12BIC - Unit 5: Transport in Mammals

Why small organisms do not need a bloodstream

They rely entirely on diffusion to transport materials because they have a large surface area to volume ratio (SA:V) and short diffusion distances.

Mass flow system

A transport system found in larger animals where a pump moves substances through the bulk flow of a fluid.

Closed double circulation

A system where blood passes through the heart twice for each complete circuit of the body, divided into pulmonary circulation (right side to lungs) and systemic circulation (left side to rest of body).

The 4 heart chambers

Right atrium, right ventricle, left atrium, and left ventricle; the right side handles deoxygenated blood to the lungs, while the left side handles oxygenated blood to the body.

Tunica intima

The innermost layer of blood vessels, covered in a smooth squamous epithelium (endothelium) to minimize friction.

Tunica media

The middle layer of blood vessels, containing smooth muscle, collagen, and elastic fibres.

Tunica externa

The outermost layer of blood vessels, containing elastic and collagen fibres.

Artery structure and function

Carries blood away from the heart under high pressure; features thick, elastic walls that expand during heart contraction and recoil during relaxation to even out pulsating blood flow.

Arterioles

Small arteries that are more muscular and less elastic; they contract (vasoconstriction) or dilate (vasodilation) to control blood flow into capillary beds.

Capillary structure and function

Formed by branched arterioles; walls are 1 layer of epithelial cells thick with tiny pores, a 7 µm diameter (bringing RBCs within 1 µm of the wall), and a large SA:V ratio for rapid diffusion.

Capillary beds

Arranged networks of capillaries where blood pressure decreases to allow dissolved gases and substances to exchange with tissues.

Vein structure and function

Carries low-pressure blood towards the heart; formed by joining venules, features a thin tunica media, a larger lumen, less muscle/elastic tissue, and relies on skeletal muscle contraction to move blood.

Semilunar valves in veins

Half-moon folds of the endothelium that prevent the backflow of low-pressure blood.

Plan diagram identification under a microscope

Arteries have thick walls and small lumens; veins have thin walls and large lumens; capillaries are extremely small, single-layered vessels often found in networks.

Graticule measurement of vessels

An eyepiece tool calibrated against a stage micrometer used to accurately measure the actual diameter and wall thickness of arteries and veins under a light microscope.

Erythrocytes (RBC) structure and function

Biconcave discs with a 7 µm diameter and flexible membranes that increase SA:V ratio for rapid diffusion; packed with haemoglobin close to the surface, though membranes are fragile and can rupture in tight spots.

Leukocytes (WBC) division

Divided into Granulocytes (with visible granules: neutrophils, eosinophils, basophils) and Agranular cells (without visible granules: monocytes, lymphocytes).

Neutrophils

Granular microphages that act as phagocytes; they are the most abundant white blood cells and serve as first responders to infection.

Monocytes

Agranular cells that mature into macrophages; they act as phagocytes to 'clean' the circulatory and lymphatic systems.

Lymphocytes

Agranular cells involved in adaptive immunity; T-lymphocytes regulate immune cells and destroy infected cells, while B-lymphocytes produce antibodies.

Eosinophils

Granular white blood cells that release toxins and trigger inflammatory responses during allergic reactions.

Basophils

Granular white blood cells that produce histamine (for allergies) and heparin (to prevent blood clotting).

Origin of blood cells

Almost all blood cells are derived from bone marrow, but lymphocytes are matured and multiplied by lymphatic tissue.

Platelets

Small cell fragments with no nucleus that are essential for blood clotting.

Blood plasma

A pale yellow liquid consisting of 90% water, containing nutrients, waste products, and plasma proteins.

Blood clotting mechanism

Platelets and fibrinogen interact; the enzyme thrombin converts soluble fibrinogen into insoluble fibrin, which links together and activates platelets to form a clot, preventing blood loss and pathogen entry.

Magnification calculation of blood cells

Magnification = Image size / Actual size ($M = \frac{I}{A}$), used to find the scale of drawn or photographed red or white blood cells.

Tissue fluid formation

Liquid between cells formed when high blood pressure on the arterial side of a capillary bed forces blood plasma, white blood cells, and some proteins out through capillary pores.

Tissue fluid return

The osmotic drawing force of remaining plasma proteins pulls tissue fluid back into the capillaries at the venule end, where blood pressure is lower.

Oedema

The accumulation of fluid in body tissues caused when blood pressure is too high, forcing excessive fluid out of capillary beds.

Differences between blood, tissue fluid, and lymph

Blood contains plasma, RBCs, WBCs, platelets, and large proteins; tissue fluid contains plasma, WBCs, and small proteins but lacks RBCs/platelets; lymph is excess tissue fluid drained into lymphatic vessels containing more fats and lymphocytes.

Haemoglobin structure

A globular protein consisting of 4 polypeptide chains, each containing a haem group, capable of binding up to 4 oxygen molecules.

Oxyhaemoglobin loading

In the lungs where partial pressure of oxygen ($pO_2$) is high, haemoglobin has a high affinity for oxygen and binds with it ($Hb + 4O_2 \rightarrow HbO_8$).

Oxyhaemoglobin unloading

At respiring tissues where $pO_2$ is low, haemoglobin's affinity for oxygen decreases, causing it to release/unbind its oxygen.

Cooperative binding

A shape change in haemoglobin; it is slow/difficult for the first oxygen molecule to bind, but after it binds, the shape changes to make the next molecules bind much easier (forming an S-shaped curve). It slows down again near saturation due to limited free sites.

Carbon dioxide transport pathways

5% is carried as dissolved $CO_2$ in plasma; 10% binds to the terminal groups ($-NH_2$) of haemoglobin to form carbaminohemoglobin; 85% is converted into hydrogen carbonate ions ($HCO_3^-$).

Carbonic anhydrase role

An enzyme in the RBC cytoplasm that catalyses $CO_2 + H_2O \rightarrow H_2CO_3$ (carbonic acid), which then dissociates into $H^+$ and $HCO_3^-$.

Haemoglobin as a buffer

Haemoglobin readily combines with free hydrogen ions ($H^+$) to form haemoglobinic acid ($HHb$), removing excess acid to maintain a near-neutral blood pH.

Chloride shift

The movement of chloride ions ($Cl^-$) into the RBC to balance the electrical charge as hydrogen carbonate ions ($HCO_3^-$) diffuse out into the plasma, preventing positive charge accumulation.

The Bohr effect

At high partial pressures of carbon dioxide ($pCO_2$), $CO_2$ forms carbonic acid and lowers blood pH; this reduces haemoglobin's affinity for oxygen, shifting the oxygen dissociation curve to the right and causing faster oxygen unloading at respiring tissues.

High altitude effect on RBC count

At high altitudes, atmospheric $pO_2$ is low, triggering the body to increase its red blood cell count over time to maximize oxygen absorption and transport.

Carbon monoxide effect

Carbon monoxide binds irreversibly to haemoglobin to form carboxyhaemoglobin, permanently blocking oxygen binding sites and severely reducing the blood's oxygen-carrying capacity.

External and internal heart structure

Features a thick muscular wall, 4 chambers (atria above, ventricles below), a central septum, atrioventricular (AV) valves between atria and ventricles, and semilunar valves at the bases of the aorta and pulmonary artery.

Chamber wall thickness differences

Atria walls are thin because they only pump blood a short distance into ventricles; the right ventricle wall is moderately thick to pump blood against low resistance to the lungs; the left ventricle wall is the thickest because it must overcome high resistance to pump blood to the entire body.

Diastole

The phase of the cardiac cycle where cardiac muscles relax, semilunar valves close making the "dub" sound, and blood flows passively from the veins into the atria and ventricles (70% flows passively).

Atrial systole

The phase where the atria contract, fully opening the AV valves and forcing the remaining blood into the ventricles.

Ventricular systole

The phase where ventricles contract from the base upwards; blood pressure forces the AV valves closed making the "lub" sound, opens the semilunar valves, and forces blood out into the arteries.

Sinoatrial (SA) Node

A patch of specialized muscle in the right atrium that acts as the pacemaker; it sets the rhythm by sending an electrical excitation wave through the atrial walls, causing them to contract simultaneously.

Atrioventricular (AV) Node

A node that intercepts the excitation wave from the atria, delays it slightly to allow atria to finish contracting, and then passes the wave down into the Purkyne tissue.

Purkyne tissue

Conducting fibres located in the septum that carry the excitation wave down to the base of the heart and up through the outer ventricular walls, ensuring ventricles contract from the bottom up.

ECG / EKG wave interpretation

An electrical recording of the heart: the P wave represents atrial contraction (systole), the QRS complex represents ventricular contraction (systole), and the T wave represents heart muscle relaxation (diastole).