transport in mammals

• Circulatory systems can be open (e.g., in insects) or closed (like in fish and mammals where the blood is confined to blood vessels only)
• Closed circulatory systems in mammals come in two forms:

  • Single: heart with two chambers, blood passes through the heart once for every circuit of the body
  • Double: heart with four chambers, blood passes through the heart twice for every circuit of the body
• Mammals have a closed double circulatory system consisting of the heart, blood vessels, and blood
• Arteries are adapted to carrying blood away from the heart, with thick walls to withstand high blood pressure, elastic tissue for stretching and recoil, smooth muscle for varying blood flow, and lined with smooth endothelium to reduce friction and ease blood flow
• Arterioles branch off arteries, have thinner walls, and feed blood into capillaries
• Capillaries are the smallest blood vessels, site of metabolic exchange, one cell thick for fast exchange of substances, adapted for efficient diffusion with a narrow lumen, large surface area, and slow blood flow
• Venules are larger than capillaries but smaller than veins
• Tissue fluid contains dissolved oxygen and nutrients, enabling exchange of substances between blood and cells
• Hydrostatic pressure forces blood fluid out of capillaries to form tissue fluid, with small enough substances escaping through capillary gaps including dissolved nutrients and oxygen
• Water potential gradient causes water to move from tissue fluid to blood by osmosis
• Remaining tissue fluid not pushed back into capillaries is carried back via the lymphatic system, containing lymph fluid similar to tissue fluid but with less oxygen and nutrients, mainly carrying waste products
• Lymphatic system contains lymph nodes that filter out bacteria and foreign material with the help of lymphocytes as part of the immune system defenses
• Water is the main component of tissue fluid and blood, acting as a solvent and having a high specific heat capacity for efficient transport
• The mammalian heart's main blood vessels include the aorta, pulmonary artery, pulmonary vein, and vena cava
• The ventricle pumps blood at high pressure, with the left ventricle wall thicker than the right to ensure blood reaches the rest of the body at high pressure
• The heart is myogenic and has a sinoatrial node as the pacemaker, initiating a wave of electrical stimulation causing the atria to contract at the same time before the ventricles contract
• The ventricles do not start contracting until the atria have finished due to the presence of tissue at the base of the atria which is unable to conduct the wave of excitation
• The electrical wave eventually reaches the atrioventricular node located between the two atria which passes on the excitation to ventricles, down the bundle of His to the apex of the heart
• The bundle of His branches into the Purkyne fibres which carry the wave upwards, causing the ventricles to contract and empty
• There are 3 stages of the cardiac cycle:

  • Atrial systole: atria contract, forcing the atrioventricular valves open and blood flows into the ventricles
  • Ventricular systole: contraction of the ventricles causes the atrioventricular valves to close and semi-lunar valves to open, allowing blood to leave the ventricles
  • Cardiac diastole: atria and ventricles relax, elastic recoil lowers the pressure inside the heart chambers, and blood is drawn from the arteries and veins
• Haemoglobin is a water-soluble globular protein consisting of two beta polypeptide chains and a haem group, carrying oxygen in the blood by binding to the haem group and releasing oxygen when required
• The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen, with higher partial pressure increasing the affinity, causing oxygen to bind tightly to haemoglobin in the lungs during loading and releasing in respiring tissues during unloading
• Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes, showing that high partial pressure leads to high saturation of haemoglobin with oxygen
• Carbonic anhydrase is an enzyme that helps haemoglobin dissociate from oxygen and bind to carbon dioxide to form carbaminohaemoglobin, catalyzing a reaction between carbon dioxide and water to produce carbonic acid
• Fetal haemoglobin has a higher affinity for oxygen compared to adult haemoglobin to better absorb oxygen at low partial pressure, ensuring the survival of the fetus
• At high altitudes, red blood cell count increases due to lower oxygen partial pressure, leading to more red blood cells being made to increase the amount of haemoglobin available for oxygen binding
• The chloride shift in red blood cells helps maintain the cell's electrical balance by:

  • Blood reaching lung tissue with low carbon dioxide concentration
  • Carbonic anhydrase breaking down carbonic acid into water and carbon dioxide
  • Bicarbonate diffusing into red blood cells and reacting with hydrogen ions to form carbonic acid
  • Free carbon dioxide being released when carbonic acid is broken down, diffusing into the lungs
  • Chloride ions moving from red blood cells into the plasma down an electrochemical gradient