Human Physiology: Blood Pressure and Urinary System

Cardiovascular System II: The Blood Vessels

• Blood vessels transport blood to tissues for gas, nutrient, and waste exchange, returning it back to the heart.
• Blood vessels regulate blood flow to tissues.
• Blood vessels control blood pressure.
• Blood vessels secrete various chemicals.

Overview of Arteries and Veins

• The cardiovascular system is composed of arteries, capillaries, and veins.
• Arteries are the distribution system; as they move away from the heart, they branch into progressively smaller diameters, supplying the body's tissues with blood.
• Capillaries—the exchange system—are very small-diameter vessels forming branching networks (capillary beds). Gases, nutrients, wastes, and molecules are quickly exchanged between cells and blood via capillary walls.
• Veins are the collection system, draining blood from capillary beds and returning it to the heart. They follow an opposite pattern to arteries, with smaller veins merging into progressively larger vessels toward the heart.

Structure and Function of Arteries and Veins

• All blood vessels are tubular organs with a central lumen surrounded by tissue layers (tunics).
• Three tunics of the blood vessel wall are: tunica intima, tunica media, and tunica externa.
• Most arteries have thicker tunica media than veins, reflecting their role in blood pressure and flow control.
• Internal and external elastic laminae are more extensive in arteries than in veins, reflecting higher pressure.

Table 18.1 Types of Arteries and Veins

• Elastic arteries (e.g., aorta): large diameter; extensive elastic laminae; conduct high-pressure blood to organs.
• Muscular arteries (e.g., brachial artery): thick tunica media; control blood flow to organs; regulate blood pressure.
• Arterioles (e.g., terminal arterioles): thin walls, smooth muscle cells; control blood flow to tissues; feed capillary beds.
• Venules: thin walls, little smooth muscle; drain capillary beds.
• Veins: thin walls, large lumen, little smooth muscle, valves; return blood to the heart.

Atherosclerosis

• Atherosclerosis is a leading cause of death in developed countries.
• It affects large and medium-sized muscular arteries.
• Characterized by the formation of atherosclerotic plaques (buildups of lipids, cholesterol, calcium salts, and cellular debris within the arterial intima).
• Plaques tend to form in regions where blood flow changes in velocity or direction.
• Treatment focuses on reducing factors that injure endothelium (e.g., dietary modification, physical activity, cholesterol management, blood glucose control, smoking cessation, and blood pressure management).

Physiology of Blood Flow

Introduction to Hemodynamics

• Blood pressure is the outward force exerted by blood on blood vessel walls.
• Expressed in millimeters of mercury (mm Hg).
• Varies significantly in different parts of the vasculature, highest in large systemic arteries and lowest in large systemic veins.

Factors That Determine Blood Pressure

• Peripheral resistance: any factor hindering blood flow within the vasculature.

• Vessels near the heart contribute little to overall resistance; resistance is greatest further from the heart (in the body's periphery).

• Peripheral resistance increases blood pressure.

• Blood vessel radius inversely affects peripheral resistance.

  • Increased radius results in decreased resistance.

• Blood viscosity is the liquid's inherent resistance to flow; blood has relatively high viscosity due to its protein and cell content.

• Blood vessel length has a direct relationship to peripheral resistance: Longer vessels have higher resistance.

• Cardiac output: product of stroke volume (amount of blood pumped with each heartbeat) and heart rate (number of beats per minute).

• When cardiac output increases, blood pressure increases.

• The relationship between blood pressure (AP), cardiac output (CO), and peripheral resistance (PR) is: AP = CO × PR

• Blood volume is the total volume of blood.

• Increased blood volume increases blood pressure.

Blood Pressure in Different Portions of the Circulation

• Blood pressure is fairly low in pulmonary circulation.
• Blood pressure changes significantly in systemic circulation.
  • Averages around 15 mm Hg in pulmonary and 95 mm Hg in systemic circuits (varies based on location in circuit).
• Systemic arterial pressure—pressure profile of entire systemic circuit is highest in the aorta, decreasing slightly as it reaches muscular arteries.
• Heart pulsates, generating a pressure gradient that rises during ventricular systole and declines during diastole; leading to two separate pressures in arteries.
  • Systolic pressure—pressure in arteries during ventricular contraction.
  • Diastolic pressure—pressure in arteries during ventricular relaxation.
  • Pulse pressure—difference between systolic and diastolic pressure.
• Mean arterial pressure (MAP) is the average pressure in the arterial circuit.
• Venous return is assisted by venous valves and skeletal muscle pumps.

Varicose Veins

• Varicose veins are a common condition characterized by dilated, bulging, and hardened veins, typically in the lower limbs.
• Conditions that decrease venous return increase blood pooling in lower limb veins.
• Lower limb superficial veins lack support from skeletal muscle pumps, leading to enlarged regions visible beneath the skin.

Maintenance of Blood Pressure

Short-Term Maintenance of Blood Pressure

• Mean arterial pressure (MAP) is maintained around 95 mm Hg.
• Nervous system (e.g., sympathetic and parasympathetic responses):
  • Sympathetic stimulation: increases heart rate, contractility, and vasoconstriction, increasing blood pressure.
  • Parasympathetic stimulation: decreases heart rate, decreasing blood pressure.
• Baroreceptor reflex: negative feedback loop responding to changes in blood pressure; stimulates appropriate nervous system responses.
• Peripheral and central chemoreceptors affect blood pressure and breathing regulation.
• Endocrine system:
  • Hormones (e.g., epinephrine, norepinephrine, angiotensin-II, atrial natriuretic peptide) influence blood pressure regulation.

Long-Term Maintenance of Blood Pressure

• Urinary system regulates blood volume by controlling the amount of water lost as urine.
• Endocrine system regulates blood volume by controlling amounts of hormones.

Capillaries and Tissue Perfusion

• Capillaries form clusters (capillary beds), facilitating exchange between blood and tissue cells.
• Blood flow to tissues is called tissue perfusion; it's tightly regulated to meet metabolic needs.

Capillary Structure and Function

• Capillary walls are extremely thin (0.2 μm).
• Capillaries are made of endothelial cells joined by tight junctions.
• Materials (nutrients, gases, ions, wastes) exchange between blood in capillaries and tissue cells by diffusion, and transcytosis.
• Types of capillaries include continuous (e.g., skin), fenestrated (e.g., kidneys), and sinusoidal (e.g., liver).

Blood Flow through Capillary Beds

• Microcirculation is the blood flow within capillary beds; it involves true capillaries (where material exchange occurs) and a central vessel.
• Precapillary sphincters control blood flow into capillaries.

Tissue Perfusion in Special Circuits

• Each organ has unique blood flow and nutrient demands.
• Metabolic controls (oxygen concentration in interstitial fluid) regulate blood flow (e.g., in the heart).
• The brain is extremely sensitive to ischemia (decreased blood flow); maintains nearly constant blood flow.
• Skeletal muscle blood flow substantially increases during exercise.
• Skin regulates blood flow depending on temperature.

Capillary Pressures and Water Movement

• Movement of water across capillaries is driven by filtration (fluid movement due to force or gravity).
  • Hydrostatic pressure (HP): outward pressure of blood which pushes water out of the capillary.
  • Osmotic pressure (OP): inward pressure caused by solutes, drawing water into the capillary.
• Edema is the excessive accumulation of fluid in interstitial spaces.

Anatomy of the Systemic Arteries

• The aorta is the major systemic artery.
• Aorta branches into four divisions.
  • Ascending aorta: supplies the heart with blood.
  • Aortic arch.
  • Descending thoracic aorta.
  • Descending abdominal aorta.
• Arteries of the head and neck, upper limbs, and lower limbs.
• Circle of Willis: circular anastomosis of arteries of the brain.

Cerebrovascular Accident (CVA)

• CVA is damage to the brain caused by disruption of blood flow.
• Fourth most common cause of death in the US.
• Causes: blockage of brain arteries due to clots or hemorrhage (ruptured cerebral artery).
• Symptoms: may affect one or both sides of the body (paralysis, vision loss, speech difficulty, headaches).
• Risk factors: hypertension, atherosclerosis, diabetes, cigarette smoking, high cholesterol, atrial fibrillation.
• Treatment: clot-dissolving medications, surgery, depending on the cause.

Pulse Points

• Pulse is the rhythmic expansion and recoil of arteries with each heartbeat.
• Pulse points are superficial arteries where the pulse can be felt.

Anatomy of the Systemic Veins

• Deoxygenated blood returns to the heart through systemic veins.
• Superior vena cava: collects blood from upper body, head, neck, and thorax.
• Inferior vena cava: collects blood from lower body.
• Hepatic portal system: specialized system for nutrient and chemical monitoring by the liver.

Drugs and the Hepatic Portal System

• Drugs are metabolized by the liver via the hepatic portal system.
• This process impacts drug dosage and effectiveness.
• Some drugs are extensively metabolized, so injection is necessary to avoid hepatic metabolism.

Vein Grafting

• Vein grafting is a surgical procedure to bypass blocked arteries with veins.
• The great saphenous vein is commonly used in these procedures.

The Big Picture of Blood Vessel Anatomy

• Diagram illustrating systemic blood flow through the body's major arteries and veins.