Blood system

• Shit that is transported around the body in blood:

  • Oxygen
  • Nutrients
  • Antibodies
  • Hormones
  • Heat
  • CO2
  • Urea
• This is achieved through the beating of the heart pumping the blood through the network capillaries.
• Components of blood:

  • Plasma
  • dissolves / carries all other components of blood, nutrients, wastes
  • Erythrocytes (Red blood cell)
  • Transport oxygen in haemoglobin molecules
  • Leucocytes (White blood cell)
  • Phagocytes (“eat” pathogens & dead cells)
  • Lymphocytes (B cells, T cells) for the immune response
  • Platelets
  • Clotting of blood after dmg to cells/ erythrocytes
• 3 main types of blood vessels:

  • From the heart
  • Arteries carry high pressure blood away from heart to tissues that need it
  • Very near to cells
  • Capillaries are very small (< 10 μm diameter) & ∴ can penetrate virtually every tissue in the body. Blood moves slowly through them under low pressure providing opportunities for xΔ of substances.
  • Back to the heart
  • Veins carry low pressure blood back to heart using valves to ensure blood flows in correct direction.
• Arteries & veins ≈ are large structures, smaller arteries aka arterioles and smaller veins aka venules.
• Arteries:

  • Relatively (to the wall) small lumen to maintain high blood pressure.
  • Thick muscular wall & fibrous outer layer to help artery withstand high pressure
  • Muscle contracts to decrease the size of the lumen. This causes an increase blood pressure and therefore maintains high blood pressure between the pulses of high pressure blood travelling from the heart.
  • Elastic fibres stretch to increase the lumen with each pulse of blood. After the pulse of blood passes the fibres recoil decreasing the lumen size and therefore helping to maintain a high blood pressure.
• Capillaries:

  • The smallest blood vessels & are adapted for the xΔ of substances to & from the blood. This lets tissues gain nutrients & molecules such as oxygen & rid themselves of waste material easily.
  • Capillaries also allow substances to enter & leave the organism, e.g. gas xΔ of O2 & CO2 in the lungs.
  • Due to the large number of capillaries present & the small lumen the SA available for the xΔ of substances is very large. 
  • Walls & membrane can contain pores to ↑ diffusion of substances
• Veins:

  • Large lumen (compared to arteries & thickness of the wall) means that the blood is under low pressure.
  • Because there is less pressure to resist, walls of the veins are thinner & ↓ elastic than arteries. They also contain ↓ muscle than arteries.
  • Because of low pressure, valves are req to prevent back-flow of blood & ∴ ensure that blood moves towards to heart.
• Blood circulation process:

  1. Deoxygenated blood (low O2, high CO2) returns to heart via right atrium. It is pumped from the right ventricle to the lungs, where CO2 is offloaded & O2 is picked up.
  2. It is now oxygenated blood (high O2, low CO2). It enters the left atrium & is pumped from the left ventricle to the body, where O2 is used for respiration & CO2 is collected as a waste product. 
  3. Now it's deoxygenated, it makes its way back to the right atrium and the cycle continues.
• Beating of the heart is due to myogenic muscle contraction. This means that the myocyte (muscle cell) itself is the origin of the contraction & is not controlled externally. A region of myocytes called the “sinoatrial node” (pacemaker) controls the rate of the heartbeat. A wave of excitations is sent from the sinoatrial node, causing the atria to contract. This excitation is conducted to the atrioventricular node, where it is passed through the nerves to the muscles of the ventricles causing them to contract. 
• Heart rate can be controlled by the autonomic nervous system (the part of the nervous system that responds automatically to Δ in the body conditions). Where myocardial contraction maintains the beating of the heart, we may need to speed up/ slow down the heart rate.
• When exercising, ↑ CO2 is present in the blood. This is detected by chemoreceptors in the brain’s medulla oblongata → nerve signal being sent to the SA node to speed the heart rate via the sympathetic nerve. When CO2 levels ↓, another nerve (vagus), ↓ heart rate. The hormone adrenaline causes a rapid ↑ in heart rate in fight/ flight responses, preparing the body for action. This effect can be mimicked by stimulant drugs. 
• Atherosclerosis (X):

  Degenerative disease (areas of the artery wall become dmg). Macrophages release growth factors, encouraging growth of fibrous tissues. Cholesterol builds up in dmg areas → plaque formation & artery wall ↓ elasticity. As build-ups of cholesterol & plaque form, the lumen narrows ↓ blood flow. If plaque ruptures, blood clotting is triggered. X → blood clots, and if these clots occur in the myocardial tissue we call that coronary heart disease. Coronary muscle tissue dies as a result of a lack of blood and O2.
• Risk factors:

  • Genetic
  • Some people predisposed for high cholesterol levels/ high blood pressure
  • Age
  • Older people ↑ risk/ ↓ elasticity in arteries
  • Sex
  • Males at ↑ risk than females
  • Smoking
  • Constricts blood vessels/ ↑ blood pressure/heart-rate / ↓ oxygenation of heart muscle/ ↑ fibrinogen & platelets in blood → ↑ clotting
  • Diet
  • ↑ fat/cholesterol/LDL in blood / → plaque formation in arteries
  • Exercise
  • Lack of exercise ↑ risk due to weakened circulation
  • Obesity
  • ↑ in blood pressure → plaque formation in arteries
  • Stress
  • → ↑ cortisol hormones in blood → ↑ atherosclerosis