Circulatory Systems: Open vs. Closed

Open and Closed Circulatory Systems

• A circulatory system is composed of a circulatory fluid, interconnected vessels, and a muscular pump, i.e., the heart
• The circulatory system facilitates the exchange of gases and nutrients and eliminates wastes between cells and organs
• The two types of circulatory systems are open and closed

Open Circulatory Systems

• In insects, arthropods, and mollusks, hemolymph directly bathes organs
• This is referred to as an open circulatory system

Closed Circulatory Systems

• Blood is confined to vessels and distinct from interstitial fluid
• Annelids, cephalopods, and vertebrates have closed circulatory systems

Organization of Vertebrate Circulatory Systems

• Humans and other vertebrates posses a closed cardiovascular system
• Arteries, veins, and capillaries are the three main types of blood vessels
• Blood can only flow one way through the blood vessels

Types of Blood Vessels

• Arteries branch into arterioles, carrying blood away from the heart, to capillaries
• Networks of capillaries, called capillary beds, mediate chemical exchange between blood and interstitial fluid
• Blood is returned from capillaries to the heart via venules that converge into veins

Vertebrate Hearts

• Vertebrate hearts consist of two or more chambers
• Blood enters through atria and exits through ventricles

Single Circulation

• Bony fishes, rays, and sharks exhibit single circulation, with a two-chambered heart
• Blood passes through two capillary beds before returning in single circulation

Double Circulation

• Amphibians, reptiles, and mammals, blood is pumped separately to the right and left sides of the heart
• Amphibians possess double circulation

Pulmonary Circuit

• Oxygen-poor blood traveling through lungs to pick up oxygen in reptiles and mammals

Pulmocutaneous Circuit

• Oxygen-poor blood flows through a pulmocutaneous circuit to pick up oxygen through both the lungs and skin, in amphibians

Systemic Circuit

• Oxygen-rich blood is sent to deliver oxygen through the systemic circuit
• Double circulation leads to a higher blood pressure to organs versus single circulation

Evolutionary Variation in Double Circulation

• Some vertebrates exhibiting double circulation are intermittent breathers
• Amphibians and many reptiles may rely on gas exchange from the skin or another tissue without gas exchange for prolonged amounts of time

Amphibian Hearts

• Three-chambered heart: two atria and one ventricle
• Most oxygen-rich blood is diverted into the systemic circuit
• Most oxygen-poor blood is diverted into the pulmocutaneous circuit via a ridge in the ventricle
• Blood flow to the lungs in amphibians is almost completely shut off when underwater

Reptilian Hearts

• Turtles, snakes, and lizards all have a three-chambered heart
• The hearts have two atria and one ventricle partially divided by an incomplete septum
• Alligators, caimans, and other crocodilians have a septum which divides the ventricles
• The pulmonary and systemic circuits connect where the arteries exit the heart

Mammalian and Avian Hearts

• Mammals and birds breathe continuously and possess a four-chambered heart composed of two atria and ventricles
• The left side of the heart, only pumps and receives oxygen-rich blood
• The right side of the heart pumps and receives only oxygen-poor blood
• Mammals and birds are endotherms, needing more O2 than ectotherms

Mammalian Circulation

• Contraction of the right ventricle pumps blood to the lungs via the pulmonary arteries
• Blood flowing through capillary beds of the left and right lungs loads O2 and unloads CO2
• Oxygen-rich blood enters from the lungs back through the pulmonary veins to the left atrium of the heart
• Oxygen-rich blood flows into the left ventricle and is pumped out to body tissues via the systemic circuit
• Blood leaving the left ventricle travels through the aorta to the arteries throughout the body
• The first branches off the aorta, the coronary arteries, supply the heart muscle with blood
• Branches then lead to capillary beds in the head and forelimbs
• Further branches serve the abdominal organs and hind limbs with blood
• O2 diffuses from blood to tissues, and CO2 diffuses from tissues to blood
• Capillaries fuse to form venules, and blood makes its way to veins
• Oxygen-poor blood is channeled into the superior vena cava
• The inferior vena cava drains blood from the trunk and hind limbs
• The two venae cavae transport their blood into the right atrium
• Oxygen-poor blood flows to the right ventricle

The Mammalian Heart

• The human heart, about the size of a clenched fist, is cardiac muscle
• The two atria have relatively thin walls that act as collection chambers
• Thick ventricular walls contract with a lot of force

The Cardiac Cycle

• The heart contracts and relaxes rhythmically in the cardiac cycle
• Systole is the contraction/pumping phase
• Diastole is the relaxation/filling phase

Cardiac Output

• Cardiac output, dependent on heart rate and stroke volume, constitutes the volume of blood per minute into the systemic circulation
  • 72 beats x 70 ml = 5 liters
• Heart rate represents the number of beats per minute (normally 72 bpm)
• Stroke volume refers to the amount of blood pumped in a single contraction, normally 70ml

Heart Valves

• Four valves act prevent blood backflow in the heart
• The atrioventricular (AV) valves separates each atrium and ventricle
• The semilunar valves control blood flow to the aorta and the pulmonary artery
• The “lub-dup” heart beat sound is caused by blood recoil against the AV valves (lub) and semilunar (dup) valves
• Heart murmurs are caused by backflow of blood through defective valves

Maintaining the Heart’s Rhythmic Beat

• Cardiac muscle cells are autorhythmic and contract absent any nervous system signaling
• The sinoatrial (SA) node, i.e., pacemaker, governs cardiac muscle contraction rate and timing
• Located near where the superior vena cava joins the right atrium
• Muscle cells are electrically coupled through gap junctions

SA Node Impulses

• Impulses from the SA node move to the atrioventricular (AV) node in the wall between the right and left atria
• Here, 0.1 second of delay occurs and then the impulses travels to the Purkinje fibers
• This causes the ventricles to contract
• Cardiac cycle impulses can be recorded via an electrocardiogram (ECG/EKG)

Pacemaker Regulation

• The pacemaker is tuned by sympathetic and parasympathetic nervous system divisions
• The sympathetic division accelerates the pacemaker
• The parasympathetic division slows the pacemaker
• Hormones and temperature can also modify the pacemaker

Blood Vessel Structure and Function

• The vertebrate circulatory system relies on blood vessels that are related to their structure and function
• All blood vessels feature a central lumen lined with an epithelial layer, i.e., the endothelium
• The endothelium is smooth and minimizes flow resistance
• Capillaries facilitate material exchange because they are thin-walled and slightly wider than red blood cells

Arteries and Veins

• Arteries and veins consist of endothelium, smooth muscle, and connective tissue
• The arteries have thick, elastic walls to manage any high pressure blood being pumped from the heart
• Arteries protrude out because of the high blood pressure that occurs after every heart beat
• In thinner walled veins, blood flows back to the heart because of muscle action
• Veins have valves to ensure unidirectional blood flow, unlike arteries

Blood Flow Velocity

• Physical laws dictate the flow and pressure of blood
• Water remains uncompressed under pressure
• Blood flow velocity slows down in the capillary beds due to high resistance, and large total cross-sectional area, i.e., 500 times slower

Blood Pressure

• Blood flow stems from higher to lower pressure areas
• Blood pressure consists of a force exerted in all directions, including against the blood vessel walls
• Force applied sideways stretches the arterial walls
• It's the recoil of arterial walls that's thought to help keep blood pressure up
• The narrow width of tiny capillaries and arterioles helps dissipate some blood pressure

Blood Pressure During the Cardiac Cycle

• Systolic pressure represents the arterial pressure created through ventricular systole, indicating peak arterial pressure
• A pulse encompasses the rhythmic bulging of artery walls, which aligns with every heartbeat
• Diastolic pressure reflects arterial pressure during diastole, which is lower respective to systolic pressure

Blood Pressure Regulation

• Homeostatic mechanisms manage arterial blood pressure by adjusting arteriolar diameter
• Vasoconstriction, increasing blood pressure, occurs through contraction of smooth arteriolar muscle
• Vasodilation, decreasing blood pressure, occurs through relaxation of smooth arteriolar muscle

Factors that Affect Vasodilation and Vasoconstriction

• Nitric oxide (NO) promotes vasodilation
• Endothelin, a peptide, elevates vasoconstriction
• Vasodilation and vasoconstriction are linked to cardiac output changes that impact blood pressure
• Cardiac output increases and arterioles dilate

Blood Pressure and Gravity

• Blood pressure is typically measured in an artery in the arm at heart-level
• A healthy 20-year-old human has a blood pressure of 120 mm Hg during systole, and 70 mm Hg during diastole
• Blood pressure is vastly affected by gravity
• Brain blood pressure is 27 mm Hg lower when you stand up

Gravity Problems

• Fainting is typically triggered by inadequate blood flow to the head
• Giraffes need very high systolic pressure to maintain high blood flow against the force of gravity, i.e., 250 mm Hg
• One-way valves in veins prevent backflow of blood that’s caused by low blood pressure
• Blood return is aided by smooth muscle contraction of venules and contraction of skeletal muscles

Capillary Function

• Only 5–10% of body capillaries are actively conducting blood at any time
• Major organ capillaries are usually full (brain, heart, kidneys, liver)
• Blood supply differs across body locations like skin and digestive tract
• There are two mechanisms that regulate blood distribution throughout the capillary beds

Mechanisms for Capillary Bed Distribution

• Arteriolar constriction or dilation
• Precapillary sphincters function, which encircle smooth muscle that controls blood flow between arterioles and venules
• Regulatory factors include: nerve impulses, hormones, and local chemicals
• Histamine released from wounds causes vasodilation

Fluid Dynamics in Capillaries

• Exchange of substances between blood and interstitial fluid occurs across capillary walls
• This happens either through cells or microscopic pores
• Hydrostatic blood pressure pushes fluid out of capillaries
• Osmotic pressure from blood proteins pulls fluid back
• There is a net fluid loss by capillaries when averaged out

Lymphatic Return

• The lymphatic system is critical, as it enables the return of remaining leaked fluid from capillaries, equaling 4-8 liters per day
• Lymph constitutes the fluid lost by capillaries
• The lymphatic system helps drains fluids into the neck
• valves support the return flow of fluids to the heart

Edema

• Edema arises from swelling due to blocked lymph flow
• Lymph nodes function as lymphatic fluid filtration points, as well as having an important role in the body’s defense
• When the body combats an infection, lymph nodes can become swollen and tender

Blood

• Open circulation fluid is continuous with the fluid surrounding cells
• In contrast, closed circulatory systems of vertebrates employ a more specialized fluid known as blood

Blood composition

• The connective tissue, blood, features a liquid made 45% by cells and cell fragments in a liquid matrix known as plasma

Blood Plasma

• Blood plasma contains inorganic salts like electrolytes
• Blood pH and osmotic balance between the blood vessels, and interstitial fluid is maintained by blood protein in the plasma
• Certain blood proteins in plasma also aide with immunity, blood clotting, and lipid transport
• Plasma concentrations are similar to interstitial fluid but high in protein

Cellular Elements

• The two types of cells suspended in blood plasma are the red and white blood cells, i.e., erythrocytes and leukocytes
• Platelets are cell fragments that participate in clotting

Erythrocytes

• Red blood cells are the most abundant in blood
• Erythrocytes are disk-shaped
• Hemoglobin, an iron-containing protein, is contained in red blood cells and this transports O2
• Each hemoglobin molecule binds 4 O2 molecules
• Mammalian erythrocytes lack both the nuclei and mitochondria

Sickle-Cell Disease

• Hemoglobin proteins leading to aggregates
• Aggregates deform what erythrocytes remain into a sickle shape
• Sickled cells may stop blood from flowing

Leukocytes

• Five types of white blood cells exist, termed leukocytes
• Leukocytes work for defense mechanisms either by phagocytosis, or by mounting against foreign substances
• Leukocytes are located both inside and outside the circulatory system

Platelets

• Blood clotting is provided by platelets

Stem Cells and Cellular Replacement

• Erythrocytes, leukocytes, and platelets arise stem cells in bone marrow (i.e., ribs, vertebrate, sternum, pelvis)
• O2 delivery levels influence the production of erythropoietin (EPO)
• Recombinant EPO can be therapeutically introduced to resolve anemia

Blood Clotting

• Fluid blood is converted into solid blood via coagulation
• Inactive fibrinogen forms a clot by converting into fibrin through a cascade of complex reactions
• This blood clot forms into thrombus, which blocks blood flow

Cardiovascular Disease

• Arises from heart and blood vessel disorders
• Includes vein/heart function with life-threatening disruptions of blood flow to the heart/brain

Atherosclerosis

• Occurs when plaques form in arteries, stemming from fatty deposits
• Cholesterol concentration helps in development of atherosclerosis

Role of Lipoproteins

• Cholesterol is supplied for membrane production through low-density lipoproteins (LDL)
• Excess cholesterol is removed through high-density lipoprotein (HDL)
• Elevated LDL relative to HDL ratio suggests a high risk of heart disease
• Inflammation caused by leukocytes taking up lipid plaques

Disruption of Blood Supply

• A thrombus, caused by plaque rupture, leads to stroke or heart attack/myocardial infarction
• Stroke is the death of nervous tissue in the brain due to rupture or blockage of brain arteries
• Angina pectoris arises from chest pain caused by blockage of coronary arteries

Treatment for Cardiovascular Disease

• A high LDL/HDL poses a high risk for cardiovascular disease
• Reduction of LDL can be facilitated by exercise, reduced smoking, and avoiding trans fats
• LDL can be therapeutically reduced via the use of statins
• Aspirin inhibits inflammation which can disrupt blood function
• Hypertension, or high blood pressure, i.e., >140 mm Hg and diastolic pressure >90 mm Hg, can contribute to heart attacks/stroke with endothelial lining damaged by plaques
• Improved dietary changes, exercise, and medication help treat hypertension