Cardiovascular System: Blood Vessels and Circulation

Blood Vessel Structure and Function

Types of Blood Vessels

The cardiovascular system consists of three main types of blood vessels, each with distinct roles in circulation:

• Arteries: Always carry blood away from the heart.

• Veins: Always carry blood back to the heart.

• Capillaries: Serve as the site of exchange between blood and tissues.

• Most cells are within 0.1 mm of a capillary, except for cartilage.

• O2 and nutrients are deposited in the interstitial fluid; CO2 and waste products are removed.

Vessel Wall Anatomy

Layers of Blood Vessel Walls

Blood vessels are composed of three main layers, each contributing to their function:

• Tunica intima (interna): Consists of endothelium and elastic fibers; only layer in direct contact with blood.

• Tunica media: Thickest layer; contains elastic fibers (providing high compliance) and smooth muscle (enabling contraction).

• Tunica externa: Anchors vessel to surrounding tissue; made of loose connective tissue with collagen.

• Elasticity: Vessels stretch easily and resist tearing.

• Contractility: Vessels can shorten to return to original length.

• Pressure reservoir: Arteries act as pressure reservoirs, maintaining blood flow during cardiac cycles.

Arteries

Classification of Arteries

Arteries are classified based on their size and function:

• Elastic arteries: Large vessels near the heart; tunica media is mostly elastic fibers, which dampen pressure oscillations.

• Muscular arteries: Majority of named arteries; tunica media is mostly smooth muscle, allowing control of blood distribution and maintenance of blood pressure.

• Arterioles: Small arteries; regulate blood flow into capillary beds via dilation and constriction. Precapillary sphincters (circular muscles) can constrict to prevent blood flow into capillary beds.

Capillaries

Types and Structure of Capillaries

Capillaries are specialized for exchange and vary in structure:

• Vascular shunt/thoroughfare channel: Directs blood from arterioles to venules.

• True capillaries: Primary site of exchange.

• Tight junctions: Regulate permeability.

• Continuous capillaries: Found in skin, lungs, muscles, and CNS; least permeable.

• Fenestrated capillaries: Areas of filtration, absorption, and endocrine hormone secretion; have pores (fenestrations) for greater exchange.

• Sinusoid capillaries: Located in liver, spleen, and bone marrow; have large lumens and incomplete basement membranes, allowing passage of large molecules and cells.

Veins

Structure and Function of Veins

Veins return blood to the heart and have unique features:

• Venules: Small veins.

• Veins: Have the same tunics as arteries, but with larger lumens and valves to prevent backflow.

• Venous sinuses: (e.g., coronary and dural venous sinuses) consist only of endothelium, supported by surrounding tissue.

• Anastomoses: Most organs receive blood from more than one artery, providing collateral circulation; venous anastomoses are more common.

• Varicose veins: Caused by incompetent, leaky valves.

Physiology of Circulation

Blood Flow and Resistance

Blood flow is the volume of blood moving through vessels, organs, or the entire circulation per unit time. It is determined by pressure differences and resistance:

• Blood flow: $\text{Blood flow} = \frac{\text{Pressure difference}}{\text{Resistance}}$

• Total blood flow: Equals cardiac output; varies among organs.

• Resistance: Opposition to blood flow, measured as friction within vessels. Influenced by blood viscosity, vessel length, and vessel diameter (arterioles are main contributors to peripheral resistance).

• Flow equation: $\text{Flow} = \frac{\text{Blood pressure}}{\text{Resistance}}$

Blood Pressure

Blood pressure is the force exerted on vessel walls by blood. It drives blood through tissues and is measured in mmHg. Blood moves from high to low pressure:

• Greater pressure differences increase flow; greater friction decreases flow.

• Typical values: aorta (110 mmHg), arterioles (35 mmHg), venules (16 mmHg), vena cava (0 mmHg).

• Arterial blood pressure depends on compliance of elastic arteries and blood volume.

• Systolic pressure: Pressure during heart contraction.

• Diastolic pressure: Pressure during heart relaxation.

• Pulse pressure: $\text{Pulse pressure} = \text{Systolic} - \text{Diastolic pressure}$

• Mean arterial pressure (MAP): Average pressure in arteries; propels blood to tissues.

  • $\text{MAP} = \frac{2 \times \text{Diastolic pressure} + \text{Systolic pressure}}{3}$

• Capillary blood pressure: 35 mmHg entering, 17 mmHg exiting; low pressure allows filtration.

• Venous blood pressure: Too low for adequate return; aided by skeletal muscle pump, respiratory pump, and sympathetic venoconstriction.

Blood Pressure Regulation

Variables Affecting Blood Pressure

Blood pressure is regulated by several variables:

• Cardiac output: Increased CO or resistance increases pressure; variables compensate to maintain constant MAP.

• Peripheral resistance: Decreased vessel diameter, increased blood viscosity, and increased vessel length raise resistance.

• Blood volume: Changes in blood volume affect pressure.

Regulatory Mechanisms

• Neural controls: Glossopharyngeal and vagus nerves detect vessel stretching and adjust diameter. The cardiovascular center in the medulla regulates cardiac and vasomotor activity. Baroreceptor reflexes (carotid sinus, aortic arch) adjust MAP via vasoconstriction/dilation. Chemoreceptor reflexes respond to CO2, pH, and O2 changes. Higher brain centers (cortex, hypothalamus) can override medulla signals.

• Short-term hormonal controls:

  • Adrenal medulla hormones (epinephrine, norepinephrine) increase CO and vasoconstriction; vasodilation in skeletal/cardiac muscle.

  • RAA system (renin, angiotensin, aldosterone) increases blood pressure.

  • Atrial natriuretic peptide (ANP) decreases blood volume and BP via vasodilation.

  • Antidiuretic hormone (ADH) and erythropoietin also influence BP.

• Long-term regulation: Renal mechanisms maintain blood volume near 5L. Increased blood volume raises BP (salt retention); decreased volume lowers BP (dehydration). Kidneys filter more with increased volume and reabsorb more with decreased volume.

• Intrinsic controls: Myogenic response (smooth muscle contracts when stretched), local decreases in O2 or increases in CO2/H+ increase blood flow. Vasodilating chemicals (K+, H+, lactic acid, adenosine, NO) and vasoconstricting chemicals (prostaglandins) regulate flow. Injury causes initial decrease, then increase in flow.

Homeostatic Imbalances

Hypertension

Hypertension is chronically elevated blood pressure, often asymptomatic for years. It strains the heart and damages arteries, leading to heart failure, stroke, and arterial disease. Causes include heredity, diet (high Na+, Ca++, fat; low K+, Mg++), obesity, age, diabetes, stress, smoking, and certain drugs (diuretics, ACE inhibitors). Secondary hypertension results from other conditions (kidney disease, endocrine disorders).

Hypotension

Low blood pressure is usually not concerning unless it reduces blood flow to tissues. Causes include orthostatic hypotension, aging, hypothyroidism, or severe malnutrition.

Circulatory Shock

Occurs when blood vessels lose blood, reducing flow and causing organ damage:

• Hypovolemic shock: Due to large blood/fluid loss (hemorrhage, vomiting, diarrhea, burns).

• Vascular shock: Normal volume, but excessive vasodilation (e.g., anaphylaxis from histamine).

• Cardiogenic shock: Heart too weak to pump blood.

Other Imbalances

• Plasma protein changes: Increased or decreased protein levels affect fluid balance.

• Edema: Swelling due to fluid accumulation, seen in prolonged standing, pregnancy, or obesity.

Circulatory Routes

Major Circulatory Pathways

Blood follows distinct routes through the body:

• Systemic: Heart → arteries → body capillaries → veins → right atrium.

• Hepatic portal: Heart → arteries → digestive organ capillaries → splenic/superior mesenteric vein → hepatic portal vein → liver capillaries → hepatic vein → right atrium. This route filters harmful substances before systemic circulation.

• Coronary: Left coronary artery → anterior interventricular artery (feeds anterior ventricles), circumflex artery (feeds left ventricle/atrium); right coronary artery → marginal artery (feeds right ventricle), posterior interventricular artery (feeds posterior ventricles).

• Fetal circulation: Fetal internal iliac arteries → umbilical arteries → placenta → capillary bed → umbilical vein → hepatic portal vein → liver → hepatic vein or ductus venosus → inferior vena cava. Special features include foramen ovale and ductus arteriosus. At birth, these structures close and become ligaments.

Table: Types of Blood Vessels and Their Features

Vessel Type	Main Function	Wall Structure	Special Features
Artery	Carry blood away from heart	Thick tunica media (elastic/smooth muscle)	High pressure, pressure reservoir
Vein	Carry blood to heart	Thin walls, large lumen, valves	Low pressure, valves prevent backflow
Capillary	Exchange of gases/nutrients	Single layer endothelium	Permeability varies (continuous, fenestrated, sinusoid)

Example: Regulation of Blood Pressure

During exercise, sympathetic stimulation increases heart rate and contractility, raising systolic pressure. Vasodilation in skeletal muscles ensures adequate blood flow, while vasoconstriction in other areas maintains MAP.

Additional info: The notes have been expanded to include definitions, examples, and equations for clarity and completeness. The table summarizes vessel types and their features for comparison.