MAMMALIAN HEART: STRUCTURE, ATERIES, CAPILLARIES, TRANSPORT MECHANISM, BLOOD CIRCULATION, TRANSPORT IN HIGHER PLANTS, SIMILARITIES AND DIFFERENCES BETWEEN TRANSPORT IN PLANTS AND ANIMALS

The human heart is a muscular organ which pumps blood to all parts of the body throughout life. The heart is located roughly in the chest cavity. It is surrounded by a protective membrane called the Pericardium.

When at rest, the heart contracts and relaxes about 70 times per minute. Thus, the contraction and relaxation of the heart is called Heart Beat. During exercise, the rate of the heart beat is very high. The muscular wall of the heart is made up of the Cardiac muscle. This muscle of the heart is involuntary.

Arteries carry blood away from the heart while veins return blood into the heart. Substances in the blood enter or leave the tissue through the Capillaries.

STRUCTURE OF THE MAMMALIAN HEART

The mammalian heart is four chambered, two auricles above and two ventricles below. Both auricles and ventricles have muscular walls which contract to pump blood. The left ventricle is thicker than the right ventricle because it pumps blood with sufficient pressure round the body. The Vena Cava enters the right auricle and the pulmonary vein enters the left auricle. The Pulmonary Artery leaves the right ventricle and the Aorta leaves the left ventricle. The right auricle is connected to the right ventricle, and the Tricuspid valve guards the entrance. The left auricle is connected to the left ventricle and the entrance is guarded by the bicuspid valve. Semilunar valves guard the exit to the pulmonary artery and the aorta. These valves maintain the one way flow of blood.

ARTERIES
An artery has thick muscular wall, smaller lumen, no valves and an elastic wall.

VEIN
A vein has thin muscular wall, valves, large lumen and the wall is less elastic.

CAPILLARIES
A capillary has a very thin wall, very narrow lumen, no pulse and no valve.

DIFFERENCES BETWEEN ARTERIES AND VEINS
ARTERIES VEINS
Have thick, elastic and muscular wall Have a thinner, less elastic and less muscular walls
Carry blood away from the heart Carry blood from the tissues to the heart
Carry oxygenated blood except the pulmonary artery Carry deoxygenated blood except the pulmonary vein
Have a pulse due to the presence of heart contraction Have no pulse
Have no valves Have valves to prevent backflow of blood
Are located deeper in the body Are located at the surface
Blood flows due to pressure from heart Blood flows due to contraction of the body muscle
Have smaller lumen hence high blood pressure Have larger lumen hence low blood pressure
Pink in colour Reddish in colour

MECHANISM OF TRANSPORT

TRANSPORT IN UNICELLULAR ORGANISMS
In amoeba and paramecium, the food is transported by cytoplasmic streaming and diffusion.

TRANSPORT IN MULTI CELLULAR ORGANISMS
In some invertebrates, e.g. snails, insects (grasshopper, cockroach), it is an open blood system while in other animals, e.g. earthworm, squid and vertebrates it’s a closed transport system.

(a) In a closed transport system, the blood is always retained within the system of blood vessels.

(b) In an open transport system, the blood is pumped by the heart through vessels into body spaces or cavities.

TRANSPORT IN HIGHER ANIMALS
In higher animals such as mammals, materials are transported by blood which is made to circulate by the pumping action of the heart.

NOTE
1. All veins carry blood in the heart. Most veins contain deoxygenated blood with the exception of the pulmonary veins which contain oxygenated blood.
2. All arteries carry blood away from the heart. Most arteries contain oxygenated blood with the exception of the pulmonary arteries which contain deoxygenated blood.
3. Blood passes through the heart twice before it completes one circuit through the body. This type of blood circulation is known as Double Circulation.

PULMONARY CIRCULATION AND SYSTEMIC CIRCULATION
Mammals display every pattern of double circulation, which involves the pulmonary and systemic circulation. In Pulmonary circulation, the blood flows from the heart to lungs and back to the heart, while in systemic circulation, blood flows from the heart to all parts of the body besides the lungs and back to the heart. The systemic circulation therefore transports products of digestion, oxygen, hormones, and other substances to all parts of the body and collects excretory products for removal. Thus in double circulation, the blood passes through the heart twice before it completes one circuit through the body.

CIRCULATION OF BLOOD
Immediately before a heart beats, the right auricle receives deoxygenated blood from two anterior venae cavae and a posterior vena cava from all parts of the body except the lungs while the left auricle receives oxygenated blood from the pulmonary veins arising from the two lungs.
A heart beat occur in two stages. At the first stage called Diastole, the two auricles contract. Hence, deoxygenated blood enters the right ventricle through the Tricuspid valve and oxygenated blood enters the left ventricle through the bicuspid valve.
At the second stage called Systole, the two ventricles contract, causing the deoxygenated blood to pass from the right ventricle into the pulmonary artery through the semi-lunar valve to the lungs and the oxygenated blood from the left ventricle passes into the aorta through the semilunar valve from where it is transported to all parts of the body.
The bicuspid and tricuspid valve prevent backward flow of blood into the auricles when the ventricles contract while the semilunar valves prevent backward flow of blood into the auricles when the ventricles relax.

NOTE
1. Hepatic portal vein transports digested food from the alimentary canal to the liver for distribution and storage.
2. The hepatic portal vein has a capillary at both ends.

TRANSPORT IN HIGHER PLANTS

Transport in higher plants involves these four stages as highlighted below;
1. Absorption and transport of water. (Root and Suction Pressures).
2. Absorption and transport of mineral salts.
3. Transpiration
4. Transportation of manufactured food (translocation).

ABSORPTION AND TRANSPORT OF MINERAL SALTS
Mineral salts in form of ions in soil water enter into the root hairs by diffusion since they are in higher concentrations in the soil solution than in the root cells. The ions from the root hairs move from cell to cell in the root hairs by diffusion across the root cortex to the xylem vessels. Mineral salts can also be drawn by the roots through active transport which involves the use of energy.

TRANSPIRATION
This is the loss of water vapour from the aerial parts of the plant to the atmosphere. It takes place mainly through the Stomata and may also take place through the lenticels and Cuticle.
As water is lost from leaves, it is replaced by water from the xylem vessels in the veins of the leaf and so more water is transported by xylem from below. This creates a stream-like water movement called the Transpiration Pull. Thus evaporation of water causes the transportation of more water upwards.

Transpiration is important to plants because it sets up the transpiration pull or stream that enables
a. Water to move up the xylem vessels, for distribution to all parts of the plant cells for cellular activities ; and
b. Mineral salts dissolved in water to be transported to the leaves for the synthesis of carbohydrates,
c. Transpiration also helps to cool plants in hot days since latent heat of vaporisation removed from the cells of the leaves brings about cooling.

Excess transpiration (this is when water loss exceeds its uptake by the root) may cause wilting of plants.

FACTORS THAT AFFECTS TRANSPIRATION
1. Air movement.
2. Air temperature.
3. Atmospheric humidity.
4. Light.

TRANSPORT OF FOOD (TRANSLOCATION)
1. Glucose produced during photosynthesis is translocated in the phloem by diffusion through the cytoplasm of the sieve cells to all parts of the plant where it is utilized for energy or stored in form of starch.
2. Amino acids produced in the leaves and in the roots are transported downwards from leaves or upwards from root to the cells where they are needed for growth and repairs.

SIMILARITIES BETWEEN TRANSPORT IN PLANTS AND ANIMALS
1. Tubular or cylindrical vessels are necessary in both plants and animals.
2. There is a liquid medium required for transportation in both plants and animals.
3. Materials or food nutrients and hormones transported are in dissolved forms or liquid solution forms in both plants and animals.
4. Diffusion plays a good role in transportation between both plants and animals.

DIFFERENCES BETWEEN TRANSPORT IN PLANTS AND ANIMALS
PLANTS ANIMALS
Cell sap medium for transport Blood medium is used for transpiration

Root pressure or transpiration generates transpiration pull Heart generates forces for transportation
Water or mineral salts and manufactured food transported through different vessels or xylem and phloem vessels Water, food substances and mineral nutrients are transported in the same vessel
Transport medium is not tissues Transport medium is made up of cells of different types or tissues

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