Pulmonary Circulation

Cardiovascular System

• The cardiovascular system consists of blood vessels, heart, and circulation.

Blood Vessel Structure and Function

• Arteries:
  • Thicker walls, narrower lumen
  • Carry oxygenated blood away from heart
  • Divide into smaller arterioles
• Veins:
  • Larger lumen, thinner walls
  • Carry deoxygenated blood back to heart
  • Have valves to prevent backflow
• Capillaries:
  • Composed of endothelium (very thin cells)
  • Allow for easy diffusion of substances
  • Connected to arterioles and venules in networks
  • Three types: continuous, fenestrated, and sinusoids

Capillary Exchange

• Movement of substances between blood and interstitial fluid
• Three methods:
  1. Diffusion:
     • Most important method
     • Substances move down their concentration gradient
     • Oxygen and nutrients move from blood to interstitial fluid to body cells
     • Carbon dioxide and wastes move from body cells to interstitial fluid to blood
  2. Transcytosis:
     • Small quantity of material
     • Substances in blood plasma become enclosed in pinocytotic vesicles
     • Important for large, lipid-insoluble molecules
  3. Bulk flow:
     • Passive process
     • Move together in the same direction
     • Based on pressure gradient
     • Filtration and reabsorption occur in capillaries

Venous Return

• Volume of blood flowing back to heart through systemic veins
• Occurs due to pressure generated by constriction of left ventricle
• Aided by:
  1. Skeletal muscle pump
  2. Respiratory pump

Skeletal Muscle Pump

• Proximal and distal valves
• Contraction of muscle:
  • Pushes blood through valves
  • Closes distal valve
  • Opens proximal valve
• Relaxation of muscle:
  • Opens distal valve
  • Closes proximal valve
  • Fills with blood from foot

Respiratory Pump

• Diaphragm movement:
  • Inhalation: diaphragm moves downward, decreases thoracic pressure, and increases abdominal pressure
  • Exhalation: diaphragm moves upward, increases thoracic pressure, and decreases abdominal pressure
• Suction of the heart:
  • Contraction of the heart during systole
  • Increases atrial cavity
  • Atrioventricular valve pulled downward
  • Facilitates suction of blood from superior and inferior vena cava

Blood Flow through Blood Vessels

• Cardiac output (CO) = stroke volume (SV) x heart rate (HR)
• Blood pressure (BP) = pressure exerted by blood on walls of blood vessels
• Resistance: opposition to blood flow due to friction between blood and walls of blood vessels

Regulation of Blood Pressure

• Interconnected negative feedback systems
• Control blood pressure by adjusting heart rate, stroke volume, systemic vascular resistance, and blood volume
• Factors that increase blood pressure:
  • Increased heart rate
  • Increased stroke volume
  • Increased systemic vascular resistance
  • Increased blood volume

Role of Cardiovascular Center

• Location: medulla oblongata
• Function: helps regulate heart rate and stroke volume, controls neural, hormonal, and local negative feedback systems that regulate blood pressure and blood flow to specific tissues

Neural Regulation of Blood Pressure

• Reflexes:
  • Baroreceptors: monitor pressure changes and stretch in blood vessel walls
  • Proprioceptors: monitor movements of joints and muscles
  • Chemoreceptors: monitor concentration of various chemicals in the blood
• Output from cardiovascular centre flows along neurons of autonomic nervous system (ANS)
• Sympathetic (stimulatory) opposes parasympathetic (inhibitory)

Hormonal Regulation of Blood Pressure

• Hormones:
  • Renin-angiotensin-aldosterone (RAA) system
  • Epinephrine and norepinephrine
  • Antidiuretic hormone (ADH)
  • Atrial natriuretic peptide (ANP)
• Actions:
  • Help regulate blood pressure and blood flow
  • Alter cardiac output
  • Change systemic vascular resistance
  • Adjust total blood volume

Renin-Angiotensin-Aldosterone (RAA) System

• Renin: released by kidney when blood volume falls or blood flow decreases
• Angiotensin converting enzyme (ACE): acts on substrates to produce active hormone angiotensin II
• Angiotensin II: raises blood pressure by vasoconstriction and secretion of aldosterone
• Aldosterone: increases water reabsorption in kidneys to raise blood volume and pressure

Epinephrine and Norepinephrine

• Released by adrenal medulla in response to sympathetic stimulation
• Increase cardiac output by increasing rate and force of heart contractions
• Raise blood pressure

Antidiuretic Hormone (ADH) or Vasopressin

• Released by cells of atria
• Lowers blood pressure by causing vasodilation and promoting loss of salt and water in urine
• Reduces blood volume

Atrial Natriuretic Peptide (ANP)

• Released by cells of atria
• Lowers blood pressure by causing vasodilation and promoting loss of salt and water in urine
• Reduces blood volume

Circulatory Routes

• Circulation of blood through the body

• Includes the heart, arteries, capillaries, and veins### Hemodynamics: Factors Affecting Blood Flow

• Blood flow is the volume of blood that flows through a tissue in a given period of time (in mL/min).

• Total blood flow equals cardiac output (CO), which is the volume of blood that circulates through systemic or pulmonary blood vessels each minute.

• CO = heart rate (HR) x stroke volume (SV).

• Distribution of CO depends on:

  • Pressure differences that drive blood through tissue (flows from higher to lower pressure).
  • Resistance to blood flow in specific blood vessels (higher resistance means smaller blood flow).

Circulatory Routes

• There are two main routes: systemic and pulmonary circulation.
• Systemic circulation:
  • Oxygenated blood travels from the heart throughout the body, deoxygenating as it goes.
  • All systemic arteries branch from the aorta.
  • All systemic veins empty into the superior vena cava, inferior vena cava, or the coronary sinus.
  • Deoxygenated blood returns to the heart.
• Subdivisions of systemic circulation:
  • Coronary circulation
  • Hepatic portal circulation
• Other circulatory routes:
  • Pulmonary circulation (blood flow to the lungs for gas exchange)
  • Cerebral circulation (route to the brain)
  • Fetal circulation (between the developing fetus and mother)

The Anatomy of Blood Vessels

• Arteries and veins have three layers:
  • Tunica intima (innermost layer)
  • Tunica media (middle layer)
  • Tunica adventitia (outer layer)
• Lumen is the blood vessel cavity.
• Arteries are thicker and stronger than veins.
• Arterioles are small arteries that attach to capillaries.
• Capillaries allow for gas, nutrient, and waste exchange.
• Venules connect capillaries to veins.
• Veins are less elastic than arteries and contain valves.

Major Arteries and Veins

• Aorta and its branches:
  • Ascending aorta
  • Aortic arch
  • Thoracic aorta
  • Abdominal aorta
• Circulatory routes:
  • Aorta + branches
  • Arch of aorta
  • Pelvis and lower limbs
  • Principle veins

Pulmonary Circulation

• Carries blood from the right side of the heart to the lungs for gas exchange.
• Route:
  • Right ventricle ! Pulmonary trunk ! R+L pulmonary arteries ! Both lungs ! Carry “blue blood” low in O2 and high in CO2
  • Pulmonary capillaries: gas exchange ! R and L pulmonary veins ! L atrium
  • Carry “red blood” (high in O2 and low in CO2)

Hepatic Portal Circulation

• Route travels back from the intestine of the digestive tract to the liver.
• To store excess sugars from digestion into liver after a meal and to release stored sugar as glycogen between meals to maintain glucose levels.
• Portal vein: transports blood from one organ’s capillary bed to another.
• GI organs ! Splenic and superior mesenteric veins ! Hepatic portal vein (“blue blood”) ! Sinusoids (“leaky capillaries” in liver) ! Mixes “blue blood” with “red blood”
• Hepatic vein ! Inferior vena cava (IVC) ! Heart

Fetal Circulation

• Exists only between the developing fetus and mother.
• Specialized for exchange of materials with maternal blood and bypass of lungs.
• Exchange in placenta ! Umbilical vein
• ! Ductus venosus (bypasses liver)
• ! Inferior vena cava ! R atrium (mixes with deoxygenated blood from lower body)
• ! Foramen ovale ! L atrium

Changes at Birth

• Umbilical arteries ! Medial umbilical ligaments
• Umbilical vein ! Ligamentum teres
• Ductus venosus ! Ligamentum venosum
• Placenta expelled after birth
• Foramen ovalis closes ! Fossa ovalis
• Ductus arteriosus ! Ligamentum arteriosum

Aging

• Stiffening of aorta
• Loss of cardiac muscle strength
• Reduced cardiac output (CO) and increased systolic pressure
• Higher risk for:
  • Coronary artery disease (CAD)
  • Congestive heart failure (CHF)
  • Atherosclerosis

Checking Circulation: Pulse

• Pulse in arteries = heart rate (HR)
• Sites used:
  • Radial artery (thumb side of wrist)
  • Carotid artery (neck)
  • Brachial artery (arm)
• Tachycardia: rapid resting HR (>100 bpm)
• Bradycardia: slow resting HR (<60 bpm)