Capillaries are built for exchange, not for speed

A capillary is a microscopic blood vessel linking arterioles to venules, with a wall thin enough for exchange between blood and surrounding cells. It deals with a basic multicellular problem: cells buried inside a body cannot all touch the external environment, so blood must carry materials close enough for diffusion.

Capillary networks branch heavily. That branching creates a very large total surface area, and since each capillary is narrow, blood passes through many tiny channels rather than one large pipe. In practice, most active cells sit only a short diffusion distance from the blood.

A capillary wall consists of a single layer of endothelium, a tissue lining blood vessels and made of flattened epithelial cells. A basement membrane is an extracellular mesh of protein fibres that supports the endothelium and works as a selective filter. With such thin walls, oxygen, glucose, carbon dioxide and other small solutes have a shorter diffusion pathway.

Small gaps between endothelial cells let fluid and dissolved substances leave the blood, while red blood cells and most large plasma proteins stay inside. Some capillaries are fenestrated capillaries, capillaries with larger pores that allow especially rapid filtration or exchange. This design appears where high-volume exchange is useful, such as in kidney filtration surfaces.

Recognising arteries and veins in micrographs

An artery is a blood vessel that carries blood away from the heart under relatively high pressure. A vein is a blood vessel that carries blood towards the heart under relatively low pressure. Flow direction is what defines them — not whether the blood is oxygenated. For example, pulmonary arteries carry deoxygenated blood away from the heart.

In transverse sections, arteries and veins don’t look the same because they work under different pressures. Arteries usually have a thicker wall relative to the diameter of the lumen, a more rounded outline, a smaller lumen, and visible elastic or muscle layers. Veins usually have a thinner wall, a wider lumen, and may look collapsed or irregular because the pressure inside is low.

A lumen is the internal space of a tube through which fluid flows. In micrographs, compare wall thickness with lumen size every time. A thick wall and narrow lumen strongly suggest an artery; a thin wall and wide or flattened lumen strongly suggest a vein.

A plan diagram is a low-detail biological drawing that shows the arrangement of tissues without drawing individual cells. For vessel micrographs, use clean outlines for the lumen and wall layers, keep proportions realistic, and label the artery or vein using visible structural evidence.

Arteries withstand pressure and keep blood moving

Arteries take blood directly or indirectly from ventricular contraction, so their walls have to cope with pressure that is both high and constantly changing. An artery wall has three main layers: the tunica intima, a smooth endothelial lining that reduces friction with blood; the tunica media, a middle layer with plenty of smooth muscle and elastic tissue; and the tunica externa, an outer connective tissue layer containing collagen fibres for strength.

Elastic tissue is stretchable connective tissue containing elastin fibres that recoil after being stretched. As the ventricle contracts, arterial pressure rises and the artery wall stretches. Between beats, elastic recoil squeezes the blood forwards. Flow and pressure are therefore maintained even while the ventricle is relaxing — a lovely example of form smoothing out a pulsed pump.

Smooth muscle is involuntary muscle tissue made of spindle-shaped cells that can contract slowly and sustain contraction. In arteries and arterioles, circular smooth muscle alters the lumen diameter. Vasoconstriction is narrowing of a blood vessel caused by contraction of smooth muscle; it reduces local blood flow. Vasodilation is widening of a blood vessel caused by relaxation of smooth muscle; it increases local blood flow.

Collagen fibres give the artery wall strength, preventing it from bulging or rupturing under high pressure. The relatively narrow lumen also helps maintain high blood pressure as blood is carried away from the heart. Pressure differences move the blood, but the artery wall makes high-pressure transport safe and useful.

Measuring heart rate from a pulse

A pulse is the pressure wave that travels through an artery after each ventricular contraction. One pressure wave is produced with each heartbeat, so pulse rate can be used to estimate heart rate. Heart rate is the number of heart contractions per minute, usually recorded in beats $\text{min}^{-1}$.

To find the radial pulse, rest two or three fingertips lightly on the thumb side of the wrist. For the carotid pulse, place the fingertips gently in the groove beside the windpipe. Don’t use your thumb, as it has its own detectable pulse and may confuse the count. Count for 60 seconds, or, if the rhythm is regular, count for 30 seconds and double the value.

A pulse oximeter is an electronic device that estimates pulse rate and blood oxygen saturation by detecting changes in red and infrared light transmitted through a fingertip. To compare fingertip counting with a digital meter fairly, use repeated trials, the same person, the same posture, the same timing period, and ideally take the measurements simultaneously or immediately one after another. Repeated measurements help you judge reliability; comparison with a trusted device helps judge accuracy.

Veins return blood at low pressure

Veins carry blood back to the heart after it has moved through capillary networks. By this stage, much of the pressure produced by the heart has been lost. So veins don’t need the same structure as arteries: their walls are thinner, with less elastic tissue and less smooth muscle, and their lumens are wider.

For veins, the main problem is not bursting. It’s slow flow and backflow. Many veins contain valves, flap-like structures that let fluid move in one direction but close when the fluid starts to reverse. In veins, pocket valves open as blood moves towards the heart and shut if blood slips backwards.

A vein’s thin, flexible wall is part of the adaptation. When nearby skeletal muscles contract, they squeeze the veins. Because a vein can be compressed, blood gets pushed along it. Valves ensure that this muscle action moves blood towards the heart, not back towards the capillaries. That is why walking, or even fidgeting, helps venous return, especially from the legs.

Coronary arteries supply the heart muscle

A coronary artery is an artery that branches from the aorta and supplies oxygenated blood to cardiac muscle. The heart wall does real muscular work, so it needs a blood supply of its own; the blood sitting inside the chambers does not adequately nourish the thick myocardium.

An occlusion is a blockage of a tube that prevents or reduces flow through it. In coronary arteries, occlusion is often linked to atheroma, a fatty plaque that forms in the artery wall and narrows the lumen. These plaques contain lipids, including cholesterol, and may harden when calcium salts build up in them. If the plaque surface becomes roughened, it can promote thrombosis, which is the formation of a blood clot inside a blood vessel.

When blood flow falls, part of the heart muscle gets less oxygen and glucose. Chest pain may appear during exertion, because cardiac muscle cannot increase aerobic respiration enough. If the blockage is severe or persists, cardiac muscle cells die. A myocardial infarction is death of heart muscle tissue caused by an inadequate blood supply. Coronary heart disease is a group of conditions caused by narrowed or blocked coronary arteries.

Risk factors include hypertension, smoking, high intake of saturated fat, high blood cholesterol, obesity, high salt intake, excessive alcohol intake, sedentary lifestyle, genetic predisposition and ageing. Be careful with the word “cause”: epidemiology often begins by showing association, not proof.

Epidemiology is the study of the distribution and determinants of disease in populations. Epidemiological data on coronary heart disease may compare variables such as blood pressure, diet, smoking frequency or age with disease incidence or death rate. A correlation coefficient is a numerical measure of the strength and direction of association between two variables. Values close to $+1$ show strong positive correlation, values close to $-1$ show strong negative correlation, and values close to $0$ show weak or no linear correlation.

Low or absent correlation can count against a proposed hypothesis. A strong correlation, such as between a dietary factor and coronary heart disease incidence, gives useful evidence of association, but it does not by itself prove causation. For a causal claim, you need a plausible biological mechanism, controls for confounding variables, and ideally several independent lines of evidence.

Transpiration pull moves water up $\text{xylem}$

Xylem is plant vascular tissue that transports water and dissolved mineral ions from roots towards shoots and leaves. In a transpiring plant, roots absorb water; it moves through xylem, evaporates from moist leaf cell walls, and diffuses out through stomata.

Transpiration is the loss of water vapour from aerial parts of a plant, mainly through stomata in leaves. As water evaporates from mesophyll cell walls, the water left behind is pulled through the tiny spaces in the cellulose wall. Capillary action is movement of liquid through narrow spaces caused by adhesion to the surface and cohesion within the liquid. In this case, water adheres to hydrophilic cellulose and is drawn through leaf cell walls from nearby xylem vessels.

Removing water from the top of the xylem creates tension, which is a pulling force associated with negative pressure potential. That tension passes down the continuous water column in the xylem. Transpiration pull is the upward movement of xylem sap caused by tension generated in transpiring leaves.

The system relies on cohesion, which is attraction between molecules of the same substance. Water molecules cohere because of hydrogen bonding, so a continuous column can be pulled upward without breaking easily. If the column breaks, gas bubbles interrupt transport; this is cavitation. In healthy xylem, cohesion makes water act a bit like a tiny rope pulled from the top.

The pressure idea runs through the whole topic: arteries mostly use positive pressure from the heart, but xylem during transpiration is drawn upward by negative pressure generated in leaves.

Xylem vessels are dead tubes strengthened for tension

A xylem vessel is a long water-conducting tube made from dead cells joined end to end, with most end walls removed. By maturity, xylem vessel elements have no cytoplasm, no plasma membrane and no organelles, so water moves through them passively with little obstruction.

Removing, or only partly retaining, the end walls lowers resistance to flow. It’s like drinking through one continuous straw rather than through a stack of straws separated by paper partitions — the partitions would slow the water down.

Xylem vessels have to be strong as well. During transpiration, the pressure inside the vessels can become very low, which puts them at risk of collapsing. Lignin is a waterproof strengthening polymer deposited in plant cell walls. Once the walls are lignified, they can withstand tension and keep the vessel open.

Lignin may be laid down as rings, spirals or more extensive thickening, depending on the age and position of the tissue. Water does not pass easily through lignified thickening, so vessels have pits, thin unthickened regions of xylem wall that let water enter or leave sideways. These pits matter because xylem must supply water to living tissues along the route, not only to the very top of the plant.

Dicot stems have vascular bundles arranged near the outside

A dicotyledonous plant is a flowering plant whose seed embryo has two cotyledons, or seed leaves. In a young dicot stem, the vascular tissue usually forms a ring of vascular bundles close to the outside of the stem.

A vascular bundle is a group of xylem and phloem tissues arranged together for transport. In a typical dicot stem, xylem sits towards the inside of each bundle, while phloem sits towards the outside. Between them there may be cambium, which is meristematic tissue that produces new xylem and phloem. The epidermis forms the outer protective cell layer of the stem, and the cortex is the tissue between the epidermis and vascular bundles; it often supports the stem and may photosynthesise. The central pith is ground tissue filling the middle of the stem.

When you draw a plan diagram from a stem micrograph, don’t draw individual cells. Use single, clear outlines to show the relative positions of the epidermis, cortex, vascular bundles, xylem and phloem. Add function labels: xylem transports water, phloem transports sugars and other assimilates, epidermis protects, cortex supports, and vascular bundles organise transport tissue.

Dicot roots place vascular tissue in the centre

Dicot roots are arranged differently from dicot stems. In a transverse section of a typical young dicot root, the vascular tissue sits in the centre rather than forming a ring near the edge.

The xylem often makes a star-shaped central region, with phloem between the arms of the xylem. Much of the space between the vascular tissue and the outer epidermis is cortex. The epidermis may have root hairs, increasing the surface area for absorption of water and mineral ions.

In micrographs, xylem vessels often look larger, rounder and thicker-walled than phloem cells. If the section is stained, lignified xylem may appear a different colour from the surrounding unlignified tissues. For a root plan diagram, include epidermis, cortex, vascular bundle, xylem and phloem in the correct relative positions. Keep it as a plan diagram: tissue boundaries and proportions, not cell-by-cell artwork.

Tissue fluid forms by pressure filtration

Blood plasma is the liquid part of blood that carries suspended blood cells. It contains water, dissolved ions, glucose, amino acids, hormones, gases, plasma proteins and other transported substances.

Tissue fluid is extracellular fluid formed from blood plasma. It surrounds body cells and allows exchange between cells and capillaries. It forms by pressure filtration, where blood pressure pushes fluid out of capillaries through the capillary wall.

At the arteriole end of a capillary bed, blood pressure is relatively high. Plasma is pushed out through gaps in the capillary wall and basement membrane. Red blood cells and most large plasma proteins stay in the capillary, while the fluid that leaves becomes tissue fluid.

At the venule end, blood pressure has fallen. Some tissue fluid drains back into the capillaries and becomes plasma again. Release and reuptake, then, are not two separate locations in the body; they are two tendencies across a capillary bed, driven mainly by pressure differences from arteriole to venule.

Tissue fluid is the immediate environment of most body cells

Plasma and tissue fluid are similar, but they’re not the same. Plasma has blood cells suspended in it, along with relatively high concentrations of plasma proteins. Tissue fluid has no blood cells and far fewer large proteins, since the capillary wall mostly keeps these inside the blood. Both contain water and small dissolved substances such as glucose, amino acids, ions, oxygen, carbon dioxide and hormones.

Most cells exchange materials with tissue fluid rather than directly with blood. Oxygen diffuses from tissue fluid into cells because aerobic respiration keeps the oxygen concentration lower inside respiring cells. Carbon dioxide moves out of cells because respiration produces it. Glucose and amino acids can enter cells through membrane transport proteins, including by active transport when concentration gradients require it.

As tissue fluid passes through a tissue, cells take up useful substances from it and release waste products into it. When the fluid re-enters capillaries, those wastes go back into the bloodstream. Carbon dioxide is carried to gas exchange surfaces for removal, while many nitrogenous or toxic wastes are processed by organs such as the liver and kidneys.

The lymphatic system returns excess tissue fluid to blood

Some tissue fluid does not re-enter capillaries. If this extra fluid remained in the tissues, swelling would occur.

Oedema

is abnormal swelling caused by accumulation of excess tissue fluid.

Lymph

is tissue fluid that has entered lymphatic vessels.

Lymph ducts

are thin-walled vessels of the lymphatic system that return lymph to the blood circulation. Gaps between their endothelial cells let excess tissue fluid enter easily. Because lymph pressure is low, valves in lymph vessels help keep the flow one-way.

Small lymph vessels join and form larger ducts. Eventually, lymph returns to the blood circulation by entering large veins near the shoulders, then flowing towards the vena cava and the right side of the heart. For the syllabus, stick to the key points: thin walls with gaps, valves, and return to blood.

One circuit in fish, two circuits in mammals

A single circulation is a circulatory arrangement in which blood passes through the heart once during one complete circuit of the body. In bony fish, the route is $\text{heart} \to \text{gills} \to \text{body tissues} \to \text{heart}$. Blood is oxygenated at the gills, then it travels on to systemic tissues before returning to the heart.

A double circulation is a circulatory arrangement in which blood passes through the heart twice during one complete circuit of the body. In mammals, blood moves through two connected circuits: $\text{right side of heart} \to \text{lungs} \to \text{left side of heart}$, and $\text{left side of heart} \to \text{body tissues} \to \text{right side of heart}$.

This mammalian arrangement lets the two circuits run at different pressures. Blood goes to the lungs in the pulmonary circuit at lower pressure, which protects the delicate gas exchange surfaces. In the systemic circuit, higher pressure sends blood around the body quickly, supplying organs and supporting processes such as filtration in kidneys. Keeping the two sides separate also prevents oxygenated and deoxygenated blood from mixing.

The heart is a double pump with one-way valves

The mammalian heart is a muscular organ that pumps blood through pulmonary and systemic circulations. Its structure fits that role: it generates pressure, keeps oxygenated and deoxygenated blood apart, and maintains one-way flow.

Cardiac muscle is specialised muscle tissue in the heart wall that contracts rhythmically and spreads electrical excitation between connected cells. These contractions are myogenic, meaning they start in the muscle tissue itself rather than needing motor neuron stimulation for each beat.

An atrium is a thin-walled heart chamber that receives blood from veins and passes it to a ventricle. A ventricle is a thick-walled heart chamber that pumps blood into arteries. Ventricles need thicker walls because they produce the pressure that drives blood through circuits; the left ventricle is especially thick because systemic circulation needs higher pressure than pulmonary circulation.

An atrioventricular valve is a valve between an atrium and a ventricle that prevents blood flowing back into the atrium during ventricular contraction. A semilunar valve is a valve between a ventricle and an artery that prevents blood flowing back into the ventricle during ventricular relaxation. The valves turn pressure changes into directed flow: when a chamber contracts, blood moves the correct way.

A pacemaker is a region of cardiac tissue that initiates each heartbeat by spontaneous electrical activity. In mammals, the sinoatrial node in the right atrium acts as the pacemaker. A septum is a wall that separates the left and right sides of the heart, preventing mixing of oxygenated and deoxygenated blood. Coronary vessels are blood vessels in the heart wall that supply cardiac muscle with oxygen and nutrients and remove wastes.

You should be able to trace flow on a frontal heart diagram: $\text{vena cava} \to \text{right atrium} \to \text{right atrioventricular valve} \to \text{right ventricle} \to \text{pulmonary semilunar valve} \to \text{pulmonary artery} \to \text{lungs} \to \text{pulmonary veins} \to \text{left atrium} \to \text{left atrioventricular valve} \to \text{left ventricle} \to \text{aortic semilunar valve} \to \text{aorta}$. The structures enforce that one-way pathway.

The cardiac cycle is a repeating pressure sequence

The cardiac cycle is the repeating sequence of contraction and relaxation events in the heart during one heartbeat. Systole is the phase when a heart chamber contracts. Diastole is the phase when a heart chamber relaxes and fills.

The sinoatrial node starts the heartbeat. Electrical excitation spreads through the atria, which causes atrial systole. Blood moves through the open atrioventricular valves into the ventricles. After a short delay, excitation passes through the ventricles and brings on ventricular systole.

Early in ventricular systole, ventricular pressure rises quickly, closing the atrioventricular valves. The semilunar valves stay closed at first because ventricular pressure has not yet exceeded arterial pressure. Once ventricular pressure becomes higher than arterial pressure, the semilunar valves open and blood is ejected into the arteries.

During ventricular diastole, the ventricles relax. Ventricular pressure drops below arterial pressure, so the semilunar valves close. When ventricular pressure falls below atrial pressure, the atrioventricular valves open, and blood flows passively from veins through atria into ventricles. The next atrial systole then tops up the ventricles, and the cycle repeats.

Systolic blood pressure is the maximum arterial pressure reached during ventricular contraction. Diastolic blood pressure is the minimum arterial pressure reached before the next ventricular contraction. On pressure graphs, you work out valve events by comparing pressures: an atrioventricular valve opens when atrial pressure exceeds ventricular pressure, while a semilunar valve opens when ventricular pressure exceeds arterial pressure.

This section also covers the “cycles” linking idea within this topic: at organ level, the heart works as a repeated cycle of electrical initiation, pressure change, valve movement and blood flow.

Root pressure pushes when transpiration pull is weak

Root pressure is positive pressure potential in xylem generated by active transport of mineral ions into xylem, followed by osmosis of water. It matters most when transpiration is too low to provide much pull, such as in very humid conditions, at night when stomata may be closed, or in spring before deciduous leaves have opened.

Living root cells beside xylem vessels actively transport mineral ions into the xylem. Active transport is movement of substances across a membrane using energy to move them against their concentration gradient. The xylem vessels themselves are dead, so they do not pump ions; neighbouring living cells do that job.

Adding ions lowers the water potential of xylem sap. Water then moves into the xylem by osmosis, which is the net movement of water across a partially permeable membrane from higher water potential to lower water potential. As water enters, pressure builds in the xylem and can push sap upward through roots and stems.

Root pressure does not mainly explain how water reaches the tops of tall trees during active transpiration; transpiration pull does. Even so, root pressure helps refill xylem and move water when the leaf-driven tension system is weak or absent.

Phloem translocates sap from sources to sinks

Phloem is plant vascular tissue that carries dissolved organic compounds, especially sucrose, between sources and sinks. Translocation means the movement of organic solutes in phloem sap from source tissues to sink tissues. A source is plant tissue that loads carbon compounds into the phloem, for example a photosynthesising leaf. A sink is plant tissue that takes carbon compounds out of the phloem for use or storage, such as a growing root or developing fruit.

A sieve tube element is a living phloem cell specialised for mass flow of sap, joined end to end with other sieve tube elements. It has reduced cytoplasm, lacks many organelles, and has no nucleus. That creates more space inside the cell and gives the phloem sap fewer obstructions as it flows.

Sieve plates are perforated end walls between sieve tube elements. They let phloem sap move from one element to the next, keeping the pathway continuous while helping brace the tube against pressure. Sieve tube elements still retain a plasma membrane, which is essential because loading and unloading rely on controlled movement of solutes.

A companion cell is a nucleated phloem cell closely linked to a sieve tube element, and it supports the sieve tube element metabolically. Companion cells have many mitochondria, so they can supply ATP for active processes such as loading sucrose at sources and unloading it at sinks. Plasmodesmata are cytoplasmic channels through plant cell walls that connect neighbouring cells; between companion cells and sieve tube elements, they allow substances and metabolic support to pass between the cells.

At sources, active loading of sucrose and other carbon compounds into the phloem increases solute concentration. Water enters by osmosis, so hydrostatic pressure rises. At sinks, unloading reduces solute concentration, water leaves, and pressure drops. This pressure difference drives sap from source to sink. In both animals and plants, pressure differences move materials — but plants use xylem tension, root pressure and phloem pressure flow in different contexts.