'AB' blood group individuals have both the antigens A and B. Therefore they do not produce antibodies against 'A' group blood 'B' group blood or 'O' group (No antigens). Hence people with AB blood group are compatible with other groups (A, B and O) and are called "Universal Recipients".

Coronary artery supply blood to the heart.

Semilunar valves associated with the major arteries leaving the ventricles.

RBCs impart red colour to the blood due to the presence of the respiratory pigment hemoglobin.

Blood Groups
(i) The concept of blood grouping was discovered by Karl Landsteiner. He identified blood groups A, B and O. AB blood group was recognized by Decastello and Steini.
(ii) Human blood contains certain specific substances called agglutinogens or antigens (Ag) and agglutinins or antibodies (Ab). Antigens are found on the membrane surface of RBC. Antibodies are present in blood plasma. Based on the presence or absence of antigen and antibodies human blood group is classified into four groups called A, B, AB, and O. An individual has one of the four blood groups.
(iii) 'A' group individuals: Antigen 'A' is present on the surface of RBC and antibody b (anti-b) is present in the plasma.
(iv) 'B' group individuals: Antigen 'B' is present on the surface of RBC and antibody an (anti-a) is present in the plasma.
(v) 'AB' group individuals: Antigens A and B are present on the surface of RBC and antibodies are absent in the plasma.
(vi) 'O' group individuals: No antigen A or B are absent on the surface of RBC. However, the plasma contains both antibodies 'a' and 'b' (anti a and b).
Blood donation :
(i) In blood transfusion one must consider the antigen and antibody compatibility (matching) between the donor and the person receiving blood (recipient). When an individual receives a mismatched blood group from the donor agglutination (clumping) of blood occurs in the body which leads to death.

Each cardiac cycle lasts for about 0.8 seconds. The events during a single cardiac cycle involves,
a) Atrial systole                     - 0.1 sec
(Contraction of auricles)
b) Ventricular systole            - 0.3 sec
( Contraction of ventricles)
c) Ventricular diastole           - 0.4 sec
(Relaxation of ventricles)
Total time                              - 0.8 sec

Sino-atrial node is called the 'pacemaker' of heart because it is capable of initiating impulse which can stimulate the heart muscles to contract thereby pushing the blood into the ventricles.

(i) There are millions of root hairs on the tip of the root which absorb water and minerals by diffusion
(ii) Root hairs are thin walled, slender extension of epidermal cell that increase the surface area of absorption.
Pathway of water absorbed by roots:
(i) Once the water enters the root hairs, the concentration of water molecules in the root hair cells become more than that of the cortex.
(ii) Thus, water from the root hair moves to the cortical cells by osmosis and then reaches the xylem.
(iii) From there, the water is transported to the stem and leaves.

(i) During transpiration, water is lost from leaves.
(ii) Still transpiration is essential for the movement of water and minerals from the root to the healthy parts of the plant.
(iii) But excess transpiration may result in drying up of the leaves or wilting and loss of soil water.
(iv) Hence it is termed as a necessary evil.