known as the tunica adventitia, is the outermost layer. It is primarily composed of collagen fibers with some elastin. Its functions include anchoring the vessel to surrounding tissues, providing protection, and preventing overextension. In veins, the tunica externa is generally the thickest layer and blends with surrounding connective tissue to prevent vessels from "rolling."

The Arterial System and Resistance Vessels

Arteries transport blood away from the heart and are categorized into three main types based on their size and composition. Elastic arteries, known as conducting vessels, have a diameter greater than $10\,mm$ and contain more elastin than smooth muscle. These are found nearest the heart, such as the aorta and its immediate branches. Muscular arteries, or distribution vessels, range from $0.1\,mm$ to $10\,mm$ in diameter and contain more smooth muscle than elastin.

Arterioles, often called resistance vessels, have lumens smaller than $0.03\,mm$. They are primarily composed of smooth muscle with very little elastin and lead directly to the capillary beds. Their primary function is to regulate blood flow into the capillaries through constriction and dilation.

Capillaries and Microcirculation

Capillaries serve as the functional connection between the arterial and venous systems. They supply blood directly to tissues through perfusion and are the primary site of nutrient and waste exchange. Their walls consist of a single layer of simple squamous epithelium and a sparse basal lamina. There are three types of capillaries based on their degree of "leakiness":

1. Continuous capillaries are the most abundant and are found in almost all vascularized tissues. They have an uninterrupted endothelium with tight junctions that are often incomplete, creating intercellular clefts.
2. Fenestrated capillaries are found where rapid fluid exchange occurs, such as in the kidneys, small intestine, and endocrine organs. Their endothelium contains numerous pores or fenestrations that allow larger molecules to pass.
3. Sinusoid capillaries are found in the liver, bone marrow, and spleen. They have larger fenestrations and an incomplete basement membrane, allowing whole cells to leave circulation and enter tissues.

Microcirculation refers to the flow of blood through collateral arteries, arterioles, metarterioles, and capillary beds. Precapillary sphincters, which are rings of smooth muscle, open and close to regulate blood flow into specific tissue areas.

The Venous System and Capacitance

Veins are referred to as capacitance vessels or blood reservoirs because approximately $60\text{-}64\%$ of the total blood volume is located in the venous system at any given time. They transport blood toward the heart. Many veins, particularly in the limbs and inferior to the heart, contain valves formed from folds of the tunica intima to prevent the backflow of blood against gravity.

Venules drain capillary beds into veins and possess thin tunica media and externa layers. Veins have thin walls, large irregular lumens, and low blood pressure. They are easily distensible and contain smooth muscle in the tunica externa for venoconstriction, which helps return the venous reserve to the heart quickly. Specialized flattened veins with very thin walls are called venous sinuses, such as the coronary sinus and dural sinuses.

Physiology of Circulation: Flow, Pressure, and Resistance

Blood flow (F), or tissue perfusion, is the volume of blood flowing through a vessel, organ, or the entire circulation per unit of time, typically measured in $mL/min$. Cardiovascular centers in the brain adjust this flow based on physical activity and cardiac output. Blood pressure (BP) is the force per unit area exerted on a vessel wall by blood, measured clinically in $mmHg$.

Arterial blood pressure is measured as systolic and diastolic. Systolic pressure is the peak pressure during ventricular contraction, and diastolic pressure is the minimum pressure at the end of ventricular relaxation. The average clinical reading is $120/80\,mmHg$. Pulse pressure is the difference between these two values:

$Pulse\,Pressure = Systolic\,BP - Diastolic\,BP$

Mean Arterial Pressure (MAP) represents the average force driving blood into the vessels and can be approximated by the following formula:

$MAP = Diastolic\,BP + \frac{Systolic\,BP - Diastolic\,BP}{3}$

Normal MAP ranges from $70\text{-}110\,mmHg$; values below $60\,mmHg$ can lead to ischemia.

Variables Affecting Blood Flow and Pressure

Several physical factors determine blood pressure and resistance. Cardiac Output (CO) is calculated as:

$CO = Heart\,Rate \times Stroke\,Volume$

An increase in CO leads to an increase in BP. Compliance refers to the ability of a vessel to expand; increased compliance results in decreased BP. Blood volume is directly proportional to BP. Blood viscosity, or thickness, contributes to resistance; higher viscosity increases resistance and BP.

Vessel length and diameter also play critical roles. Vessel length is directly proportional to resistance: longer vessels have higher resistance. Vessel diameter is inversely proportional to resistance: increased diameter decreased resistance. Diameter is the most variable factor, regulated by neural and chemical signals triggering vasodilation or vasoconstriction.

Venous Return and Capillary Exchange Mechanisms

Two pumps assist in returning blood to the heart against gravity. The Skeletal Muscle Pump utilizes the contraction of muscles surrounding veins to push blood upward. The Respiratory Pump uses pressure changes in the thorax and abdomen during inhalation and exhalation to create a pressure gradient that drives blood toward the heart.

Capillary exchange involves the movement of gases, nutrients, and wastes. Small lipid-soluble molecules cross the membrane via simple diffusion, while larger or polar molecules (like glucose and ions) use facilitated diffusion through transport proteins or intercellular clefts. Bulk flow is managed by two competing pressures:

1. Capillary Hydrostatic Pressure (CHP) is the pressure exerted by blood that pushes fluid out of the vessel (filtration).
2. Blood Colloid Osmotic Pressure (BCOP) is the osmotic pressure exerted by plasma proteins that sucks fluid into the vessel (reabsorption).

Net Filtration Pressure (NFP) is calculated as:

$NFP = CHP - BCOP$

At the arterial end, $CHP (35\,mmHg) > BCOP (25\,mmHg)$, resulting in a net filtration of $+10\,mmHg$. At the venous end, $CHP (18\,mmHg) < BCOP (25\,mmHg)$, resulting in a net reabsorption of $-7\,mmHg$. Because filtration is always slightly greater than reabsorption, the lymphatic system is required to drain excess interstitial fluid.

Homeostatic Regulation and Clinical Considerations

The cardiovascular center in the medulla oblongata regulates vascular homeostasis using inputs from baroreceptors (stretch/pressure sensors) and chemoreceptors (sensors for $O_2$, $CO_2$, and $pH$). Endocrine regulation involves various hormones:

• Epinephrine and Norepinephrine: Increase BP.
• Anti-Diuretic Hormone (ADH): Increases water retention and BP.
• Renin-Angiotensin II-Aldosterone: Increases BP.
• Erythropoietin (EPO): Stimulates red blood cell production.
• Atrial Natriuretic Hormone (ANH): Promotes fluid loss to decrease BP.

Clinical conditions affecting this homeostasis include hypertension, hemorrhage, and various types of circulatory shock. These shocks include hypovolemic (low volume), cardiogenic (heart failure), vascular (excessive vasodilation), septic (infection), neurogenic (nerve damage), anaphylactic (allergic reaction), and obstructive (physical blockage).

Circulatory Pathways and Fetal Circulation

Blood travels through the pulmonary circuit starting from the right ventricle into the pulmonary trunk, which forks into two pulmonary arteries. Oxygenated blood returns via the pulmonary veins to the left atrium. The systemic circuit begins at the aorta, which has three regions: ascending aorta, aortic arch, and descending aorta (thoracic and abdominal).

Key arterial structures include the Circle of Willis (arterial circle) serving the brain, the carotid arteries serving the head and neck, and the hepatic portal system, which delivers blood from the digestive organs to the liver for processing.

Fetal circulation differs due to three shunts that bypass non-functional lungs and the liver:

1. Foramen ovale: Shunts blood from the right atrium to the left atrium.
2. Ductus arteriosus: Connects the pulmonary trunk to the aorta.
3. Ductus venosus: Links the umbilical vein to the inferior vena cava.