B3.2 Transport

Types of blood vessels

Arteries, arterioles, capillaries, venules, veins

Capillaries

The capillary wall is made from a single layer of endothelial cells, reducing the diffusion distance for gas exchange. The endothelial cells of some capillaries have fenestrations for blood plasma to leak out and form tissue fluid. They have a lumen with a small diameter

Arteries

Arteries transport blood away from the heart at high pressure. The walls consist of three layers: an endothelial layer, smooth muscle cells and a thick layer of elastic tissue, and collagen and elastic fibres

Endothelial layer of arteries

One cell thick and lines the lumen of all blood vessels. It is smooth and reduces friction for blood flow

Smooth muscle cells and thick layer of elastic tissue of arteries

The muscle strengthens the arteries so they can withstand high pressure and can contract or relax to control the diameter and regulate blood pressure. The elastic tissue helps to maintain blood pressure, it stretches and recoils

Collagen and elastic fibres of arteries

Collagen protects blood vessels from damage by over-stretching, preventing the arterial wall from rupturing

Systolic pressure

The peak pressure point reached in the arteries as blood is forced out of the ventricles at high pressure. The arterial walls are forced outwards by the stretching of elastic fibres

Diastolic pressure

The lowest pressure point reached in the artery as the heart relaxes. The stretched elastic fibres recoil and force the blood onward through the arteries

Veins

Transports blood to the heart at low pressure. The middle layer is thinner in veins and the walls of veins are flexible. Veins contain valves which prevent black flow and have wide lumen

Pulse

The contraction of the ventricles forces a large volume of blood through the arteries, which expands. The carotid artery (side of the neck) or radial artery (passes over the wrist bone) is used

How to measure pulse rate using hands?

Place two fingers on the radial or carotid artery and gently compress. Count the number of pulses for 60 seconds

Occlusion

The narrowing of the arteries due to a blockage, which may be blocked by atherosclerosis

Atherosclerosis

Begins when there is damage to the arterial due to high blood pressure. This leads to the build up of fatty deposits (atheromas) under the endothelium, which narrows the lumen. Fibrous tissue is produced to repair the damage but lowers the elasticity of the arterial wall

Blood clotting

Further damage to the arterial wall by atherosclerosis leads to the rupturing of blood vessel walls, resulting in blood clotting. Thrombus is clots formed in a blood vessel, embolus is clots in the circulatory system

Coronary heart disease

If atherosclerosis occurs in the coronary arteries then parts of the heart muscle die, which may stop the heart from pumping blood and lead to myocardial infarction (heart attack). This can be bypassed by heart bypass surgery

Transpiration stream

The loss of water from the xylem vessels generates tension (negative pressure) within the xylem, creating a pulling force known as transpiration pull which allows water to be moved against gravity

Vascular tissues

Tubes used for transport in plants: xylem and phloem

Adaptations of xylem vessels

Mature xylem vessels are long, continuous, hollow tubes that lack cell content and end walls, allowing for unimpeded flow. The walls have cellulose and are strengthened with lignin which means they are tough and can withstand tension without collapsing. The walls also have pits which allow water to enter and move sideways between vessels

Draw a plan diagram of the distribution of tissues in a transverse section of a dicotyledonous stem

Epidermis - Prevents water loss and provides protection

Cortex and pith - Storage for starch and substances

Xylem - Transports water and dissolved mineral ions from roots to leaves

Phloem - Transports organic solutes from leaves to other parts of the plant

Draw a plan diagram of the distribution of tissues in a transverse section of a dicotyledonous root

Plasma

Largely composed of water. As blood passes through capillaries some plasma is forced out through gaps, creating tissue fluid. Tissue fluid contains fewer proteins and cells (no RBC, platelets)

Pressure filtration

The process by which tissue fluid is forced out of the arterial end of the capillary at high pressure

Reuptake of tissue fluid

At the venous end of the capillary tissue, tissue fluid drains back into the capillaries. It is eventually collected by lymph vessels and returned to the circulatory system

Closed circulatory system

Blood is contained within a system of blood vessels

Single circulation

Blood moves through the heart once during each complete circuit. The heart has 2 chambers

Double circulation

Blood flows through the heart twice for each complete circuit. The heart has 4 chambers. The right side pumps deoxygenated blood to the lungs for gas exchange (pulmonary circulation). The blood returns to the left side to be pumped around the body (systemic circulation)

Advantages of the mammalian double circulation system

Oxygenated and deoxygenated blood is separated, so cells receive blood with high oxygen for respiration, maintaining a high pressure for blood around the body, and low pressure for lungs so vessels are not damaged

Atria

Two top chambers of the heart. They receive blood from veins that return from the lungs and body. They are surrounded by a thin layer of muscle

Ventricles

Two bottom chambers of the heart. They receive blood from the atria and have thick muscle walls to create high pressure. The left ventricle has thicker muscle as it pumps all around the body

Septum

A wall of muscular tissue separating the left and right sides of the heart

SA node (sinoatrial node)

A small region of tissue in the wall of the right atrium. It initiates the heart beat by sending a wave of excitation across the atria. The impulse travels to the base of the ventricles

Two blood vessels bringing blood to the heart

Vena cava and pulmonary vein

Two blood vessels taking blood away from the heart

Aorta and pulmonary artery

When do valves open and close?

Valves open when the pressure of blood behind them is greater than the pressure in front of them. They close when the pressure of blood in front of them is greater than the pressure behind them

Atrioventricular valves

Valves between the atria and the ventricles, they prevent blood from flowing back into the atria. The right is separated by the tricuspid valve and the left by the bicuspid valve

Semilunar valves

Valves between the ventricles and the arteries. The right ventricle and pulmonary artery is separated by the pulmonary valve and the left ventricle and aorta are separated by the aortic valve

Coronary arteries

Supply the heart muscle with oxygenated blood

Myogenic

Means that the heart will beat without any external stimulus from the nervous system

Cardiac cycle

The SA node initiates depolarisation that causes the atria to contract. A region of non-conducting tissue prevents depolarisation spreading straight to the ventricles. Depolarisation is carried to the atrioventricular node which carries the wave of excitation down the septum. The ventricles contract.

Systole

Contraction of the heart

Diastole

Relaxation of the heart

Translocation

The transport of organic solutes in the phloem. The phloem sap is the liquid that is being transported, consisting of sucrose, water, and amino acids

Sources

Regions of plants in which organic solutes originate (eg. leaves and stems, storage organs)

Sinks

Regions of plants where organic compounds are required for growth (eg. meristems, roots, young leaves, seeds, fruits)

Adaptations of the phloem

Sieve tube cells line up to form a continuous tube. The cells have reduced cytoplasm and few organelles to allow free flow of phloem sap. Companion cells aid with the loading and unloading of dissolved substances, they have mitochondria for ATP

Process of translocation

Active transport is used to load organic compounds into the phloem. Water moves into the phloem by osmosis which generates a hydrostatic pressure gradient, the contents of the phloem flow towards the sink