Physiology of Circulation, Respiration, and Plant Transport

Bohr Shift

In the cytoplasm of red blood cells, the enzyme carbonic anhydrase catalyzes the following reaction: CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid). The carbonic acid then dissociates: H₂CO₃ ⇌ H⁺ + HCO₃^-. Hemoglobin readily combines with the hydrogen ions, forming haemoglobinic acid (HHb), which releases the oxygen it is carrying.

The Cardiac Cycle

• 1. Atrial systole: Both atria contract. Pressure forces the atrioventricular valves open, and blood flows from the atria into the ventricles.
• 2. Ventricular systole: The ventricles contract. Atrioventricular valves are pulled shut as ventricular pressure exceeds atrial pressure. Semilunar valves are forced open, and blood rushes into the arteries.
• 3. Ventricular diastole: The heart muscle relaxes. Semilunar valves shut, and blood from the veins flows into the atria.

The Pacemaker and Heart Excitation

Each cardiac cycle begins in the right atrium. A small patch of muscle tissue called the sinoatrial node (SAN) automatically contracts and relaxes. As the SAN contracts, it produces an electrical excitation wave that causes the atrial walls to contract. The wave reaches the atrioventricular node (AVN), which delays the impulse for a fraction of a second before it travels into the ventricles. The excitation wave moves swiftly down the septum through Purkyne tissue. Once the wave arrives at the base of the ventricles, they contract, then relax, and the sequence repeats.

Water Movement Through the Leaf

Water vapor diffuses from an air space through an open stoma, a process called transpiration. It is carried away from the leaf surface by air movements, reducing water potential inside the leaf. Water evaporates from a mesophyll cell wall into the air space. Water moves through the mesophyll cell wall or out of the mesophyll cytoplasm into the cell wall. Water leaves a xylem vessel through a non-lignified area (a pit) and may enter the cytoplasm or cell wall of a mesophyll cell. Water moves up the xylem vessels to replace the water lost from the leaf.

Cohesion and Adhesion

The movement of water up through xylem vessels occurs by mass flow, where water molecules and dissolved solutes move together. This is facilitated by cohesion (hydrogen bonding between water molecules) and adhesion (attraction to cellulose and lignin in xylem walls). These forces maintain a continuous column of water.

Symplastic and Apoplastic Pathways

Symplastic Pathway

1. Water enters the cytoplasm by osmosis through the partially permeable cell surface membrane.
2. Water moves into the sap in the vacuole through the tonoplast by osmosis.
3. Water may move from cell to cell through the plasmodesmata.
4. Water may move from cell to cell through adjacent cell surface membranes and cell walls.

Apoplastic Pathway

1. Water enters the cell wall.
2. Water moves through the cell wall.
3. Water may move from cell wall to cell wall through intercellular spaces.
4. Water may move directly from cell wall to cell wall.

Blood Pressure and Composition

Blood pressure drops in arterioles from 120 to 85 mmHg as arteries branch out. Upon entering a capillary, the pressure drops to about 35 mmHg. Within the capillary, blood pressure further decreases (35 to 10 mmHg), allowing for slower flow and efficient material exchange.

Blood Composition

In 5 dm³ of blood (approx. 5 kg), the composition includes:

• Red Blood Cells (Erythrocytes): 2.5 x 10¹³
• Platelets (Thrombocytes): 6 x 10¹²
• White Blood Cells (Leukocytes): 5 x 10¹¹