Block E

• Cardiovascular system


  Responsible for the transport of nutrients and other materials, via the blood, to and from various parts of the body
• Cardiovascular system function


  Blood is continuously circulated, which is achieved by the heart (pump) and blood vessels (system of tubes)
• Closed cardiovascular system


  • Blood is contained within the vessels and is physically separated from interstitial fluid
  • The blood and interstitial fluid differ in components and solute composition
• Pulmonary circuit


  1. Deoxygenated blood returns to the heart via the vena cava, and blood flows into the right atrium, through the tricuspid valve into the right ventricle, then flows through the pulmonic valve into the pulmonary artery before being delivered to the lungs

  2. In the lungs, blood diverges into the numerous pulmonary capillaries where gas exchange occurs
• Systemic circuit


  Oxygenated blood is carried from the left ventricle, through the arteries, to the capillaries in the tissues of the body
• Open circulatory system


  • Fluid that is pumped by 1 or more contractile hearts into the body cavity (hemocoel) of an animal
  • The fluid in the blood vessels and interstitial fluids that surround cells mingle in 1 large body compartment (hemolymph)
  • Nutrients and waste are exchanged by diffusion between hemolymph and body cells before returning to the heart through vessels/ostia
  • O2 and CO2 aren't transported in this system
  • Hemolymph is emptied in bulk into hemocoel, it doesn't allow selective delivery to regions with high metabolic requirements
• Blood distribution at rest

  • Some organs (kidney, liver, GI tract) receive blood in excess of their metabolic needs
  • The brain can least tolerate a disrupted blood supply. Irreparable damage occurs if blood supply is disrupted >4 mins
  • Neurons are susceptible to insufficient nutrients
• Blood


  A fluid connective tissue that communicates information between places
• Components of blood


  • Plasma
  • Leucocytes
  • Erythrocytes
  • Platelets
• Plasma


  • 30-60% of total blood volume
  • Water, containing organic and inorganic nutrients (95%)
  • Dissolved O2
  • Waste products of metabolism (CO2, urea)
  • Hormones (steroid/peptide - endocrine system)
  • Proteins (important for blood clotting and it promotes movement of liquid back into cells - osmoregulation)
  • pH buffers
• Leucocytes


  • Mainly white blood cells that develop from specialised connective tissues (marrow) of certain bones
  • They function to defend the body against infection and disease. They make up the majority of the immune system
• Erythrocytes


  • Haematocrit refers the to volume of blood that is composed of RBC (40-65%)
  • There's an average of ~250 mil haemoglobin molecules, and each molecule contains 4 protein subunits (2 𝛼- and 2 𝛽-chains). Each protein subunit contains 1 heme molecule that binds to 1 O2 molecule
  • In total, each haemoglobin can bind to 4 O2 molecules
  • Biconcave (~6.8-7.2 μm) in structure to increase surface area, which increases the efficiency of gas exchange
  • RBC is needed because the amount of O2 dissolved in the plasma is insufficient to support a vertebrates basal metabolic rate, let alone strenuous activity
  • O2 is poorly soluble in plasma
  • Capillaries are the site of gas and nutrient exchange, so it's important that they are the width of a RBC (smallest) and one cell thick
• Platelets


  • Derived from bone marrow - cell fragments without a nucleus
  • Known as thrombocytes in other vertebrates
  • It plays a crucial role in the formation of blood clots
  • When a blood vessel is injured, platelets secrete a substance that causes them to clump together and bind to the collagen fibres in the surrounding connective tissue at the wound (plug), to prevent blood loss
  • Other platelet secretions interact with plasma proteins, the precipitate from the solution of fibrous protein fibrin form a meshwork of threadlike-fibres that wrap between platelets and RBC
• Heart anatomy


  1. Blood pathway: vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta

  2. The contraction of the atria and ventricles make a heartbeat - "lub-DUB" sound

  3. 2 phases of heart pumping cycle - systole, when the heart contracts and pushes blood out of its chambers; and diastole, the period between contractions when the heart (myocardium) relaxes and the chambers fill with blood
• Cardiac muscle


  • Rectangular - 20μm
  • When cellular [Ca2+] increases, it signals contraction in the cardiac, smooth and skeletal muscles
• Human heart


  • Myogenic, meaning it has signalling mechanisms (stimulus originate within myocardium) that initiates heart contractions, and is not reliant on neural stimulation
  • Myogenic initiation is due to excitation waves (electrical impulse/action potentials) that are conducted by specialised cardiac muscle cells that travel along the myocardium layer of the heart
• Cardiac conduction system


  1. Sinoatrial node (SAN) - specialised patch of muscle near the opening of the vena cava, in the right atrial wall that initiates the excitation wave that acts as the stimulus for heart contraction

  2. The excitation wave travels across the atrial walls at 1ms-1, causing them to contract immediately, almost simultaneously (atrial systole)

  3. There's a fibrous ring (non-conducting tissue) that prevents the flow of excitation waves from atria to ventricles

  4. Atrioventricular node (AVN) - specialised patch of muscle in the septum of the atrioventricular wall that receives the excitation wave from the SAN (0.1s delay - allowing atria to empty and for ventricles to fill, preventing overlap of atrial and ventricular systole)

  5. Atrioventricular bundle - Purkinje tissue that connects the atrium and ventricle, running along the septum

  6. Bundle of His - continuation of atrioventricular bundle, found in the septum, it transmits the electrical impulse from the AV bundle to the Purkinje fibres in the ventricles

  7. Purkinje fibres - specialised muscle fibres that carry excitation wave from AV node, down the septum, and up the walls of the ventricles in 5ms-1 (ventricular systole)
• Neurogenic heart


  • Relies on the nerve from the brain to initiate cardiac movement
  • Often found in lower invertebrates like annelids and arthropods
  • The heartbeat is initiated by the ganglion situated near the heart
• Electrocardiogram (ECG)


  • Records the excitation impulse trace
  • Magnitude - number of muscle cells that are alive
  • Pattern - direction of movement of excitation wave
  • An abnormal ECG is an indication of more muscle cells (higher voltage)/less muscle cells (low voltage)/damaged SAN or AVN (abnormal pattern)
  • P-wave (atrial depolarisation) - excitation wave through atrial wall (magnitude is smaller as atrial walls are thin due to lesser muscle cells)
  • QRS-wave (ventricular depolarisation) - excitation wave down then up ventricles (magnitude higher as ventricles have more muscle cells)
  • T-wave (ventricular repolarisation) - residual excitation waves at the top of Purkinje fibres that are absorbed by fibrous tissue
• Blood pressure


  • Pressure is exerted by blood pushing against the vessel walls
  • F = Flow/Cardiac output (ΔP ∝ R)
  • ΔP = Pressure gradient/mean arterial pressure (only the LHS, RHS is negligible)
  • R = Resistance/total peripheral resistance
• Factors affecting blood pressure

  1. Cardiac output

  2. Blood volume

  3. Vessel elasticity
• Peripheral vascular resistance


  • Diameter of blood vessels
• Vascular tree


  • Arteries - carry blood away from the heart to tissues
  • Arterioles - smaller branches of arteries
  • Capillaries - smaller branches of arterioles; smallest of vessels across which all exchanges are made with surrounding cells
  • Venules - formed when capillaries rejoin, and work to return blood to the heart
  • Veins - formed when venules merge, and work to return blood to the heart
• Arteries


  • Serve as rapid-transit passageways for blood from heart to organs
  • Due to its large radius, arteries offer little resistance to blood flow, and rather act as pressure reservoirs to provide a driving force for blood when heart is relaxing
  • Arterial connective tissue contains - collagen fibres (tensile strength) and elastin fibres (elasticity to arterial walls)
• Arterioles


  • Major resistance vessels that converts pulsatile systolic to diastolic pressure swings in the arteries into the non-fluctuating pressure present in the capillaries
  • Radius of arteriole can be adjusted to distribute cardiac output among systemic organs, depending on body's momentary needs and to help regulate arterial blood pressure
• Arteriolar vasoconstriction


  Increased myogenic activity, O2, endothelin and sympathetic stimulation by vasopressin; angiotensin II (cold) and decreased CO2 and other metabolites
• Arteriolar vasodilation


  Decreased myogenic activity, O2, sympathetic stimulation, histamine release (heat); and increased CO2, other metabolites and nitric oxide
• Intrinsic vascular control


  • Control of local blood flow (contraction) - Vascular smooth muscle intrinsic myogenic tone stimulates smooth muscle tone in response to pressure
  • Control of local blood flow (vasodilation) - Vascular endothelium releases nitric oxide, prostacyclin, EDHF to relax smooth muscle
• Thermoregulation


  • Internal core temperature (37c) - Abdominal and thoracic organs, central nervous system, skeletal muscles
  • Outer shell - Consists of skin and subcutaneous fat
  • Skin temperature varies between 20c and 40c without damage
• Increase in internal core temperature

  • Increase in speed of cellular chemical reactions
  • Overheating (more serious that cooling)
  • Nerve malfunction, irreversible protein denaturation
• Vascular regulation


  1. Intrinsic control

  2. Control of local blood flow (contraction)

  3. Control of local blood flow (vasodilation)
• Vascular smooth muscle


  • Intrinsic myogenic tone stimulates smooth muscle tone in response to pressure
• Vascular endothelium


  • Nitric oxide
  • Prostacyclin
  • EDHF
• Thermoregulation


  Regulation of heat exchange in the body
• Internal core temperature


  • Abdominal and thoracic organs
  • Central nervous system
  • Skeletal muscles
• Outer shell


  Skin and subcutaneous fat
• Increase in internal core temperature

  • Increase in speed of cellular chemical reactions
  • Overheating
  • Nerve malfunction, irreversible protein denaturation
• Decrease in internal core temperature


  • Decrease in speed of cellular chemical reactions
  • Pronounced, prolonged fall in body temperature that slows metabolism down to a fatal level
• Mechanisms of heat transfer

  • Radiation
  • Conduction
  • Convection
  • Evaporation
• Hypothalamus


  Acts as a "thermostat" that speeds up heat loss or heat production as needed